Infection prevalence, intensity, and gill coverage by the parasitic flatworm, Bdelloura candida, in the American horseshoe crab (Limulus polyphemus)

Parasitic infections can have profound implications on host fitness, yet there is minimal information on parasites of the American horseshoe crab (Limulus polyphemus), a species that has experienced recent population declines. Therefore, we aimed to quantify the prevalence, intensity, and gill surface area coverage of the ectoparasite Bdelloura candida in adult (n = 58), sub-adult (n = 7) and juvenile (n = 32) horseshoe crabs (HSCs) collected from coastal waters of Long Island, NY in 2019 and 2022. Sub samples of horseshoe crab gill tissue (10%) were collected from live specimens and B. candida cocoon intensities from individual lamellae were enumerated microscopically. B. candida was present in all adult and sub-adult crabs (100%), whereas juveniles exhibited 6.2% prevalence. 4.0–94.0% of gill lamellae harbored cocoons, with intensities ranging from 0 to 805 cocoons per sub sample. Cocoon covered between 0.06%–14.51% of the gill sub sample surface area, with higher cocoon intensities observed in the ventral-most gill quartiles relative to the dorsal-most regions. Sex was strongly supported as a primary driver behind B. candida infection intensities with adult females harboring higher intensities. These results provide novel insight into B. candida infection dynamics across HSC demographics, but further research is necessary to quantify the physiological impacts of the infection on L. polyphemus.