Scallop Ecology: Distributions and Behaviour
Abstract This chapter discusses aspects of the ecology of scallops concerned with their distributions and behaviour. Geographical and local distributions of scallop populations are reviewed, concentrating on the commercial species. The geographical distribution of each species is described with, where possible, illustrations showing the location of the main fishing grounds. All scallops have aggregated distributions within their geographical range. Scallops distributions at different spatial scales are described, and absolute densities, the establishment and maintenance of spatial pattern, temporal and spatial variability, spatial differences in population parameters such as age structures, growth rates and breeding cycles, genetic variation, recruitment mechanisms and the extent to which populations on different grounds are self sustaining are discussed. Temperature is the primary factor determining the overall geographical range of a species but it interacts with water depth, substrate type, currents, food availability, turbidity, salinity and the occurrence of competitors and predators in determining the local distribution. Scallops are unusual among bivalve molluscs as many of them can swim, using a type of jet propulsion. Post larval scallops are byssally attached and some species retain the byssus throughout life. Others lose the byssus and become free living, using swimming to control their orientation, recess into the seabed and escape from predators. The behavioural mechanisms associated with byssus attachment, orientations and movements affect adult distribution. The occurrence, mechanism and function of these types of behaviour in different species are discussed, together with changes during ontogeny. Swimming is predominantly an escape mechanism but may serve other functions. Scallops react to contact by a predator by closing the shell, jumping or swimming, three distinct responses of increasing intensity. There is a trade off between the mechanical protection of a heavy shell and swimming ability. In most species, juvenile scallops are better swimmers than the adults. With growth, the thicker shell provides better protection against attack by predators but swimming capacity decreases. Adult scallops eventually reach a size where they are not easily consumed by their common predators, reducing the need for an escape strategy. Many scallops can differentiate between potential predators and harmless species: this minimises feeding interruptions and prevents energy being wasted making unnecessary escapes. Scallops swim for short distances, become rapidly fatigued and take a long time to recover after swimming. They are well adapted for making short distance escape reactions but not for extensive migrations. In areas of strong current flow swimming scallops are carried downstream. Downstream dispersals account for the majority of scallop movements but a few species may undertake limited migrations.