Position of horseshoe crabs in estuarine food webs: N and C stable isotopic study of foraging ranges and diet composition

To discern the position of horseshoe crabs as a potentially important predator in estuarine food webs, we determined where they foraged and what they ate. We used N and C stable isotopes to link adult horseshoe crabs to their oraging locations and potential food sources in Pleasant Bay, Cape Cod. The δ¹⁵N in tissues of horseshoe crabs and their potential foods suggest crabs were loyal to local foraging sites and did not forage substantially in subestuaries receiving >110 kg N ha⁻¹ year⁻¹. Among locations where crabs foraged, δ¹³C values in potential foods showed that food webs in subestuaries subject to higher N loads were supported by algal producers, while food webs in subestuaries with lower N loads were also supported by Spartina. δ¹³C values in horseshoe crab tissue did not change with load, suggesting they ate a mixed diet, regardless of N load. N and C isotopes in horseshoe crab feces were similar to signatures of estimated diet, suggesting low assimilation efficiency, perhaps due to ingestion of low quality organic matter. Although horseshoe crabs were relatively opportunistic in foraging habits, conservation or culture of horseshoe crabs may require habitats with higher water quality, ample particulate organic matter, and supporting a variety of prey.