Swimming in Amusium pleuronectes (Bivahia: Pectinidae)*

Abstract

Aspects of functional design in the Pectinidae have led scientists to speculate that scallops of the Amusium group (Hertlein, 1969) are among the best swimmers. This study of Amusium pleuronectes confirms this view, with average swimming speeds ranging from 37–45 cm/second. A speed of 73 cm/second was recorded from one scallop. Swimming times varied, on average, between 8–10 seconds; one scallop, however, swam for 18 seconds, covering a distance of 10 metres.This study has also investigated pertinent aspects of the functional design of A. pleuronectes, including the adaptations to the shell, mantle and musculature that increase swimming efficiency: These include: (1) A thin, very smooth, biconvex, rounded shell; (2) Internal strengthening ribs; (3) A small valve convexity; (4) A more central point of maximum valve convexity; (5) Upturned antero‐ and postero‐lateral upper valve margins; (6) Emarginated anterior and posterior shell margins; (7) A highly elastic ligament; (8) A single large, centrally located, adductor muscle; (9) Small perpendicularly oriented “slow” and large obliquely oriented “quick” adductor muscle components; (10) Expulsion of water both antero‐and postero‐dorsally; (11) Highly muscular middle and inner mantle folds acting as a (mechanical) “valve” to direct and regulate water flow.It has occasionally been reported in the literature that small scallops swim “better” than large specimens of the same species; this has been investigated for A. pleuronectes and the general conclusion is that large scallops can swim just as well as small specimens (indeed distances covered are greater in animals of the former category), but that the threshold stimulus necessary for swimming to occur is raised. This is true also of the escape reaction; small scallops are much more likely to perform an escape response of between one to three adductions than large specimens which seem to rely more on sustained adduciion for protection. Earlier authors have advocated ontogenetic allometric growth changes in the position and obliquity of the “quick” component of the adductor muscle to explain how larger scallops overcome the increasingly negative forces of gravity and drag. No evidence of such allometric changes have been recorded for A. pleuronectes and the present author believes that the increasingly negative forces of drag and gravity are easily overcome by scallops of any size. Increasing weight with age and physiological ageing of the adductor muscle is probably sufficient to account for differences in swimming ability between small and large specimens of A. pleuronectes.It is finally concluded that swimming in A. pleuronectes is not a response to predation. Circumstantial evidence is presented which, when considered with the efficiency of swimming and aspects of functional design in this species, suggests that swimming is concerned with active and seasonal migration, possibly linked to reproduction.