Size-Normalized Fecundity Reveals Reversed Geographic Patterns in the Porcelain Crab Pachycheles laevidactylus (Crustacea: Decapoda: Porcellanidae)

Abstract

Fecundity in pleocyemate crustaceans is defined as the number of eggs produced per spawning event and is strongly dependent on female body size, which can limit direct comparisons among populations. To address this limitation, we evaluated geographic variation in fecundity in the porcellanid crab Pachycheles laevidactylus along the southwestern Atlantic, comparing populations from Ubatuba (UB: southeastern Brazil) and Torres and Tramandaí (TO and TR: southern Brazil) based on both absolute average fecundity ( F ) and size-normalized fecundity ( F’ ). Ovigerous females were measured for carapace length ( CL ), and egg number ( EN ) was quantified for each individual. Carapace length and absolute fecundity followed the same pattern among localities (TR > TO > UB). However, after removing the effect of body size, this pattern reversed (UB > TO > TR), indicating that the higher fecundity observed in southern populations may have been driven by differences in the population size structure of ovigerous females. Size normalization also resulted in a sharp reduction in variability, with the coefficient of variation decreasing by approximately 50% compared to absolute fecundity. These results demonstrate that population structure can mask underlying differences in reproductive patterns, supporting the use of size-corrected metrics in interpopulation comparisons. This approach provides a more consistent and biologically meaningful framework for assessing geographic variation in decapod crustaceans.