Haemolymph parasite of the shore crab Carcinus maenas: pathology, ultrastructure and observations on crustacean haplosporidians.

A protozoan parasite with some features of haplosporidians is described from the European shore crab Carcinus maenas. The parasite establishes a systemic infection through the haemal sinuses and connective tissues. Intracellular stages of the parasite were found within reserve inclusion, connective tissue, and muscle cells, while free forms were present in all haemal spaces. A uninucleate stage appeared to develop to a multinucleate plasmodial stage following multiple mitotic divisions of the nucleus. Histopathology also indicated that nuclear division may occur to form multinucleate plasmodia, in connective tissue, reserve inclusion and muscle cells, the multinucleate plasmodium being enclosed in the host-cell plasma membrane. It appears that the multinucleate plasmodium may then undergo internal cleavages which result in plasmodial fragmentation to form many uninucleate stages. Both stages, but particularly the uninucleate stage, contained cytoplasmic, large, ovoid, dense vesicles (DVs), some of which contained an internal membrane separating the medulla from the cortex, as in haplosporosomes. Golgi-like cisternae, closely associated with the nuclear membrane, formed DVs and haplosporosome-like bodies (HLBs), superficially resembling viruses. Infrequently, HLBs may condense to form haplosporosomes. The DVs, as in spores of some Haplosporidium spp. and paramyxeans, may give rise to, and are homologous with, haplosporosomes. Other features, such as the presence of an intranuclear mitotic spindle, lipid droplets, and attachment of DVs and haplosporosomes to the nuclear membrane, indicate that the C. maenas parasite is a haplosporidian. A similar organism reported from the haemolymph of spot prawns Pandalus spp., and haplosporidians reported from prawns Penaeus vannamei and crabs Callinectes sapidus may belong to this group. It is concluded that the well-characterised haplosporidians of molluscs and some other invertebrates may not be characteristic of the whole phylum, and that morphologically and developmentally similar organisms may also be haplosporidians, whether they have haplosporosomes or not.