Taxonomy is the science of classification; it allows us to group organisms with similar traits and common ancestors into categories so we can more easily differentiate and compare living things. This article will focus entirely on explaining the classification of corals.

The use of taxonomy to classify all organisms originated in the 1730's when the Swedish biologist, Carolus Linnaeus, developed the system of binomial nomenclature for naming species. This naming system utilizes two words for each species name; the first word is the genus (e.g Homo) and the second word is the specific epithet (e.g. sapiens). All organisms are grouped into seven primary taxonomic ranks: kingdom, phylum, class, order, family, genus, and species. There are often other subdivisions of these ranks.

"Corals belong to class Hydrozoa and Anthozoa, although most corals are anthozoans."

Corals belong to kingdom Animalia and phylum Cnidaria. Cnidarians include three classes: (1) Hydrozoa: solitary and colonial polyp organisms (e.g. Hydra sp., Portuguese Man of War, and some corals), (2) Scyphozoa: true jellyfish, and (3) Anthozoa: anemones and corals. Corals belong to class Hydrozoa and Anthozoa, although most corals are anthozoans. 

Hydrozoa

There are two types of corals that belong to class Hydrozoa: fire corals (family Milleporidae) and lace corals (family Stylasteridae). Hydrocorals are often mistaken as stony corals because they have a hard, calcareous skeleton, but they are only similar in appearance. Fire corals have extremely powerful stinging cells called nematocysts on their tentacles that can cause an intense burning sensation as well as redness, rashes and welts if touched by bare skin. Both fire corals and lace corals have thousands of pinhole-sized pores covering their skeletons through which the tiny polyps extend. They have feeding polyps (gastrozooids) and sensory/stinging polyps (dactylozooids); the gastrozooids are short and are surrounded by five to fifteen dactylozooids, which are much longer and thinner. Lace corals, which are often shades of purple, lavender or burgundy are much more colorful than fire corals, which are typically tan to mustard colored.

Figure 1: Fire corals (Family Milleporidae, lower center) have extremely powerful nematocysts (stinging cells) on their tentacles which extend through thousands of tiny pinhole-sized pores (photograph: Leo Roest).

Anthozoa

All other corals belong to the Anthozoa class. Organisms that belong to class Anthozoa can be grouped into one of two major subclasses: Hexacorallia and Octocorallia. Each of these subclasses is divided into multiple orders and subsequent families.

Figure 2: An overview of the current consensus of coral taxonomy. The subclasses Octocorallia and Hexacorallia are in bold, orders are in semibold and suborders are in regular text format (image: Tim Wijgerde).

- subclass Hexacorallia

Subclass Hexacorallia includes stony corals, black coals, some soft corals, and a variety of anemones. There is quite a bit of variation in hexacoral polyps, but they tend to exhibit symmetry in multiples of six; hexacorals can be solitary or colonial. Subclass Hexacorallia is split up into six orders. Two of these orders have well-developed skeletons: Scleractinia, stony corals with calcareous skeletons, and Antipatharia, black corals with organic skeletons. The other four orders are composed of organisms without solid skeletons: Actinaria - true anemones, Ceriantharia - tube anemones, Corallimorpharia - mushroom corals, Zoanthidae -zoanthids or button polyps.

- order Scleractinia

The Scleractinia are generally referred to as stony corals or hard corals. There are over 1300 species of scleractinians; these corals are extremely diverse and have an aragonite skeleton where each polyp is housed within its own calcium carbonate cup called a corallite that protects its soft body. Colonial hard corals are called reef-building or hermatypic corals because they contribute substantial amounts of limestone to reef structures. Most hermatypic corals are zooxanthellate as well, although some deep water species lack zooxanthellae. Aquarists group stony corals into two primary categories: SPS (small polyp stony) corals and LPS (large polyp stony) corals. Some common SPS corals are Acropora, Montipora, Stylophora, Porites, and Pocillopora. Some common LPS corals are Euphyllia, Trachyphyllia, Favites, Galaxea, and Tubastrea.

"The Scleractinia are generally referred to as stony corals or hard corals. There are over 1300 species of scleractinians; these corals are extremely diverse and have an aragonite skeleton ."

- order Antipatharia

The order Antipatharia is made up of black corals. They resemble gorgonians (which are categorized in the Octocorallia subclass) but have six simple tentacles on each polyp instead of eight pinnate tentacles, and they don’t have a sclerite-filled rind like octocorals have. Black coral polyps are permanently extended and cannot withdraw into the coenenchyme, which is the colonial tissue between polyps. This order contains several  families such as the Antipathidae, and many genera such as Antipathes, Cirrhipathes and Leiopathes. Black corals from this last genus may exhibit extreme longevity; Leiopathes glaberrima has a lifespan in excess of 4,000 years, making it the oldest known marine organism in existence.  Many species of black corals are threatened or endangered because they are collected to make jewelry and have been completely wiped out in parts of the tropics due to high demand in the deep diving industry.

Figure 3: Black corals, such as this Antipathes sp., have a black skeleton and non-retractable polyps. This species grows in a spiral-shaped pattern, characteristic for these corals (photograph: Poppe Images).

- order Actiniaria

There are over 1,000 species of sea anemones in the order Actiniaria. These “true” anemones are soft-bodied, solitary anthozoans which don’t secrete a calcareous skeleton. They have an adhesive pedal disk with which they anchor themselves to the substrate, and an oral disk with a mouth and tentacles. Many of them can deliver a powerful sting by discharging nematocysts located in their tentacles. Most actiniarians harbor symbiotic zooxanthellae like hermatypic corals do, which greatly supplement their nutrition. Two well-known actiniarians in the aquarium hobby are the ever-popular bubble tip anemone, Entacmaea quadricolor, and the dreaded glass anemone, genus Aiptasia.

- order Ceriantharia

The order Ceriantharia represents the tube-dwelling anemones. They live in leathery tubes that extend far into the substrate, offering them quick cover from predators. Their ability to move so quickly is a unique characteristic of this order that is possible because of the sphincter muscle they have at the bottom of their oral disk. They are represented by two families: Arachnanthidae and Cerianthidae.

- order Corallimorpharia

The order Corallimorpharia includes the mushroom corals, which are actually taxonomically considered to be anemones (often called disc anemones). As their name suggests, corallimorph polyps resemble stony corals in their anatomy, but they lack a skeleton. There are four families represented by corallimorphs: Actinodiscidae, which are smooth with tiny tentacles (e.g. Actinodiscus, Rhodactis), Corallimorphidae, which have elongated tentacles (e.g. Corallimorphus, Corynactis), and Discomatidae and Ricordeidae, which are brightly-colored, zooxanthellate, warm water species with bump-shaped tentacles (e.g. Discosoma, Ricordea). 

- order Zoanthidea

The order Zoanthidea is comprised of anthozoans that lack a secreted skeleton; they have small anemone-like polyps with tentacles surrounding the periphery of the oral disk. Zoanthids are almost always colonial. There are three families of zoanthids: Epizoanthidae, Parazoanthidae, and Zoanthidae. In the aquarium hobby, the family Zoanthidae is the most commonly represented out of the three, with many species of Zoanthus and Palythoa being extremely popular corals. 

"The classification of organisms in the subclass Octocorallia is much more ambiguous than the Hexacorallia subclass and is currently in a state of flux."

- subclass Octocorallia

The classification of organisms in the subclass Octocorallia is much more ambiguous than the Hexacorallia subclass and is currently in a state of flux. Gorgonians are the most common and diverse of the octocorals and are characterized by a tree-like body plan. Octocoral polyps have eight pinnate tentacles; a network of gastrovascular canals (solenia) allows food/nutrient sharing throughout an entire colony. Traditionally, octocorals are divided into three orders: Helioporacea - blue coral (Heliopora coerulea), Pennatulacea - sea pens and sea pansies, and Alcyonacea - the rest of the octocorals.

- order Alcyonacea

The current single order Alcyonacea was historically represented by four orders instead of one: Alcyonacea, Gorgonacea, Stolonifera, Telestacea. The most current arrangement has instead grouped these four octocoral orders into the single order Alcyonacea and divided this order into five suborders: Calcaxonia, Holaxonia, Alcyoniina, Scleraxonia, and Stolonifera.

Figure 4: This yellow unidentified gorgonian is a member of the Scleraxiona suborder, order Alcyonacea and subclass Octocorallia. They often live with symbiotic snails, which may be parasitic (photograph: Poppe Images).

Suborders Calcaxonia, Holaxonia and Scleraxonia are all gorgonians. Gorgonians are a diverse group of octocorals that have a skeleton made of gorgonin (a complex fibrous protein) and/or calcium carbonate. Their skeleton is covered with a rind much like that of soft corals, with an extremely high concentration of sclerites and they can completely withdraw their polyps into the coenenchyme.

- suborder Stolonifera

The suborder Stolonifera is made up of corals that lack a coenenchyme; these “simple” corals typically resemble large hydrozoan colonies such as the genus Carijoa (often referred to as telestaceans), but also include the unique organ-pipe coral (Tubipora). Alcyoniina are the true soft corals and they include xeniids, neptheids, leather corals, colt corals, etc. They have a thick, fleshy coenenchyme with many sclerites in it. This type of tissue allows soft corals to take on a wide variety of shapes. Soft corals have siphonozooids, which are responsible for inflating the colony and for reproduction, and grastrozooids for eating.

- order Helioporacea

The order Helioporacea (blue coral) contains only two families (Helioporidae and Lithothelestidae) and three genera. These octocorals possess a skeleton of fibrocrystalline aragonite and their characteristic blue color is derived from iron salts. The genus Heliopora is often referred to as a living fossil because it has existed in the same general form since the Cretaceous (150-65 mya).

"Sea pens are extremely unique octocorals that have three different types of zooids: primary zooids, gastrozooids, and siphonozooids."

- order Pennatulacea

The order Pennatulacea is made up of fourteen families and encompasses a complex group of octocorals (primarily sea pens) that are easily-distinguishable from other octocoral clades. The physiologic structure of sea pens is quite different from other octocorals. They have three types of zooids: (1) primary zooids form the center axis of a colony called the rachis and have a root-like base or peduncle that is used to anchor the colony into sediment. (2) Gastrozooids are the feeding polyps and (3) siphonozooids function as water intake structures. Sea pens inhabit deeper water with less turbulence of 2,000 meters or more, and some can bioluminesce. 

Figure  5: Sea pens are extremely unique octocorals that have three different types of zooids: primary zooids, gastrozooids, and siphonozooids. They often harbour unique symbiotic animals, such as this unidentified crab species (photograph: Poppe Images).

Concluding remarks

Many changes have occurred in coral taxonomy over the last decade and will continue to occur as we learn more about the interrelatedness of corals through genetic studies. While reading this article please keep in mind that there are some discrepancies in coral taxonomy, especially with regard to octocorals, and that there are several conflicting schools of thought on their “correct” classification.

References

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Borneman, E. H. 2001. Aquarium Corals: Selection, Husbandry and Natural History. Microcosm/TFH, Neptune City. pp 464.

Dubinsky, Z. (Ed.) 1990. Coral Reefs: Ecosystems of the World. Volume 25. Elsevier Sci. Publ., Amsterdam.

Fabricius, K. and P. Alderslade. 2001. Soft Corals and Sea Fans. AIMS, Townsville. pp 264.

Humann, P. and N. Deloach. 2006. Reef Coral Identification: Florida, Caribbean, Bahamas. New World, Jacksonville, FL.

Veron, J.E.N. 2000. Corals of the World. Australian Institute of Marine Science, Townsville. 3 Volumes.

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