This group of cnidarians, the anthozoans (Anthozoa), has only a polyp phase, and includes more than 6,000 exclusively marine species, among which we find anemones, corals, gorgonians or sea pens.
Generally, anthozoans have a cylindrical body, of which the aboral part is the one that allows them to attach to the substrate, while the apical part is where the mouth and tentacles surround it. The tentacles have stinging cells (cnidocytes), which can be very powerful. Anthozoans are carnivores, capturing their prey with their tentacles and immobilizing them with their stinging cells. Many species supplement their diet with sugars generated by unicellular algae (zooxanthellae), which are inside the body of anthozoans forming a symbiotic relationship with them. These anthozoans that possess zooxanthellae usually live in shallow and well illuminated waters forming coral reefs. We can consider this group of species as the main creators of reefs, since there are species that form an exoskeleton composed of calcium carbonate, which when accumulated forming large colonies, form coral reefs. Those species lacking symbiotic algae are usually found at greater depths.
Anthozoans have a radial symmetry, and three major subclasses can be distinguished.
- Hexacoralarians or subclass Hexacorallia: They have a symmetry based on 6 rays. Among the hexacoralarians we find species such as the anemones (order Actiniaria), the so-called black corals (order Antipatharia), the so-called false corals (order Corallimorpharia), the zoantharians (order Zoantharia) and the stony corals or also called hard corals (order Scleractinia).
The Octocoralaria or subclass Octocorallia: They have a symmetry based on 8 rays. They are subclassified into three orders: soft corals and gorgonians (Alcyonacea), helioporaceans or sometimes known as blue corals (Helioporacea) and sea pens (Pennatulacea).
The cerianthids or subclass (Ceriantharia): Although these species resemble the anemones, they are a separate draught.
Reproduction of anthozoans may be asexual or sexual. In anemones and hard corals, the polyp releases the gametes, these are transformed into planulae larvae after fertilization, which live pelagically until they find a place to settle. Asexual reproduction is usually by budding.