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Parasite infection dynamics can have profound implications on a host’s fitness; yet, there is a dearth of information on parasites in the American horseshoe crab (Limulus polyphemus) (Linnaeus 1758), a species that has experienced population declines in recent decades. Therefore, we aimed to quantify the prevalence, intensity, and gill surface area coverage of the ectoparasitic flatworm (cocoon and adult stages), Bdelloura candida in adult (n = 29), sub-adult (n = 7) and juvenile (n = 32) horseshoe crabs collected from Moriches Bay, NY (40.7810° N, 72.7171° W) in 2019 and 2020. Subsamples of horseshoe crab gill tissue (10%) were collected from live specimen, then B. candida cocoons were enumerated across the gill subsamples using microscopy while the extent of tissue damage was quantified with histology. B. candida was present in all adult and sub-adult crabs (100%), whereas juveniles exhibited 6.2% prevalence. Cocoon intensities per sample ranged from 28 to 805 cocoons, with 4.0–94.0% of gill lamellae harboring cocoons. In infected individuals, the total cocoon surface area coverage on gill tissues ranged from 0.06–14.51%, with higher cocoon intensities observed in the ventral-most gill quartiles relative to the dorsal-most gill regions. Sex was strongly supported as a primary driver behind B. candida infection intensities with adult females harboring higher intensities. Among infected gill lamellae, cocoon intensity was lower in mitochondrial-rich regions relative to mitochondrial-poor regions. These results provide novel insight into B. candida infection dynamics across horseshoe crab demographics, but further research is necessary to quantify the physiological impacts of the infection on L. polyphemus.

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