This study reports on the prevalence and severity of infections caused by the parasitic dinoflagellate, Hematodinium in juvenile edible crabs (Cancer pagurus) found in 2 intertidal survey sites (Mumbles Head and Oxwich Bay) in the Bristol Channel, UK. Crabs were assessed for the presence and severity of Hematodinium infections by the histological examination of infected tissues. Such infections were found to exhibit a seasonal trend in the 2 study areas with high numbers of animals (ca. 30%) infected in the spring to summer but with low severity. Conversely, in November only ca. 10% of crabs were infected but these animals had large numbers of parasites in their haemolymph and other tissues. At this time, the carapace and underlying tissues of infected crabs had the chalky, pinkish-orange appearance that is characteristic of this disease. Hematodinium-infected crabs ranged in size from 12 to 74 mm carapace width. Overall, it is concluded that the high prevalence of infection of juvenile crabs in this area may have implications for the sustainability of the edible crab fishery in the Bristol Channel.

bThis study reports on an emerging fungal disease of the edible crab, Cancer pagurus. Juvenile (prerecruit) crabs were found to be subject to this disease condition during the months of May to September at two intertidal sites in South Wales, United Kingdom. Histopathology revealed that the fungi overwhelm the host response in the tissues, leading to progressive septicemia. The causative agent of this infection was isolated and grown in pure culture and was identified as a member of the Ophiocordyceps clade by sequencing of the small subunit of the fungal ribosomal DNA (rDNA). Of the crabs naturally infected with the fungus, 94% had a coinfection with the parasitic dinoflagellate Hematodinium species. To determine if there was any interaction between the two disease-causing agents, apparently fungus-free crabs, both with and without natural Hematodinium infections, were challenged with the fungal isolate. The presence of Hematodinium caused a significant reduction in fungal multiplication in the hemocoel of the crabs in comparison to that in Hematodinium-free individuals. Histopathology of coinfected crabs showed a systemic multiplication of Hematodinium within host tissues, leading to a rapid death, while Hematodinium-free crabs experimentally infected with the fungal isolate died due to fungal sepsis (septicemia) with the same characteristic pathology as seen in natural infections.

Juvenile edible crabs, Cancer pagurus L., were surveyed from Mumbles Head and Oxwich Bay in South Wales, UK, and the number of heterotrophic bacteria and vibrios in the hemolymph was determined. The percentage of crabs with hemolymph containing bacteria was variable over the survey with higher numbers of animals affected in summer than in winter. Post-moult crabs contained significantly higher numbers of heterotrophic bacteria in the hemolymph than pre- and intermoult animals. Crabs with cuticular damage to the gills also had significantly higher numbers of bacteria in the hemolymph. Crabs were found to have a high prevalence of infection by the dinoflagellate, Hematodinium. Such animals had significantly fewer bacteria in the blood in comparison with Hematodinium-free animals. Of the 463 crabs surveyed, only 3 individuals had hemolymph containing 2000 + CFU mL(-1). Based on 16S rRNA gene sequences, two of these crabs contained a Vibrio pectenicida-like isolate, while the other had a mixed assemblage of vibrios. Although 59% of the crabs surveyed had culturable bacteria in the hemolymph, the majority only had small numbers (<2000 CFU mL(-1) ), suggesting that such infections may be of limited importance to the sustainability of the crab fishery in this region.