Habitat selection and adult-larvae interactions in settling larvae of soft-shell clam Mya arenaria

Abstract

Like many benthic taxa, the soft-shell clam Mya arenaria L. is patchily distributed in nature. Near Halifax, Canada (Eastern Passage), M. arenaria and several other species (Polydora cornuta, Pygospio elegans, Gemma gemma, Hydrobia sp.) occur in higher densities at a sheltered site than at an exposed site ~300 m away with similar grain size but different sediment organic texture. Total faunal densities at the exposed site were comparatively low. To evaluate whether larval settlement plays a role in establishing M. arenaria patterns, laboratory flume experiments were conducted with competent M. arenaria larvae. Natural cores from the sheltered and exposed sites with resident infauna intact, as well as cores from the same sites that had been defaunated by freezing, were inserted flush with the flume bottom. Highest settlement was observed in faunated cores from the sheltered site where M. arenaria are more common. Significantly lower settlement was observed in other treatments, including defaunated cores from the sheltered site. For corresponding treatments, settlement in sediment from the exposed site was less than that at the sheltered site. Of the abundant taxa in intact flume cores, Gemma gemma densities were a significant, albeit weak predictor of M. arenaria settlement. We propose that G. gemma influence interface sediment characteristics which, in turn, result in differential larval response. Settlement patterns contradict a previous study (Emerson and Grant 1991; Limnol Oceanogr 36:1288-1300) that reported an absence of recently settled M. arenaria spat at the sheltered site, despite high numbers at the nearby exposed site. The contrast in results likely reflects our focus on settlement and their focus on spat abundance, and these results considered in tandem suggest that both pre- and post-settlement processes likely determine soft-shell clam distributions at Eastern Passage.