The mode of life of Montacuta elevata, a bivalve 'commensal' with Clymenella torquata (Polychaeta)

Abstract

The small erycinacean bivalve, Montacuta elevata, is found attached by a byssus to the lower end of the sandy tubes of Clymenella torquata. Samples of M. elevata were collected monthly in 1965 from an intertidal sand-flat in Barnstable Harbor, Mass. The bivalve was found to be a protandrous hermaphrodite, with its gonads ripening in summer, and to brood the larvae, within the mantle cavity, to a shelled veliger stage. Later stages were found in the plankton; the largest ones could be linked directly to the smallest individuals found associated with C. torquata. Although no clear pattern could be detected from a frequency distribution of measurements of the size of M. elevata in the samples, a simple analysis of the annual growth-checks on the shells distinguished at least five year-classes.Some laboratory experiments with these highly motile bivalves established that they show a direct chemoorthokinesis to the 'host'. They also respond to certain physical factors of their microenvironment, showing a direct orthokinesis to light and moving uphill as a negative geotaxis on hard surfaces, but burrowing downwards into sand. A very characteristic response of circling in a velocity gradient of water issuing from fine-bore tubing is interpreted as a 'direct klinokinesis'; this response seems important in finally aggregating the bivalves to the lower end of the worm-tube where similar current conditions exist. The responses are shown to function in an integrated way in maintaining the association. Since young (postlarval) stages showed the same responses, it is concluded that behavior similar to that of adults probably operates in initiating the association.