Ultrastructure of the holdfast of Phrixocephalus cincinnatus (Wilson), a blood-feeding parasitic copepod of flatfishes.

Phrixocephalus cincinnatus is a parasitic copepod inhabiting the eye of flatfishes. Specimens of infected Pacific sanddabs Citharichthyes sordidus were collected from Santa Monica Bay, California, and the copepods were fixed and examined using standard light and transmission electron microscopy. The holdfast consisted of numerous, intertwined rootlets originating from the cephalothorax of the parasite and embedded in the choroid of the host's eye. A large hematoma, extravasated blood, and necrotic host cells surrounded the holdfast. Electron microscopy revealed that the rootlets were encased with a trilaminar cuticle consisting of an outer epicuticle and an external and internal procuticle. The outer surface of the epicuticle was covered with microvillosities that contacted host tissue. In regions where host macrophages contacted the cuticle, the external layer of procuticle was expanded by large, cholesterol-like crystal deposits. Internally, the rootlets were comprised of at least 3 types of large cells. Epithelial cells contacted the internal layer of procuticle and contained numerous mitochondria. The cytoplasm of stellate cells was dominated by rough endoplasmic reticulum or large vacuoles. Ovoid cells contained distinct proteinaceous granules of unknown chemical composition. These observations revealed that the rootlets are structurally complex and suggest that the holdfast may be actively involved with molecular exchange between parasite and host.