The polychaete Hydroides elegans (Serpulidae, Lophotrochozoa) is a problematic marine fouling organism in most tropical and subtropical coastal environment. Competent larvae of H. elegans undergo the transition from the swimming larval stage to the sessile juvenile stage with substantial morphological, physiological, and behavior changes. This transition is often referred to as larval settlement and metamorphosis. In this study, we examined the possible involvement of calmodulin (CaM) - a multifunctional calcium metabolism regulator, in the larval settlement and metamorphosis of H. elegans. A full-length CaM cDNA was successfully cloned from H. elegans (He-CaM) and it contained an open reading frame of 450 bp, encoding 149 amino acid residues. It was highly expressed in 12h post-metamorphic juveniles, and remained high in adults. In situ hybridization conducted in competent larvae and juveniles revealed that He-CaM gene was continuously expressed in the putative growth zones, branchial rudiments, and collar region, suggesting that He-CaM might be involved in tissue differentiation and development. Our subsequent bioassay revealed that the CaM inhibitor W7 could effectively inhibit larval settlement and metamorphosis, and cause some morphological defects of unsettled larvae. In conclusion, our results revealed that CaM has important functions in the larval settlement and metamorphosis of H. elegans.
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