Gregarious and Nongregarious Larval Settlement in the Aggregation-Forming Annelid <i>Ficopomatus enigmaticus</i>

Gregarious larval settlement, a phenomenon in which cues associated with conspecifics induce larval settlement, plays a role in the growth of existing aggregations of many aggregation-forming sessile marine invertebrates. The formation of new aggregations, however, requires larvae to settle in response to other cues. The mechanism underlying this variation in larval settlement responses is unknown for most species with gregarious settlement. In this study we first present evidence that larvae of the serpulid annelid Ficopomatus enigmaticus settle gregariously. In no-choice, still-water experiments, a much higher percentage of larvae settled after 24 h of exposure to conspecific tube than after exposure to mussel shell collected from the same habitat. We then tested the hypothesis that larvae of F. enigmaticus display a genetically determined dimorphism in settlement behavior like that of the serpulid Hydroides dianthus, with most larvae capable of settling only in response to a conspecific cue but a small percentage of larvae capable of settling only in response to a biofilm cue. If this hypothesis is correct, the sum of the percentages of larvae that settle in response to a conspecific cue and those that settle in response to a biofilm cue cannot exceed 100% (since each larva can accept only one of the two cue types throughout its competent period). Our data on F. enigmaticus are not consistent with this prediction, suggesting that individual larvae can respond to multiple types of settlement cues during their competent period. This has significant implications for how frequently larvae can form new aggregations, a topic of special importance for F. enigmaticus and H. dianthus, both of which are well-known invasive species in marine habitats around the globe.