Sponge Zoanthid

The polyps of Umimayanthus parasiticus superficially resemble rather small sea anemones. They grow either singly or in small groups of two or three on the surface of certain species of sponge. The polyps are joined by a thin layer of tissue called coenenchyme and occur at densities of 3 to 10 per square centimetre (20 to 60 per square inch). The distances between the polyps widens as they spread out over the surface of the host sponge by budding. The polyp's column is encrusted with sand particles and calcareous material and is white. The oral disc is up to 6 millimetres (0.24 in) in diameter and there are two rings of tentacles round the edge with about 14 tentacles in each. These are brown and contain symbiotic zooxanthellae, unicellular photosynthetic algae which supply the polyp with nutrients while benefiting from its nitrogenous waste.

Umimayanthus parasiticus is found in the Caribbean Sea, the Gulf of Mexico, Florida, Bermuda and the Bahamas at depths varying from 7.5 to 30 metres (25 to 98 ft). It is common where its host sponges are found. These include Cliona deletrix and other Cliona spp., Gelloides ramosa, Callyspongia vaginalis and Spheciospongia spp..