HYDRAULIC BURROWING IN THE BIVALVE MYA ARENARIA LINNAEUS (MYOIDEA) AND ASSOCIATED LIGAMENTAL ADAPTATIONS

Author： CHECA ANTONIO G. CADÉE GERHARD C.

ISSN： 1464-3766

Source： Journal of Molluscan Studies, Vol.63, Iss.2, 1997-05, pp. : 157-171

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Abstract

The burrowing behaviour of the bivalve Mya arenaria from tidal flats of the Dutch Wadden Sea has been observed and recorded. Compared to other bivalves, M. arenaria is a very slow burrower, its burrowing behaviour being unique among bivalves since it is based essentially on the ejection of water through the pedal gape, with little assistance by the foot, which performs only an anchoring function. Water ejection is specially powerful and individual jets may last several seconds, thus constituting an effective means of removing sand from below the animal during digging. This hydraulic burrowing is more effective in loose sandy than in cohesive muddy substrates. Water ejection is provided by the ability of the bivalve to rock its valves across a dorsoventral axis. This rocking motion implies special modifications of the ligamental area. The ligament is conical in appearance and runs dorso-ventrally between the two chondrophores which are placed in two planes parallel to the cardinal axis. During rocking the whole ligament acts in torsion and the lamellar layer of the ligament opposes closing of the anterior part of the valves. During normal adduction of the valves the ligament acts in bending, the axis of motion being placed internally with respect to the cardinal axis. This leads to approaching of the umbones with complete adduction and to resorption of the left umbo. From the adaptive point of view, the slow hydraulic mode of burrowing is sufficient to cope with the slow sedimentation and erosion rates of the tidal flats in which M. arenaria lives. This burrowing mode implies the existence of a tiny foot, which leaves room for other organs within the mantle cavity. This, together with anterior divarication of the valves permits a large volume of water to be ejected from the mantle cavity, but, in the case of M. arenaria, also the existence of an enormous stomach, possibly as an adaptation for food processing.