Geographic Variation in Size Structure and Size at Maturity in the Crab Pilumnus Sayi (Crustacea: Decapoda: Xanthidae) in the Northern Gulf of Mexico

Female body size is a major determinant of individual reproductive output in many decapod crustaceans. As a result, population structure (e.g., female size at maturity, population size structure) strongly influences population-level reproductive output and potential population growth rates. Size structure and size at maturity are often variable within a species and may be subject to environmental influences. For example, latitudinal variation in population structure has been observed in many species of decapods, and is usually attributed to the effects of temperature on growth rate. Here, we document extraordinary differences in size structure and female size at maturity between two populations of the crab Pilumnus sayi Rathbun, 1897 in the northeastern Gulf of Mexico. We suggest that this spatial variation in population structure is caused by critical differences in the size of available shelter between the two locations. We also discuss the implications of these findings for understanding decapod life history patterns and population dynamics and potential consequences for the management of exploited species.