Marine life

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General characteristics of a large marine ecosystem (Gulf of Alaska)
Killer whales (orca) are marine apex predators that hunt many large species. Oceans also contain microscopic marine life, such as bacteria and phytoplankton

Marine life, or sea life or ocean life, is the plants, animals and other organisms that live in the salt water of the sea or ocean, or the brackish water of coastal estuaries. At a fundamental level, marine life affects the nature of the planet. Marine organisms produce oxygen. Shorelines are in part shaped and protected by marine life, and some marine organisms even help create new land.

Most life forms evolved initially in marine habitats. By volume, oceans provide about 90 percent of the living space on the planet.[1] The earliest vertebrates appeared in the form of fish, which live exclusively in water.[2] Some of these evolved into amphibians which spend portions of their lives in water and portions on land. Other fish evolved into land mammals and subsequently returned to the ocean as seals, dolphins or whales. Plant forms such as kelp and algae grow in the water and are the basis for some underwater ecosystems. Plankton, and particularly phytoplankton, are key primary producers forming the general foundation of the ocean food chain.

Marine invertebrates exhibit a wide range of modifications to survive in poorly oxygenated waters, including breathing tubes as in mollusc siphons. Fish have gills instead of lungs, although some species of fish, such as the lungfish, have both. Marine mammals, such as dolphins, whales, otters, and seals need to surface periodically to breathe air.

A total of 230,000 documented marine species exist, including about 20,000 species of marine fish, with some two million marine species yet to be documented.[3] Marine species range in size from the microscopic, including plankton and phytoplankton which can be as small as 0.02 micrometres, to huge cetaceans (whales, dolphins and porpoises), including the blue whale – the largest known animal reaching up to 33 metres (108 ft) in length.[4][5] Marine microorganisms, including bacteria and viruses, constitute about 70% of the total marine biomass.

Water[edit]

Elevation histogram showing the percentage of the Earth's surface above and below sea level

There is no life without water.[6] It has been described as the universal solvent for its ability to dissolve many substances,[7][8] and as the solvent of life.[9] Water is the only common substance to exist as a solid, liquid, and gas under conditions normal to life on Earth.[10] The Nobel Prize winner Albert Szent-Györgyi referred to water as the mater und matrix: the mother and womb of life.[11]

The abundance of surface water on Earth is a unique feature in the Solar System. Earth's hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000 metres (6,600 ft) The deepest underwater location is Challenger Deep of the Mariana Trench in the Pacific Ocean, having a depth of 10,900 metres (6.8 mi).[note 1][12]

The mass of the oceans is 1.35×1018 metric tons, or about 1/4400 of Earth's total mass. The oceans cover an area of 3.618×108 km2 with a mean depth of 3682 m, resulting in an estimated volume of 1.332×109 km3.[13] If all of Earth's crustal surface was at the same elevation as a smooth sphere, the depth of the resulting world ocean would be about 2.7 kilometres (1.7 mi).[14][15]

The Earth's water cycle

About 97.5% of the water on Earth is saline; the remaining 2.5% is fresh water. Most fresh water – about 69% – is present as ice in ice caps and glaciers.[16] The average salinity of Earth's oceans is about 35 grams (1.2 oz) of salt per kilogram of seawater (3.5% salt).[17] Most of the salt in the ocean comes from the weathering and erosion of rocks on land.[18] Some salts are released from volcanic activity or extracted from cool igneous rocks.[19]

The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.[20] Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.[21] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation.[22]

Jupiter's moon Europa may have an underground ocean which supports life.

Altogether the ocean occupies 71 percent of the world surface,[1] averaging nearly 3.7 kilometres (2.3 mi) in depth.[23] By volume, the ocean provides about 90 percent of the living space on the planet.[1] The science fiction writer Arthur C. Clarke has pointed out it would be more appropriate to refer to planet Earth as planet Ocean.[24][25]

However water is found elsewhere in the solar system. Europa, one of the moons orbiting Jupiter, is slightly smaller than the Earth's moon. There is a strong possibility a large saltwater ocean exists beneath its ice surface.[26] It has been estimated the outer crust of solid ice is about 10–30 km (6–19 mi) thick and the liquid ocean underneath is about 100 km (60 mi) deep.[27] This would make Europa's ocean over twice the volume of the Earth's ocean. There has been speculation Europa's ocean could support life,[28][29] and could be capable of supporting multicellular microorganisms if hydrothermal vents are active on the ocean floor.[30]

Evolution[edit]

The Earth is about 4.54 billion years old.[31][32][33] The earliest undisputed evidence of life on Earth dates from at least 3.5 billion years ago,[34][35] during the Eoarchean Era after a geological crust started to solidify following the earlier molten Hadean Eon. Microbial mat fossils have been found in 3.48 billion-year-old sandstone in Western Australia.[36][37][38] Other early physical evidence of a biogenic substance is graphite in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland[39] as well as "remains of biotic life" found in 4.1 billion-year-old rocks in Western Australia.[40][41] According to one of the researchers, "If life arose relatively quickly on Earth … then it could be common in the universe."[40]

All organisms on Earth are descended from a common ancestor or ancestral gene pool.[42][43] Highly energetic chemistry is thought to have produced a self-replicating molecule around 4 billion years ago, and half a billion years later the last common ancestor of all life existed.[44] The current scientific consensus is that the complex biochemistry that makes up life came from simpler chemical reactions.[45] The beginning of life may have included self-replicating molecules such as RNA[46] and the assembly of simple cells.[47] In 2016 scientists reported a set of 355 genes from the last universal common ancestor (LUCA) of all life, including microorganisms, living on Earth.[48]

Current species are a stage in the process of evolution, with their diversity the product of a long series of speciation and extinction events.[49] The common descent of organisms was first deduced from four simple facts about organisms: First, they have geographic distributions that cannot be explained by local adaptation. Second, the diversity of life is not a set of completely unique organisms, but organisms that share morphological similarities. Third, vestigial traits with no clear purpose resemble functional ancestral traits and finally, that organisms can be classified using these similarities into a hierarchy of nested groups—similar to a family tree.[50] However, modern research has suggested that, due to horizontal gene transfer, this "tree of life" may be more complicated than a simple branching tree since some genes have spread independently between distantly related species.[51][52]

Past species have also left records of their evolutionary history. Fossils, along with the comparative anatomy of present-day organisms, constitute the morphological, or anatomical, record.[53] By comparing the anatomies of both modern and extinct species, paleontologists can infer the lineages of those species. However, this approach is most successful for organisms that had hard body parts, such as shells, bones or teeth. Further, as prokaryotes such as bacteria and archaea share a limited set of common morphologies, their fossils do not provide information on their ancestry.

EuryarchaeotaNanoarchaeotaCrenarchaeotaProtozoaAlgaePlantaeSlime moldsAnimalFungusGram-positive bacteriaChlamydiaeChloroflexiActinobacteriaPlanctomycetesSpirochaetesFusobacteriaCyanobacteriaThermophilesAcidobacteriaProteobacteria
Evolutionary tree showing the divergence of modern species from their common ancestor in the centre.[54] The three domains are coloured, with bacteria blue, archaea green and eukaryotes red.

More recently, evidence for common descent has come from the study of biochemical similarities between organisms. For example, all living cells use the same basic set of nucleotides and amino acids.[55] The development of molecular genetics has revealed the record of evolution left in organisms' genomes: dating when species diverged through the molecular clock produced by mutations.[56] For example, these DNA sequence comparisons have revealed that humans and chimpanzees share 98% of their genomes and analysing the few areas where they differ helps shed light on when the common ancestor of these species existed.[57]

Prokaryotes inhabited the Earth from approximately 3–4 billion years ago.[58][59] No obvious changes in morphology or cellular organisation occurred in these organisms over the next few billion years.[60] The eukaryotic cells emerged between 1.6–2.7 billion years ago. The next major change in cell structure came when bacteria were engulfed by eukaryotic cells, in a cooperative association called endosymbiosis.[61][62] The engulfed bacteria and the host cell then underwent coevolution, with the bacteria evolving into either mitochondria or hydrogenosomes.[63] Another engulfment of cyanobacterial-like organisms led to the formation of chloroplasts in algae and plants.[64]

Phylogenetic and symbiogenetic tree of living organisms, showing a view of the origins of eukaryotes and prokaryotes

The history of life was that of the unicellular eukaryotes, prokaryotes and archaea until about 610 million years ago when multicellular organisms began to appear in the oceans in the Ediacaran period.[58][65] The evolution of multicellularity occurred in multiple independent events, in organisms as diverse as sponges, brown algae, cyanobacteria, slime moulds and myxobacteria.[66] In 2016 scientists reported that, about 800 million years ago, a minor genetic change in a single molecule called GK-PID may have allowed organisms to go from a single cell organism to one of many cells.[67]

Soon after the emergence of these first multicellular organisms, a remarkable amount of biological diversity appeared over a span of about 10 million years, in an event called the Cambrian explosion. Here, the majority of types of modern animals appeared in the fossil record, as well as unique lineages that subsequently became extinct.[68] Various triggers for the Cambrian explosion have been proposed, including the accumulation of oxygen in the atmosphere from photosynthesis.[69]

About 500 million years ago, plants and fungi started colonising the land. Evidence for the appearance of the first land plants occurs in the Ordovician, around 450 million years ago, in the form of fossil spores.[70] Land plants began to diversify in the Late Silurian, from around 430 million years ago.[71] The colonisation of the land by plants was soon followed by arthropods and other animals.[72] Insects were particularly successful and even today make up the majority of animal species.[73] Amphibians first appeared around 364 million years ago, followed by early amniotes and birds around 155 million years ago (both from "reptile"-like lineages), mammals around 129 million years ago, homininae around 10 million years ago and modern humans around 250,000 years ago.[74][75][76] However, despite the evolution of these large animals, smaller organisms similar to the types that evolved early in this process continue to be highly successful and dominate the Earth, with the majority of both biomass and species being prokaryotes.[77]

Estimates on the number of Earth's current species range from 10 million to 14 million,[78] of which about 1.2 million have been documented and over 86 percent have not yet been described.[79]

Microorganisms[edit]

microbial mats
Microbial mats are the earliest form of life on Earth for which there is good fossil evidence. The image shows a cyanobacterial-algal mat.
Stromatolites are formed from microbial mats as microbes slowly move upwards to avoid being smothered by sediment.

Microorganisms make up about 70% of the marine biomass.[80] A microorganism, or microbe, is a microscopic organism too small to be recognised with the naked eye. It can be single-celled[81] or multicellular. Microorganisms are diverse and include all bacteria and archaea, most protozoa such as algae, fungi and certain microscopic animals such as rotifers.

Many macroscopic animals and plants have microscopic juvenile stages. Some microbiologists also classify viruses (and viroids) as microorganisms, but others consider these as nonliving.[82][83]

Microorganisms are crucial to nutrient recycling in ecosystems as they act as decomposers. Some microorganisms are pathogenic, causing disease and even death in plants and animals.[84] As inhabitants of the largest environment on Earth, microbial marine systems drive changes in every global system. Microbes are responsible for virtually all the photosynthesis that occurs in the ocean, as well as the cycling of carbon, nitrogen, phosphorus and other nutrients and trace elements.[85]

The range of sizes shown by prokaryotes (bacteria and archaea) and viruses relative to those of other organisms and biomolecules


Marine microorganisms   

Viruses Bacteriophage2.png

  Prokaryotes  

Bacteria Prokaryote cell.svg

Archaea     Haloquadratum walsbyi00 (cropped).png

Eukaryotes

Protists Diatomee - Diatom (fossile) - Stephanopyxis sp. - 630x (14117161428).jpg

Microanimals   SEM image of Milnesium tardigradum in active state - journal.pone.0045682.g001-2.png

Microscopic life undersea is diverse and still poorly understood, such as for the role of viruses in marine ecosystems.[86] Most marine viruses are bacteriophages, which are harmless to plants and animals, but are essential to the regulation of saltwater and freshwater ecosystems.[87] They infect and destroy bacteria in aquatic microbial communities, and are the most important mechanism of recycling carbon in the marine environment. The organic molecules released from the dead bacterial cells stimulate fresh bacterial and algal growth.[88] Viral activity may also contribute to the biological pump, the process whereby carbon is sequestered in the deep ocean.[89]

Sea spray containing marine microorganisms can be swept high into the atmosphere and may travel the globe before falling back to earth.
Under a magnifier, a splash of seawater teems with life

A stream of airborne microorganisms circles the planet above weather systems but below commercial air lanes.[90] Some peripatetic microorganisms are swept up from terrestrial dust storms, but most originate from marine microorganisms in sea spray. In 2018, scientists reported that hundreds of millions of viruses and tens of millions of bacteria are deposited daily on every square meter around the planet.[91][92]

Microscopic organisms live throughout the biosphere. The mass of prokaryote microorganisms — which includes bacteria and archaea, but not the nucleated eukaryote microorganisms — may be as much as 0.8 trillion tons of carbon (of the total biosphere mass, estimated at between 1 and 4 trillion tons).[93] Single-celled barophilic marine microbes have been found at a depth of 10,900 m (35,800 ft) in the Mariana Trench, the deepest spot in the Earth's oceans.[94][95] Microorganisms live inside rocks 580 m (1,900 ft) below the sea floor under 2,590 m (8,500 ft) of ocean off the coast of the northwestern United States,[94][96] as well as 2,400 m (7,900 ft; 1.5 mi) beneath the seabed off Japan.[97] The greatest known temperature at which microbial life can exist is 122 °C (252 °F) (Methanopyrus kandleri).[98] In 2014, scientists confirmed the existence of microorganisms living 800 m (2,600 ft) below the ice of Antarctica.[99][100] According to one researcher, "You can find microbes everywhere — they're extremely adaptable to conditions, and survive wherever they are."[94]

Marine viruses[edit]

Viruses are small infectious agents that do not have their own metabolism and can replicate only inside the living cells of other organisms.[101] Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea.[102] The linear size of the average virus is about one one-hundredth that of the average bacterium. Most viruses cannot be seen with an optical microscope so electron microscopes are used instead.[103]

Viruses are found wherever there is life and have probably existed since living cells first evolved.[104] The origin of viruses is unclear because they do not form fossils, so molecular techniques have been used to compare the DNA or RNA of viruses and are a useful means of investigating how they arise.[105]

Viruses are now recognised as ancient and as having origins that pre-date the divergence of life into the three domains.[106] But the origins of viruses in the evolutionary history of life are unclear: some may have evolved from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity.[107]

Bacteriophages (phages)
Multiple phages attached to a bacterial cell wall at 200,000x magnification
Diagram of a typical tailed phage
These are cyanophages, viruses that infect cyanobacteria (scale bars indicate 100 nm)

Opinions differ on whether viruses are a form of life or organic structures that interact with living organisms.[108] They are considered by some to be a life form, because they carry genetic material, reproduce by creating multiple copies of themselves through self-assembly, and evolve through natural selection. However they lack key characteristics such as a cellular structure generally considered necessary to count as life. Because they possess some but not all such qualities, viruses have been described as replicators[109] and as "organisms at the edge of life".[110]

In terms of individual counts, tailed phage are the most abundant biological entities in the sea.

Bacteriophages, often just called phages, are viruses that parasite bacteria and archaea. Marine phages parasite marine bacteria and archaea, such as cyanobacteria.[111] They are a common and diverse group of viruses and are the most abundant biological entity in marine environments, because their hosts, bacteria, are typically the numerically dominant cellular life in the sea. Generally there are about 1 million to 10 million viruses in each mL of seawater, or about ten times more double-stranded DNA viruses than there are cellular organisms,[112][113] although estimates of viral abundance in seawater can vary over a wide range.[114][115] Tailed bacteriophages appear to dominate marine ecosystems in number and diversity of organisms.[111] Bacteriophages belonging to the families Corticoviridae,[116] Inoviridae[117] and Microviridae[118] are also known to infect diverse marine bacteria.

Microorganisms make up about 70% of the marine biomass.[80] It is estimated viruses kill 20% of this biomass each day and that there are 15 times as many viruses in the oceans as there are bacteria and archaea. Viruses are the main agents responsible for the rapid destruction of harmful algal blooms,[119] which often kill other marine life.[120] The number of viruses in the oceans decreases further offshore and deeper into the water, where there are fewer host organisms.[89]

There are also archaean viruses which replicate within archaea: these are double-stranded DNA viruses with unusual and sometimes unique shapes.[121][122] These viruses have been studied in most detail in the thermophilic archaea, particularly the orders Sulfolobales and Thermoproteales.[123]

Viruses are an important natural means of transferring genes between different species, which increases genetic diversity and drives evolution.[107] It is thought that viruses played a central role in the early evolution, before the diversification of bacteria, archaea and eukaryotes, at the time of the last universal common ancestor of life on Earth.[124] Viruses are still one of the largest reservoirs of unexplored genetic diversity on Earth.[89]

Marine bacteria[edit]

Vibrio vulnificus, a virulent bacterium found in estuaries and along coastal areas

Bacteria constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste,[125] and the deep portions of Earth's crust. Bacteria also live in symbiotic and parasitic relationships with plants and animals.

Once regarded as plants constituting the class Schizomycetes, bacteria are now classified as prokaryotes. Unlike cells of animals and other eukaryotes, bacterial cells do not contain a nucleus and rarely harbour membrane-bound organelles. Although the term bacteria traditionally included all prokaryotes, the scientific classification changed after the discovery in the 1990s that prokaryotes consist of two very different groups of organisms that evolved from an ancient common ancestor. These evolutionary domains are called Bacteria and Archaea.[126]

The ancestors of modern bacteria were unicellular microorganisms that were the first forms of life to appear on Earth, about 4 billion years ago. For about 3 billion years, most organisms were microscopic, and bacteria and archaea were the dominant forms of life.[127][128] Although bacterial fossils exist, such as stromatolites, their lack of distinctive morphology prevents them from being used to examine the history of bacterial evolution, or to date the time of origin of a particular bacterial species. However, gene sequences can be used to reconstruct the bacterial phylogeny, and these studies indicate that bacteria diverged first from the archaeal/eukaryotic lineage.[129] Bacteria were also involved in the second great evolutionary divergence, that of the archaea and eukaryotes. Here, eukaryotes resulted from the entering of ancient bacteria into endosymbiotic associations with the ancestors of eukaryotic cells, which were themselves possibly related to the Archaea.[62][130] This involved the engulfment by proto-eukaryotic cells of alphaproteobacterial symbionts to form either mitochondria or hydrogenosomes, which are still found in all known Eukarya. Later on, some eukaryotes that already contained mitochondria also engulfed cyanobacterial-like organisms. This led to the formation of chloroplasts in algae and plants. There are also some algae that originated from even later endosymbiotic events. Here, eukaryotes engulfed a eukaryotic algae that developed into a "second-generation" plastid.[131][132] This is known as secondary endosymbiosis.

The largest known bacterium, the marine Thiomargarita namibiensis, can be visible to the naked eye and sometimes attains 0.75 mm (750 μm).[134][135]

Marine archaea[edit]

Archaea were initially viewed as extremophiles living in harsh environments, such as the yellow archaea pictured here in a hot spring, but they have since been found in a much broader range of habitats.[136]

The archaea (Greek for ancient[137]) constitute a domain and kingdom of single-celled microorganisms. These microbes are prokaryotes, meaning they have no cell nucleus or any other membrane-bound organelles in their cells.

Archaea were initially classified as bacteria, but this classification is outdated.[138] Archaeal cells have unique properties separating them from the other two domains of life, Bacteria and Eukaryota. The Archaea are further divided into multiple recognized phyla. Classification is difficult because the majority have not been isolated in the laboratory and have only been detected by analysis of their nucleic acids in samples from their environment.

Archaea and bacteria are generally similar in size and shape, although a few archaea have very strange shapes, such as the flat and square-shaped cells of Haloquadratum walsbyi.[139] Despite this morphological similarity to bacteria, archaea possess genes and several metabolic pathways that are more closely related to those of eukaryotes, notably the enzymes involved in transcription and translation. Other aspects of archaeal biochemistry are unique, such as their reliance on ether lipids in their cell membranes, such as archaeols. Archaea use more energy sources than eukaryotes: these range from organic compounds, such as sugars, to ammonia, metal ions or even hydrogen gas. Salt-tolerant archaea (the Haloarchaea) use sunlight as an energy source, and other species of archaea fix carbon; however, unlike plants and cyanobacteria, no known species of archaea does both. Archaea reproduce asexually by binary fission, fragmentation, or budding; unlike bacteria and eukaryotes, no known species forms spores.

Archaea are particularly numerous in the oceans, and the archaea in plankton may be one of the most abundant groups of organisms on the planet. Archaea are a major part of Earth's life and may play roles in both the carbon cycle and the nitrogen cycle.

Marine protists[edit]

All living organisms can be grouped as prokaryotes or eukaryotes. Life originated as single-celled prokaryotes (bacteria and archaea) and later evolved into more complex eukaryotes. Eukaryotes are the more developed life forms known as plants, animals, fungi and protists. In contrast to the cells of prokaryotes, the cells of eukaryotes are highly organised.

The term "protist" is came into use historically as a term of convenience for eukaryotes that cannot be classified as plants or as animals or as fungi. They are not a part of modern cladistics, because they are paraphyletic (lacking a common ancestor). However, protists can be put into three groups depending on whether their nutrition is animal-like or plant-like or fungus-like

  • Algae (see below) are the plant-like protists. They are autotrophs that make their own food without needing to consume other organisms, usually by using photosynthesis
  • Protozoa are the animal-like protists. They are heterotrophs that need to get their food by consuming other organisms
  • Slime moulds and water moulds are the fungus-like protists. Like protozoa they are heterotrophs, but they get their food from the remains of life forms that have broken down and decayed.

Protists include amoebae, ciliates, red algae, euglena, slime molds, and phytoplankton such as diatoms and dinoflagellates. They are a highly diverse group of organisms currently organised into 18 phyla, but they are not easy to classify.[142][143] Studies have shown a high protist diversity exists in oceans, deep sea-vents and river sediments, which suggests a large number of eukaryotic microbial communities have yet to be discovered.[144][145]

Plants, animals and fungi are usually multi-celled and are typically macroscopic. Most protists are single-celled and microscopic. But there are exceptions. Some single-celled marine protists are macroscopic. Some marine slime molds have unique life cycles that involve switching between unicellular, colonial, and multicellular forms.[146] Other marine protist are neither single-celled nor microscopic, such as seaweed.

In common usage the term protist is often used loosely to refer to unicellular eukaryotic microorganisms, even though technically some protists are multicellular and/or macroscopic.

Marine microanimals[edit]

As juveniles, animals develop from microscopic stages, which can include spores, eggs and larvae. At least one microscopic animal group, the parasitic cnidarian Myxozoa, is unicellular in its adult form, and includes marine species. Other adult marine microanimals are multicellular. Microscopic adult arthropods are more commonly found inland in freshwater, but there are marine species as well. Microscopic adult marine crustaceans include some copepods, cladocera and water bears. Some marine nematodes and rotifers are also too small to be seen with the naked eye, as are many loricifera, including the recently discovered anaerobic species that spend their lives in an anoxic environment.[149][150] Copepods contribute more to the secondary productivity and carbon sink of the world oceans than any other group of organisms.

Fungi[edit]

Lichen on a rock in a marine splash zone. Lichens are mutualistic associations between a fungus and an alga or cyanobacterium.

Over 1500 species of fungi are known from marine environments.[151] These are parasitic on marine algae or animals, or are saprobes feeding on dead organic matter from algae, corals, protozoan cysts, sea grasses, wood and other substrata.[152] Spores of many species have special appendages which facilitate attachment to the substratum.[153] They can also be found in sea foam. A diverse range of unusual secondary metabolites is produced by marine fungi.[154]

Mycoplankton are saprotropic members of the plankton communities of marine and freshwater ecosystems.[155][156] They are composed of filamentous free-living fungi and yeasts that are associated with planktonic particles or phytoplankton.[157] Similar to bacterioplankton, these aquatic fungi play a significant role in heterotrophic mineralization and nutrient cycling.[158] Mycoplankton can be up to 20 mm in diameter and over 50 mm in length.[159]

In a typical milliliter of seawater, there are approximately 103 to 104 fungal cells.[160] This number is greater in coastal ecosystems and estuaries due to nutritional runoff from terrestrial communities. The greatest diversity and number of species of mycoplankton is found in surface waters (< 1000 m), and the vertical profile depends on the abundance of phytoplankton.[161][162] Furthermore, this difference in distribution may vary between seasons due to nutrient availability.[163] Marine fungi survive in a constant oxygen deficient environment, and therefore depend on oxygen diffusion by turbulence and oxygen generated by photosynthetic organisms.[164]

Marine fungi can be classified as:[164]

Lichens are mutualistic associations between a fungus, usually an ascomycete, and an alga or a cyanobacterium. Several lichens are found in marine environments.[165] Many more occur in the splash zone, where they occupy different vertical zones depending on how tolerant they are to submersion.[166]

According to fossil records, fungi date back to the late Proterozoic era 900-570 million years ago. Fossil marine lichens 600 million years old have been discovered in China.[167] It has been hypothesized that mycoplankton evolved from terrestrial fungi, likely in the Paleozoic era (390 million years ago).[168]

Invertebrate animals[edit]

Dickinsonia may be the earliest animal. They appear in the fossil record 571 million to 541 million years ago.

The earliest animals were marine invertebrates, that is, vertebrates came later. Animals are multicellular eukaryotes,[note 2] and are distinguished from plants, algae, and fungi by lacking cell walls.[169] Marine invertebrates are animals that inhabit a marine environment apart from the vertebrate members of the chordate phylum; invertebrates lack a vertebral column. Some have evolved a shell or a hard exoskeleton.

The earliest animal fossils may belong to the genus Dickinsonia,[170] 571 million to 541 million years ago.[171] Individual Dickinsonia typically resemble a bilaterally symmetrical ribbed oval. They kept growing until they were covered with sediment or otherwise killed,[172] and spent most of their lives with their bodies firmly anchored to the sediment.[173] Their taxonomic affinities are presently unknown, but their mode of growth is consistent with a bilaterian affinity.[174]

Apart from Dickinsonia, the earliest widely accepted animal fossils are the rather modern-looking cnidarians (the group that includes jellyfish, sea anemones and Hydra), possibly from around 580 Ma[175] The Ediacara biota, which flourished for the last 40 million years before the start of the Cambrian,[176] were the first animals more than a very few centimetres long. Like Dickinsonia, many were flat with a "quilted" appearance, and seemed so strange that there was a proposal to classify them as a separate kingdom, Vendozoa.[177] Others, however, have been interpreted as early molluscs (Kimberella[178][179]), echinoderms (Arkarua[180]), and arthropods (Spriggina,[181] Parvancorina[182]). There is still debate about the classification of these specimens, mainly because the diagnostic features which allow taxonomists to classify more recent organisms, such as similarities to living organisms, are generally absent in the Ediacarans. However, there seems little doubt that Kimberella was at least a triploblastic bilaterian animal, in other words, an animal significantly more complex than the cnidarians.[183]

Small shelly fauna are a very mixed collection of fossils found between the Late Ediacaran and Middle Cambrian periods. The earliest, Cloudina, shows signs of successful defense against predation and may indicate the start of an evolutionary arms race. Some tiny Early Cambrian shells almost certainly belonged to molluscs, while the owners of some "armor plates," Halkieria and Microdictyon, were eventually identified when more complete specimens were found in Cambrian lagerstätten that preserved soft-bodied animals.[184]

Body plans and phyla[edit]

Kimberella, an early mollusc important for understanding the Cambrian explosion. Invertebrates are grouped into different phyla (body plans).
Opabinia, an extinct stem group arthropod appeared in the Middle Cambrian[185]:124–136

Invertebrates are grouped into different phyla. Informally phyla can be thought of as a way of grouping organisms according to their body plan.[186][187]:33 A body plan refers to a blueprint which describes the shape or morphology of an organism, such as its symmetry, segmentation and the disposition of its appendages. The idea of body plans originated with vertebrates, which were grouped into one phylum. But the vertebrate body plan is only one of many, and invertebrates consist of many phyla or body plans. The history of the discovery of body plans can be seen as a movement from a worldview centred on vertebrates, to seeing the vertebrates as one body plan among many. Among the pioneering zoologists, Linnaeus identified two body plans outside the vertebrates; Cuvier identified three; and Haeckel had four, as well as the Protista with eight more, for a total of twelve. For comparison, the number of phyla recognised by modern zoologists has risen to 35.[187]

Historically body plans were thought of as having evolved rapidly during the Cambrian explosion,[188] but a more nuanced understanding of animal evolution suggests a gradual development of body plans throughout the early Palaeozoic and beyond.[189] More generally a phylum can be defined in two ways: as described above, as a group of organisms with a certain degree of morphological or developmental similarity (the phenetic definition), or a group of organisms with a certain degree of evolutionary relatedness (the phylogenetic definition).[189]

In the 1970s there was already a debate about whether the emergence of the modern phyla was "explosive" or gradual but hidden by the shortage of Precambrian animal fossils.[184] A re-analysis of fossils from the Burgess Shale lagerstätte increased interest in the issue when it revealed animals, such as Opabinia, which did not fit into any known phylum. At the time these were interpreted as evidence that the modern phyla had evolved very rapidly in the Cambrian explosion and that the Burgess Shale's "weird wonders" showed that the Early Cambrian was a uniquely experimental period of animal evolution.[190] Later discoveries of similar animals and the development of new theoretical approaches led to the conclusion that many of the "weird wonders" were evolutionary "aunts" or "cousins" of modern groups[191]—for example that Opabinia was a member of the lobopods, a group which includes the ancestors of the arthropods, and that it may have been closely related to the modern tardigrades.[192] Nevertheless, there is still much debate about whether the Cambrian explosion was really explosive and, if so, how and why it happened and why it appears unique in the history of animals.[193]

Arthropods total about 1,113,000 described extant species, molluscs about 85,000 and chordates about 52,000.[194][195]

Marine sponges[edit]

Sponges have no nervous, digestive or circulatory system

Sponges are animals of the phylum Porifera (Modern Latin for bearing pores[196]). They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them, consisting of jelly-like mesohyl sandwiched between two thin layers of cells. They have unspecialized cells that can transform into other types and that often migrate between the main cell layers and the mesohyl in the process. Sponges do not have nervous, digestive or circulatory systems. Instead, most rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes.

Sponges are similar to other animals in that they are multicellular, heterotrophic, lack cell walls and produce sperm cells. Unlike other animals, they lack true tissues and organs, and have no body symmetry. The shapes of their bodies are adapted for maximal efficiency of water flow through the central cavity, where it deposits nutrients, and leaves through a hole called the osculum. Many sponges have internal skeletons of spongin and/or spicules of calcium carbonate or silicon dioxide. All sponges are sessile aquatic animals. Although there are freshwater species, the great majority are marine (salt water) species, ranging from tidal zones to depths exceeding 8,800 m (5.5 mi).

While most of the approximately 5,000–10,000 known species feed on bacteria and other food particles in the water, some host photosynthesizing micro-organisms as endosymbionts and these alliances often produce more food and oxygen than they consume. A few species of sponge that live in food-poor environments have become carnivores that prey mainly on small crustaceans.[197]

Linnaeus mistakenly identified sponges as plants in the order Algae.[198] For a long time thereafter sponges were assigned to a separate subkingdom, Parazoa (meaning beside the animals).[199] They are now classified as a paraphyletic phylum from which the higher animals have evolved.[200]

Marine cnidarians[edit]

Cnidarians are the simplest animals with cells organised into tissues. Yet the starlet sea anemone contains the same genes as those that form the vertebrate head.

Cnidarians (Greek for nettle) are distinguished by the presence of stinging cells, specialized cells that they use mainly for capturing prey. Cnidarians include corals, sea anemones, jellyfish and hydrozoans. They form a phylum containing over 10,000[201] species of animals found exclusively in aquatic (mainly marine) environments. Their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick. They have two basic body forms: swimming medusae and sessile polyps, both of which are radially symmetrical with mouths surrounded by tentacles that bear cnidocytes. Both forms have a single orifice and body cavity that are used for digestion and respiration.

Fossil cnidarians have been found in rocks formed about 580 million years ago. Fossils of cnidarians that do not build mineralized structures are rare. Scientists currently think cnidarians, ctenophores and bilaterians are more closely related to calcareous sponges than these are to other sponges, and that anthozoans are the evolutionary "aunts" or "sisters" of other cnidarians, and the most closely related to bilaterians.

Cnidarians are the simplest animals in which the cells are organised into tissues.[202] The starlet sea anemone is used as a model organism in research.[203] It is easy to care for in the laboratory and a protocol has been developed which can yield large numbers of embryos on a daily basis.[204] There is a remarkable degree of similarity in the gene sequence conservation and complexity between the sea anemone and vertebrates.[204] In particular, genes concerned in the formation of the head in vertebrates are also present in the anemone.[205][206]

Marine worms[edit]

Many marine worms are related only distantly, so they form a number of different phyla. The worm shown is an arrow worm, found worldwide as a predatory component of plankton.

Worms (Old English for serpent) form a number of phylums. Their body plan typically involves long cylindrical tube-like bodies and no limbs. Marine worms vary in size from microscopic to over 1 metre (3.3 ft) in length for some marine polychaete worms (bristle worms)[212] and up to 58 metres (190 ft) for the marine nemertean worm (bootlace worm).[213] Some marine worms occupy a small variety of parasitic niches, living inside the bodies of other animals, while others live more freely in the marine environment or by burrowing underground.

Different groups of marine worms are related only distantly, so they are found in several different phyla such as the Annelida (segmented worms), Chaetognatha (arrow worms), Hemichordata, and Phoronida (horseshoe worms). Many of these worms have specialized tentacles used for exchanging oxygen and carbon dioxide and also may be used for reproduction. Some marine worms are tube worms, such as the giant tube worm which lives in waters near underwater volcanoes and can withstand temperatures up to 90 degrees Celsius.

Platyhelminthes (flatworms) form another worm phylum which includes a class Cestoda of parasitic tapeworms. The marine tapeworm Polygonoporus giganticus, found in the gut of sperm whales, can grow to over 30 m (100 ft).[214][215]

Nematodes (roundworms) constitute a further worm phylum with tubular digestive systems and an opening at both ends.[216][217] Over 25,000 nematode species have been described,[218][219] of which more than half are parasitic. It has been estimated another million remain undescribed.[220] They are ubiquitous in marine, freshwater and terrestrial environments, where they often outnumber other animals in both individual and species counts. They are found in every part of the earth's lithosphere, from the top of mountains to the bottom of oceanic trenches.[221] By count they represent 90% of all animals on the ocean floor.[222] Their numerical dominance, often exceeding a million individuals per square meter and accounting for about 80% of all individual animals on earth, their diversity of life cycles, and their presence at various trophic levels point at an important role in many ecosystems.[223]

Marine molluscs[edit]

Bigfin reef squid displaying vivid iridescence at night. Cephalopods are the most neurologically advanced invertebrates.[224]

Molluscs (Latin for soft) form a phylum with about 85,000 extant recognized species.[225] They are the largest marine phylum in terms of species count, comprising about 23% of all the named marine organisms.[226] Molluscs have more varied forms than other invertebrate phylums. They are highly diverse, not just in size and in anatomical structure, but also in behaviour and in habitat. The majority of species still live in the oceans, from the seashores to the abyssal zone, but some form a significant part of the freshwater fauna and the terrestrial ecosystems.

The mollusc phylum is divided into 9 or 10 taxonomic classes, two of which are extinct. These classes include gastropods, bivalves and cephalopods, as well as other lesser-known but distinctive classes. Gastropods with protective shells are referred to as snails, whereas gastropods without protective shells are referred to as slugs. Gastropods are by far the most numerous molluscs in terms of classified species, accounting for 80% of the total.[194] Bivalves include clams, oysters, cockles, mussels, scallops, and numerous other families. There are about 8,000 marine bivalves species (including brackish water and estuarine species), and about 1,200 freshwater species.

Cephalopods include octopus, squid and cuttlefish. About 800 living species of marine cephalopods have been identified,[227] and an estimated 11,000 extinct taxa have been described.[228] They are found in all oceans, but there are no fully freshwater cephalopods.[229]

Generalized or hypothetical ancestral mollusc

Molluscs have such diverse shapes that many textbooks base their descriptions of molluscan anatomy on a generalized or hypothetical ancestral mollusc. This generalized mollusc is unsegmented and bilaterally symmetrical with an underside consisting of a single muscular foot.[234][235]:484–628 Beyond that it has three further key features. Firstly, it has a muscular cloak called a mantle covering its viscera and containing a significant cavity used for breathing and excretion. A shell secreted by the mantle covers the upper surface.[235] Secondly (apart from bivalves) it has a rasping tongue called a radula used for feeding. Thirdly, it has a nervous system including a complex digestive system using microscopic, muscle-powered hairs called cilia to exude mucus. The generalized mollusc has two paired nerve cords (three in bivalves). The brain, in species that have one, encircles the esophagus. Most molluscs have eyes and all have sensors detecting chemicals, vibrations, and touch. The simplest type of molluscan reproductive system relies on external fertilization, but more complex variations occur. All produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. The depiction is rather similar to modern monoplacophorans, and some suggest it may resemble very early molluscs.[234]:284–291[234]:298–300[236][237]

Good evidence exists for the appearance of marine gastropods, cephalopods and bivalves in the Cambrian period 541 to 485.4 million years ago. However, the evolutionary history both of molluscs' emergence from the ancestral Lophotrochozoa and of their diversification into the well-known living and fossil forms are still subjects of vigorous debate among scientists.

Marine arthropods[edit]

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Segments and tagmata of an arthropod[238] The head and thorax are fused in some arthropods, such as crabs and lobsters.
Pneumodesmus newmani is the first known invertebrate to colonise land.[239] It lived in the Early Devonian

Arthropods (Greek for jointed feet) have an exoskeleton (external skeleton), a segmented body, and jointed appendages (paired appendages). They form a phylum which includes insects, arachnids, myriapods, and crustaceans. Arthropods are characterized by their jointed limbs and cuticle made of chitin, often mineralised with calcium carbonate. The arthropod body plan consists of segments, each with a pair of appendages. The rigid cuticle inhibits growth, so arthropods replace it periodically by moulting. Their versatility has enabled them to become the most species-rich members of all ecological guilds in most environments.

Extant marine arthropods range in size from the microscopic crustacean Stygotantulus to the Japanese spider crab. Arthropods' primary internal cavity is a hemocoel, which accommodates their internal organs, and through which their haemolymph - analogue of blood - circulates; they have open circulatory systems. Like their exteriors, the internal organs of arthropods are generally built of repeated segments. Their nervous system is "ladder-like", with paired ventral nerve cords running through all segments and forming paired ganglia in each segment. Their heads are formed by fusion of varying numbers of segments, and their brains are formed by fusion of the ganglia of these segments and encircle the esophagus. The respiratory and excretory systems of arthropods vary, depending as much on their environment as on the subphylum to which they belong.

Arthropod vision relies on various combinations of compound eyes and pigment-pit ocelli: in most species the ocelli can only detect the direction from which light is coming, and the compound eyes are the main source of information. Arthropods also have a wide range of chemical and mechanical sensors, mostly based on modifications of the many setae (bristles) that project through their cuticles. Arthropod methods of reproduction are diverse: terrestrial species use some form of internal fertilization while marine species lay eggs using either internal or external fertilization. Arthropod hatchlings vary from miniature adults to grubs that lack jointed limbs and eventually undergo a total metamorphosis to produce the adult form.

The evolutionary ancestry of arthropods dates back to the Cambrian period. The group is generally regarded as monophyletic, and many analyses support the placement of arthropods with cycloneuralians (or their constituent clades) in a superphylum Ecdysozoa. Overall however, the basal relationships of animals are not yet well resolved. Likewise, the relationships between various arthropod groups are still actively debated.

Echinoderms[edit]

Starfish larvae
Starfish larvae are bilaterally symmetric, whereas the adults have fivefold symmetry

Echinoderms (Greek for spiny skin) is a phylum which contains only marine invertebrates. The adults are recognizable by their radial symmetry (usually five-point) and include starfish, sea urchins, sand dollars, and sea cucumbers, as well as the sea lilies.[247] Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone. The phylum contains about 7000 living species,[248] making it the second-largest grouping of deuterostomes (a superphylum), after the chordates (which include the vertebrates, such as birds, fishes, mammals, and reptiles).

Echinoderms are unique among animals in having bilateral symmetry at the larval stage, but fivefold symmetry (pentamerism, a special type of radial symmetry) as adults.[249]

The echinoderms are important both biologically and geologically. Biologically, there are few other groupings so abundant in the biotic desert of the deep sea, as well as shallower oceans. Most echinoderms are able to regenerate tissue, organs, limbs, and reproduce asexually; in some cases, they can undergo complete regeneration from a single limb. Geologically, the value of echinoderms is in their ossified skeletons, which are major contributors to many limestone formations, and can provide valuable clues as to the geological environment. They were the most used species in regenerative research in the 19th and 20th centuries.

It is held by some scientists that the radiation of echinoderms was responsible for the Mesozoic Marine Revolution. Aside from the hard-to-classify Arkarua (a Precambrian animal with echinoderm-like pentamerous radial symmetry), the first definitive members of the phylum appeared near the start of the Cambrian.

Marine chordates[edit]

The chordate phylum has three subphylums, one of which is the vertebrates (see below). The other two subphylums are marine invertebrates: the tunicates (salps and sea squirts) and the cephalochordates (such as lancelets). Invertebrate chordates are close relatives to vertebrates. In particular, there has been discussion about how closely some extinct marine species, such as Pikaiidae, Palaeospondylus, Zhongxiniscus and Vetulicolia, might relate ancestrally to vertebrates.

Other phyla[edit]

Other phyla containing marine invertebrates include tardigrades and comb jellies, as well as extinct phyla such as lobopodians. Lobopodians are an extinct group of worm-like taxa with stubby legs that resemble modern velvet worms. They date from the Lower Cambrian.

Vertebrate animals[edit]

Marine tetrapod (sperm whale)
Skeletal structures showing the vertebral column running from the head to the tail

Vertebrates (Latin for joints of the spine) are a subphylum of chordates. They are chordates that have a vertebral column (backbone). The vertebral column provides the central support structure for an internal skeleton. The internal skeleton gives shape, support, and protection to the body and can provide a means of anchoring fins or limbs to the body. The vertebral column also serves to house and protect the spinal cord that lies within the column.

Marine vertebrates can be divided into marine fish and marine tetrapods.

Marine fish[edit]

Fish fall into two main groups: fish with bony internal skeletons and fish with cartilaginous internal skeletons. Fish anatomy and physiology generally includes a two-chambered heart, eyes adapted to seeing underwater, and a skin protected by scales and mucous. They typically breathe by extracting oxygen from water through gills. Fish use fins to propel and stabilise themselves in the water. Over 33,000 species of fish have been described as of 2017,[256] of which about 20,000 are marine fish.[257]

Jawless fish[edit]

Hagfish form a class of about 20 species of eel-shaped, slime-producing marine fish. They are the only known living animals that have a skull but no vertebral column. Lampreys form a superclass containing 38 known extant species of jawless fish.[258] The adult lamprey is characterized by a toothed, funnel-like sucking mouth. Although they are well known for boring into the flesh of other fish to suck their blood,[259] only 18 species of lampreys are actually parasitic.[260] Together hagfish and lampreys are the sister group to vertebrates. Living hagfish remain similar to hagfish from around 300 million years ago.[261] The lampreys are a very ancient lineage of vertebrates, though their exact relationship to hagfishes and jawed vertebrates is still a matter of dispute.[262] Molecular analysis since 1992 has suggested that hagfish are most closely related to lampreys,[263] and so also are vertebrates in a monophyletic sense. Others consider them a sister group of vertebrates in the common taxon of craniata.[264]

Pteraspidomorphi is an extinct class of early jawless fish ancestral to jawed vertebrates. The few characteristics they share with the latter are now considered as primitive for all vertebrates.

Cartilaginous fish[edit]

Cartilaginous fish, such as sharks and rays, have jaws and skeletons made of cartilage rather than bone. Megalodon is an extinct species of shark that lived about 28 to 1.5 Ma. It looked much like a stocky version of the great white shark, but was much larger with fossil lengths reaching 20.3 metres (67 ft).[265] Found in all oceans[266] it was one of the largest and most powerful predators in vertebrate history,[265] and probably had a profound impact on marine life.[267] The Greenland shark has the longest known lifespan of all vertebrates, about 400 years.[268] Some sharks such as the great white are partially warm blooded and give live birth.

Bony fish[edit]

Bony fish have jaws and skeletons made of bone rather than cartilage. About 90% of the world's fish species are bony fish. Bony fish also have hard, bony plates called operculum which help them respire and protect their gills, and they often possess a swim bladder which they use for better control of their buoyancy.

Bony fish can be further divided into those with lobe fins and those with ray fins. Lobe fins have the form of fleshy lobes supported by bony stalks which extend from the body.[271] Lobe fins evolved into the legs of the first tetrapod land vertebrates, so by extension an early ancestor of humans was a lobe-finned fish. Apart from the coelacanths and the lungfishes, lobe-finned fishes are now extinct. The rest of the modern fish have ray fins. These are made of webs of skin supported by bony or horny spines (rays) which can be erected to control the fin stiffness.


Marine tetrapods[edit]

Tiktaalik, an extinct lobe-finned fish, developed limb-like fins that could take it onto land

A tetrapod (Greek for four feet) is a vertebrate with limbs (feet). Tetrapods evolved from ancient lobe-finned fishes about 400 million years ago during the Devonian Period when their earliest ancestors emerged from the sea and adapted to living on land.[272] This change from a body plan for breathing and navigating in gravity-neutral water to a body plan with mechanisms enabling the animal to breath in air without dehydrating and move on land is one of the most profound evolutionary changes known.[273][274] Tetrapods can be divided into four classes: amphibians, reptiles, birds and mammals.

Marine tetrapods are tetrapods that returned from land back to the sea again. The first returns to the ocean may have occurred as early as the Carboniferous Period[275] whereas other returns occurred as recently as the Cenozoic, as in cetaceans, pinnipeds,[276] and several modern amphibians.[277]

Amphibians[edit]

Amphibians (Greek for both kinds of life) live part of their life in water and part on land. They mostly require fresh water to reproduce. A few inhabit brackish water, but there are no true marine amphibians.[278] There have been reports, however, of amphibians invading marine waters, such as a Black Sea invasion by the natural hybrid Pelophylax esculentus reported in 2010.[279]

Reptiles[edit]

Reptiles (Late Latin for creeping or crawling) do not have an aquatic larval stage, and in this way are unlike amphibians. Most reptiles are oviparous, although several species of squamates are viviparous, as were some extinct aquatic clades[280] — the fetus develops within the mother, contained in a placenta rather than an eggshell. As amniotes, reptile eggs are surrounded by membranes for protection and transport, which adapt them to reproduction on dry land. Many of the viviparous species feed their fetuses through various forms of placenta analogous to those of mammals, with some providing initial care for their hatchlings.

Some reptiles are more closely related to birds than other reptiles, and many scientists prefer to make Reptilia a monophyletic group which includes the birds.[281][282][283][284] Extant non-avian reptiles which inhabit or frequent the sea include sea turtles, sea snakes, terrapins, the marine iguana, and the saltwater crocodile. Currently, of the approximately 12,000 extant reptile species and sub-species, only about 100 of are classed as marine reptiles.[285]

Except for some sea snakes, most extant marine reptiles are oviparous and need to return to land to lay their eggs. Apart from sea turtles, the species usually spend most of their lives on or near land rather than in the ocean. Sea snakes generally prefer shallow waters nearby land, around islands, especially waters that are somewhat sheltered, as well as near estuaries.[286][287] Unlike land snakes, sea snakes have evolved flattened tails which help them swim.[288]

Some extinct marine reptiles, such as ichthyosaurs, evolved to be viviparous and had no requirement to return to land. Ichthyosaurs resembled dolphins. They first appeared about 245 million years ago and disappeared about 90 million years ago. The terrestrial ancestor of the ichthyosaur had no features already on its back or tail that might have helped along the evolutionary process. Yet the ichthyosaur developed a dorsal and tail fin which improved its ability to swim.[289] The biologist Stephen Jay Gould said the ichthyosaur was his favourite example of convergent evolution.[290] The earliest marine reptiles arose in the Permian. During the Mesozoic many groups of reptiles became adapted to life in the seas, including ichthyosaurs, plesiosaurs, mosasaurs, nothosaurs, placodonts, sea turtles, thalattosaurs and thalattosuchians. Marine reptiles were less numerous after mass extinction at the end of the Cretaceous.

Birds[edit]

Waterbird food web in Chesapeake Bay

Marine birds are adapted to life within the marine environment. They are often called seabirds. While marine birds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking convergent evolution, as the same environmental problems and feeding niches have resulted in similar adaptations. Examples include albatross, penguins, gannets, and auks.

In general, marine birds live longer, breed later and have fewer young than terrestrial birds do, but they invest a great deal of time in their young. Most species nest in colonies, which can vary in size from a few dozen birds to millions. Many species are famous for undertaking long annual migrations, crossing the equator or circumnavigating the Earth in some cases. They feed both at the ocean's surface and below it, and even feed on each other. Marine birds can be highly pelagic, coastal, or in some cases spend a part of the year away from the sea entirely. Some marine birds plummet from heights, plunging through the water leaving vapour-like trails, similar to that of fighter planes.[291] Gannets plunge into the water at up to 100 kilometres per hour (60 mph). They have air sacs under their skin in their face and chest which act like bubble-wrap, cushioning the impact with the water.

The first marine birds evolved in the Cretaceous period, and modern marine bird families emerged in the Paleogene.

Mammals[edit]

Sea otter, a classic keystone species which controls sea urchin numbers

Mammals (from Latin for breast) are characterised by the presence of mammary glands which in females produce milk for feeding (nursing) their young. There are about 130 living and recently extinct marine mammal species such as seals, dolphins, whales, manatees, sea otters and polar bears.[292] They do not represent a distinct taxon or systematic grouping, but are instead unified by their reliance on the marine environment for feeding. Both cetaceans and sirenians are fully aquatic and therefore are obligate water dwellers. Seals and sea-lions are semiaquatic; they spend the majority of their time in the water, but need to return to land for important activities such as mating, breeding and molting. In contrast, both otters and the polar bear are much less adapted to aquatic living. Their diet varies considerably as well: some may eat zooplankton; others may eat fish, squid, shellfish, and sea-grass; and a few may eat other mammals.

In a process of convergent evolution, marine mammals, especially cetaceans redeveloped their body plan to parallel the streamlined fusiform body plan of pelagic fish. Front legs became flippers and back legs disappeared, a dorsal fin reappeared and the tail morphed into a powerful horizontal fluke. This body plan is an adaptation to being an active predator in a high drag environment. A parallel convergence occurred with the now extinct marine reptile ichthyosaur.[293]

Primary producers[edit]

Composite image showing the global distribution of photosynthesis, including both oceanic phytoplankton and terrestrial vegetation. Dark red and blue-green indicate regions of high photosynthetic activity in the ocean and on land, respectively.

Primary producers are the autotroph organisms that make their own food instead of eating other organisms. This means primary producers become the starting point in the food chain for heterotroph organisms that do eat other organisms. Some marine primary producers are specialised bacteria and archaea which are chemotrophs, making their own food by gathering around hydrothermal vents and cold seeps and using chemosynthesis. However most marine primary production comes from organisms which use photosynthesis on the carbon dioxide dissolved in the water. This process uses energy from sunlight to convert water and carbon dioxide[296]:186–187 into sugars that can be used both as a source of chemical energy and of organic molecules that are used in the structural components of cells.[296]:1242 Marine primary producers are important because they underpin almost all marine animal life by generating most of the oxygen and food that provide other organisms with the chemical energy they need to exist.

The principal marine primary producers are cyanobacteria, algae and marine plants. The oxygen released as a by-product of photosynthesis is needed by nearly all living things to carry out cellular respiration. In addition, primary producers are influential in the global carbon and water cycles. They stabilize coastal areas and can provide habitats for marine animals. The term division has been traditionally used instead of phylum when discussing primary producers, although the International Code of Nomenclature for algae, fungi, and plants now accepts the terms as equivalent.[297]

Cyanobacteria[edit]

Cyanobacteria from a microbial mat. Cyanobacteria were the first organisms to release oxygen via photosynthesis

Cyanobacteria are a phylum (division) of bacteria which range from unicellular to filamentous and include colonial species. They are found almost everywhere on earth: in damp soil, in both freshwater and marine environments, and even on Antarctic rocks.[298] In particular, some species occur as drifting cells floating in the ocean, and as such were amongst the first of the phytoplankton.

The first primary producers that used photosynthesis were oceanic cyanobacteria about 2.3 billion years ago.[299][300] The release of molecular oxygen by cyanobacteria as a by-product of photosynthesis induced global changes in the Earth's environment. Because oxygen was toxic to most life on Earth at the time, this led to the near-extinction of oxygen-intolerant organisms, a dramatic change which redirected the evolution of the major animal and plant species.[301]

Prochlorococcus marinus

The tiny marine cyanobacterium Prochlorococcus, discovered in 1986, forms today part of the base of the ocean food chain and accounts for more than half the photosynthesis of the open ocean[302] and an estimated 20% of the oxygen in the Earth's atmosphere.[303] It is possibly the most plentiful genus on Earth: a single millilitre of surface seawater may contain 100,000 cells or more.[304]

Originally, biologists thought cyanobacteria was algae, and referred to it as "blue-green algae". The more recent view is that cyanobacteria is a bacteria, and hence is not even in the same Kingdom as algae. Most authorities exclude all prokaryotes, and hence cyanobacteria from the definition of algae.[305][306]

Algae[edit]

Algae is an informal term for a widespread and diverse group of photosynthetic eukaryotic organisms which are not necessarily closely related and are thus polyphyletic. Algae can be divided into six groups:

  • green algae, an informal group containing about 8,000 recognized species.[307] Many species live most of their lives as single cells or are filamentous, while others form colonies made up from long chains of cells, or are highly differentiated macroscopic seaweeds.
  • euglenophytes, a phylum of unicellular flagellates with only a few marine members

Unlike higher plants, algae lack roots, stems, or leaves. They can be classified by size as microalgae or macroalgae.

Microalgae[edit]

Microalgae are the microscopic types of algae, not visible to the naked eye. They are mostly unicellular species which exist as individuals or in chains or groups, though some are multicellular. Microalgae are important components of the marine protists discussed above, as well as the phytoplankton discussed below. They are very diverse. It has been estimated there are 200,000-800,000 species of which about 50,000 species have been described.[311] Depending on the species, their sizes range from a few micrometers (µm) to a few hundred micrometers. They are specially adapted to an environment dominated by viscous forces.

Macroalgae[edit]

Kelp forests are among the most productive ecosystems on Earth.

Macroalgae are the larger, multicellular and more visible types of algae, commonly called seaweeds. Seaweeds usually grow in shallow coastal waters where they are anchored to the seafloor by a holdfast. Seaweed that becomes adrift can wash up on beaches. Kelp is a large brown seaweed that forms large underwater forests covering about 25% of the world coastlines.[312] They are among the most productive and dynamic ecosystems on Earth.[313] Some Sargassum seaweeds are planktonic (free-floating). Like microalgae, macroalgae (seaweeds) are technically marine protists since they are not true plants.

Marine plants[edit]

Evolution of mangroves and seagrasses

Back in the Silurian, some phytoplankton evolved into red, brown and green algae. These algae then invaded the land and started evolving into the land plants we know today. Later, in the Cretaceous, some of these land plants returned to the sea as mangroves and seagrasses.[314]

Marine plants can be found in intertidal zones and shallow waters, such as seagrasses like eelgrass and turtle grass, Thalassia. These plants have adapted to the high salinity of the ocean environment. Plant life can also flourish in the brackish waters of estuaries, where mangroves or cordgrass or beach grass beach grass might grow.

The total world area of mangrove forests was estimated in 2010 as 134,257 square kilometres (51,837 sq mi) (based on satellite data).[316][317] The total world area of seagrass meadows is more difficult to determine, but was conservatively estimated in 2003 as 177,000 square kilometres (68,000 sq mi).[318]

Mangroves and seagrasses provide important nursery habitats for marine life, acting as hiding and foraging places for larval and juvenile forms of larger fish and invertebrates.

  • Spalding, M. (2010) World atlas of mangroves, Routledge. ISBN 9781849776608. doi:10.4324/9781849776608.

Plankton and trophic interactions[edit]

Six relatively large variously-shaped organisms with dozens of small light-colored dots all against a dark background. Some of the organisms have antennae that are longer than their bodies.
Plankton are drifting or floating organisms that cannot swim against a current, and include organisms from all the domains of life: bacteria, archaea, algae, protozoa and animal.

Plankton (from Greek for wanderers) are a diverse group of organisms that live in the water column of large bodies of water but cannot swim against a current. As a result, they wander or drift with the currents.[319] Plankton are defined by their ecological niche, not by any phylogenetic or taxonomic classification. They are a crucial source of food for many marine animals, from forage fish to whales. Plankton can be divided into a plant-like component and an animal component.

Phytoplankton[edit]

Widely distributed coccolithophores named after the BBC TV documentary series
The Blue Planet

Phytoplankton are the plant-like components of the plankton community ("phyto" comes from the Greek for plant). They are autotrophic (self-feeding), meaning they generate their own food and do not need to consume other organisms.

Phytoplankton consist mainly of microscopic photosynthetic eukaryotes which inhabit the upper sunlit layer in all oceans. They need sunlight so they can photosynthesize. Most phytoplankton are single-celled algae, but other phytoplankton are bacteria and some are protists.[320] Phytoplankton can be categorised into cyanobacteria (also called blue-green algae/bacteria), various types of algae (red, green, brown, and yellow-green), diatoms, dinoflagellates, euglenoids, coccolithophorids, cryptomonads, chrysophytes, chlorophytes, prasinophytes, and silicoflagellates. They form the base of the primary production that drives the ocean food web, and account for half of the current global primary production, more than the terrestrial forests.[321]

Zooplankton[edit]

Radiolarian protist drawn by Ernst Haeckel
in 1904

Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek for animal). They are heterotrophic (other-feeding), meaning they cannot produce their own own food and must consume instead other plants or animals as food. In particular, this means they eat phytoplankton.

Zooplankton are generally larger than phytoplankton, mostly still microscopic but some can be seen with the naked eye. Many protozoans (single-celled eukaryotes) are zooplankton, including dinoflagellates, zooflagellates, foraminiferans, and radiolarians. Some of these, such as dinoflagellates, can also be classified as phytoplankton; the distinction between plants and animals often breaks down in very small organisms. Other zooplankton include cnidarians, ctenophores, chaetognaths, molluscs, arthropods, urochordates, and annelids such as polychaetes.

Larger zooplankton can be predatory and eat smaller zooplankton.

Many marine animals begin life as zooplankton in the form of eggs or larvae, before they develop into adults.

Marine food web[edit]

Whale pump nutrient cycle

In 2010 researchers found whales carry nutrients from the depths of the ocean back to the surface using a process they called the whale pump.[323] Whales feed at deeper levels in the ocean where krill is found, but return regularly to the surface to breathe. There whales defecate a liquid rich in nitrogen and iron. Instead of sinking, the liquid stays at the surface where phytoplankton consume it. In the Gulf of Maine the whale pump provides more nitrogen than the rivers.[324]

Marine food web

If phytoplankton dies before it is eaten, it descends through the euphotic zone and settles into the depths of sea. In this way, phytoplankton sequester about 2 billion tons of carbon dioxide into the ocean each year, causing the ocean to become a sink of carbon dioxide holding about 90% of all sequestered carbon.[325]

Zooplankton make up most of the marine animal biomass, and as primary consumers are the crucial link between primary producers (mainly phytoplankton) and the rest of the marine food web (secondary consumers).[326]

Land interactions[edit]

The drainage basins of the principal oceans and seas of the world are marked by continental divides. The grey areas are endorheic basins that do not drain to the ocean.

Biogeochemical cycles[edit]

Energy gathering mechanism in marine bacteria via Proteorhodopsin[328]

In 2000 a team of microbiologists led by Edward DeLong made a crucial discovery in the understanding of the marine carbon and energy cycles. They discovered a gene in several species of bacteria[329][330] responsible for production of the protein rhodopsin, previously unheard of in the domain Bacteria. These proteins found in the cell membranes are capable of converting light energy to biochemical energy due to a change in configuration of the rhodopsin molecule as sunlight strikes it, causing the pumping of a proton from inside out and a subsequent inflow that generates the energy.[331]

Anthropogenic impacts[edit]

Fishing down the foodweb
Overfishing of high trophic fish like tuna can result in
them being replaced by low trophic organisms, like jellyfish
"It is almost as though we use our military to fight the animals in the ocean. We are gradually winning this war to exterminate them."

Daniel Pauly, pioneer on human impacts on global fisheries, [332]

Ocean acidification is the increasing acidification of the oceans, caused by the uptake of carbon dioxide from the atmosphere.[333] The rise in atmospheric carbon dioxide due to the burning of fossil fuels leads to more carbon dioxide dissolving in the ocean. When carbon dioxide dissolves in water it forms hydrogen and carbonate ions. This in turn increases the acidity of the ocean and makes survival increasingly harder for shellfish and other marine organisms that depend on calcium carbonate to form their shells.[334] Increasing acidity is also thought to have a range of potentially harmful consequences for marine organisms, such as depressing metabolic rates and immune responses in some organisms, and causing coral bleaching.[335] Ocean acidification has been compared to anthropogenic climate change and called the "evil twin of global warming"[336] and "the other CO
2
problem".[337]

Marine pollution results from the entry into the ocean of industrial, agricultural, and residential wastes.[338] Pathways for this pollution include agricultural runoff into rivers and wind-blown debris and dust. Nutrient pollution is a primary cause of eutrophication of surface waters, in which excess nutrients, usually nitrates or phosphates, stimulate algae growth. Toxic chemicals can adhere to tiny particles which are then taken up by plankton and benthic animals, most of which are either deposit feeders or filter feeders. In this way, toxins are concentrated upward within ocean food chains. Many particles combine chemically in a manner which depletes oxygen, causing estuaries to become anoxic. Pesticides and toxic metals are similarly incorporated into marine food webs, harming the biological health of marine life. Many animal feeds have a high fish meal or fish hydrolysate content. In this way, marine toxins are transferred back to farmed land animals, and then to humans.

Phytoplankton concentrations have increased over the last century in coastal waters, and more recently have declined in the open ocean. Increases in nutrient runoff from land may explain the increases in coastal phytoplankton, while warming surface temperatures in the open ocean may have strengthened stratification in the water column, reducing the flow of nutrients from the deep that open ocean phytoplankton find useful.[339]

Estimates suggest something like 9 million tonnes of plastic is added to the ocean every year. It is thought this plastic will need 450 years or more to biograde. Once in the ocean, plastics are shredded by marine amphipods into microplastics. There are now beaches where 15 percent of the sand are grains of microplastic. In the oceans themselves, microplastics float in surface waters amongst the plankton, where they are ingested by plankton eaters.[340]

Overfishing is occurring in one third of world fish stocks, according to a 2018 report by the Food and Agriculture Organization of the United Nations.[341] In addition, industry observers believe illegal, unreported and unregulated fishing occurs in most fisheries, and accounts for up to 30% of total catches in some important fisheries.[342] In a phenomenon called fishing down the foodweb, the mean trophic level of world fisheries has declined because of overfishing high trophic level fish.[343]

Habitat loss is occurring in seagrass meadows, mangrove forests, coral reefs and kelp forests, which are in global decline due to human disturbances. Seagrasses have lost 30,000 km2 (12,000 sq mi) during recent decades, one fifth of the world's mangrove forests have been lost since 1980,[344] and another fifth of coral reefs have been lost.[345] The most pressing threat to kelp forests may be the overfishing of coastal ecosystems, which by removing higher trophic levels facilitates their shift to depauperate urchin barrens.[346]

Shifting baselines arise in research on marine ecosystems because changes must be measured against some previous reference point (baseline), which in turn may represent significant changes from an even earlier state of the ecosystem.[347] For example, radically depleted fisheries have been evaluated by researchers who used the state of the fishery at the start of their careers as the baseline, rather than the fishery in its unexploited or untouched state. Areas that swarmed with a particular species hundreds of years ago may have experienced long term decline, but it is the level a few decades previously that is used as the reference point for current populations. In this way large declines in ecosystems or species over long periods of time were, and are, masked. There is a loss of perception of change that occurs when each generation redefines what is natural or untouched.[347]

Biodiversity and extinction events[edit]

Apparent marine fossil diversity during the Phanerozoic[348]
Extinction intensity.svgCambrianOrdovicianSilurianDevonianCarboniferousPermianTriassicJurassicCretaceousPaleogeneNeogene
Marine extinction intensity during the Phanerozoic
%
Millions of years ago
Extinction intensity.svgCambrianOrdovicianSilurianDevonianCarboniferousPermianTriassicJurassicCretaceousPaleogeneNeogene
Apparent extinction intensity, i.e. the fraction of genera going extinct at any given time as reconstructed from the fossil record (excluding the current Holocene extinction event)

Biodiversity is the result of over three billion years of evolution. Until approximately 600 million years ago, all life consisted of archaea, bacteria, protozoans and similar single-celled organisms. The history of biodiversity during the Phanerozoic (the last 540 million years), starts with rapid growth during the Cambrian explosion – a period during which nearly every phylum of multicellular organisms first appeared. Over the next 400 million years or so, invertebrate diversity showed little overall trend and vertebrate diversity shows an overall exponential trend.[349]

However, more than 99 percent of all species that ever lived on Earth, amounting to over five billion species,[350] are estimated to be extinct.[351][352] These extinctions occur at an uneven rate. The dramatic rise in diversity has been marked by periodic, massive losses of diversity classified as mass extinction events.[349] Mass extinction events occur when life undergoes precipitous global declines. Most diversity and biomass on earth is found among the microorganisms, which are difficult to measure. Recorded extinction events are therefore based on the more easily observed changes in the diversity and abundance of larger multicellular organisms, rather than the total diversity and abundance of life.[353] Marine fossils are mostly used to measure extinction rates because of their superior fossil record and stratigraphic range compared to land organisms.

Based on the fossil record, the background rate of extinctions on Earth is about two to five taxonomic families of marine animals every million years. The Great Oxygenation Event was perhaps the first major extinction event. Since the Cambrian explosion five further major mass extinctions have significantly exceeded the background extinction rate.[354] The worst was the Permian-Triassic extinction event, 251 million years ago. Vertebrates took 30 million years to recover from this event.[355] In addition to these major mass extinctions there are numerous minor ones, as well as the current ongoing mass-extinction caused by human activity, the Holocene extinction sometimes called the "sixth extinction".

Marine biology[edit]

In the fourth century BC Aristotle gave accurate descriptions of the embryological development of the hound shark Mustelus mustelus

During the sixth century BC, the Greek philosopher Xenophanes (570-475 BC) recognised that some fossil shells were remains of shellfish. He used this to argue that what was at the time dry land was once under the sea.[356] This was an important step in advancing from simply stating an idea to backing it with evidence and observation.[357]

Later, during the fourth century BC, another Greek philosopher Aristotle (384–322 BC) attempted a comprehensive classification of animals which included systematic descriptions of many marine species,[358][359] and particularly species found in the Mediterranean Sea.[360] These pioneering works include History of Animals, a general biology of animals, Parts of Animals, a comparative anatomy and physiology of animals, and Generation of Animals, on developmental biology. The most striking passages are about the sea-life visible from observation on Lesbos and available from the catches of fishermen. His observations on catfish, electric fish (Torpedo) and angler-fish are detailed, as is his writing on cephalopods, namely, Octopus, Sepia (cuttlefish) and the paper nautilus (Argonauta argo). His description of the hectocotyl arm, used in sexual reproduction, was widely disbelieved until its rediscovery in the 19th century. He separated aquatic mammals from fish, and knew that sharks and rays were part of a group he called Selachē (selachians).[361] He gave accurate descriptions of the ovoviviparous embryological development of the hound shark Mustelus mustelus.[362] His classification of living things contains elements which were still in use in the 19th century. What the modern zoologist would call vertebrates and invertebrates, Aristotle called "animals with blood" and "animals without blood" (he did not know that complex invertebrates do make use of hemoglobin, but of a different kind from vertebrates). He divided animals with blood into live-bearing (mammals), and egg-bearing (birds and fish). Invertebrates ("animals without blood") he divided into insects, crustacea (further divided into non-shelled – cephalopods – and shelled) and testacea (molluscs).[363][364]

In contemporary times, marine life is a field of study both in marine biology and in biological oceanography. In biology many phyla, families and genera have some species that live in the sea and others that live on land. Marine biology classifies species based on the environment rather than on taxonomy. For this reason marine biology encompasses not only organisms that live only in a marine environment, but also other organisms whose lives revolve around the sea. Biological oceanography is the study of how organisms affect and are affected by the physics, chemistry, and geology of the oceanographic system. Biological oceanography mostly focuses on the microorganisms within the ocean; looking at how they are affected by their environment and how that affects larger marine creatures and their ecosystem.[365] Biological oceanography is similar to marine biology, but it studies ocean life from a different perspective. Biological oceanography takes a bottom up approach in terms of the food web, while marine biology studies the ocean from a top down perspective. Biological oceanography mainly focuses on the ecosystem of the ocean with an emphasis on plankton: their diversity (morphology, nutritional sources, motility, and metabolism); their productivity and how that plays a role in the global carbon cycle; and their distribution (predation and life cycle).[365][366][367] Biological oceanography also investigates the role of microbes in food webs, and how humans impact the ecosystems in the oceans.[365]

See also[edit]

Notes[edit]

  1. ^ This is the measurement taken by the vessel Kaikō in March 1995 and is considered the most accurate measurement to date. See the Challenger Deep article for more details.
  2. ^ Myxozoa were thought to be an exception, but are now thought to be heavily modified members of the Cnidaria. Jímenez-Guri, Eva; Philippe, Hervé; Okamura, Beth; Holland, Peter W. H. (6 July 2007). "Buddenbrockia Is a Cnidarian Worm". Science. 317 (5834): 116–118. Bibcode:2007Sci...317..116J. doi:10.1126/science.1142024. ISSN 0036-8075. PMID 17615357. Retrieved 3 September 2008.

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