An unusual symbiotic relationship between a cyclostome bryozoan and a thecate hydroid

Previous examples of symbiotic associations between bryozoans and hydrozoans mainly involve cheilostome bryozoans and athecate hydroids belonging to the Family Zancleidae, in which only the hydrorhizal stolons become covered by a calcareous layer secreted by the bryozoan. The aim of this study is to describe an unusual symbiotic relationship between the cyclostome bryozoan Disporella densiporoides and the stolonal thecate hydroid Filellum bouvetensis (Family Lafoeidae) from Isla de los Estados, Southwest Atlantic. We found more than 120 chimney-like calcareous tubes covering most of the hydrozoan symbiont produced by only one bryozoan colony. Similar chimney-like tubes built by other cyclostome bryozoans have been attributed to the association with sedentary polychaetes or mistakenly interpreted as an ooeciostome, i.e. the aperture of a bryozoan reproductive polymorph. This is the first example of a symbiotic relationship in which most of the hydrozoan colony becomes covered by the bryozoan skeleton. The brood-chamber of D. densiporoides, previously unknown, is described and illustrated.