Observations on burrowing in Glycymeris glycymeris (L.) (Bivalvia, Arcacea)

Abstract

The burrowing movements of Glycymeris glycymeris (L.) consist of a series of downward steps, the digging cycle, repeated cyclically during a digging period. The basic pattern of the digging cycle is similar to that of other bivalves but there are important differences in detail. In particular, dilation of the foot during stage (i) of the cycle is maintained by contraction of strongly developed transverse muscles in the visceral mass and pedal anchorage is aided by spreading of the lateral flaps of the deeply cleft foot. Adduction, at stage (iii), which further causes a decrease of a half in gape angle of the shell, results in ejection of water from the mantle cavity into the sand, but has little effect on pedal anchorage. The duration of a digging cycle varies from 0.5 to 3.0 min in both sand and gravel, the time per cycle increasing as the digging period proceeds. Fewer digging cycles are performed in sand than in gravel and the rate of penetration into gravel (0.8 mm/cycle) is slower than in sand (2 mm/cycle). The force involved in retraction is less in gravel than in sand indicating weaker pedal anchorage in gravel. In both sand and gravel, smaller individuals dig by means of a greater number of short steps repeated more rapidly than by larger individuals.