Disarticulated bivalve shells as substrates for encrustation by the bryozoan Cribrilina puncturata in the Plio-Pleistocene Red Crag of eastern England

Cribrilina puncturata (Wood, 1844) forms small, patch-like colonies encrusting the concave (almost invariably lower) surface of disarticulated bivalve mollusc shells in the Plio-Pleistocene Red Crag of eastern England. The species is restricted to the deeper central part of each shell, away from the margin. Peripheral abrasion of the shell's surface, which might have removed marginal settlement, is rejected as the major agent in producing the observed distribution. A number of larval behaviour mechanisms that might have been responsible are therefore considered. All are discounted except one: that the larva crept up the slope of the shell's inner surface towards the highest point (geonegative movement) before fixation. This hypothesis seems to explain many details of the settlement pattern. C. puncturata apparently exploited the shell's major concavity as a refuge from physical disturbance in a high-energy environment, and its larval settlement behaviour appears to have been specialized for concavo-convex (bivalve) substrates. A second category of refuge, minor local concavities anywhere on the shell's surface, was occupied during the vulnerable early astogenetic stages of two other bryozoan species, both with a runner-like colony morphology.