DO SYMBIOTIC PEA CRABS DECREASE GROWTH RATE IN MUSSELS?

Abstract

ABsTRACr Pea crabs living within the mantle cavities ofa variety ofbivalve hosts have several adverse effects. In blue mussels (Mytilus edulis L.), the crab Pinnotheres maculatus (Say) steals food strands and causes gil lesions. We studied the long-term stress of P. maculatus on its host by measuring shell accretion in the field, and by numerically characterizing shell shape. Shell form in M. edulis is presumed to reflect environmental and physiological history. We computed growth increments in infested populations of mussels over a three-month period at two sites with high and low nutrient regimes. When growth was measured by change in shell length, significant differences between mussels with and without large pea crabs occurred at the low nutrient site, but not where mussels enjoyed a high nutrient regime. To integrate very long-term disparity in growth rates associated with infestation, we used mussels from a robust, naturally occurring population. We recorded and analyzed mussel sithouette shapes with a video digitizer. Elliptic Fourier approximation completely characterized the two-dimensional outlines of shells in such a way that the allometric dependence of shape variables on shell size could be easily removed. At this evidently favorable site, infested mussels displayed significant shell shape distortion characteristic of reduced growth rates. Thus, even in apparently benign environments, pea crab infestation appears to be a chronic stress to M. edulis.