Morphological changes and role of substrate during early bivalve development

Substrate characteristics can be important in controlling the post-settlement development of both epifaunal and infaunal bivalve species. Overall objectives of this study were (1) to describe the ontogeny of key pallial structures in terms of shell height in two epifaunal species, the bay scallop Argopecten irradians and sea scallop Placopecten magellanicus and to determine scallop sizes at which substrate associated pedal-feeding on benthic algae is most likely to occur, and (2) to determine the influence of substrate (sediment type and benthic film) on the softshell clam, Mya arenaria, postlarval and juvenile growth, shell morphology, survival and behaviour.;The effects of different sediment types versus the absence of sediment were tested on Mya arenaria postlarvae and juveniles fed mixed algal diet. Results demonstrated that sediment plays a role in controlling growth rate and shell morphology of Mya arenaria. Furthermore, substrate needs changed during post-metamorphic development and one key size was determined at ∼829 μm shell length (SL). This size coincided with the fusion of the mantle lobes and therefore a change from epifaunal to infaunal species. Softshell clams grew better in the absence of sediment when epifaunal, but grew better in sediment when infaunal. The effects of a Fragilaria familica benthic film indicated that the association of both suspended and settled algae enhanced the growth of softshell clams until they attained a shell length of ∼829 μm. Although postlarvae in the benthic film alone showed much lower growth rates than in treatments in which algae were suspended, significant positive growth of soft tissues provided evidence of pedal-feeding during early post-settlement.;This study described pronounced ontogenetic changes during postlarval development of two scallop species that have important implications for feeding function and confirmed that substrate plays an important role during early stages of infaunal species.;The ontogeny and function of feeding organs in post-settlement bivalves is poorly understood. Scanning electron microscopy of bay and sea scallop postlarvae was used to characterize the development of feeding organs, including the gill, foot, labial palps, lips and mantle. Gill development followed a sequence of events similar to that previously described in other pectinids, with evolution from a homorhabdic to a heterorhabdic condition. Suspension-feeding may be relatively ineffective until the gill is reflected and food tracts are formed, and thus pedal-feeding might provide an alternate early feeding mode. The foot evolved from a simple to a more complex form, with a pronounced basal hump and a distinct ciliary tract from the base of the palps to the mantle margin. A second mantle ciliary tract was located anteriorly and ended at the byssal notch. It is proposed that both foot and mantle ciliary tracts are involved in particle rejection at stages when hydrodynamic expulsion by clapping, the mechanism described in adult scallops, is not fully functional. The labial palps and arborescent lips suggested to aid in particle retention during clapping, do not attain their adult form until fairly late in development when a progressive reduction of both foot and mantle ciliary tracts was observed. Thus pedal-feeding may be a transient but important phase especially in sea scallops that have more protracted development. The foot and mantle ciliated tracts may facilitate particle rejection in early postlarvae before known adult rejection mechanisms become fully effective.