The importance of larval choice and hydrodynamics in creating aggregations of Hydroides elegans (Polychaeta: Serpulidae)

Aggregations of the sessile, serpulid tubeworm Hydroides elegans are commonly encountered on hard surfaces in Pearl Harbor, Hawai'i, and a bio-organic film has been found to be one requisite for larval settlement in this species. To determine if additional pre- or postsettlement factors influence larval distribution, we recorded settlement frequency, location, and survival on biofilmed settlement plates, either unoccupied as controls, or occupied by live juvenile or adult worms, tubes of dead adult worms, or plastic mimics of adult worm tubes. Early post-settlement mortality was extremely low in all trials (<1.0%), and thus not responsible for structuring populations of H. elegans. The hypothesis that larvae settle preferentially on or near conspecific individuals (i.e., gregariously) was falsified: larvae did not settle faster (laboratory) or in greater numbers (field) on surfaces already occupied by H. elegans or their tubes. In the laboratory, settling larvae of H. elegans did not actively select crevices created where adult worm tubes or worm-tube mimics intersected with the substratum; in the field, however, there was significantly more settlement in tube crevices than expected by chance. Because this pattern appeared only in moving water, it probably results from hydrodynamics. In flowing water, larvae are entrained in eddies shed on the leeward side of roughness elements, such as the tubes of adult worms or their mimics, and attach there if these sites are otherwise acceptable (i.e., biofilmed). A similar pattern of enhanced settlement in crevices adjacent to tubes occurred on the plates with juvenile worms on the last day of field trial 1, when the diameter of their tubes (0.6 mm) was large enough to create eddies capable of entraining conspecific larvae. Dense aggregations of H. elegans found on hard surfaces in bays and estuaries most likely result from passive deposition of larvae in crevices beside tubes of conspecific individuals, followed by selective attachment in these locations if the bio-organic film is acceptable.