Published online by Cambridge University Press: 11 May 2009
The inability of Lima Mans Gmelin to enclose the soft tissues of the body within the shell is correlated with the development of other defensive adaptations.
L, Mans can automize entire pallial tentacles or parts of tentacles which subsequently secrete a viscous mucus distasteful to potential predators. Autotomy takes place at transverse septa along the length of the tentacles and mucus is secreted by epidermal gland cells.
The transverse septa of the tentacles also regulate the hydrostatic pressure of blood within the tentacles and permit the performance of complex locomotory movements. It is suggested that the septa were originally developed to serve this locomotory function and, secondarily, function as planes of weakness at which autotomy may take place.
L. Mans builds protective nests. The nests are constructed by burrowing into a gravel substratum and consolidating the walls of the burrow with byssal threads. When disturbed in the nest L. Mans performs locomotory movements which lead to enlargement of the nest. If the nest is broken the locomotory activities result in free swimming followed by attempts to burrow into the substratum to form a new nest. It is unlikely that L. Mans swims freely in nature except when displaced from the nest.
Introduction
Most bivalve molluscs react to unfavourable stimuli by retracting the soft parts of the body within the shell valves and contracting the adductor muscles to oppose the valve margins. In Lima hians Gmelin the mantle margin, even with its numerous long tentacles fully retracted, cannot be accommodated within the shell valves. In this paper the inability of L. hians to retract the soft parts within the valves is correlated with two types of defensive behaviour: (i) the secretion of mucus by the tentacles of the mantle margin and the autotomy of these tentacles, and (ii) the interrelated activities of swimming and nest-building.