Unusual cohabitation: sipunculan‐associated solitary corals

Abstract

Heterocyathus aequicostatus and Heteropsammia cochlea are two species of solitary Anthozoan corals endemic to the Indo-Pacific. These species live along sandy bottoms at depths equal to or greater than approximately 20 m; where present, they can be locally dense within small areas. Each of the two corals may engage in a unique form of commensalism with a sipunculan worm (Aspidosiphon sp), which moves the coral about on substrates and prevents it from becoming buried in sediment or toppled upside-down. The symbiosis begins when the coral larva settles on a small mollusk shell already inhabited by a sipunculan worm. The developing coral then grows over the shell, but leaves open an orifice so that the worm may survive and perform its commensal duties. Given that the two coral species belong to different taxonomic families, this is a possible example of convergent coevolution.