CONSTRAINT ON REPRODUCTIVE OUTPUT IN BRACHYURAN CRABS - PINNOTHERIDS TEST THE RULE

Brood weight and number of eggs per brood are primarily determined by female body size in brachyuran crabs. Brood weight exhibits an isometric relationship spanning more than four orders of magnitude in body weight among 33 non-pinnotherid species, with brood size being constrained to about 10% of female body weight by the space available for yolk accumulation within the cephalothorax. In contrast, two pinnotherids (Pinnotheres ostreum, Fabia subquadrata) have relative brood sizes which are 66% and 97%, respectively, of body weight. The allometric constraint is circumvented in these extraordinarily large brood sizes by two pinnotherid features which allow more space for yolk accumulation. Unlike other brachyurans, their ovaries extend out of the cephalothorax into the abdomen, and calcification of their exoskeleton is greatly reduced, potentially allowing distension of the body. As in crabs, pinnotherid egg size is highly variable among species, which tends to increase the variance among species in fecundity per brood, making pinnotherid fecundities not extraordinary. The number of broods produced per year in these two species is limited by short reproductive seasons, which tends to bring their annual reproductive output in line with other brachyurans. The trade-offs among reproductive variables in pinnotherids is adaptive for the small body size and parasitic niche of these species.