The order Zoantharia, commonly known as zoanthids or colonial anemones, comprises benthic cnidarians that exhibit characteristics intermediate between sea anemones (Actinaria) and soft corals. They are distinguished primarily by their cylindrical polyps, arranged in colonies, which attach to the substrate through an encrusting base. One of their most notable features is the presence of a coenenchyme, a common tissue connecting the polyps that, unlike hard corals, lacks a calcareous skeleton, though some species can incorporate mineral particles or detritus to reinforce their structure. Approximately 300 species of Zoantharia have been described, distributed in tropical, subtropical, and temperate seas, typically associated with coral reefs, rocky bottoms, or even living organisms such as sponges and corals.
Morphologically, zoanthids possess small to medium-sized polyps, typically ranging from 5 to 20 mm in diameter, with tentacles arranged in two concentric rings around the mouth, usually in multiples of six, giving them hexaradial symmetry. Their coloration is highly variable, with shades of brown, yellow, green, or orange, influenced both by their own pigments and by the presence of symbiotic algae (zooxanthellae) in their tissues. Unlike hard corals (Scleractinia), they do not form massive skeletons, but some species can consolidate surrounding sediment or incorporate calcareous fragments for greater stability.
In terms of trophic ecology, most zoanthids are mixotrophic. They maintain a close symbiotic relationship with zooxanthellae, which provide a significant portion of their energy through photosynthesis, explaining their predominance in clear, shallow waters. They complement their diet by capturing plankton and dissolved organic matter, using their cnidocyte-bearing tentacles to immobilize small prey. Deep-water species lacking zooxanthellae rely exclusively on heterotrophic feeding.
The sub-classification of the order Zoantharia includes several main groups. Encrusting zoanthids (suborder Macrocnemina, families such as Zoanthidae and Epizoanthidae) are characterized by extensive colonies that cover rocks, with polyps partially embedded in the coenenchyme. Free or non-encrusting zoanthids (suborder Brachycnemina, families such as Sphenopidae) feature more exposed individual polyps and often inhabit crevices or live on other organisms, such as scleractinian corals. Finally, symbiotic zoanthids (family Parazoanthidae) form close associations with sponges, hydrozoans, or gorgonians, benefiting from the protection and support provided by these hosts.
Zoanthids play an important ecological role in marine ecosystems, contributing to benthic coverage and structural complexity in reefs. Some species produce potent toxins, such as palytoxin, considered one of the most toxic non-protein substances known, which provides defensive advantages against predators. Their morphological diversity and symbiotic adaptations make them a key group for studying evolutionary and ecological processes in marine environments.
