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

The burrowing behaviour of the bivalve Mya arenaria from tidal flats o f the Dutch Wadden Sea has been observed and recorded. Compared to oth er bivalves, M. arenaria is a very slow burrower, its burrowing behavi our being unique among bivalves since it is based essentially on the e jection of water through the pedal gape, with little assistance by the foot, which performs only an anchoring function. Water ejection is sp ecially powerful and individual jets may last several seconds, thus co nstituting an effective means of removing sand from below the animal d uring digging. This hydraulic burrowing is more effective in loose san dy than in cohesive muddy substrates. Water ejection is provided by th e 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 dorsoventrally be tween 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 ligamen t acts in bending, the axis of motion being placed internally with res pect to the cardinal axis. This leads to approaching of the umbones wi th complete adduction and to resorption of the left umbo. From the ada ptive point of view, the slow hydraulic mode of burrowing is sufficien t to cope with the slow sedimentation and erosion rates of the tidal f lats in which M. arenaria lives. This burrowing mode implies the exist ence of a tiny foot, which leaves room for other organs within the man tle cavity. This, together with anterior divarication bf the valves pe rmits a large volume of water to be ejected from the mantle cavity, bu t, in the case of M. arenaria, also the existence of an enormous stoma ch, possibly as an adaptation for food processing.