STIMULATION OF METAMORPHOSIS IN THE POLYCHAETE HYDROIDS ELEGANS HASWELL (SERPULIDAE)

The serpulid polychaete Hydroides elegans is a comment cosmopolitan wa rm-water biofouling organism. Competent larvae of H. elegans metamorph ose rapidly after induction by marine biofilms. Only 15 min after comi ng in contact with the metamorphic cue, larvae have completed secretio n of the primary tube; secretion of the secondary, calcareous tube beg ins 1.5 h after the primary tube has been deposited. Metamorphosis is characterized by disappearance of the prototroch and differentiation o f the tentacular crown in the head region, the collar and thoracic mem brane in the thoracic region, and the pygidium at the tip of the abdom en. These morphogenetic events were used to gauge the responses of lar vae to biofilms, as well as to the artificial inducers Cs+ and K+ A ma ximal metamorphic response to the two ions requires exposure to differ ent 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 respon se to Cs+ or K+ is much slower than the response to biofilms, demonstr ating that the tissues respond differently to artificial inducers. The differences in the kinetics of the responses to the natural and catio nic inducers suggest that the induction mechanisms are not the same. W hen these artificial inducers were used, some, but not all, of the met amorphosed juveniles never attached to the substratum or secreted a pr imary tube, probably as a result of secondary effects of the ions on p rocesses of tube formation. The exact mechanisms by which Cs+ and exce ss K+ induce metamorphosis are still unclear, although we assume, as d o others, that these agents act by depolarizing the membranes of excit able sensory cells and not by interacting with specific receptors.