The costs of being bored: Effects of a clionid sponge on the gastropod Littorina littorea (L)

Mollusc shells provide a major defense against predators, but changes in shell properties may alter the effectiveness of this defense. An endolithic boring sponge, Cliona sp., infests many shells of the gastropod Littorina littorea in the southern Gulf of Maine. Cliona excavates ramifying chambers within the shell but does not affect tissue directly. Field surveys showed that 22–83% of snails were infected with Cliona at the Isles of Shoals; the prevalence of infection declined with increasing tidal height and increased with increasing snail size. We also investigated potential costs to L. littorea of infection by Cliona. L. littorea appeared to respond to sponge boring by laying down additional material on the interior of the shell, which reduced interior shell volume and was associated with smaller dry body mass for a given shell length. Biomechanical studies showed that sponge boring weakened shells: the mean compressive force required to break severely bored L. littorea was 29% lower than that required to break unbored snails. Cliona infection increased vulnerability of snails to predation by the crab, Cancer borealis Stimpson, which attacks molluscs by crushing or peeling shells. Unlike uninfected snails, predation risk did not decline with increasing size in bored L. littorea. In addition, bored snails were generally crushed by the crab, regardless of snail size, whereas large unbored snails could only be attacked by peeling. Thus, costs to L. littorea of Cliona boring included the following: (1) increased vulnerability to predation due to structural changes in the shell; (2) reduction or elimination of the size-refuge advantage; and (3) a potential decrease in growth and fecundity due to diversion of energy to supplemental interior layers to the shell and the consequent reduction in interior shell living space and body mass.