Infection of Cancer pagurus by a parasitic dinoflagellate of the genus Hematodinium is described for the first time in Ireland. An industry-based monitoring programme was established to determine seasonality of infection intensity and prevalence in the country's 3 largest brown crab fisheries in the southwest, north and southeast. The parasite was present in all areas for the majority of sampling periods, with highest prevalences recorded in pre-recruit animals of both sexes. Microscopic examination of haemolymph revealed trophont, plasmodial and dinospore stages of the parasite. Overall prevalence in males (16%) was higher than in females (9%). Prevalence of Hematodinium sp. infection ranged from 0 to 51%, but a distinct seasonal trend was not apparent. Infection intensity was seasonal with significantly higher peaks occurring in late autumn/early winter months than in other quarters, corresponding to industry reports of moribund and dead pink-shelled crabs in commercial catches. We postulate that seawater temperature or a temperature-linked process is a key factor in triggering the final stages of infection, as significant autumn peaks were followed by a reduction in infection intensity as temperature decreased in the late winter/early spring months with no increase in intensity again until the following autumn. We propose that infection intensity, rather than prevalence, provides a more appropriate indication of the period when there is greatest potential for biological and economic impacts; the parameter's application as a fisheries management tool is discussed.

Ní Chualáin, C., and Robinson, M. 2011. Comparison of assessment methods used to diagnose Hematodinium sp. infections in Cancer pagurus. – ICES Journal of Marine Science, 68: . Endoparasitic dinoflagellates of the genus Hematodinium have recently gained attention as significant pathogens of the brown crab Cancer pagurus in Ireland. Patent infections, which are characterized by a hyperpigmented carapace and moribund condition, are limited to the discrete periods when macroscopic identification is possible. Three methods are assessed for diagnosing Hematodinium sp. infections in brown crab at the times when macroscopic identification is not always possible. Haemolymph smears, histological sections of gill, heart, midgut, hepatopancreas, muscle, and gonad, and a polymerase chain reaction (PCR) assay provided virtually equivalent accuracy in gauging infection prevalence, regardless of season. Sequences of PCR amplicons from the 18S ribosomal RNA gene confirmed the identity of the parasite as belonging to the genus Hematodinium. Infection intensity values (<1–87%) obtained from haemolymph smears underscored infection levels within tissues, 90% of which contained advanced levels of infection. Alterations to tissues of infected crabs included haemocytopoenia, oedema, which caused dilation of the haemal sinuses resulting in pressure necrosis to the connective tissues around the oocytes, myocardial bundles, and hepatopancreatic tubules. The claw muscle of infected animals contained the fewest parasites.