The mode of life of Mysella cuneata, a bivalve 'commensal' with Phascolion strombi (Sipunculoidea)

Abstract

The minute erycinacean bivalve, Mysella cuneata, lives in the burrow system of the sipunculid, which occupies discarded gastropod shells. Specimens were collected monthly from dredgings in Buzzards Bay, Mass. in 1965. The bivalve was found to be a protandrous hermaphrodite, becoming ripe in summer. The larvae are brooded to a shelled veliger stage within the mantle cavity of the adult; later larval stages are most probably plank tonic. Although plotting a size/frequency distribution of the M. cuneata, found associated with Phascolion strombi in the dredgings, did not show any clear pattern, an analysis of measurements of the annual growth-checks on the shells distinguished at least four year-classes.In the laboratory, bivalves isolated from their 'host' were quite motile and would reassociate with the worm. Experiments established that the bivalves respond by a direct orthokinesis to an unknown chemical factor from the sipunculid. Trials also demonstrated a positive geotaxis. A positive klinolaxis in uniform and gradient water-flow, and a response of circling which may be a 'klinokinesis', was demonstrated in some experiments in a trough with conditions simulating the 'host' microenvironment; under these conditions, too, well-defined turning movements were shown at a chemical boundary. The responses are shown to function in reestablishing their association with the 'host' in the laboratory, and to account for their posture in the worm-burrow. Although such reassociation perhaps occurs only rarely in nature, the behavior could certainly operate to initiate the association by a free-living larva.A preliminary review of behavior in the Erycinacea is included.