Diet Composition of Juvenile Horseshoe Crabs: Implications for Growth and Survival of Natural and Cultured Stocks

Horseshoe crabs are valued for economic, ecological, and educational purposes. These values have raised interest in managing natural stocks and culturing crabs for conservation, research, and education. To inform these efforts, we used N and C stable isotopes to define the natural diet of juvenile horseshoe crabs and then assessed effects of different diets on growth and survival of juveniles in culture. In the natural environment, N and C isotope ratios in juvenile horseshoe crabs changed as crabs grew, with larger crabs consuming larger prey. Linear mixing analyses suggested young crabs were supported by high quantities of benthic and suspended particulate organic matter (POM), shifting between marine algae and salt marsh-based food webs, depending on size. In culture, we tested the relative importance of algae, as a proxy for POM, in horseshoe crab diet by feeding juvenile crabs different percentages of algae and prey animals. Initially,juvenile crabs showed a significant increase in size when fed diets >70% protein, but showed a decrease in survival compared to algae-rich diets. Overall, growth rates and survival declined through time during the 128-day study, regardless of diet composition. These data Suggest horseshoe crabs require foods from a combination of plant and animal sources. Successful culture or conservation of horseshoe crabs will depend on understanding the relative importance of different food sources at different life stages as well as discerning the balance between factors that increase growth, but reduce survival.