MEPS 224:103-114 (2001)  -  doi:10.3354/meps224103

ABSTRACT: We conducted still-water time-course experiments on cultured larvae of the serpulid polychaete Hydroides dianthus (Verrill, 1873) to examine timing and patterns of gregarious and nongregarious settlement to better understand the conditions under which larvae of a gregarious species colonize new habitats. We first confirmed that these worms are aggregated in the field, and quantified patterns of association with a number of other common intertidal species. Patterns of negative association with spatial competitors is unlikely to have resulted from larval avoidance during settlement, because we found no difference in larval settlement in response to conspecifics with or without the solitary tunicate Ascidia interrupta present. We then examined the settlement of competent larvae in response to biofilm and conspecifics to determine the conditions under which the larvae of this gregarious species settle nongregariously. Larvae settled concurrently in response to both biofilm and conspecifics, starting approximately 4 d after fertilization, and response was of similar magnitude to both biofilm and to conspecifics for the first couple of days post-competency. While settlement in response to conspecifics continued in subsequent assays carried out over 70 d (at which time larvae began to die in culture), settlement in response to biofilm decreased abruptly after the peak, and ceased by Day 14. Settlement patterns were qualitatively similar regardless of previous exposure to substrata; similar results were obtained whether the whole population of larvae in culture was denied access to or daily provided access to 1 or both experimental substrata. Furthermore, these patterns of settlement did not change as larval planktonic period was prolonged. Of larvae settling during 24 h sample assays, the majority of larvae metamorphosed in response to biofilm during the first 6 h, whereas metamorphosis in the final 6 h (between 18 and 24 h) of the assays was primarily in response to conspecifics. These results are opposite to the pattern predicted by the desperate larva hypothesis, because some larvae settled in response to biofilm even after rejecting conspecifics the previous day. In fact, cultures of competent larvae appear to become more specific in terms of larval settlement in response to live adult conspecifics as the planktonic period is artificially prolonged.

KEY WORDS: Colonization · Gregarious settlement · Habitat choice · Hydroides dianthus · Polychaete