Ultrastructure of Didymocystis semiglobularis (Didymozoidae, Digenea) cysts in the gills of Pacific bluefin tuna (Thunnus orientalis)

Tuna are active pelagic fish with an extraordinary migratory activity, well known for their unique physiology reflected in high metabolic rates. However, knowledge of microbial and environmental diseases is still limited. We have analyzed the ultrastructure of the digenean trematode Didymocystis semiglobularis isolated from the gill arch of Pacific bluefin tuna (Thunnus orientalis) by transmission electron microscopy (TEM). The parasite is demarcated from the rest of host tissue in a sac of host origin, composed of active fibroblast and scattered bundles of collagen tissue, with no macrophage accumulations. TEM micrographs reported in this study reveal a wide multilayered tissue isolating the host from the parasite capsule and more internally complex and compact layers dividing the parasite capsule from the body itself, which encapsulates eggs at different developmental stages. Since the size and shape of the parasite would imply host tissue activation at the site of infection, no histopathological changes were observed in the architecture of the tuna superficial layer. No degeneration or necrosis was observed in the upper layer of the host tissue.