Marine life

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As of 2023[update], more than 242,000 marine species have been documented, and perhaps two million marine species are yet to be documented. An average of 2,332 new species per year are being described.^[4][5]

Marine species range in size from the microscopic like

There is no life without water.^[10] It has been described as the universal solvent for its ability to dissolve many substances,^[11][12] and as the solvent of life.^[13] Water is the only common substance to exist as a solid, liquid, and gas under conditions normal to life on Earth.^[14] The Nobel Prize winner Albert Szent-Györgyi referred to water as the mater und matrix: the mother and womb of life.^[15]

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][16]}

Conventionally, the planet is divided into five separate oceans, but these oceans all connect into a single

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.^[21] The average salinity of Earth's oceans is about 35 grams (1.2 oz) of salt per kilogram of seawater (3.5% salt).^[22] Most of the salt in the ocean comes from the weathering and erosion of rocks on land.^[23] Some salts are released from volcanic activity or extracted from cool igneous rocks.^[24]

The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.

Altogether the ocean occupies 71 percent of the world surface,^[2] averaging nearly 3.7 kilometres (2.3 mi) in depth.^[28] By volume, the ocean provides about 90 percent of the living space on the planet.^[2] The science fiction writer Arthur C. Clarke has pointed out it would be more appropriate to refer to planet Earth as planet Ocean.^[29][30]

However, water is found elsewhere in the Solar System.

All organisms on Earth are descended from a common ancestor or ancestral gene pool.^[47][48] Highly energetic chemistry is thought to have produced a self-replicating molecule around 4 billion years ago, and half a billion years later the

Current species are a stage in the process of evolution, with their diversity the product of a long series of speciation and extinction events.[54] 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 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.^[55] 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.^[56][57]

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.^[58] 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.^[60] 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.^[61] 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.^[62]

Prokaryotes inhabited the Earth from approximately 3–4 billion years ago.^[63][64] No obvious changes in morphology or cellular organisation occurred in these organisms over the next few billion years.^[65] The eukaryotic cells emerged between 1.6 and 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.^[66][67] The engulfed bacteria and the host cell then underwent coevolution, with the bacteria evolving into either mitochondria or hydrogenosomes.^[68] Another engulfment of cyanobacterial-like organisms led to the formation of chloroplasts in algae and plants.^[69]

The history of life was that of the

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.^[73] Various triggers for the Cambrian explosion have been proposed, including the accumulation of oxygen in the atmosphere from photosynthesis.^[74]

About 500 million years ago, plants and fungi started colonising the land. Evidence for the appearance of the first

Estimates on the number of Earth's current species range from 10 million to 14 million,^[83] of which about 1.2 million have been documented and over 86 percent have not yet been described.^[84]

Microorganisms

microbial mats

cyanobacterial-algal mat.

Stromatolites

are formed from microbial mats as microbes slowly move upwards to avoid being smothered by sediment.

Many macroscopic animals and plants have microscopic juvenile stages. Some microbiologists also classify viruses (and viroids) as microorganisms, but others consider these as nonliving.^[86][87]

Microorganisms are crucial to nutrient recycling in

Microscopic life undersea is diverse and still poorly understood, such as for the role of viruses in marine ecosystems.^[90] Most marine viruses are bacteriophages, which are harmless to plants and animals, but are essential to the regulation of saltwater and freshwater ecosystems.^[91]: 5 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.^[91]: 593 Viral activity may also contribute to the biological pump, the process whereby carbon is sequestered in the deep ocean.^[92]

A stream of airborne microorganisms circles the planet above weather systems but below commercial air lanes.^[93] 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.^[94][95]

Microscopic organisms live throughout the

Viruses are found wherever there is life and have probably existed since living cells first evolved.[107] 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.^[108]

Viruses are now recognised as ancient and as having origins that pre-date the divergence of life into the

Bacteriophages (phages)

Multiple phages attached to a bacterial cell wall at 200,000× magnification

Diagram of a typical tailed phage

Opinions differ on whether viruses are a form of life or organic structures that interact with living organisms.^[111] 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^[111] and as "organisms at the edge of life".^[112]

Microorganisms make up about 70% of the marine biomass.[8] 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,^[115] which often kill other marine life.^[121] The number of viruses in the oceans decreases further offshore and deeper into the water, where there are fewer host organisms.^[92]

There are also

Once regarded as

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.^[65][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

• 
  The marine Thiomargarita namibiensis, the largest known bacterium
• 
  Cyanobacteria blooms can contain lethal cyanotoxins.
• 
  The

  endosymbiotic origin from cyanobacteria.^[132]
• 
  Bacteria can be beneficial. This

  Pompeii worm, an extremophile found only at hydrothermal vents

  , has a protective cover of bacteria.

The largest known bacterium, the marine Thiomargarita namibiensis, can be visible to the naked eye and sometimes attains 0.75 mm (750 μm).^[133][134]

Marine archaea

The

.

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.

• 
  Halobacteria

  , found in water near saturated with salt, are now recognised as archaea.
• 
  Flat, square-shaped cells of the archaea Haloquadratum walsbyi
• 
  Methanosarcina barkeri, a marine archaea that produces methane
• 
  Thermophiles, such as Pyrolobus fumarii, survive well over 100 °C.
• 
  Drawing of another marine thermophile, Pyrococcus furiosus

Marine protists

Main article: Marine protists

paraphyletic (lacking a common ancestor). Protists can be broadly divided into four groups depending on whether their nutrition is plant-like, animal-like, fungus-like,^[139] or a mixture of these.^[140]

Protists according to how they get food
Type of protist		Description	Example		Other examples
Plant-like	Algae (see below)	Autotrophic protists that make their own food without needing to consume other organisms, usually by using photosynthesis		Red algae, Cyanidium sp.	Green algae, brown algae, diatoms and some dinoflagellates. Plant-like protists are important components of phytoplankton discussed below.
Animal-like	Protozoans	Heterotrophic protists that get their food consuming other organisms		Radiolarian protist as drawn by Haeckel	amoebae, ciliates and flagellates .
Fungus-like	slime nets	Saprotrophic protists that get their food from the remains of organisms that have broken down and decayed		Marine slime nets form labyrinthine networks of tubes in which amoeba without pseudopods can travel	Marine lichen
Mixotropes	Various	osmotrophic protists that get their food from a combination of the above		Euglena mutabilis, a photosynthetic flagellate	Many marine mixotropes are found among protists, including among ciliates, Rhizaria and dinoflagellates [141]

micrograph

cell schematic

Choanoflagellates, unicellular "collared" flagellate protists, are thought to be the closest living relatives of the animals.^[142]

---

Getting to know our single-celled ancestors - MicroCosmos

Protists are highly diverse organisms currently organised into 18 phyla, but are not easy to classify.

mixotrophic protists, but recent studies in marine environments found mixotrophic protests contribute a significant part of the protist biomass.^[141]

• Single-celled and microscopic protists
• 
  Diatoms are a major algae group generating about 20% of world oxygen production.^[147]
• 
  Diatoms have glass like cell walls made of

  silica and called frustules.^[148]
• 
  Fossil diatom frustule from 32 to 40

  mya
• 
  Radiolarian
• 
  Single-celled alga, Gephyrocapsa oceanica
• 
  Two dinoflagellates
• 
  Zooxanthellae is a photosynthetic algae that lives inside hosts like coral.
• 
  A single-celled

  endosymbiotically

  .
• 
  Euglenoid
• 
  This ciliate is digesting cyanobacteria. The cytostome or mouth is at the bottom right.

In contrast to the cells of prokaryotes, the cells of eukaryotes are highly organised. Plants, animals and fungi are usually

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.^[149] Other marine protist are neither single-celled nor microscopic, such as seaweed

.

• Macroscopic protists (see also unicellular macroalgae →)
• 
  The single-celled

  giant amoeba has up to 1000 nuclei

  and reaches lengths of 5 mm (0.20 in).
• 
  testate amoeba which makes mud trails. Its diameter is up to 3.8 cm (1.5 in).^[150]
• 
  foraminiferan with an appearance and lifestyle that mimics a sponge

  , grows to 5 cm long.
• 
  The

  xenophyophore, another single-celled foraminiferan, lives in abyssal zones. It has a giant shell up to 20 cm (7.9 in) across.^[151]
• 
  Giant kelp, a brown algae

  , is not a true plant, yet it is multicellular and can grow to 50m.

Protists have been described as a taxonomic grab bag where anything that doesn't fit into one of the main biological kingdoms can be placed.^[152] Some modern authors prefer to exclude multicellular organisms from the traditional definition of a protist, restricting protists to unicellular organisms.^[153][154] This more constrained definition excludes seaweeds and slime molds.^[155]

Marine microanimals

See also:

Microanimal and Ichthyoplankton

External videos
Copepods: The Diatom-Devouring King of Plankton - Journey to the Microcosmos

As juveniles, animals develop from microscopic stages, which can include

secondary productivity and carbon sink of the world oceans than any other group of organisms.^[158][159] While mites are not normally thought of as marine organisms, most species of the family Halacaridae live in the sea.^[160]

• Marine microanimals
• 
  Over 10,000 marine species are copepods, small, often microscopic crustaceans
• 
  Darkfield photo of a gastrotrich

  , a worm-like animal living between sediment particles
• 
  Armoured Pliciloricus enigmaticus, about 0.2 mm long, live in spaces between marine gravel.
• 
  Drawing of a tardigrade (water bear) on a grain of sand
• 
  Rotifers, usually 0.1–0.5 mm long, may look like protists but have many cells and belongs to the Animalia.

Fungi

ascomycetous

fungi.

See also: Marine fungi, Mycoplankton, and Evolution of fungi

Over 1500 species of

hydrothermal areas of the ocean.^[164] A diverse range of unusual secondary metabolites is produced by marine fungi.^[165]

nutrient cycling.^[169] Mycoplankton can be up to 20 mm in diameter and over 50 mm in length.^[170]

A typical milliliter of seawater contains about 10³ to 10⁴ fungal cells.[171] This number is greater in coastal ecosystems and estuaries due to nutritional runoff from terrestrial communities. A higher diversity of mycoplankton is found around coasts and in surface waters down to 1000 metres, with a vertical profile that depends on how abundant phytoplankton is.^[172][173] This profile changes between seasons due to changes in nutrient availability.^[174] Marine fungi survive in a constant oxygen deficient environment, and therefore depend on oxygen diffusion by turbulence and oxygen generated by photosynthetic organisms.^[175]

Marine fungi can be classified as:^[175]

• Lower fungi - adapted to marine habitats (zoosporic fungi, including mastigomycetes: oomycetes and chytridiomycetes)
• Higher fungi - filamentous, modified to planktonic lifestyle (hyphomycetes, ascomycetes, basidiomycetes). Most mycoplankton species are higher fungi.^[172]

ascomycete, and an alga or a cyanobacterium. Several lichens are found in marine environments.^[176] Many more occur in the splash zone, where they occupy different vertical zones depending on how tolerant they are to submersion.^[177] Some lichens live a long time; one species has been dated at 8,600 years.^[178] However their lifespan is difficult to measure because what defines the same lichen is not precise.^[179] Lichens grow by vegetatively breaking off a piece, which may or may not be defined as the same lichen, and two lichens of different ages can merge, raising the issue of whether it is the same lichen.^[179]

The sea snail Littoraria irrorata damages plants of Spartina in the sea marshes where it lives, which enables spores of intertidal ascomycetous fungi to colonise the plant. The snail then eats the fungal growth in preference to the grass itself.^[180]

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.^[181] It has been hypothesized that mycoplankton evolved from terrestrial fungi, likely in the Paleozoic era (390 million years ago).^[182]

Origin of animals

Further information:

Evolutionary origin of animals, Avalon explosion, and Cambrian explosion

The earliest

shell or a hard exoskeleton

.

The earliest animal fossils may belong to the genus Dickinsonia,^[184] 571 million to 541 million years ago.^[185] Individual Dickinsonia typically resemble a bilaterally symmetrical ribbed oval. They kept growing until they were covered with sediment or otherwise killed,^[186] and spent most of their lives with their bodies firmly anchored to the sediment.^[187] Their taxonomic affinities are presently unknown, but their mode of growth is consistent with a bilaterian affinity.^[188]

Apart from Dickinsonia, the earliest widely accepted animal fossils are the rather modern-looking

Vendozoa.^[191] Others, however, have been interpreted as early molluscs (Kimberella^[192][193]), echinoderms (Arkarua^[194]), and arthropods (Spriggina,^[195] Parvancorina^[196]). 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.^[197]

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.^[198]

Body plans and phyla

Invertebrates are grouped into different

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.^[200]

WoRMS, 18 October 2019.^[201][202]

Middle Cambrian.^[203]

^{: 124–136}

Historically body plans were thought of as having evolved rapidly during the

phylogenetic definition).^[205]

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.^[198] 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.^[206] 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^[207]—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.^[208] 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.^[209]

Earliest animals

Further information: Animal § Phylogeny

The

Porifera, Ctenophora, Placozoa and Cnidaria. No member of these clades exhibit body plans with bilateral symmetry

.

	Choanoflagellata   unicellular protists thought to be the closest living relatives of animals 950 mya        Animals          Porifera       sponges – asymmetric   Ctenophora       comb jellies – biradial symmetry   Placozoa       simplest animals – asymmetric   Cnidaria       have tentacles with stingers – radial symmetry   bilaterians     all remaining animals – bilateral symmetry  →  760 mya

There has been much controversy over which invertebrate phyla, sponges or comb jellies, is the most basal.^[210] Currently, sponges are more widely considered to be the most basal.^[211][212]

Marine sponges

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

Monorhaphis chuni living about 11,000 years.^[214][215]

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.^[216]

• 
  Sponge biodiversity. There are four sponge species in this photo.
• 
  Branching vase sponge
• 
  Venus' flower basket at a depth of 2572 meters
• 
  Barrel sponge
• 
  The long-living

  Monorhaphis chuni

Linnaeus mistakenly identified sponges as plants in the order Algae.^[217] For a long time thereafter sponges were assigned to a separate subkingdom, Parazoa (meaning beside the animals).^[218] They are now classified as a paraphyletic phylum from which the higher animals have evolved.^[219]

Ctenophores

cilia (hairs or combs).^[220] Coastal species need to be tough enough to withstand waves and swirling sediment, but some oceanic species are so fragile and transparent that it is very difficult to capture them intact for study.^[221] In the past ctenophores were thought to have only a modest presence in the ocean, but it is now known they are often significant and even dominant parts of the planktonic biomass.^[222]

^: 269

The phylum has about 150 known species with a wide range of body forms. Sizes range from a few

crustaceans

but beroids prey on other ctenophores.

• 
  Light diffracting along the comb rows of a cydippid, left tentacle deployed, right retracted
• 
  Deep-sea ctenophore trailing tentacles studded with

  tentilla

  (sub-tentacles)
• 
  Egg-shaped

  cydippid

  ctenophore
• 
  Group of small benthic

  symbiotically

  on a starfish.
• 
  Lobata sp. with paired thick lobes
• 
  The

  sea walnut has a transient anus which forms only when it needs to defecate.^[223]

beroid

ctenophore, mouth gaping, preys on other ctenophores.

Early writers combined ctenophores with

biradial symmetry (half-turn rotational symmetry).^[224] The position of the ctenophores in the evolutionary family tree of animals has long been debated, and the majority view at present, based on molecular phylogenetics, is that cnidarians and bilaterians are more closely related to each other than either is to ctenophores.^[222]

^: 222

External videos
Iridescent red ctenophore — EVNautilus

Placozoa

undescribed species.^[230]

Placozoa have the simplest structure of all animals.

T. adhaerens

Trichoplax is a small, flattened, animal about one mm across and usually about 25 µm thick. Like the

cilia which the animal uses to help it creep along the seafloor.^[232]

Trichoplax feed by engulfing and absorbing food particles – mainly microbes and organic detritus – with their underside.

Marine cnidarians

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

anthozoans

are the evolutionary "aunts" or "sisters" of other cnidarians, and the most closely related to bilaterians.

protocol has been developed which can yield large numbers of embryos on a daily basis.^[236] There is a remarkable degree of similarity in the gene sequence conservation and complexity between the sea anemone and vertebrates.^[236] In particular, genes concerned in the formation of the head in vertebrates are also present in the anemone.^[237][238]

• 
  Sea anemones are common in

  tidepools

  .
• 
  Their tentacles sting and paralyse small fish.
• 
  Close up of polyps on the surface of a coral, waving their tentacles.
• 
  If an island sinks below the sea, coral growth can keep up with rising water and form an atoll.
• 
  The mantle of the

  red paper lantern jellyfish crumples and expands like a paper lantern.^[239]

• 
  The

  siphonophore
• 
  Marrus orthocanna another colonial siphonophore, assembled from two types of zooids.
• 
  hydroids^[240]
• 
  largest known jellyfish^[241]
• 
  Turritopsis dohrnii achieves biological immortality by transferring its cells back to childhood.^[242][243]
• 
  The

  sea wasp is the most lethal jellyfish in the world.^[244]

Bilateral invertebrate animals

Some of the earliest

bilaterian may have been a bottom dwelling worm with a single body opening.^[245] A bilaterian body can be conceptualized as a cylinder with a gut running between two openings, the mouth and the anus. Around the gut it has an internal body cavity, a coelom or pseudocoelom.^[a] Animals with this bilaterally symmetric body plan have a head (anterior) end and a tail (posterior) end as well as a back (dorsal) and a belly (ventral); therefore they also have a left side and a right side.^[246][247]

Having a front end means that this part of the body encounters stimuli, such as food, favouring

parasitic worms have extremely simplified body structures.^[246][247]

←  bilaterians	Xenacoelomorpha       basal bilaterians (lack a true gut)[245] Nephrozoa protostomes     develops mouth first  →  610 mya      deuterostomes     develops anus first  →  650 mya

Protostomes

See also: Embryological origins of the mouth and anus

embryos develop. In protostomes the first opening that develops becomes the mouth, while in deuterostomes it becomes the anus.^[251][252]

Protostomes	Ecdysozoa     mud dragons   arthropods   mainly crustaceans   roundworms   >529 mya Spiralia Gnathifera   rotifers   arrow worms   Platytrochozoa     flatworms   Lophotrochozoa     bivalves and cephalopods   ringed worms   550 mya   580 mya
(extant)

Marine worms

Further information: Marine worm and Sea worm

arrow worm

, found worldwide as a predatory component of plankton.

Hemichordata

. All worms, apart from the Hemichordata, are protostomes. The Hemichordata are deuterostomes and are discussed in their own section below.

The typical body plan of a worm involves long cylindrical tube-like bodies and no

Polygonoporus giganticus, found in the gut of sperm whales, can grow to over 30 m (100 ft).^[255][256]

Nematodes (roundworms) constitute a further worm phylum with tubular digestive systems and an opening at both ends.^[257][258] Over 25,000 nematode species have been described,^[259][260] of which more than half are parasitic. It has been estimated another million remain undescribed.^[261] 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.^[262] By count they represent 90% of all animals on the ocean floor.^[263] 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.^[264]

• 
  Giant tube worms cluster around hydrothermal vents

  .
• 
  pseudocoelomates

  which can parasite marine plants and animals.
• 
  Bloodworms

  are typically found on the bottom of shallow marine waters.

Marine molluscs

Hypothetical ancestral mollusc

See also: Evolution of molluscs and Evolution of cephalopods

anatomical

structure, but also in behaviour and in habitat.

NOAA

)

The mollusc phylum is divided into 9 or 10

ocean quahog clam has been reported as having lived 507 years^[270] making it the longest recorded life of all animals apart from colonial animals, or near-colonial animals like sponges.^[214]

• bivalves
• 
  Marine

  gastropods are sea snails or sea slugs. This nudibranch

  is a sea slug.
• 
  The sea snail Syrinx aruanus has a shell up to 91 cm long, the largest of any living gastropod.
• 
  Molluscs usually have eyes. Bordering the edge of the mantle of a

  bivalve mollusc, can be over 100 simple eyes

  .
• 
  Common mussel

  , another bivalve

Cephalopods include octopus, squid and cuttlefish. About 800 living species of marine cephalopods have been identified,^[271] and an estimated 11,000 extinct taxa have been described.^[272] They are found in all oceans, but there are no fully freshwater cephalopods.^[273]

• Cephalopods
• 
  The nautilus is a living fossil little changed since it evolved 500 million years ago as one of the first cephalopods.^{[274][275][276]}
• 
  Reconstruction of an

  mya

  .
• 
  Cephalopods, like this

  mantle cavity for jet propulsion

  .
• 
  Colossal squid, the largest of all invertebrates^[277]

Molluscs have such diverse shapes that many textbooks base their descriptions of molluscan anatomy on a generalized or

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.^[278][279]

Good evidence exists for the appearance of marine gastropods,

cephalopods and bivalves in the Cambrian period 538.8 to 485.4

million years ago.

Marine arthropods

Head

___________

Thorax

___________

Abdomen

___________

Segments and tagmata of an arthropod^{[278]: 518–52} The thorax bears the main locomotory appendages. The head and thorax are fused in some arthropods, such as crabs and lobsters.

Pneumodesmus newmani,^[280] lived in the Early Devonian.^[281]

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.

The evolutionary ancestry of arthropods dates back to the

basal relationships of arthropods with extinct phyla such as lobopodians have recently been debated.^[282][283]

Panarthropoda	tardigrades     water bears   Lobopodia   velvet worms     (terrestrial) arthropods   mainly crustaceans

Some palaeontologists think Lobopodia represents a basal grade which lead to an arthropod body plan.^[284]

Tardigrades (water bears) are a phylum of eight-legged, segmented microanimals able to survive in extreme conditions.

• Arthropod fossils and living fossils
• 
  Fossil

  Ma. They were highly successful and were found everywhere in the ocean for 270 Ma.^[285]
• 
  The Anomalocaris ("abnormal shrimp") was one of the first apex predators and first appeared about 515 Ma.
• 
  The largest known arthropod, the

  estuarine strata from about 390 Ma. It was up to 2.5 m (8.2 ft) long.^[286][287]
• 
  Xiphosurans, the group including modern Horseshoe crabs appeared around 480 Ma.^[288]

Extant marine arthropods range in size from the microscopic

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.

• Modern crustaceans
• 
  Many crustaceans are very small, like this tiny

  amphipod, and make up a significant part of the ocean's zooplankton

  .
• 
  The Japanese spider crab has the longest leg span of any arthropod, reaching 5.5 metres (18 ft) from claw to claw.^[289]
• 
  The Tasmanian giant crab is long-lived and slow-growing, making it vulnerable to overfishing.^[290]
• 
  Mantis shrimp have the most advanced eyes in the animal kingdom,^[291] and smash prey by swinging their club-like raptorial claws.^[292]

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.

Deuterostomes

See also: Evolution of brachiopods

In

protostomes.^[251][252] It is once considered that the earliest known deuterostomes are Saccorhytus fossils from about 540 million years ago.^[293] However, another study considered that Saccorhytus is more likely to be an ecdysozoan.^[294]

←  deuterostomes	ambulacrarians        echinoderms     hemichordates   chordates   cephalochordates     tunicates     vertebrates  →
(extant)

Echinoderms

bilateral symmetry. This is why they are in the Bilateria

.

chordates

.

Adult echinoderms are recognizable by their

pentamerism, a special type of radial symmetry) as adults.^[297]

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.

• 
  Echinoderm literally means "spiny skin", as this water melon sea urchin illustrates.
• 
  The

  keystone predator to be studied. They limit mussels which can overwhelm intertidal communities.^[298]
• 
  Colorful sea lilies in shallow waters

• 
  Sea cucumbers filter feed on plankton and suspended solids.
• 
  The sea pig, a deep water sea cucumber, is the only echinoderm that uses legged locomotion.
• 
  A benthopelagic and bioluminescent swimming sea cucumber, 3200 metres deep

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

.

Hemichordates

Gill (pharyngeal) slits

Gill slits have been described as "the foremost morphological innovation of early deuterostomes".^[299][300] In aquatic organisms, gill slits allow water that enters the mouth during feeding to exit. Some invertebrate chordates also use the slits to filter food from the water.^[301]

acorn worms and the Pterobranchia. Pterobranchia form a class containing about 30 species of small worm-shaped animals that live in secreted tubes on the ocean floor. Acorn worms form a class containing about 111 species that generally live in U-shaped burrows on the seabed, from the shoreline to a depth of 3000 metres. The worms lie there with the proboscis sticking out of one opening in the burrow, subsisting as deposit feeders or suspension feeders. It is supposed the ancestors of acorn worms used to live in tubes like their relatives, the Pterobranchia, but eventually started to live a safer and more sheltered existence in sediment burrows.^[302]

Some of these worms may grow to be very long; one particular species may reach a length of 2.5 metres (8 ft 2 in), although most acorn worms are much smaller.

Acorn worms are more highly specialised and advanced than other worm-like organisms. They have a circulatory system with a heart that also functions as a kidney. Acorn worms have gill-like structures they use for breathing, similar to the gills of fish. Therefore, acorn worms are sometimes said to be a link between classical invertebrates and vertebrates. Acorn worms continually form new gill slits as they grow in size, and some older individuals have more than a hundred on each side. Each slit consists of a branchial chamber opening to the pharynx through a U-shaped cleft. Cilia push water through the slits, maintaining a constant flow, just as in fish.^[303] Some acorn worms also have a postanal tail which may be homologous to the post-anal tail of vertebrates.

The three-section body plan of the acorn worm is no longer present in the vertebrates, except in the anatomy of the frontal neural tube, later developed into a brain divided into three parts. This means some of the original anatomy of the early chordate ancestors is still present in vertebrates even if it is not always visible. One theory is the three-part body originated from an early common ancestor of the deuterostomes, and maybe even from a common bilateral ancestor of both deuterostomes and protostomes. Studies have shown the gene expression in the embryo share three of the same signaling centers that shape the brains of all vertebrates, but instead of taking part in the formation of their neural system,^[304] they are controlling the development of the different body regions.^[305]

Marine chordates

The

Pikaiidae, Palaeospondylus, Zhongxiniscus and Vetulicolia

, might relate ancestrally to vertebrates.

• Invertebrate chordates are close relatives of vertebrates
• 
  The

  cephalochordate, is one of the closest living invertebrate relative of the vertebrates.^[307][308]
• 
  Tunicates, like these fluorescent-colored sea squirts, may provide clues to vertebrate and therefore human ancestry.^[309]
• 
  bioluminescent

  tunicates made up of hundreds of individuals.
• 
  Salp chain

The larval stage of the tunicate possesses all of the features characteristic of chordates: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail.^[301]

In the adult stage of the tunicate the notochord, nerve cord, and tail disappear.^[301]

Vertebrate animals

Skeletal

structures showing the vertebral column and internal skeleton running from the head to the tail.

Main article: Marine vertebrate

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 which 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 vertebral column.

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

Marine fish

Further information: Fish, diversity of fish, and evolution of fish

Fish typically breathe by extracting oxygen from water through

eyes well adapted to seeing underwater, as well as other sensory systems. Over 33,000 species of fish have been described as of 2017,^[310] of which about 20,000 are marine fish.^[311]

←  vertebrates	jawless fish    hagfish  lampreys jawed fish cartilaginous fish bony fish  →
(extant)

Jawless fish

Early fish had no

monophyletic sense. Others consider them a sister group of vertebrates in the common taxon of craniata.^[318]

The

jawless fish in the lineage of the lamprey,^[321][322] while in 2023 other researchers found 3D fossils scans did not support those conclusions.^[323]

• 
  Hagfish are the only known living animals with a skull but no vertebral column.
• 
  Lampreys are often parasitic and have a toothed, funnel-like sucking mouth.
• 
  The extinct

  jawed vertebrates

vertebrates

.

Around the start of the Devonian, fish started appearing with a deep remodelling of the vertebrate skull that resulted in a jaw.^[324] All vertebrate jaws, including the human jaw, have evolved from these early fish jaws. The appearance of the early vertebrate jaw has been described as "perhaps the most profound and radical evolutionary step in vertebrate history".^[325][326] Jaws make it possible to capture, hold, and chew prey. Fish without jaws had more difficulty surviving than fish with jaws, and most jawless fish became extinct during the Triassic period.

Cartilaginous fish

Main article:

Cartilaginous fish

fish with cartilaginous internal skeletons. 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 may looked much like a stocky version of the great white shark, but was much larger with estimated lengths reaching 20.3 metres (67 ft).^[327] Found in all oceans^[328] it was one of the largest and most powerful predators in vertebrate history,^[327] and probably had a profound impact on marine life.^[329] The Greenland shark has the longest known lifespan of all vertebrates, about 400 years.^[330] Some sharks such as the great white are partially warm blooded and give live birth. The manta ray, largest ray in the world, has been targeted by fisheries and is now vulnerable.^[331]

• Cartilaginous fishes
• 
  Cartilaginous fishes may have evolved from

  spiny sharks

  .
• 
  Stingray
• 
  Manta ray, the largest ray
• 
  endangered.^[332]

• 
  The extinct megalodon resembled a giant great white shark.
• 
  The Greenland shark lives longer than any other vertebrate.
• 
  The largest

  extant fish, the whale shark, is now a vulnerable species

  .

Bony fish

Further information:

Bony fish

Bony fish have jaws and skeletons made of

ray fins. The approximate dates in the phylogenetic tree are from Near et al., 2012^[333] and Zhu et al., 2009.^[334]

←  bony fish	lobe fins     coelacanths     lungfish     tetrapods  →  419 mya   ray fins      chondrosteans        (sturgeon, paddlefish, bichir, reedfish)   neopterygians     holosteans       (bowfin, gars) 275 mya   teleosts     all remaining fish (about 14,000 marine species) 310 mya 360 mya 400 mya
(extant)

Lobe fins have the form of fleshy lobes supported by bony stalks which extend from the body.^[335] Guiyu oneiros, the earliest-known bony fish, lived during the Late Silurian 419 million years ago. It has the combination of both ray-finned and lobe-finned features, although analysis of the totality of its features place it closer to lobe-finned fish.^[334] 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 remaining bony 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.

• The main distinguishing feature of the
  chondrosteans (sturgeon, paddlefish, bichir and reedfish) is the cartilaginous nature of their skeletons. The ancestors of the chondrosteans are thought to be bony fish, but the characteristic of an ossified skeleton was lost in later evolutionary development, resulting in a lightening of the frame.^[336]
• Neopterygians (from Greek for new fins) appeared sometime in the Late Permian, before dinosaurs. They were a very successful group of fish, because they could move more rapidly than their ancestors. Their scales and skeletons began to lighten during their evolution, and their jaws became more powerful and efficient.^[337]

Teleosts

homocercal tails

.

Main article: Teleost

About 96% of all modern fish species are teleosts,

cancellous bones of holostean fish.^[341]

Teleosts are found in almost all

gobies 8mm long ^[343] to ocean sunfish weighing over 2,000 kg.^[344]

The following images show something of the diversity in the shape and colour of modern marine teleosts...

• 
  Sailfish
• 
  Mahi-mahi
• 
  Eel
• 
  Seahorse

• 
  Ocean sunfish
• 
  Anglerfish
• 
  Pufferfish
• 
  Clown triggerfish
• 
  Mandarin dragonet

Nearly half of all extant vertebrate species are teleosts.^[345]

Marine tetrapods

See also:

Tetrapods and evolution of tetrapods

A

Devonian Period when their earliest ancestors emerged from the sea and adapted to living on land.^[346] 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.^[347][348] Tetrapods can be divided into four classes: amphibians, reptiles, birds and mammals

.

←  tetrapods	amphibians (there are no true marine amphibians) amniotes mammals sauropsids lepidosaurs (lizards, including snakes) archosaurs (turtles, crocodiles & birds)

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

Pelophylax esculentus reported in 2010.^[353]

Reptiles

Main article: Marine reptile

See also: Evolution of reptiles

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^[354] — 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

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.^[359]

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.^[360][361] Unlike land snakes, sea snakes have evolved flattened tails which help them swim.^[362]

• 
  Marine iguana
• 
  Leatherback sea turtle
• 
  Saltwater crocodile
• 
  Marine snakes have flattened tails.
• 
  The ancient

  Ichthyosaurus communis

  independently evolved flippers similar to dolphins.

Some

mass extinction at the end of the Cretaceous

.

Birds

Main article: Seabird

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,

bubble-wrap

, cushioning the impact with the water.

• 
  European herring gull attack herring schools from above.
• 
  Gentoo penguin swimming underwater
• 
  Albatrosses range over huge areas of ocean and some even circle the globe.
• 
  Gannets "divebomb" at high speed

The first marine birds evolved in the

period, and modern marine bird families emerged in the Paleogene

.

Mammals

Main article: Marine mammal

See also: Evolution of cetaceans, Evolution of sirenians, and List of marine mammal species

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

ichthyosaur.^[367]

• 
  Endangered blue whale, the largest living animal^[368]
• 
  The

  encephalization of any animal after humans^[369]
• 
  Beluga whale
• 
  Dugong grazing on seagrass
• 
  Walrus
• 
  Polar bear

Primary producers

Main article: marine primary production

See also: evolution of photosynthesis

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^{[370]: 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.^{[370]: 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, but the International Code of Nomenclature for algae, fungi, and plants now accepts both terms as equivalents.^[371]

Cyanobacteria

Cyanobacteria

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

The cyanobacterium genus Prochlorococcus is a major contributor to atmospheric oxygen.

Cyanobacteria were the first organisms to evolve an ability to turn sunlight into chemical energy. They form 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.^[372] 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.

dramatic change which redirected the evolution of the major animal and plant species.^[375]

The tiny marine cyanobacterium Prochlorococcus, discovered in 1986, forms today part of the base of the ocean food chain and accounts for much of the photosynthesis of the open ocean^[376] and an estimated 20% of the oxygen in the Earth's atmosphere.^[377] It is possibly the most plentiful genus on Earth: a single millilitre of surface seawater may contain 100,000 cells or more.^[378]

Originally, biologists classified cyanobacteria as algae, and referred to it as "blue-green algae". The more recent view is that cyanobacteria are bacteria, and hence are not even in the same

prokaryotes, and hence cyanobacteria from the definition of algae.^[379][380]

Algae

Diatoms

silica shell (frustule

) with radial (centric) or bilateral (pennate) symmetry.

Dinoflagellates

Armoured

Unarmoured

Traditionally dinoflagellates have been presented as armoured or unarmoured.

polyphyletic

. Marine algae can be divided into six groups:

• green algae, an informal group containing about 8,000 recognised species.^[381] 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.
• multicellular and including many notable seaweeds.^[382][383]
• multicellular and including many seaweeds, including kelp
• diatoms, a (disputed) phylum containing about 100,000 recognised species of mainly unicellular algae. Diatoms generate about 20 percent of the oxygen produced on the planet each year,^[147] take in over 6.7 billion metric tons of silicon each year from the waters in which they live,^[385] and contribute nearly half of the organic material found in the oceans. The shells (frustules) of dead diatoms can reach as much as half a mile deep on the ocean floor.^[386]
• phagotrophy).^[388] Some species are endosymbionts of marine animals and play an important part in the biology of coral reefs
  . Others predate other protozoa, and a few forms are parasitic.
• 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

macroalgae

.

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.^[389]

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.

• 
  Chlamydomonas globosa, a unicellular green alga with two

  flagella

  just visible at bottom left
• 
  endosymbiosis with a ciliate^[390]
• 
  Centric diatom
• 
  Dinoflagellates

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.^[391] They are among the most productive and dynamic ecosystems on Earth.^[392] Some Sargassum seaweeds are planktonic (free-floating). Like microalgae, macroalgae (seaweeds) are technically marine protists

since they are not true plants.

• 
  A seaweed is a macroscopic form of

  brown or green algae

  .
• 
  Sargassum seaweed is a planktonic brown alga with air bladders that help it float.
• 
  Sargassum fish are camouflaged to live among drifting Sargassum seaweed.

• Unicellular macroalgae (see also macroscopic protists ←)
• 
  The unicellular

  tidal zones. It can have a 4 cm diameter.^[393]
• 
  The unicellular

  mermaid's wineglass

  are mushroom-shaped algae that grow up to 10 cm high.
• 
  Killer algae are single-celled organisms, but look like ferns and grow stalks up to 80 cm long.^[394]

Unicellular organisms are usually microscopic, less than one tenth of a millimeter long. There are exceptions.

Mermaid's wineglass, a genus of subtropical green algae, is single-celled but remarkably large and complex in form with a single large nucleus, making it a model organism for studying cell biology.^[395] Another single celled algae, Caulerpa taxifolia, has the appearance of a vascular plant including "leaves" arranged neatly up stalks like a fern. Selective breeding in aquariums to produce hardier strains resulted in an accidental release into the Mediterranean where it has become an invasive species known colloquially as killer algae.^[396]

Origin of plants

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 marine plants, such as mangroves and seagrasses.^[397]

Marine plants can be found in

beach grass

might grow.

• 
  Mangroves
• 
  Seagrass meadow
• 
  Sea dragons camouflaged to look like floating seaweed live in kelp forests and seagrass meadows.^[398]

The total world area of mangrove forests was estimated in 2010 as 134,257 square kilometres (51,837 sq mi) (based on satellite data).[399]^[400] 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).^[401]

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.^[402]

Plankton and trophic interactions

Further information: Plankton, Bacterioplankton, Ichthyoplankton, and Mycoplankton

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.^[403] 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

autotrophic

(self-feeding), meaning they generate their own food and do not need to consume other organisms.

Phytoplankton consist mainly of microscopic photosynthetic

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.^[405]

Coccolithophores

...have plates called coccoliths

...extinct fossil

Coccolithophores build calcite skeletons important to the marine carbon cycle.^[406]

• Phytoplankton
• 
  Phytoplankton are the foundation of the ocean food chain.
• 
  Phytoplankton come in many shapes and sizes.
• 
  Diatoms are one of the most common types of phytoplankton.
• 
  Colonial phytoplankton

• 
  The cyanobacterium Prochlorococcus accounts for much of the ocean's primary production.
• 
  Green cyanobacteria scum washed up on a rock in California
• 
  Gyrodinium, one of the few naked dinoflagellates which lack armour
• 
  Zoochlorellae (green) living inside the ciliate

  Stichotricha secunda

• 
  Coccolithophores named after the BBC documentary series. The Blue Planet
• 
  The coccolithophore Emiliania huxleyi
• 
  Algae bloom

  of Emiliania huxleyi off the southern coast of England
• 
  Guinardia delicatula, a diatom responsible for algal blooms in the North Sea and the English Channel^[407]

Zooplankton

Radiolarians

Drawings by Haeckel 1904

heterotrophic

(other-feeding), meaning they cannot produce their own food and must consume instead other plants or animals as food. In particular, this means they eat phytoplankton.

Foraminiferans

...can have more than one nucleus

...and defensive spines

Foraminiferans are important unicellular zooplankton protists, with calcium shells.

Turing and radiolarian morphology

Shell of a spherical radiolarian

Shell micrographs

Computer simulations of Turing patterns on a sphere closely replicate some radiolarian shell patterns.^[408]

Zooplankton are generally larger than phytoplankton, mostly still microscopic but some can be seen with the naked eye. Many

molluscs, arthropods and tunicates, as well as planktonic arrow worms and bristle worms

.

Radiolarians are unicellular protists

with elaborate silica shells

Microzooplankton: major grazers of the plankton

• 
  Radiolarians come in many shapes.
• 
  Group of planktic

  foraminiferans
• 
  Copepods eat phytoplankton. This one is carrying eggs.
• 
  The dinoflagellate, Protoperidinium extrudes a large feeding veil to capture prey.

Larger zooplankton can be predatory on smaller zooplankton.

Macrozooplankton

---

• 
  Moon jellyfish
• 
  ctenophore
• 
  Arrow worm
• 
  Tomopteris, a planktonic segmented worm with unusual yellow bioluminescence^[409]
• 
  Marine

  amphipod
• 
  Krill
• 
  Pelagic sea cucumber

External videos
Venus Girdle - Youtube

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

meroplanktic

, that is, they are planktonic for only part of their life.

• Larvae and juveniles
• 
  Salmon larva hatching from its egg
• 
  Ocean sunfish larva
• 
  Juvenile planktonic squid
• 
  Larva stage of a spiny lobster

Mixotrophic plankton

Lingulodinium polyedrum

A suggested explanation for glowing seas^[410]

See also: Mixotrophic dinoflagellate

Dinoflagellates are often

mixotrophic or live in symbiosis

with other organisms.

• Mixoplankton
• 
  Tintinnid ciliate Favella
• 
  Euglena mutabilis, a photosynthetic flagellate
• 
  Noctiluca scintillans, a bioluminescence dinoflagellate

Some dinoflagellates are

cytoplasmic bodies which contain dinoflagellate luciferase, the main enzyme involved in the luminescence. The luminescence, sometimes called the phosphorescence of the sea, occurs as brief (0.1 sec) blue flashes or sparks when individual scintillons are stimulated, usually by mechanical disturbances from, for example, a boat or a swimmer or surf.^[413]

Marine food web

See also: Marine food web

Compared to terrestrial environments, marine environments have biomass pyramids which are inverted at the base. In particular, the biomass of consumers (copepods, krill, shrimp, forage fish) is larger than the biomass of primary producers. This happens because the ocean's primary producers are tiny phytoplankton which tend to be

K-strategists

that grow and reproduce slowly, so a much larger mass is needed to achieve the same rate of primary production.

Because of this inversion, it is the zooplankton that make up most of the marine animal

secondary consumers).^[414]

If phytoplankton dies before it is eaten, it descends through the

euphotic zone as part of the marine snow 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.^[415]

In 2010 researchers found whales carry nutrients from the depths of the ocean back to the surface using a process they called the

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.^[417]

Other interactions

Biogeochemical cycles

Marine biogeochemical cycles

The dominant feature of the planet viewed from space is water – oceans of liquid water flood most of the surface while water vapour swirls in atmospheric clouds and the poles are capped with ice.

Further information: Marine biogeochemical cycles, biological pump, and blue carbon

Taken as a whole, the oceans form a single marine system where water – the "universal solvent" ^[418] – dissolves nutrients and substances containing elements such as oxygen, carbon, nitrogen and phosphorus. These substances are endlessly cycled and recycled, chemically combined and then broken down again, dissolved and then precipitated or evaporated, imported from and exported back to the land and the atmosphere and the ocean floor. Powered both by the biological activity of marine organisms and by the natural actions of the sun and tides and movements within the Earth's crust, these are the marine biogeochemical cycles.^[419][420]

• 
  Marine carbon cycle^[421]
• 
  Oxygen cycle

• 
  Marine nitrogen cycle
• 
  Marine phosphorus cycle

Sediments and biogenic ooze

See also:

Protist shells

Sediments at the bottom of the ocean have two main origins, terrigenous and biogenous. Terrigenous sediments account for about 45% of the total marine sediment, and originate in the erosion of rocks on land, transported by rivers and land runoff, windborne dust, volcanoes, or grinding by glaciers.

Challenger Expedition.^[422] A biogenic ooze is a pelagic sediment

containing at least 30 percent from the skeletal remains of marine organisms.

Main types of biogenic ooze
type	mineral forms	protist responsible		name of skeleton	description
Siliceous ooze	SiO2 quartz glass opal chert	diatoms		frustule	Individual diatoms range in size from 0.002 to 0.2 mm.[423]
		radiolarians		skeleton	Radiolarians are protozoa with diameters typically between 0.1 and 0.2 mm that produce intricate mineral skeletons, usually made of silica
Calcareous ooze	CaCO3 calcite aragonite limestone chalk	foraminiferans		test	There are about 10,000 living species of foraminiferans,[424] usually under 1 mm in size.
		coccolithophores		coccolith	Coccolithophores are spherical cells usually less than 0.1 mm across, enclosed by calcareous plates called coccoliths. white cliffs of Dover .

• 
  An elaborate mineral skeleton of a radiolarian made of silica.
• 
  Diatoms, major components of marine plankton, also have silica skeletons called frustules.
• 
  Coccolithophores have plates or scales made with calcium carbonate called coccoliths
• 
  Calcified test

  of a planktic foraminiferan

• 
  A diatom microfossil from 40 million years ago
• 
  diatoms

  (click to magnify).
• 
  Illustration of a Globigerina ooze
• 
  Shells (tests), usually made of calcium carbonate, from a foraminiferal ooze on the deep ocean floor

Land interactions

See also: Freshwater ecosystem and Continental shelf pump

Land interactions impact marine life in many ways. Coastlines typically have

phytoplankton blooms

.

Water evaporated by the sun from the surface of the ocean can precipitate on land and eventually return to the ocean as

estuarine fish thrive. Overall, life in inland lakes can evolve with greater diversity than happens in the sea, because freshwater habitats are themselves diverse and compartmentalised in a way marine habitats are not. Some aquatic life, such as salmon and eels, migrate

back and forth between freshwater and marine habitats. These migrations can result in exchanges of pathogens and have impacts on the way life evolves in the ocean.

Anthropogenic impacts

Main article: Human impact on marine life

Human activities affect marine life and marine habitats through overfishing, pollution, acidification and the introduction of invasive species. These impact marine ecosystems and food webs and may result in consequences as yet unrecognised for the biodiversity and continuation of marine life forms.^[428]

Biodiversity and extinction events

Marine extinction intensity during Phanerozoic

%

Millions of years ago

(H)

K–Pg

Tr–J

P–Tr

Cap

Late D

O–S

Apparent extinction intensity, i.e. the fraction of

Holocene extinction event

)

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.^[430]

However, more than 99 percent of all species that ever lived on Earth, amounting to over five billion species,

stratigraphic

range compared to land organisms.

Based on the

Permian-Triassic extinction event, 251 million years ago. One generally estimates that the Big Five mass extinctions of the Phanerozoic (the last 540 million years) wiped out more than 40% of marine genera and probably more than 70% of marine species.^[436] The current Holocene extinction

caused by human activity, and now referred to as the "sixth extinction", may prove ultimately more devastating.

Investigating and Exploring Marine Life

Research and study

In order to perform research and enrich Marine Life knowledge, Scientists use various methods in-order to reach and explore the depths of the ocean. several Hi-tech instruments and vehicles are used for this purpose. [437]

• Autonomous Underwater Vehicles (AUVs)- Underwater robots used to explore the ocean. AUVs are independent robots and can explore unmanned. They are released from a ship and are operated from the surface. ^[438]
• Deep-Towed Vehicles (DTVs)- vehicles towed behind research vessels, offering a simpler alternative to more advanced underwater vehicles. They serve as versatile platforms for deploying oceanographic instruments to measure various ocean parameters, with specific models like the DTV BRIDGET used for studying hydrothermal vent plumes by moving near the ocean floor.^[439]
• Manned Submersibles- an manned underwater vehicle used for exploring, experimenting and is often used by army. ^[437]
• Research vessels (R/Vs)- a boat or ship used to conduct research over a ling period of time. It is capable of transporting a diverse range of sampling and surveying equipment. Research vessels typically feature on-board laboratory space, allowing researchers to promptly analyze the materials collected during cruises.
• Remotely Operated Vehicles (ROVs)- unmanned vehicles. able to reach greater depths under water in order to collect a wider variety of information. ^[437][440]

See also

• Blue Planet – 2001 British nature documentary television series - David Attenborough
  • Blue Planet II – 2017 British nature documentary television series
• Census of Marine Life – 10 year international marine biological program
• Colonization of land
• Marine larval ecology
• Taxonomy of invertebrates – System of classification of animals with emphasis on the invertebrates

Notes

1. Kaikō in March 1995 and is considered the most accurate measurement to date. See the Challenger Deep
   article for more details.
2. S2CID 5170702
   .

1. ^ The earliest Bilateria may have had only a single opening, and no coelom.^[245]

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Further reading

Wikivoyage has a travel guide for Marine life.

• Halpern BS, Walbridge S, Selkoe KA, Kappel CV, Micheli F, D'Agrosa C, et al. (February 2008). "A global map of human impact on marine ecosystems". Science. 319 (5865): 948–52.
  S2CID 26206024
  .
• Paleczny M, Hammill E, Karpouzi V, Pauly D (2015). "Population Trend of the World's Monitored Seabirds, 1950-2010". PLOS ONE. 10 (6): e0129342.
  PMID 26058068
  .
• Ruppert EE, Fox RS, Barnes RD (2004). Invertebrate Zoology (7th ed.). Brooks / Cole.
  ISBN 978-0-03-025982-1
  .
• "After 60 million years of extreme living, seabirds are crashing". The Guardian. 22 September 2015.

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