The American horseshoe crab (Limulus polyphemus) is a marine arthropod that serves crucial economic and ecological roles and is currently experiencing regional declines. Declines are attributed to overharvest, but there is limited knowledge regarding the impacts that natural stressors impose on horseshoe crab fitness, especially symbiont interactions. Therefore, we quantified the prevalence and intensities of the parasitic flatworm (adult and cocoon stages), Bdelloura candida, on the gills of L. polyphemus. Adult (n=29) and juvenile (n=30) horseshoe crabs were collected in Moriches Bay, NY in June 2019. B. candida was prevalent in all adult crabs (100%), whereas, juveniles exhibited 0% prevalence. To quantify infection intensity, gill sections (10%) were removed from adult crabs and B. candida cocoons were enumerated and measured to quantify the respiratory surface area occupied by the parasites. A positive correlation between adult worm intensity and number of cocoons was observed, whereas crab size did not explain the variation of worm intensities (R²>.23). Cocoon intensities per sample ranged from 28 to 805, with 4-94% of gill lamellae harboring cocoons. In infected individuals, the total cocoon surface area on gill tissues was variable as it ranged from 0.06% to 14.51% and infection intensity was positively correlated with gill surface area (ρ=0.45, p = 0.01). High infection rates may negatively affect gill functioning in L. polyphemus. Our results provide novel insight into B. candida infection dynamics, but further research is necessary to quantify the physiological impacts of the infection on L. polyphemus.