Stimulation of metamorphosis in the polychaete Hydroides elegans Haswell (Serpulidae)
E. Carpizo-Ituarte and M. G. Hadfield
The serpulid polychaete Hydroides elegans is a common, cosmopolitan warm-water biofouling organism. Competent larvae of H. elegans metamorphose rapidly after induction by marine biofilms. Only 15 min after coming in contact with the metamorphic cue, larvae have completed secretion of the primary tube; secretion of the secondary, calcareous tube begins 1.5 h after the primary tube has been deposited. Metamorphosis is characterized by disappearance of the prototroch and differentiation of the tentacular crown in the head region, the collar and thoracic membrane in the thoracic region, and the pygidium at the tip of the abdomen. These morphogenetic events were used to gauge the responses of larvae to biofilms, as well as to the artificial inducers Cs+ and K+. A maximal metamorphic response to the two ions requires exposure to different concentrations and durations, i.e., a 3-h pulse of 10 mM CsCl, or a 24-h continuous exposure to 50 mM excess KCl. The metamorphic response to Cs+ or K+ is much slower than the response to biofilms, demonstrating that the tissues respond differently to artificial inducers. The differences in the kinetics of the responses to the natural and cationic inducers suggest that the induction mechanisms are not the same. When these artificial inducers were used, some, but not all, of the metamorphosed juveniles never attached to the substratum or secreted a primary tube, probably as a result of secondary effects of the ions on processes of tube formation. The exact mechanisms by which Cs+ and excess K+ induce metamorphosis are still unclear, although we assume, as do others, that these agents act by depolarizing the membranes of excitable sensory cells and not by interacting with specific receptors.