A new type of symbiosis: Heterocyathus japonicus (Cnidaria: Scleractinia) living on Fissidentalium vernedei (Mollusca: Scaphopoda)

After introductory remarks on the solitary coral genera Heterocyathus and Heteropsammia, commonly known to be symbiotic with a sipunculan inhabiting a gastropod shell, Heterocyathus japonicus is redescribed in detail. It occurs from Taiwan to northern Honshu, Japan. It is the first of these symbiotic corals to be found in association with an alternative partner, living specimens of the large scaphopod Fissidentalium vernedei. The coral always occupies the apical end of the shell, the part that protrudes from the sediment. It is thus preserved from being buried and is maintained in an upright position (the same advantages it also gets when its partner is a sipunculan). On live F. vernedei, H. japonicus incrusts a manganese rich black mineral coating that characterizes the exposed, apical part of the shell. It is speculated about which factors may cause coral larvae to associate with the partner (sipunculan squatting a shell, scaphopod in its own shell).