A histological study of shell disease syndrome in the edible crab Cancer pagurus

Shell disease syndrome is characterised by the external manifestation of black spot lesions in the exoskeletons of crustaceans. In the present study, gills, hepatopancreas and hearts from healthy (<0.05% black spot coverage) and diseased (5 to 15% coverage) edible crabs, Cancer pagurus, were examined histologically to determine whether this disease can cause internal damage to such crabs. There was clear evidence of cuticular damage in the gills of diseased crabs leading to the formation of haemocyte plugs termed nodules. Nephrocytes found within the branchial septa of the gills showed an increase in the accumulation of dark material in their vacuoles in response to disease. In the hepatopancreas, various stages of tubular degradation were apparent that correlated with the severity of external disease. Similarly, there was a positive correlation between the number of viable bacteria in the haemolymph and the degree of shell disease severity. Approximately 21% of the haemolymph-isolated bacteria displayed chitinolytic activity. Overall, these findings suggest that shell disease syndrome should not be considered as a disease of the cuticle alone. Furthermore, it shows that in wild populations of crabs shell perforations may lead to limited septicaemia potentially resulting in damage of internal tissues. Whether such natural infections lead to significant fatalities in crabs is still uncertain.