Corals, key players in marine ecosystems, are marine animals classified within the phylum Cnidaria , which also includes jellyfish and anemones. Corals are mainly divided into two subclasses: Hexacorallia (hard corals) and Octocorallia (soft corals). Each of these categories includes corals with distinct biological characteristics. To better understand this classification, let's explore the different taxonomic levels used to group corals and the types of corals found in each category.
1. Taxonomic classification of corals
Here are the main taxonomic levels to which corals belong:
Kingdom : Animalia (Animals)
Phylum : Cnidaria (Cnidarians)
Class : Anthozoa (Anthozoa)
Subclass : Hexacorallia (hard corals)
Subclass : Octocorallia (soft corals)
2. Hexacorallia (hard or scleractinian corals)
Hexacorallia include hard corals , also called scleractinians , which are the primary reef builders. These corals have a calcareous skeleton made of calcium carbonate that forms the physical structure of coral reefs. Hexacorallia are characterized by the presence of six tentacles or a number of tentacles that are multiples of six around the mouth.
Types of hard corals:
Branching corals : Corals of the genus Acropora are the best known among branching corals. They form branches that grow rapidly, contributing to the rapid formation of reefs.
Massive corals : These corals, like those of the genus Porites , are characterized by their round and compact shape. They grow slowly but are extremely resistant to physical disturbance.
Foliate corals : Montipora is a genus of foliate corals that grow in thin, overlapping plates. These corals maximize the surface area of contact with sunlight, which is necessary for the photosynthesis of the zooxanthellae that live in their tissues.
These hard corals live in symbiosis with microscopic algae, zooxanthellae , which provide them with nutrients through photosynthesis, allowing the corals to grow rapidly and form massive colonies. They form the physical basis of tropical coral reefs.
3. Octocorals (soft corals)
Octocorallia , also called soft corals , differ from hard corals in their structure and composition. Unlike scleractinians, soft corals do not have a rigid calcium carbonate skeleton, but a softer internal skeleton that gives them some flexibility. They are characterized by the presence of eight tentacles around each polyp, hence their name "octo".
Types of soft corals:
Gorgonians : Gorgonians, belonging to the order Alcyonacea , appear as large, branched fans or whips. They are often found in strong ocean currents, where they capture nutrients suspended in the water.
Leather corals : These corals, also classified in the order Alcyonacea, have a thicker, stiffer texture, similar to leather. They are very colorful and form large colonies.
Whip corals : These corals grow in long, flexible and often sinuous branches. They are able to swing in the ocean currents, capturing food particles.
Octocorals are found in both shallow tropical areas and in deeper, even dark waters, where they can survive without direct sunlight. Unlike hard corals, some soft corals do not rely on symbiosis with zooxanthellae for energy.
4. Fire corals and other reef-building cnidarians
In addition to hard and soft corals, there are other groups of cnidarians that are often associated with coral reefs. This is the case of fire corals (genus Millepora ), which resemble hard corals but actually belong to the class Hydrozoa . These corals are characterized by their stinging cells capable of delivering painful stings. Although they do not build reefs as massive as scleractinians, they participate in the formation of some reef structures.
5. Solitary corals
Some corals, called solitary corals , do not live in colonies like reef corals, but grow individually. These corals, such as Caryophyllia , are often found in deeper, colder waters, where they can survive without sunlight. They do not participate in reef formation, but are nevertheless important in deep-sea ecosystems.
Conclusion
The classification of corals reflects the diversity and complexity of these marine animals. Hexacorallia (hard corals) are the main builders of coral reefs thanks to their calcareous skeleton. Octocorallia (soft corals), although they do not build massive reefs, play an important role in the biodiversity of reefs and contribute to the creation of habitats for a multitude of marine species. Together, these different types of corals form a fragile ecosystem, but essential to marine life and the balance of the oceans.
Scientific sources:
Veron, JEN (2000). Corals of the World . Australian Institute of Marine Science.
Sheppard, CRC, Davy, SK, & Pilling, GM (2009). The Biology of Coral Reefs . Oxford University Press.
Cairns, S.D. (2007). Deep-Water Corals: An Overview with Special Reference to Diversity and Distribution of Deep-Water Scleractinian Corals . Coral Reefs , 26(1), 125-142.
