Induction of larval settlement and metamorphosis by pharmacological and conspecific associated compounds in the serpulid polychaete Hydroides elegans

Field populations of the serpulid polychaete Hydroides elegans occur in dense aggregations. Preliminary laboratory assays showed that planktonic larvae H. elegans from Hong Kong waters did not settle and metamorphose without proper chemical cues and could remain planktonic up to 14 d in laboratory culture. Adult H. elegans capture conspecific larvae with feeding tentacles but cannot readily consume older, competent larvae. Contact between adult feeding tentacles and larvae may increase larval exposure to adult associated inductive compounds. In this study, we tested the effects of homogenates of adult worms and their tubes, as well as a variety of artificial inducers, on settlement and metamorphosis of H. elegans larvae. Conspecific adult homogenates induced 39 and 82% of larvae to settle and metamorphose within a period of 2 and 4 d, respectively. Homogenates of the adult tube alone did not induce settlement, indicating that the inducer originates from the worm, Extraction and assays on crushed adult homogenates revealed that the inductive compounds from adults are smaller than 10 000 daltons and can be bound to amberlite XAD-7. Further isolation and identification of the conspecific associated inducer will enable studies of chemoreceptors and signaling pathways involved in metamorphosis. Additionally, among 5 artificial inducers tested, isobutyl methylxanthine (IBMX) induced a high percentage of normal metamorphosis while gamma-aminobutyric acid (GABA), choline chloride, dihydroxyphenyl L-alanine (L-DOPA), and potassium chloride evoked a low percentage of settlement, but abnormal metamorphosis. Ammonia had no effect on the metamorphosis of H. elegans.