Response of competent blue mussel (Mytilus edulis) larvae to positive and negative settlement cues

Recent work on larval settlement cues has emphasized mechanisms by which larvae exploit individual, positive cues (cues that larvae move toward), often in complex flow fields. Yet in natural systems, larvae of habitat generalists probably respond to multiple settlement cues, including a mixture of positive and negative cues. First, a simple test chamber in which cue dispersal was dominated by diffusion was used to assess whether competent blue mussel (Mytilus edulis) larvae responded negatively or positively to cues from a variety of intertidal species. Second, choice experiments tested the responses of larvae offered a mixture of conflicting (positive and negative) cues from the same direction and conflicting cues from different directions. Responses to individual cues were predictable from established ecological interactions. Larvae were attracted to odors from conspecifics, tended to move toward odors from a filamentous alga, avoided odors from two predators of post-settlement mussels, and exhibited little response to odors from an herbivorous gastropod. Negative and positive cues offered from the same direction produced movement both toward and away from the mixture, while offering the combined cues from different directions resulted in net movement that was largely consistent with predictions from the individual cue responses. Larvae presented with a choice between two negative cues mainly remained where they started, in the middle of the test apparatus, and exhibited no choice. These results highlight the need to consider the combined effects of a broad range of cues when evaluating the net effect of cues on settlement in the field.