Prevalence, distribution and gravidity of the parasitic isopod Bopyrus crangorum among common prawns Palaemon serratus.

The isopod Bopyrus crangorum is a hematophagous ectoparasite of the common prawn Palaemon serratus. Bopyrid parasites are identifiable due to the distinct carapace deformity they cause to their host, though knowledge of these parasites among temperate prawn populations is limited. Herein, we carried out two large, targeted sampling campaigns: (1) site-restricted, temporal survey of a commercial prawn fishery over three cycles (August to December 2023-2025, n = 5220), and (2) a multi-site, regional survey along the Irish coast (October 2025 to February 2026, n = 11,380). We observed an average annual prevalence of ~13% (actively parasitised and scarred). Generally, fewer B. crangorum were recorded among prawn populations from the East (0-4%) versus West (up to 28%) coasts. Smaller prawns were most likely to be bopyrid-positive, with the predicted probability of parasitisation declining from ~93% at 50 mm carapace length to ~14% at 80 mm and < 1% at 100 mm. Concurrent (dual and triple) bopyrid presence was rare (n = 3 and 1, respectively). Upon maintaining control and parasitised prawns ex situ (n = 45), we determined significant differences in survival outcomes, with parasitised prawns being over 2-fold more likely to die. We present novel data on the extent and intensity of B. crangorum parasitisation of P. serratus, as well as differentiating host traits associated with disease susceptibility.