Contrasting Trophic Niche and Resource Use Dynamics Across Multiple American Horseshoe Crab (Limulus polyphemus) Populations and Age Groups

Horseshoe crabs (Limulus polyphemus) exhibit ontogenetic and geographic variability in migratory patterns, yet the implications of movement on their resource use dynamics remain poorly understood. Here, we evaluate horseshoe crab resource use across ontogeny (instars 14–18 and adults), sex, seasons, and between two spatially distinct populations that exhibit different migratory patterns. Multi-tissue stable isotope analyses of carbon, nitrogen, and sulfur were used to examine (1) ontogenetic and sex-specific variability in the reliance of resource pools; (2) variation in isotopic niche breadth within and among populations; and (3) seasonal variability in resource use within juvenile life stages. We found evidence for subtle ontogenetic shifts in resource use, with increased contributions of phytoplankton with ontogeny, but all age groups predominantly relied on sedimentary organic matter (SOM) pools. Sex-specific differences in resource use were observed with SOM serving as the predominant source assimilated in males, while females exhibited a higher dependence on phytoplankton resource pools, indicating foraging or spatial preferences may be different between sexes. Low trophic niche overlap between adult and juvenile horseshoe crabs was evident, likely arising from differences in migratory patterns and size constraints. Within juveniles, no seasonal differences in resource use were observed, indicating that dietary patterns may remain static across temporal scales, consistent with limited dispersal of that life stage. Spatial differences in resource use were, however, observed between adult crabs likely reflecting the migratory strategies of different populations. Our results are consistent with previous evidence that horseshoe crabs are dietary generalists but provide novel insights into the linkages between movement and trophic patterns.