“Marine life”的意思、由来-开放百科全书

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 were believed to have appeared in the form of fish, which live exclusively in water. ^[1]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 (see insect and mollusc siphons) and gills. 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. Some amphibians are able to absorb oxygen through their skin.

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.^[2] 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 {{convert|33|m|ft}} in length.^[3]^[4] Marine microorganisms, including bacteria and viruses, constitute about 70% of the total marine biomass.

Water

There is no life without water – the "solvent of life".^[5] The Nobel Prize winner Albert Szent-Györgyi referred to water as the mater und matrix: the mother and womb of life.^[6]

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 {{convert|2000|m|ft}} The deepest underwater location is Challenger Deep of the Mariana Trench in the Pacific Ocean, having a depth of {{convert|10911|m|mi}}.^[9]

The mass of the oceans is approximately 1.35{{e|18}} metric tons, or about 1/4400 of Earth's total mass. The oceans cover an area of {{val|3.618|e=8|u=km2}} with a mean depth of {{val|3682|u=m}}, resulting in an estimated volume of {{val|1.332|e=9|u=km3}}.^[10] 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 {{convert|2.7|km|mi}}.^[7]^[8]

About 97.5% of the water on Earth is saline; the remaining 2.5% is fresh water. Most fresh water – about 68.7% – is present as ice in ice caps and glaciers.^[9] The average salinity of Earth's oceans is about {{convert|35|g|oz}} of salt per kilogram of seawater (3.5% salt).^[14] Most of the salt in the ocean comes from the weathering and erosion of rocks on land.^[10] Some salts are released from volcanic activity or extracted from cool igneous rocks.^[16] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.^[17] Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.^[18] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation.^[19]

Altogether the ocean occupies 71 percent of the world surface,^[11] averaging nearly {{convert|3.7|km|mi}} in depth.^[12] By volume, the ocean provides about 90 percent of the living space on the planet.^[11] The science fiction writer Arthur C. Clarke has pointed out it would be more appropriate to refer to planet Earth as planet Ocean.^[13]^[14]

Evolution

The Earth is about 4.54 billion years old.^[15]^[16]^[17] The earliest undisputed evidence of life on Earth dates from at least 3.5 billion years ago,^[18]^[19] 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.^[20]^[21]^[22] Other early physical evidence of a biogenic substance is graphite in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland^[23] as well as "remains of biotic life" found in 4.1 billion-year-old rocks in Western Australia.^[24]^[25] According to one of the researchers, "If life arose relatively quickly on Earth … then it could be common in the universe."^[24]

All organisms on Earth are descended from a common ancestor or ancestral gene pool.^[29]^[30]

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.^[31] The current scientific consensus is that the complex biochemistry that makes up life came from simpler chemical reactions.^[32] The beginning of life may have included self-replicating molecules such as RNA^[33] and the assembly of simple cells.^[34] In 2016 scientists reported a set of 355 genes from the last universal common ancestor (LUCA) of all life, including microorganisms, living on Earth.^[35]

Current species are a stage in the process of evolution, with their diversity the product of a long series of speciation and extinction events.^[36] 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.^[37] 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.^[38]^[39]

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.^[40] 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.

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.^[42] 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.^[43] 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.^[44]

Prokaryotes inhabited the Earth from approximately 3–4 billion years ago.^[45]^[46] No obvious changes in morphology or cellular organisation occurred in these organisms over the next few billion years.^[47] 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.^[48]^[49] The engulfed bacteria and the host cell then underwent coevolution, with the bacteria evolving into either mitochondria or hydrogenosomes.^[50] Another engulfment of cyanobacterial-like organisms led to the formation of chloroplasts in algae and plants.^[51]

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.^[45]^[52] The evolution of multicellularity occurred in multiple independent events, in organisms as diverse as sponges, brown algae, cyanobacteria, slime moulds and myxobacteria.^[53] In January 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.^[54]

Soon after the emergence of these first multicellular organisms, a remarkable amount of biological diversity appeared over approximately 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.^[55] Various triggers for the Cambrian explosion have been proposed, including the accumulation of oxygen in the atmosphere from photosynthesis.^[56]

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 {{Ma|450}}, in the form of fossil spores.^[57] Land plants began to diversify in the Late Silurian, from around {{Ma|430}}.^[58] The colonisation of the land by plants was soon followed by arthropods and other animals.^[59] Insects were particularly successful and even today make up the majority of animal species.^[60] 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.^[61]^[62]^[63] 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.^[64]

Estimates on the number of Earth's current species range from 10 million to 14 million,^[65] of which about 1.2 million have been documented and over 86 percent have not yet been described.^[66]

Marine microorganisms

Microorganisms make up about 70% of the marine biomass.^[67] A microorganism, or microbe, is a microscopic organism too small to be recognised with the naked eye. It can be single-celled^[68] 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.^[69]^[70]

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.^[71] 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.^[72]

Microscopic life undersea is diverse and still poorly understood, such as for the role of viruses in marine ecosystems.^[73] Most marine viruses are bacteriophages, which are harmless to plants and animals, and are essential to the regulation of saltwater and freshwater ecosystems.^[74] 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.^[75] Viral activity may also contribute to the biological pump, the process whereby carbon is sequestered in the deep ocean.^[76]

A stream of airborne microorganisms circles the planet above weather systems but below commercial air lanes.^[77] 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.^[78]^[79]

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).^[80] Barophilic marine microbes have been found at more than a depth of {{convert|10,000|m|ft mi|abbr=on}} in the Mariana Trench, the deepest spot in the Earth's oceans.^[81] Single-celled life forms have been found in the deepest part of the Mariana Trench, by the Challenger Deep, at depths of {{convert|11,034|m|ft mi|abbr=on}}.^[82]^[83]^[84] Microorganisms live inside rocks up to {{convert|580|m|ft mi|abbr=on}} below the sea floor under {{convert|2590|m|ft mi|abbr=on}} of ocean off the coast of the northwestern United States,^[83]^[85] as well as {{convert|2400|m|ft mi|abbr=on}} beneath the seabed off Japan.^[86] The greatest known temperature at which microbial life can exist is {{convert|122|°C|°F|abbr=on}} (Methanopyrus kandleri).^[87] In 2014, scientists confirmed the existence of microorganisms living {{convert|800|m|ft mi|abbr=on}} below the ice of Antarctica.^[88]^[89] According to one researcher, "You can find microbes everywhere — they're extremely adaptable to conditions, and survive wherever they are."^[83]

Marine viruses

Viruses are found wherever there is life and have probably existed since living cells first evolved.^[93] 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 arose.^[94]

Viruses are now recognised as ancient and as having origins that pre-date the divergence of life into the three domains.^[95] 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.^[110]

Opinions differ on whether viruses are a form of life or organic structures that interact with living organisms.^[96] 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^[97] and as "organisms at the edge of life".^[98]

Bacteriophages, often just called phages, are viruses that are parasitic on bacteria. Marine phages are obligate parasitic agents in marine bacteria such as cyanobacteria.^[99] 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,^[100]^[101] although estimates of viral abundance in seawater can vary over a wide range.^[102]^[103] Tailed bacteriophages appear to dominate marine ecosystems in number and diversity of organisms,^[99] and bacteriophages belonging to the families Corticoviridae,^[104] Inoviridae^[105] and Microviridae^[106] are also known to infect diverse marine bacteria.

Microorganisms make up about 70% of the marine biomass.^[107] 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,^[108] which often kill other marine life.^[109]

The number of viruses in the oceans decreases further offshore and deeper into the water, where there are fewer host organisms.^[76]

There are also archaean viruses which replicate within archaea: these are double-stranded DNA viruses with unusual and sometimes unique shapes.^[110]^[111] These viruses have been studied in most detail in the thermophilic archaea, particularly the orders Sulfolobales and Thermoproteales.^[112]

Viruses are an important natural means of transferring genes between different species, which increases genetic diversity and drives evolution.^[113] 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.^[114] Viruses are still one of the largest reservoirs of unexplored genetic diversity on Earth.^[76]

Marine bacteria

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,^[115] 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.^[116]

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.^[117]^[118] 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.^[119]

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.^[49]^[122] 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.^[123]^[124] This is known as secondary endosymbiosis.

The largest known bacterium, the marine Thiomargarita namibiensis, can be visible to the naked eye and sometimes attains {{convert|0.75|mm|µm|abbr=on}}.^[126]^[127]

Marine archaea

The archaea (Greek for ancient^[129]) 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.^[130] 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.^[131] 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

All living organisms can be grouped as either prokaryotes or eukaryotes. Life originated as single-celled prokaryotes and later evolved into the more complex eukaryotes. In contrast to prokaryotic cells, eukaryotic cells are highly organised. Prokaryotes are the bacteria and archaea, while eukaryotes are the other life forms — plants, animals, fungi and protists. Plants, animals or fungi are usually multi-celled. Protists are eukaryotes that cannot be classified as plants, animals or fungi. They are usually single-celled and microscopic.

Protists include amoebae, ciliates, red algae, euglena, phytoplankton such as diatoms and dinoflagellates, and slime molds. Several algae species are multicellular protists, and some marine slime molds have unique life cycles that involve switching between unicellular, colonial, and multicellular forms.^[132] Protists are a highly diverse group of organisms currently organised into 18 phyla, but they are not easy to classify.^[133]^[134] 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.^[135]^[136]

Marine microanimals

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.^[137]^[138] Copepods contribute more to the secondary productivity and carbon sink of the world oceans than any other group of organisms.

Marine algae and plants

Microscopic algae and plants provide important habitats for life, sometimes acting as hiding and foraging places for larval forms of larger fish and invertebrates. Algal life is widespread and very diverse under the ocean. Microscopic photosynthetic algae contribute a larger proportion of the world's photosynthetic output than all the terrestrial forests combined. Most of the niche occupied by sub plants on land is actually occupied by macroscopic algae in the ocean, such as Sargassum and kelp, which are commonly known as seaweeds that create kelp forests.

Plants that survive in the sea are often found in shallow waters, such as the seagrasses (examples of which are eelgrass, Zostera, and turtle grass, Thalassia). These plants have adapted to the high salinity of the ocean environment. The intertidal zone is also a good place to find plant life in the sea, where mangroves or cordgrass or beach grass might grow.

Marine fungi

Over 1500 species of fungi are known from marine environments.^[139] These are parasitic on marine algae or animals, or are saprobes on algae, corals, protozoan cysts, sea grasses, wood and other substrata, and can also be found in sea foam.^[140] Spores of many species have special appendages which facilitate attachment to the substratum.^[141] A very diverse range of unusual secondary metabolites is produced by marine fungi.^[142]

In a typical milliliter of seawater, there are approximately 10³ to 10⁴ fungal cells.^[148] 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.^[149]^[150] Furthermore, this difference in distribution may vary between seasons due to nutrient availability.^[151] Marine fungi survive in a constant oxygen deficient environment, and therefore depend on oxygen diffusion by turbulence and oxygen generated by photosynthetic organisms.^[152]

According to fossil records, fungi date back to the late Proterozoic era 900-570 million years ago. It has been hypothesized that mycoplankton evolved from terrestrial fungi, likely in the Paleozoic era (390 million years ago).^[156]

Invertebrates

The earliest animals were marine invertebrates, that is, vertebrates came later. Animals are multicellular eukaryotes, and are distinguished from plants, algae, and fungi by lacking cell walls.^[157] 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,^[158] 571 million to 541 million years ago.^[159] Individual Dickinsonia typically resemble a bilaterally symmetrical ribbed oval. They kept growing until they were covered with sediment or otherwise killed,^[160] and spent most of their lives with their bodies firmly anchored to the sediment.^[161] Their taxonomic affinities are presently unknown, but their mode of growth is consistent with a bilaterian affinity.^[162]

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 {{ma|580|Ma}}^[163] The Ediacara biota, which flourished for the last 40 million years before the start of the Cambrian,^[164] 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.^[165] Others, however, have been interpreted as early molluscs (Kimberella^[166]^[167]), echinoderms (Arkarua^[168]), and arthropods (Spriggina,^[169] Parvancorina^[170]). 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.^[171]

Phyla

Invertebrates are grouped into different phyla. Informally phyla can be thought of as a way of grouping organisms according to their body plan.^[174]^[175]{{rp|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.^[175]

Historically body plans were thought of as having evolved rapidly during the Cambrian explosion,^[176] but a more nuanced understanding of animal evolution suggests a gradual development of body plans throughout the early Palaeozoic and beyond.^[200] 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).^[177]

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.^[172] 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.^[178] 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^[179]—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.^[180] 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.^[181]

Marine sponges

Sponges are animals of the phylum Porifera (Modern Latin for bearing pores^[184]). 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 {{convert|8800|m|mi|abbr=on}}.

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.{{sfn|Vacelet|Duport|2004|pp=179–190}}

Marine cnidarians

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^[188] 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 {{ma|580}}. 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.

Marine worms

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 {{convert|1|m|ft}} in length for some marine polychaete worms (bristle worms)^[198] and up to {{convert|58|m|ft|}} for the marine nemertean worm (bootlace worm).^[199] 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.

Echinoderms

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.^[210] Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone. The phylum contains about 7000 living species,^[211] 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.^[212]

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. Further, 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 molluscs

Molluscs (Latin for soft) form a phylum with about 85,000 extant recognized species.^[214] They are the largest marine phylum in terms of species count, comprising about 23% of all the named marine organisms.^[215] 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.^[182] 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. Cephalopod include octopus, squid and cuttlefish. They are found in all oceans, and neurologically are the most advanced of the invertebrates.^[216] About 800 living species of marine cephalopods have been identified,^[217] and an estimated 11,000 extinct taxa have been described.^[218] There are no fully freshwater cephalopods.^[219]

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.^[224]^[225]{{rp|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.^[225] 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.^[224]{{rp|284–291}}^[224]{{rp|298–300}}^[226]^[227]

Good evidence exists for the appearance of marine gastropods, cephalopods and bivalves in the Cambrian period {{ma|Cambrian|{{period end|Cambrian}}}}. 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

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 Metazoa are not yet well resolved. Likewise, the relationships between various arthropod groups are still actively debated.

Others

Vertebrates

Fish

Jawless fish

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

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 {{convert|20.3|m|ft}}.^[276] Found in all oceans^[246] it was one of the largest and most powerful predators in vertebrate history,^[247] and probably had a profound impact on marine life.^[248] The Greenland shark has the longest known lifespan of all vertebrates, about 400 years.^[249]

Bony

Bony fish have jaws and skeletons made of bone rather than cartilage. Bony fish also have a hard, bony plate covering their gills called an operculum, and can have distinct rays, or spines, in their fins. About 90% of the world's fish species are bony fish.

Marine tetrapods

Amphibians

Reptiles

Unlike amphibians, reptiles do not have an aquatic larval stage. Most reptiles are oviparous, although several species of squamates are viviparous, as were some extinct aquatic clades^[259] — 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.

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.^[260]^[261] Unlike land snakes, sea snakes have evolved flattened tails which help them swim.^[262]

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.^[263] The biologist Stephen Jay Gould said the ichthyosaur was his favourite example of convergent evolution.^[264] 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

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.^[265] 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

There are about 130 living and recently extinct marine mammal species such as seals, dolphins, whales, manatees, sea otters and polar bears.^[266] 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, sea-grass and a few may eat other mammals.

In a process of convergent evolution, marine mammals such as dolphins and whales 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 ichthyosaur.^[267]

Plankton

Plankton are a diverse group of organisms that live in the water column of large bodies of water and that cannot swim against a current.^[270] They provide a crucial source of food to many large aquatic organisms, such as fish and whales.

These organisms include drifting or floating bacteria, archaea, algae, protozoa and animals that inhabit, for example, the pelagic zone of oceans, seas, or bodies of fresh water. Essentially, plankton are defined by their ecological niche rather than any phylogenetic or taxonomic classification.

Though many planktonic species are microscopic in size, plankton includes organisms covering a wide range of sizes, including large organisms such as jellyfish.^[271]

The role of phytoplankton is better understood due to their critical position as the most numerous primary producers on Earth. Phytoplankton are categorized 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.

Zooplankton tend to be somewhat larger, and not all are microscopic. Many Protozoa are zooplankton, including dinoflagellates, zooflagellates, foraminiferans, and radiolarians. Some of these (such as dinoflagellates) are also 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.

Many larger animals begin their life as zooplankton before they become large enough to take their adult forms.

Land interactions

Food webs

Biogeochemical cycles

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^[275]^[276] 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.^[277]

Biodiversity and extinction events

However, more than 99 percent of all species that ever lived on Earth, amounting to over five billion species,^[280] are estimated to be extinct.^[281]^[282] 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.^[279] 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.^[283] 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.^[284] The worst was the Permian-Triassic extinction event, 251 million years ago. Vertebrates took 30 million years to recover from this event.^[285] 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

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.^[286] This was an important step in advancing from simply stating an idea to backing it with evidence and observation.^[287]

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,^[288]^[289] and particularly species found in the Mediterranean Sea.^[290] 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).^[291] He gave accurate descriptions of the ovoviviparous embryological development of the hound shark Mustelus mustelus.^[292] 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).^[293]^[294]

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.^[295] 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).^[295]^[296]^[297] Biological oceanography also investigates the role of microbes in food webs, and how humans impact the ecosystems in the oceans.^[295]

See also

Notes

1. ^https://www.livescience.com/46262-fossil-ancestor-jawed-vertebrates.html
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6. ^Collins J. C. (1991) [https://books.google.com/books?id=Rm_wAAAAMAAJ&q=%22materund+matrix%22&dq=%22materund+matrix%22&hl=en&sa=X&redir_esc=y The Matrix of Life: A View of Natural Molecules from the Perspective of Environmental Water] Molecular Presentations. {{ISBN|9780962971907}}.
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