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Anthozoa
Temporal range: 570–0 Ma Late Ediacaran to recent
Coral outcrop on the
Great Barrier Reef
Gorgonian with polyps expanded
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Cnidaria
Subphylum: Anthozoa
Ehrenberg, 1834
Classes

Anthozoa[1] is one of the three subphyla of Cnidaria, along with Medusozoa and Endocnidozoa.[2] It includes sessile marine invertebrates and invertebrates of brackish water, such as sea anemones, stony corals, soft corals and sea pens. Almost all adult anthozoans are attached to the seabed, while their larvae can disperse as plankton. The basic unit of the adult is the polyp, an individual animal consisting of a cylindrical column topped by a disc with a central mouth surrounded by tentacles. Sea anemones are mostly solitary, but the majority of corals are colonial, being formed by the budding of new polyps from an original, founding individual. Colonies of stony corals are strengthened by mainly aragonite and other materials, and can take various massive, plate-like, bushy or leafy forms.

Members of Anthozoa possess cnidocytes, a feature shared among other cnidarians such as the jellyfish, box jellies and parasitic Myxozoa and Polypodiozoa. The two classes of Anthozoa are class Hexacorallia, with members that have six-fold symmetry such as stony corals, sea anemones, tube anemones and zoanthids, and class Octocorallia, with members that have eight-fold symmetry, such as soft corals, gorgonians (sea pens, sea fans and sea whips), and sea pansies. Some additional species are also included as incertae sedis until their exact taxonomic position can be ascertained.

Anthozoans are carnivores, catching prey with their tentacles. Many species supplement their energy needs by making use of photosynthetic single-celled algae that live within their tissues. These species live in shallow water and many of them are hermatypic (reef-builders). Other species lack the zooxanthellae and, having no need for well-lit areas, typically live in deep-water locations.

Unlike other members of phylum Cnidaria, anthozoans do not have a medusa stage in their development. Instead, they release sperm and egg cells into the water. After fertilisation, the planula larvae form part of the plankton. When fully developed, the larvae settle on the seabed and attach to the substrate, undergoing metamorphosis into polyps. Some anthozoans can also reproduce asexually through budding or by breaking in pieces (fragmentation).

Diversity

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Deep water corals serve as habitats for fish such as the alfonsino

The name "Anthozoa" comes from the Greek words άνθος (ánthos; "flower") and ζώα (zóa; "animals"), hence ανθόζωα (anthozoa) = "flower animals", a reference to the floral appearance of their perennial polyp stage.[3]

Anthozoans live exclusively in marine and brackish water.[1] They include sea anemones, stony corals, soft corals, sea pens, sea fans and sea pansies. Anthozoa is the largest taxon of cnidarians; over six thousand solitary and colonial species have been described. They range in size from small individuals less than half a centimetre across to large colonies a metre or more in diameter. They include species with a wide range of colours and forms that build and enhance reef systems.[4][5] Although reefs and shallow water environments exhibit a great array of species, there are in fact more species of coral living in deep water than in shallow, and many taxa have shifted during their evolutionary history from shallow to deep water and vice versa.[6]

Phylogeny

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In 1995, genetic studies by J. E. N. Veron and a group of scientists that analyzed the ribosomal DNA of many species, found that the order Ceriantipatharia, now deprecated and replaced by its constituent orders Ceriantharia and Antipatharia, [7][8] was "the most representative of the ancestral Anthozoa".[9]

After Ceriantipatharia became deprecated c. 2007, newer phylogenetic studies determined that Ceriantharia is not the basal clade of Anthozoa, but of Hexacorallia, and that Antipatharia and the remaining orders of Hexacorallia are its descendants.[10]

Anthozoa is a monophyletic clade within Cnidaria. A comprehensive phylogenetic study from 2021, has indicated that its two extant subclades, Hexacorallia and Octocorallia, are also monophyletic sisters.[10]

The same study has inferred the independent appearance of key characters in different lineages of subphylum Anthozoa. The reconstructed ancestor of Anthozoa is supposed to have been a solitary polyp with bilateral symmetry, lacking a skeleton and photosymbiosis, already present in the Tonian period (1000 to ~720 Mya) when the two classes of Anthozoa, Hexacorallia and Octocorallia, have probably diverged.[10]

Octocorallia gained colonial growth and calcified elements early on. The acquisition of photosymbiosis occurred many times independently in its different subclades.[10]

In the case of Hexacorallia, its early diverging order, Ceriantharia, retained the ancestral characteristics of being solitary, and lacking a skeleton and photosymbiosis. Zoantharia was the first order to gain colonial growth. Antipatharia and Scleractinia gained colonial growth independently from other orders. From the Devonian on, all subclades of Hexacorallia acquired photosymbiosis independently, with the exception of Ceriantharia and Relicanthus that lack it completely. A similar independent origin for radial symmetry happened to orders Actiniaria, Relicanthus, Scleractinia and Corallimorpharia.[10]

Cladogram of Anthozoa according to molecular phylogenetics

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Molecular phylogenetics studies have determined that the most likely phylogeny of the extant clades of Anthozoa can be represented by the following cladogram: [10][11][12]

Anthozoa

Taxonomy

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The taxonomy of the subphylum Anthozoa is being continuously updated and modified, based on molecular biology, molecular phylogenetics, and other methods. Its most recent version can be found in the World Register of Marine Species.[1] It subdivides Anthozoa into two monophyletic, extant classes: Hexacorallia and Octocorallia, generally showing different symmetry of polyp structure, and two extinct, fossil classes: Rugosa † and Tabulata †.

Hexacorallia includes coral reef builders (Scleractinia, also called hermatypic stony corals), sea anemones (Actiniaria), and zoanthids (Zoantharia).

Octocorallia comprises the sea pens (Pennatulacea), soft corals (Octocorallia), and blue coral (Helioporacea). Sea whips and sea fans, known as gorgonians, are part of Alcyonacea and historically were divided into separate orders. [8]

Classification according to the World Register of Marine Species (WoRMS)

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†= extinct

Examples of some major anthozoan taxa

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Some major anthozoan taxa
Class Order Image Example Characteristics Distribution
Hexacorallia Actiniaria
Sea anemones		 Actinostola sp. Mostly large, solitary polyps anchored to hard substrates. Often colourful. Zooxanthellate or azooxanthellate.[4] Worldwide in shallow and deep water, with greatest diversity in tropics.[4]
Hexacorallia Antipatharia
Black coral		 Plumapathes pennacea Bushy colonies with slender branches. Axial skeleton of dark-coloured thorny branches strengthened by a unique, non-collagen protein. Azooxanthellate.[4] On vertical rock faces of reefs, or in deep water.[4]
Hexacorallia Corallimorpharia
Corallimorphs or
coral anemones		 Discosoma sp. Large, solitary polyps similar to sea anemones, but with stumpy columns and large oral discs with many short tentacles. Catch large prey and some species zooxanthellate.[4] On coral reefs, mostly tropical.[4]
Hexacorallia Rugosa
Extinct		 Stereolasma rectum Extinct order abundant in Middle Ordovician to Late Permian. Solitary or colonial, with a skeleton formed of calcite. Septa develop in multiples of four.[13] Widespread.
Hexacorallia Scleractinia
Stony corals or
hard corals		 Fungia fungites

Tubastraea coccinea		 Solitary or colonial corals in a vast assortment of sizes and shapes, the stony skeleton being composed of aragonite. Septa develop in multiples of six.[13] Zooxanthellate or azooxanthellate.[4] Shallow and deep water habitats worldwide, the greatest diversity being in tropical seas.[4]
Hexacorallia Zoantharia
Zoanthids		 "Dragon eye" coral
Zoanthus sp.		 Small, mostly colonial species joined by coensarc or stolons. No hard skeleton but some incorporate solid matter into fleshy periderm.[4] Mostly tropical, reef-dwelling species.[4]
Hexacorallia Ceriantharia (suborder Penicillaria)
Tube-dwelling anemones		 Arachnanthus sarsi Solitary individuals with two rings of tentacles living in fibrous tubes in soft sediment. Distinguished from Spirularia by anatomy and cnidom.[14] In soft sediment, worldwide.[15]
Hexacorallia Ceriantharia (suborder Spirularia)
Tube-dwelling anemones		 Cerianthus filiformis Solitary individuals with two rings of tentacles living in fibrous tubes. Distinguished from Penicillaria by anatomy and cnidom.[14] In soft sediment, worldwide.[15]
Octocorallia Alcyonacea
Soft corals
and gorgonians		 Alcyonium digitatum

Mushroom corals		 Colonial and diverse, with polyps almost completely embedded in thick fleshy coenosarc. Gorgonians have a horny skeleton. Zooxanthellate or azooxanthellate.[4] Worldwide, mostly in tropical and subtropical waters, associated with coral reefs and in deep sea.[4]
Octocorallia Helioporacea
Blue corals		 Heliopora coerulea Octocorals with a massive skeleton composed of aragonite secreted by underside of coenosarc. Zooxanthellate.[16] Heliopora coerulea is IndoPacific; other species are from the Atlantic and Madagascar.[17]
Octocorallia Pennatulacea
Sea pens,
sea feathers and
sea pansies		 Ptilosarcus gurneyi Colonial species taking pinnate, radial or club-like forms. Main axis is a single enlarged and elongated polyp. Has several types of specialist polyp. Azooxanthellate.[16] Worldwide, from lower tidal to 6,000 m (20,000 ft)[18]

Anatomy

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The basic body form of an anthozoan is the polyp. This consists of a tubular column topped by a flattened area, the oral disc, with a central mouth; a whorl of tentacles surrounds the mouth. In solitary individuals, the base of the polyp is the foot or pedal disc, which adheres to the substrate, while in colonial polyps, the base links to other polyps in the colony.[4]

Anatomy of a stony coral polyp

The mouth leads into a tubular pharynx which descends for some distance into the body before opening into the coelenteron, otherwise known as the gastrovascular cavity, that occupies the interior of the body. Internal tensions pull the mouth into a slit-shape, and the ends of the slit lead into two grooves in the pharynx wall called siphonoglyphs. The coelenteron is subdivided by a number of vertical partitions, known as mesenteries or septa. Some of these extend from the body wall as far as the pharynx and are known as "complete septa" while others do not extend so far and are "incomplete". The septa also attach to the oral and pedal discs.[4]

The body wall consists of an epidermal layer, a jellylike mesogloea layer and an inner gastrodermis; the septa are infoldings of the body wall and consist of a layer of mesogloea sandwiched between two layers of gastrodermis. In some taxa, sphincter muscles in the mesogloea close over the oral disc and act to keep the polyp fully retracted. The tentacles contain extensions of the coelenteron and have sheets of longitudinal muscles in their walls. The oral disc has radial muscles in the epidermis, but most of the muscles in the column are gastrodermal, and include strong retractor muscles beside the septa. The number and arrangement of the septa, as well as the arrangement of these retractor muscles, are important in anthozoan classification.[4]

The tentacles are armed with nematocysts, venom-containing cells which can be fired harpoon-fashion to snare and subdue prey. These need to be replaced after firing, a process that takes about forty-eight hours. Some sea anemones have a circle of acrorhagi outside the tentacles; these long projections are armed with nematocysts and act as weapons. Another form of weapon is the similarly armed acontia (threadlike defensive organs) which can be extruded through apertures in the column wall. Some stony corals employ nematocyst-laden "sweeper tentacles" as a defence against the intrusion of other individuals.[4]

Many anthozoans are colonial and consist of multiple polyps with a common origin joined by living material. The simplest arrangement is where a stolon runs along the substrate in a two dimensional lattice with polyps budding off at intervals. Alternatively, polyps may bud off from a sheet of living tissue, the coenosarc, which joins the polyps and anchors the colony to the substrate. The coenosarc may consist of a thin membrane from which the polyps project, as in most stony corals, or a thick fleshy mass in which the polyps are immersed apart from their oral discs, as in the soft corals.[4]

The skeleton of a stony coral in the order Scleractinia is secreted by the epidermis of the lower part of the polyp; this forms a corallite, a cup-shaped hollow made of calcium carbonate, in which the polyp sits. In colonial corals, following growth of the polyp by budding, new corallites are formed, with the surface of the skeleton being covered by a layer of coenosarc. These colonies adopt a range of massive, branching, leaf-like and encrusting forms.[19] Soft corals in the subclass Octocorallia are also colonial and have a skeleton formed of mesogloeal tissue, often reinforced with calcareous spicules or horny material, and some have rod-like supports internally.[20] Other anthozoans, such as sea anemones, are naked; these rely on a hydrostatic skeleton for support. Some of these species have a sticky epidermis to which sand grains and shell fragments adhere, and zoanthids incorporate these substances into their mesogloea.[4]

Biology

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Giant green anemone, a zooxanthellate species; the tentacles are armed with nematocysts to catch prey

Most anthozoans are opportunistic predators, catching prey which drifts within reach of their tentacles. The prey is secured with the help of sticky mucus, spirocysts (non-venomous harpoon cells) and nematocysts (venomous harpoon cells). The tentacles then bend to push larger prey into the mouth, while smaller, plankton-size prey, is moved by cilia to the tip of the tentacles which are then inserted into the mouth. The mouth can stretch to accommodate large items, and in some species, the lips may extend to help receive the prey. The pharynx then grasps the prey, which is mixed with mucus and slowly swallowed by peristalsis and ciliary action. When the food reaches the coelenteron, extracellular digestion is initiated by the discharge of the septa-based nematocysts and the release of enzymes. The partially digested food fragments are circulated in the coelenteron by cilia, and from here they are taken up by phagocytosis by the gastrodermal cells that line the cavity.[4]

Most anthozoans supplement their predation by incorporating into their tissues certain unicellular, photosynthetic organisms known as zooxanthellae (or zoochlorellae in a few instances); many fulfil the bulk of their nutritional requirements in this way. In this symbiotic relationship, the zooxanthellae benefit by using nitrogenous waste and carbon dioxide produced by the host while the cnidarian gains photosynthetic capability and increased production of calcium carbonate, a substance of great importance to stony corals.[21] The presence of zooxanthellae is not a permanent relationship. Under some circumstances, the symbionts can be expelled, and other species may later move in to take their place. The behaviour of the anthozoan can also be affected, with it choosing to settle in a well lit spot, and competing with its neighbours for light to allow photosynthesis to take place. Where an anthozoan lives in a cave or other dark location, the symbiont may be absent in a species that, in a sunlit location, normally benefits from one.[22] Anthozoans living at depths greater than 50 m (200 ft) are azooxanthellate because there is insufficient light for photosynthesis.[6]

White black coral Leiopathes glaberrima with white sea anemones below, both azooxanthellate, deep water species

With longitudinal, transverse and radial muscles, polyps are able to elongate and shorten, bend and twist, inflate and deflate, and extend and contract their tentacles. Most polyps extend to feed and contract when disturbed, often invaginating their oral discs and tentacles into the column. Contraction is achieved by pumping fluid out of the coelenteron, and reflation by drawing it in, a task performed by the siphonoglyphs in the pharynx which are lined with beating cilia. Most anthozoans adhere to the substrate with their pedal discs but some are able to detach themselves and move about, while others burrow into the sediment. Movement may be a passive drifting with the currents or in the case of sea anemones, may involve creeping along a surface on their base.[4]

Gas exchange and excretion is accomplished by diffusion through the tentacles and internal and external body wall, aided by the movement of fluid being wafted along these surfaces by cilia. The sensory system consists of simple nerve nets in the gastrodermis and epidermis, but there are no specialised sense organs.[4]

Anthozoans exhibit great powers of regeneration; lost parts swiftly regrow and the sea anemone Aiptasia pallida can be vivisected in the laboratory and then returned to the aquarium where it will heal. They are capable of a variety of asexual means of reproduction including fragmentation, longitudinal and transverse fission and budding.[4] Sea anemones for example can crawl across a surface leaving behind them detached pieces of the pedal disc which develop into new clonal individuals. Anthopleura species divide longitudinally, pulling themselves apart, resulting in groups of individuals with identical colouring and patterning.[23] Transverse fission is less common, but occurs in Anthopleura stellula and Gonactinia prolifera, with a rudimentary band of tentacles appearing on the column before the sea anemone tears itself apart.[24] Zoanthids are capable of budding off new individuals.[25]

Pocillopora damicornis produces yolky eggs and the planula larvae disperse widely.

Most anthozoans are unisexual but some stony corals are hermaphrodite. The germ cells originate in the endoderm and move to the gastrodermis where they differentiate. When mature, they are liberated into the coelenteron and thence to the open sea, with fertilisation being external.[4] To make fertilisation more likely, corals emit vast numbers of gametes, and many species synchronise their release in relation to the time of day and the phase of the moon.[26]

The zygote develops into a planula larva which swims by means of cilia and forms part of the plankton for a while before settling on the seabed and metamorphosing into a juvenile polyp. Some planulae contain yolky material and others incorporate zooxanthellae, and these adaptations enable these larvae to sustain themselves and disperse more widely.[4] The planulae of the stony coral Pocillopora damicornis, for example, have lipid-rich yolks and remain viable for as long as 100 days before needing to settle.[27]

Ecology

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Further information: Coral reef
Biodiverse, multi-coloured habitat

Coral reefs are some of the most biodiverse habitats on earth, supporting large numbers of species of corals, fish, molluscs, worms, arthropods, starfish, sea urchins, other invertebrates and algae. Because of the photosynthetic requirements of the corals, they are found in shallow waters, and many of these fringe land masses.[28] With a three-dimensional structure, coral reefs are very productive ecosystems; they provide food for their inhabitants, hiding places of various sizes to suit many organisms, perching places, barriers to large predators and solid structures on which to grow. They are used as breeding grounds and as nurseries by many species of pelagic fish, and they influence the productivity of the ocean for miles around.[29] Anthozoans prey on animals smaller than they are and are themselves eaten by such animals as fish, crabs, barnacles, snails and starfish. Their habitats are easily disturbed by outside factors which unbalance the ecosystem. In 1989, the invasive crown-of-thorns starfish (Acanthaster planci) caused havoc in American Samoa, killing 90% of the corals in the reefs.[30]

Corals that grow on reefs are called hermatypic, with those growing elsewhere are known as ahermatypic. Most of the latter are azooxanthellate and live in both shallow and deep sea habitats. In the deep sea they share the ecosystem with soft corals, polychaete worms, other worms, crustaceans, molluscs and sponges. In the Atlantic Ocean, the cold-water coral Lophelia pertusa forms extensive deep-water reefs which support many other species.[31]

Other fauna, such as hydrozoa, bryozoa and brittle stars, often dwell among the branches of gorgonian and coral colonies.[32] The pygmy seahorse not only makes certain species of gorgonians its home, but closely resembles its host and is thus well camouflaged.[33] Some organisms have an obligate relationship with their host species. The mollusc Simnialena marferula is only found on the sea whip Leptogorgia virgulata, is coloured like it and has sequestered its defensive chemicals, and the nudibranch Tritonia wellsi is another obligate symbiont, its feathery gills resembling the tentacles of the polyps.[34]

Dardanus calidus with Calliactis parasitica

A number of sea anemone species are commensal with other organisms. Certain crabs and hermit crabs seek out sea anemones and place them on their shells for protection, and fish, shrimps and crabs live among the anemone's tentacles, gaining protection by being in close proximity to the stinging cells. Some amphipods live inside the coelenteron of the sea anemone.[35] Despite their venomous cells, sea anemones are eaten by fish, starfish, worms, sea spiders and molluscs. The sea slug Aeolidia papillosa feeds on the aggregating anemone (Anthopleura elegantissima), accumulating the nematocysts for its own protection.[35]

Paleontology

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Several extinct orders of corals from the Paleozoic era (~540–252 million years ago) are thought to be close to the ancestors of modern Scleractinia:[36][37]

These are all corals and correspond to the fossil record time line. With readily-preserved hard calcareous skeletons, they comprise the majority of Anthozoan fossils.

RugosaScleractiniaTabulataEdiacaranCambrianCambrianOrdovicianOrdovicianSilurianSilurianDevonianDevonianCarboniferousCarboniferousPermianPermianTriassicTriassicJurassicCretaceousTertiaryPrecambrianPaleozoicMesozoicCenozoicPermian-Triassic extinctionLate Devonian extinctionCothoniidamya (unit)

Timeline of the major coral fossil record and developments from 650 m.y.a. to present.[38][39]

Interactions with humans

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Main article: Environmental issues with coral reefs

Coral reefs and shallow marine environments are threatened, not only by natural events and increased sea temperatures, but also by such man-made problems as pollution, sedimentation and destructive fishing practices. Pollution may be the result of run-off from the land of sewage, agricultural products, fuel or chemicals. These may directly kill or injure marine life, or may encourage the growth of algae that smother native species, or form algal blooms with wide-ranging effects. Oil spills at sea can contaminate reefs, and also affect the eggs and larva of marine life drifting near the surface.[40]

Corals are collected for the aquarium trade, and this may be done with little care for the long-term survival of the reef. Fishing among reefs is difficult and trawling does much mechanical damage. In some parts of the world explosives are used to dislodge fish from reefs, and cyanide may be used for the same purpose; both practices not only kill reef inhabitants indiscriminately but also kill or damage the corals, sometimes stressing them so much that they expel their zooxanthellae and become bleached.[40]

Deep water coral habitats are also threatened by human activities, particularly by indiscriminate trawling. These ecosystems have been little studied, but in the perpetual darkness and cold temperatures, animals grow and mature slowly and there are relatively fewer fish worth catching than in the sunlit waters above. To what extent deep-water coral reefs provide a safe nursery area for juvenile fish has not been established, but they may be important for many cold-water species.[41]

References

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Anthozoa

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Anthozoa is a class of the phylum consisting of sessile, polypoid that exhibit no stage in their life cycle, including sea anemones, stony corals, soft corals, sea pens, and sea fans./28:_Invertebrates/28.02:_Phylum_Cnidaria/28.2B:_Class_Anthozoa) These organisms, numbering over 7,000 across about 10 orders, possess a tubular body with an oral disk bearing tentacles equipped with nematocysts for prey capture and defense, and a gastrovascular cavity divided by mesenteries that enhance digestion and nutrient distribution. Anthozoans reproduce both sexually, via gametes leading to larvae that settle and form new polyps, and asexually through fission or , enabling colonial growth in species like corals. Ecologically significant, scleractinian corals secrete skeletons forming vast that support , while many anthozoans host symbiotic algae providing energy through , a mutualism critical to their calcification and survival in sunlit waters.

Systematics and Phylogeny

Phylogenetic Relationships

Anthozoa constitutes a monophyletic class within the phylum Cnidaria, sister to the clade Medusozoa, which encompasses taxa capable of producing medusae such as Hydrozoa, Scyphozoa, Cubozoa, and Staurozoa. This positioning aligns with the anthozoan life cycle lacking a free-swimming medusa stage, contrasting with medusozoans, and is corroborated by ribosomal RNA and mitogenomic analyses despite occasional discrepancies in early mitochondrial datasets attributed to substitution saturation. Recent phylogenomic studies using hundreds of loci further affirm the monophyly of Anthozoa and its distinction from Medusozoa, resolving prior ambiguities through expanded taxon sampling and nuclear gene data. Internally, Anthozoa divides into two reciprocally subclasses: and , differentiated by mesenterial arrangements and tentacle symmetries—six-partite in and eight-partite in . includes orders such as (stony corals), Actiniaria (sea anemones), Antipatharia (black corals), and , with Ceriantharia emerging as the earliest diverging lineage among hexacorals based on phylogenomic reconstructions. encompasses orders like (soft corals), Helioporacea (blue corals), and Pennatulacea (sea pens), whose is robustly supported by mitochondrial and nuclear markers, though familial relationships within it continue to refine with increased genomic data. Disagreements in earlier mitogenomic phylogenies, which sometimes implied anthozoan paraphyly by nesting medusozoans within, have been reconciled by demonstrating that nuclear datasets exhibit lower saturation and better resolve deep cnidarian divergences, prioritizing comprehensive phylogenomics over lineage-specific mitochondrial biases. These findings underscore Anthozoa's basal role in Cnidaria evolution, with fossil evidence from Ediacaran-like forms potentially linking to crown-group anthozoans around 540 million years ago, though molecular clocks suggest diversification post-Cambrian explosion.

Taxonomic Classification

Anthozoa constitutes a major lineage within the phylum , encompassing organisms such as sea anemones, stony and soft corals, black corals, and sea pens, with over 7,500 described species. Traditionally ranked as a class, contemporary systematic frameworks, informed by and morphological traits like arrangement and configuration, elevate its position to subphylum under , containing two primary classes: and . This division is based on 's six- or multiple-of-six-fold versus 's eight-fold , with the former typically featuring solid skeletons in some orders and the latter producing sclerites. Some classifications, such as NCBI , additionally recognize Ceriantharia (tube anemones) as a separate subclass or basal lineage within Anthozoa, reflecting unresolved phylogenetic placement from analyses.
ClassMajor OrdersCharacteristic Features and Examples
HexacoralliaActiniaria, Antipatharia, Corallimorpharia, , ZoanthariaPolyps with unbranched tentacles in multiples of six; includes sea anemones (Actiniaria), black corals (Antipatharia), mushroom corals and zoanthids (), and reef-building stony corals ( with skeletons).
Octocorallia, Helioporacea, PennatulaceaPolyps with eight pinnate tentacles; includes soft corals and gorgonians ( with proteinaceous or sclerite-based axes), blue corals (Helioporacea with hydrozoan-like calcite skeletons), and colonial sea pens (Pennatulacea).
This structure accounts for approximately 95% of anthozoan diversity, with dominating shallow-water reef ecosystems and more prevalent in deeper or soft-bottom habitats; ongoing genomic studies continue to refine boundaries, particularly for paraphyletic groups like ceriantharians.

Diversity and Recent Discoveries

Anthozoa includes approximately 7,500 described extant species, primarily divided into the subclasses and , with Ceriantharia comprising a smaller group of tube anemones. encompasses around 4,300 species across orders such as Actiniaria (sea anemones), (stony corals), Antipatharia (black corals), Corallimorpharia, , and Helioantharia. contains roughly 3,200 species in eight orders, including (soft corals and gorgonians), Helioporacea (blue corals), Pennatulacea (sea pens), and Anthomastusidae. Ceriantharia consists of about 100 species in two orders, characterized by tube-dwelling habits. Biodiversity is unevenly distributed, with dominating shallow tropical reefs and more prevalent in deeper waters; however, surveys indicate substantial undescribed diversity, particularly among zoantharians, deep-sea octocorals, and mesophotic taxa, where DNA analyses reveal novel lineages not matching known species. Phylogenetic studies highlight cryptic speciation and understudied associations, such as sponge-zoantharian symbioses, suggesting the true species count may exceed 10,000. Recent deep-sea expeditions have accelerated discoveries, including a new black coral species identified in 2022 from seamounts during a Smithsonian-NOAA survey at depths over 2,000 meters. In 2024, Anthopleura variata, a new actiniarian species, was described from intertidal zones in the Mexican Pacific, marking the seventh Anthopleura species there. Other findings include Discoactis tritentaculata, a novel sea anemone genus and family with unique tridentate tentacles, collected around . In 2025, a new Stauropathes species expanded antipatharian diversity from Antarctic collections. These additions underscore ongoing taxonomic revisions driven by molecular tools and remote habitats.

Morphology and Anatomy

Polyp Body Plan

The anthozoan polyp is the sole body form in this class, lacking any stage characteristic of other cnidarian lineages. This polyp consists of a cylindrical, sac-like structure oriented along an oral-aboral axis, with the oral end bearing a and the aboral end typically featuring a pedal disc for substrate attachment in solitary forms or integration into colonies. The body wall comprises two epithelial layers— externally and internally—sandwiched by an acellular , conferring structural support and flexibility. Externally, the oral region includes a flattened oral disc surrounding a slit-shaped , encircled by a ring of hollow tentacles armed with cnidocytes for prey capture and defense. The intervening column varies in texture, from smooth in sea anemones to encrusted with skeletal elements in corals, and may retract via longitudinal and circular musculature in the . Internally, the connects to a muscular actinopharynx, which extends into the gastrovascular cavity (coelenteron), a central chamber divided by paired mesenteries that fold inward from the body wall to enhance digestion and nutrient distribution. These mesenteries bear reproductive gonads and filaments for , with siphonozooids in colonial forms facilitating water flow. Anthozoan polyps exhibit biradial , with the mouth, , and mesenteries arranged in directives along longitudinal axes, differing from the strict radial of medusozoan polyps. In (e.g., scleractinian corals, actiniarians), mesenteries and tentacles occur in multiples of six, yielding directive , while (e.g., sea pens, soft corals) feature eight primary mesenteries and pinnate tentacles, reflecting evolutionary divergence in internal partitioning. Solitary polyps, such as those of sea anemones, can reach diameters of 10 cm or more, whereas colonial forms often comprise smaller, interconnected individuals sharing a coelenteron via canals.

Skeletal and Tissue Structures

Anthozoan polyps possess a with an outer al layer and an inner al layer separated by the , a gelatinous that provides structural support and can vary from acellular to containing amoebocytes and fibroblasts. The includes with cnidocytes for nematocyst discharge, while the lines the gastrovascular cavity and participates in nutrient absorption and processes. Mesenteries, vertical partitions extending from the body wall to the , contain retractor muscles essential for polyp retraction, with muscle morphology differing across taxa from atrophied forms in anemones to hypertrophied versions in scleractinians. Skeletal structures in Anthozoa are diverse and taxon-specific, ranging from absent in many sea anemones to elaborate or proteinaceous supports in colonial forms. Scleractinian corals (stony corals) secrete external skeletons primarily composed of , deposited extracellularly by calicoblastic ectodermal cells at the polyp base, forming massive structures through iterative polyp budding and skeletal accretion. These skeletons exhibit microstructures such as trabeculae and aligned with mesenteries, with organic matrices of proteins and modulating crystallization. In contrast, octocorals like gorgonians possess internal sclerites—microscopic spicules embedded in the for rigidity and defense—often supplemented by a central horny axis of gorgonin, a scleroprotein, enabling flexible fan or whip-like colonies. Desmocytes, specialized calicoblastic-like cells in the , anchor soft tissues to these skeletons via tonofilaments and attachments, facilitating colony integrity. corals (Antipatharia) feature proteinaceous axes reinforced by minimal sclerites, while pens (Pennatulacea) have a chitinous central rachis supporting autozooids. Variations in skeletal composition, such as in most scleractinians versus spicules in octocorals, reflect evolutionary adaptations to environmental stresses like predation and currents.

Physiology and Reproduction

Feeding Mechanisms

Anthozoans capture prey primarily through tentacles bearing nematocysts, specialized cnidocytes that discharge coiled, barbed threads to sting and immobilize organisms such as , small crustaceans, , and particulate matter. This mechanism predominates in hexacorallian orders like Actiniaria (sea anemones) and (stony corals), where extended tentacles actively or passively intercept prey in the . Upon contact, nematocyst discharge paralyzes the prey, after which tentacles flex inward, contract, or wipe across the oral disk to deliver it to the slit-like and extensible . occurs via ciliary currents directing particles into the gastrovascular cavity, where by enzymes and mesenterial filaments breaks down tissues; undigested waste is egested through the same oral opening. Some species, including certain actiniarians, employ secondary internal stinging via nematocysts within the gastrovascular cavity to further subdue ingested prey. Variations exist across taxa: octocorallians (e.g., gorgonians in ) often rely on passive suspension feeding with pinnulate s capturing fine like copepods, using rapid tentacular flexion for transfer rather than full retraction. Scleractinians may supplement capture with entrapment of "" (organic detritus), while polyphagous actiniarians opportunistically ingest diverse items including salps or inorganic particles. Deep-sea and azooxanthellate forms emphasize heterotrophy due to limited autotrophy, heightening reliance on these mechanisms. Although many anthozoans host symbiotic dinoflagellates () providing photosynthetic nutrients, heterotrophic feeding via nematocyst-armed tentacles remains essential, contributing 10–50% or more of energy budgets even in symbiotic , with proportions varying by and prey availability. Studies from 1890–2019 highlight dominance in diets, with biases toward hexacorallians underscoring gaps in octocorallian and deep-sea feeding dynamics.

Reproductive Biology

Anthozoans reproduce through both asexual and sexual mechanisms, with the prevalence of each varying across taxa such as sea anemones (Actiniaria), stony corals (Scleractinia), soft corals (Alcyonacea), and black corals (Antipatharia). Asexual reproduction predominates in clonal colonies and enables rapid population expansion in stable environments, while sexual reproduction facilitates dispersal and genetic diversity via planula larvae. Asexual reproduction occurs via budding, fission, or fragmentation, producing genetically identical polyps or colonies. In sea anemones, longitudinal fission divides the polyp into two mirror-image clones, often triggered by environmental stressors like temperature changes, with peaks observed year-round but intensified during spawning seasons in species such as Anthopleura dixoniana. Budding in scleractinian corals involves the outgrowth of daughter polyps from the parent, leading to colonial growth, while fragmentation in soft corals like alcyonaceans allows broken pieces to regenerate into new individuals. These processes are facultative in many species, co-occurring with sexual modes without direct linkage to gametogenic cycles. Sexual reproduction involves gametogenesis within the mesenteries, where oocytes and spermatocytes develop from germ cells in the gastrodermis and mesoglea. Most anthozoans are simultaneous hermaphrodites, producing both eggs and sperm, though gonochorism (separate sexes) occurs in some actiniarians and alcyonaceans, often with female-biased sex ratios. Oogenesis precedes spermatogenesis, with oocytes reaching diameters of 550–600 µm in deep-water species; gametes are released through the mouth for external fertilization. Spawning is typically annual and synchronized by lunar cycles, temperature rises, or photoperiod, as in scleractinian corals where mass events follow full moons by 3–7 days, peaking in spring or summer. Brooding, where planulae develop internally, is common in some zoanthids and actiniarians, reducing dispersal but enhancing local recruitment. Fertilized eggs form ciliated larvae that swim briefly before settling and metamorphosing into polyps, with competence lasting days to weeks depending on and conditions. In deep-sea anthozoans, such as antipatharians, remains poorly documented but involves similar gamete release, with oocytes settling rapidly post-spawning. Reproductive output correlates with colony size and environmental cues, though climate-induced disruptions like warming can desynchronize spawning and impair fertilization success.

Ecology and Distribution

Habitats and Biogeography

Anthozoa inhabit exclusively marine environments, ranging from intertidal zones to abyssal depths exceeding 10 km, with adapted to a variety of substrates including rocky reefs, soft sediments, and even hydrothermal vents or cold seeps. Most taxa are benthic and sessile or semi-sessile, attaching via a basal disc or pedal disc to hard surfaces like skeletons or rocks, while others such as ceriantharian tube anemones and pennatulacean pens burrow into or . Shallow-water scleractinian () dominate tropical reef ecosystems, typically occurring at depths less than 70 m where light supports zooxanthellate symbiosis, though azooxanthellate forms extend to over 6,000 m. , including gorgonians and soft , span tidelands to approximately 4 km, with many in mesophotic (50–150 m) and bathyal zones providing complex habitats. Antipatharian black favor deeper continental slopes, often between 200 m and 2,000 m, forming bushy colonies on hard substrates. Biogeographically, Anthozoa exhibit cosmopolitan distribution across all oceans, from polar Antarctic waters to equatorial tropics, though diversity gradients vary by subclass. The Indo-Pacific region hosts the highest species richness, particularly for reef-building scleractinians and octocorals, with biodiversity hotspots in areas like the Coral Triangle due to stable warm-water conditions and substrate availability. Actiniarian sea anemones show peak richness at mid-latitudes (30°–40° N and S), reflecting adaptations to temperate rocky shores and deeper shelves, while deep-water forms like certain octocorals and antipatharians achieve near-global uniformity, limited mainly by substrate and oxygenation rather than latitude. Cold-water species, including some antipatharians and octocorals, occur in high-latitude fjords and seamounts, contributing to biodiversity in regions like the South African slope (200–1,000 m). Environmental correlates such as temperature, salinity, and depth-driven pressure influence these patterns, with heterotrophic deep-sea taxa less constrained by light than phototrophic shallow forms.

Symbiotic Associations

Many anthozoans, particularly scleractinian corals and certain sea anemones, form mutualistic endosymbiotic relationships with dinoflagellate algae of the genus , commonly known as . These algae reside intracellularly within the gastrodermal cells of the host, performing to produce organic carbon compounds that supply up to 90% of the anthozoan's energy needs, enabling survival in nutrient-poor tropical waters. In return, the anthozoan host provides the algae with protection, inorganic nutrients such as nitrogen and phosphorus from host waste, and a stable habitat. This symbiosis, ancient and dating back to at least the period around 240 million years ago, underpins the formation of by facilitating and growth in oligotrophic environments. Disruptions, such as elevated temperatures leading to bleaching—where are expelled—can result in host starvation, as observed in mass events since the 1980s. Sea anemones in the orders Actiniaria and Corallimorpharia exhibit symbiosis with pomacentrid fishes, notably clownfishes (Amphiprion spp.), in a mutualistic association involving ten anemone species across the Indo-Pacific. The anemone's nematocysts provide protection against predators for the fish, which in turn defend the host from anemone-eating species, remove parasites, and supply nutrients via fecal matter and uneaten food remnants, enhancing anemone growth and reproduction rates by up to 52% in some studies. Clownfishes acclimate to the stinging cells through mucus production that prevents discharge, allowing residency among tentacles without harm; this relationship, obligate for most clownfish species, boosts anemone tissue regeneration and asexual reproduction. Similar protective mutualisms occur with other fishes and crustaceans, such as commensal shrimp that gain shelter while aerating the host's tissues. Certain anthozoans, including anemones and zoanthids, associate with hermit crabs in phoretic symbioses where the anthozoan adheres to the crab's shell, deterring predators for the crab while the mobile host expands the anemone's foraging range and provides defensive benefits. Bacterial microbiomes in anthozoans, comprising diverse communities dominated by Proteobacteria and Bacteroidetes, contribute to nutrient cycling, pathogen resistance, and stability; for instance, coral-associated bacteria facilitate and , with phylosymbiotic patterns mirroring host phylogeny. These microbial consortia, varying by host species and environment, influence symbiosis onset and resilience, as evidenced by stable isotope analyses showing bacterial roles in carbon transfer. While some associations border on , mutual benefits predominate, underscoring anthozoans' reliance on diverse symbionts for ecological success.

Ecological Interactions

Anthozoans function as predators in marine ecosystems, primarily capturing small prey such as , crustaceans, small , mollusks, and sea cucumbers using tentacles equipped with nematocysts that deliver paralyzing toxins. Certain large sea anemones exhibit macro-predation capabilities, engulfing sizable organisms like or crustaceans when contact occurs with their oral disc. These predatory interactions contribute to trophic dynamics, with anthozoans occasionally supplementing diets via scavenging or incidental capture of larger drifting material. Predators of anthozoans include various fish species, such as and that graze on polyps, and invertebrates like targeting scleractinian corals. Sea anemones face threats from nudibranchs, crabs, and octopuses that consume or damage tissues, often circumvented by chemical defenses including neurotoxic and cardiotoxic venoms that deter attackers. In response to predation pressure, anthozoans deploy allelopathic compounds to inhibit settler attachment or overgrowth by competitors and potential predators. As foundational species, anthozoans, particularly reef-building corals, engineer habitats that enhance by creating structural complexity; for instance, deep-sea corals support diverse assemblages of , sponges, and echinoderms, fostering resilience. Sea anemones provide refuge for commensal species like anemonefish, which gain protection from predators via the host's stinging cells, while minimally impacting the beyond occasional nutrient provision from fish waste. Competitive interactions occur among sessile anthozoans and other benthic organisms for space, mediated by overgrowth, sweeper tentacles, and that suppress rivals. These dynamics underscore anthozoans' role in maintaining community structure and facilitating higher trophic levels in reefs and soft sediments.

Evolutionary History

Fossil Record

The fossil record of Anthozoa is dominated by calcified structures from coral polyps, with the subclass (including scleractinian corals) providing the most extensive documentation due to their durable skeletons, while remains poorly represented owing to predominantly soft tissues. Potential anthozoan-like fossils, including polypoid and morphologies, appear in late () deposits dating to approximately 580 million years ago, though their assignment to Anthozoa is tentative and based on morphological similarity rather than definitive synapomorphies. More secure early records emerge in the , with mineralized coral-like forms and soft-tissue impressions of octocorals in deposits such as the , indicating a diversification of anthozoans by around 520 million years ago. In the Paleozoic Era, tabulate corals (Tabulata) originated in the Early around 485 million years ago and persisted until their extinction at the end-Permian mass extinction approximately 252 million years ago, forming colonial structures with tabular septa that contributed to early reef ecosystems. Rugose corals, often solitary and horn-shaped, similarly flourished from the through the Permian, with peak diversity in the , but these extinct groups' phylogenetic placement within Anthozoa is debated, as they lack clear homology to modern hexacorallian septa and may represent stem lineages. A notable find includes black corals (likely antipatharians) from Floian-stage deposits in , dated to about 470 million years ago, supporting molecular estimates of early anthozoan divergence while highlighting gaps in the skeletal record for soft-bodied forms. Octocorallian fossils from this era are rare, limited to sclerites and holdfasts, underscoring preservation biases against non-calcified taxa. The order , comprising modern stony corals, abruptly enters the fossil record in the approximately 240 million years ago, following the Permian-Triassic extinction, with diverse morphologies appearing in Tethyan deposits that suggest rapid post-extinction radiation rather than a long . analyses propose an origin for around 487–443 million years ago, implying pre-Triassic soft-bodied ancestors whose lack of skeletons explains the fossil gap, though this remains contentious without direct paleontological corroboration. Scleractinians subsequently diversified through the and , forming extensive reefs by the and achieving modern familial diversity by the , while enduring selective pressures from anoxic events and sea-level changes. Octocorallian fossils improve in the , with sclerites and holdfasts from Eocene to Pleistocene strata revealing deeper-water assemblages, but overall scarcity persists, likely due to taphonomic loss of axial and sclerite structures. The end-Permian extinction eliminated anthozoan clades like Tabulata and , paving the way for scleractinian dominance, with contemporary records indicating ongoing adaptation amid environmental stressors.

Key Evolutionary Adaptations

Anthozoans exhibit an exclusively polypoid body plan, having evolutionarily lost the medusa stage characteristic of other cnidarians. This adaptation supports a sessile, benthic lifestyle, with polyps attaching via a basal disc and forming colonies through asexual budding, which promotes modular growth and resilience to fragmentation. The polyp structure, featuring a cylindrical column, oral disc, and tentacles armed with nematocysts, enables efficient prey capture and environmental interaction in diverse habitats. The evolution of skeletal structures marks a major innovation, transitioning from askeletal ancestors in the Cryogenian-Tonian periods to mineralized forms. Scleractinian corals (Hexacorallia) secrete aragonite calcium carbonate skeletons via protein-mediated biomineralization, enabling erect growth, wave resistance, and reef framework construction beginning in the Triassic, though molecular evidence suggests earlier capabilities. Octocorals produce calcareous sclerites embedded in mesogleal tissue or gorgonin axes, providing flexibility and support in soft-bottom or deep-sea environments. These skeletons correlate with shifts in ocean pH and saturation states, facilitating diversification across bathymetric gradients. Photosymbiotic associations with dinoflagellates ( clade) originated by the (~383 Ma) and evolved repeatedly, supplying translocated photosynthates that fuel autotrophy, , and rapid colony expansion in nutrient-limited, illuminated waters. This mutualism enhances survival in oligotrophic reefs but has been lost in heterotrophic deep-sea lineages, highlighting adaptive plasticity. Complementary adaptations include convergent polyp retraction musculature for predator evasion and expanded photoreceptor opsins for light-mediated behaviors across photic zones.

Human Interactions

Economic and Biomedical Value

Coral reefs, primarily constructed by scleractinian anthozoans, generate substantial economic benefits through fisheries, tourism, and coastal protection, with global ecosystem services valued at approximately $375 billion annually. Healthy reefs sustain commercial and subsistence fishing that supports over half a billion people for food and income, while also driving recreation such as diving and snorkeling, which contribute hundreds of millions of dollars in revenue. In regions like the Caribbean, intact reefs could yield up to $70 billion in net benefits under healthy scenarios, including $13.9 billion from tourism and $6.2 billion from fisheries. Precious corals, including species in the genera Corallium and Paracorallium (order Antipatharia and Gorgonacea), have been harvested for their durable axial skeletons used in jewelry and ornaments since ancient times, commanding high market prices due to rarity and aesthetic appeal. Global trade in these corals persists, with historical fisheries in the Mediterranean and Pacific yielding significant , though has led to depletion in areas like the northwestern Pacific, where stocks are now rare and harvesting restricted. Annual economic productivity from reef-associated activities averages $112,000 per square kilometer, underscoring the value of anthozoan habitats despite variability across regions. Anthozoans produce diverse bioactive compounds, including terpenoids, alkaloids, and toxins, investigated for pharmaceutical applications such as , , and anticancer agents. Extracts from zoanthids (order ) yield alkaloids with potential medicinal uses, while gorgonian and alcyonacean species have provided compounds exhibiting antitumor activity in preclinical studies. Sea anemone venoms contain neurotoxins and pore-forming peptides that target ion channels, offering leads for analgesics and potential therapeutics against or cancer, as demonstrated by pharmacological profiling of families like Actiniidae. Deep-sea anemones, such as Cephalothrix cf. pilatus, show activity against and fungi, highlighting untapped potential for novel antibiotics.

Threats and Scientific Debates

Anthozoa face multiple anthropogenic and environmental pressures, with reef-building scleractinian corals particularly vulnerable due to their dependence on and with . Elevated seawater temperatures, driven by climate variability and long-term warming, induce mass bleaching events by causing the expulsion of symbiotic , leading to energy starvation and tissue if prolonged; global events in 1998, 2014–2017, and 2023–2024 affected over 80% of surveyed reefs in some regions. , resulting from increased atmospheric CO2 absorption, lowers aragonite saturation states and impairs skeletal growth in calcifying species, with laboratory studies showing up to 40% reductions in net rates under levels projected for 2100 (≈800–1000 µatm). Local stressors exacerbate these, including from coastal runoff, which elevates nutrient loads and promotes algal overgrowth, and that physically damage colonies; of herbivorous fish further shifts community dynamics toward macroalgal dominance. Non-calcifying anthozoans, such as sea anemones and zoanthids, experience less direct impact from acidification but suffer from and competitor interactions under combined pressures. Scientific debates center on the relative roles of global versus local factors in observed declines and the potential for anthozoan resilience. While consensus attributes recent bleaching severity to anthropogenic warming exceeding historical variability, some analyses contend that bleaching serves as a natural mechanism, enabling shifts in symbiont communities or host physiology in response to periodic disturbances, as evidenced by records of past recoveries without modern CO2 levels. Projections of future states vary widely due to uncertainties in rates, with systematic reviews highlighting methodological differences in modeling thresholds and , where optimistic scenarios incorporate evolutionary history showing certain deep-sea or temperate lineages with pre-adaptive traits for warmer conditions. Conservation interventions, such as assisted evolution via or introducing resilient genotypes from distant populations, spark contention over ecological risks like versus benefits in averting ; empirical trials indicate delayed erosion but not prevention of framework loss under unmitigated warming. These debates underscore the need for integrated empirical monitoring, as data-deficient —comprising over half of assessed anthozoans—may harbor underestimated risks, challenging assumptions of uniform vulnerability across taxa.

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