Size-based trophic shifts of saltmarsh dwelling blue crabs elucidated by dual stable C and N isotope analyses

Abstract

Analyses of stable C and N isotopes were used to examine trophic characteristics of blue crabs Callinectes sapidus in relation to body size (15 to 165 mm carapace width, CW) within saltmarsh habitats of the Aransas National Wildlife Refuge, Texas. Blue crab trophic position did not change with increasing body size, and all size classes appear to consume primary consumers and primary producers in relatively equal proportions. However, blue crab δ13C values increased significantly with increasing body size. As body size increased, blue crabs assimilated greater proportions of carbon ultimately derived from Spartina alterniflora. Individuals of approximately 35 mm CW assimilated carbon almost exclusively derived from C3 plants and benthic algae, whereas larger individuals (125 mm CW) assimilated similar amounts of carbon derived from benthic algae and S. alterniflora-derived detritus. Given the complex nature of saltmarsh food webs and the omnivorous diet of blue crabs, the observed size-based trends have important management and conservation applications.