Tunicates growing on zoanthids is probably nothing to worry about
We love a good reef aquarium Critter ID mystery so when Extreme Corals shared around this photo of some odd looking things growing on their zoanthids, our inner Sherlock Holmes was on the case. The colonial nature of the benthic organism growing on the outside of the large zoanthid polyps initially led us to consider it being other zoanthids, octocorals, micro anemones, hydroids, and we even briefly considered foraminiferans, but none of these really fit the bill of what we were looking at in this close up photo.
So we turned to the master of weird reef animal identification, Mr. Julian “What is That” Sprung who had already helped us to tease out the identities of the Zanclea hydroids, Halofolliculina and other numerous odd beasties over the years. Julian immediately pegged the zoanthid critter as a type of colonial tunicate, perhaps a member of Botryllus which has been documented growing in aquaria before.
This image initially caught our attention because we are fascinated with all deliberate interactions of invertebrate animals and any of the diverse cnidarians we collectively call corals. The image produced by Extreme Corals does not appear to show any damage to the zoanthid, not even any irritation, and we don’t reckon it really intentionally grew onto the zoanthid polyp since you can see other clusters of tunicates growing on the rock in the background of the image.