Direct observation, and evidence from early shell dimensions, indicate that almost all pectinids and spondylids have planktotrophic development, while some pectinids, cyclochlamidids and propeamussiids have lecithotrophic development. Propeamusiids and cyclochlamydids with small adults generally have brooded direct development. For planktonic pectinids, size at metamorphosis and development period before metamorphosis both decrease with increasing temperature. Detailed studies of larval development, physiology, behaviour and ecology have only been performed with members of the Pectinidae. These have broadcast spawning, and unprotected embryogenesis with spiral cleavage, leading to stereoblastula, gastrula, trochophore and veliger stages. The D-shaped early veliger shell eventually develops low umbones and species-specific features of the taxodont dentition of the hinge. The veliger is characterised by a velum with profuse ciliation arranged in an opposed-band feeding and locomotory array. The main musculature consists of branching velar and posterior retractors, and adductors. During veliger growth, the larva develops a battery of sense organs: an apical organ, eye spots, mantle receptors and, in the pediveliger, statocysts. These are linked to a nervous system composed of paired ganglia, connectives and diffuse fibres. The velum and many other structures are lost at metamorphosis, while gills appear and proliferate. Development is initially fuelled by yolk reserves and then by ingested phytoplankton, the quality and quantity of which determine success at metamorphosis. Laboratory and field studies of larval swimming have built a picture of an initial surface-seeking phase, followed by a feeding and dispersal phase and then a settlement phase in which the adult environment is sought. To date the precise nature of environmental triggers for metamorphosis have not been determined, though settlement appears to be promoted by features of the benthic substrate such as filamentous algae. Larval development is responsive to water temperature and salinity, and is sensitive to a wide range of pollutants. Ocean acidification may prove to be a serious threat to larval survival. Modelling of larval dispersal is starting to promise location-specific predictions of recruitment to the adult environment. Improved understanding of factors affecting larval development is increasingly guiding control of disease, nutrition and rearing conditions in aquaculture.
|Title of host publication||Scallops|
|Subtitle of host publication||biology, ecology, aquaculture and fisheries|
|Editors||Sandy E. Shumway, G. Jay Parsons|
|Number of pages||53|
|Publication status||Published - 1 Mar 2016|