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

Phrixocephalus cincinnatus is a blood-feeding copepod parasite inhabiting the choroid of the eye of the Pacific sanddab Citharichthys sordidus off the coast of southern California. The present study examined the fine structure of the midgut using both light and electron microscopy. The anterior midgut or stomach was lined with a tall columnar epithelium. The apical surfaces of the cells formed vesicles that pinched off and entered the gut lumen. Posteriorly, the midgut consisted of 2 morphologically distinct cell types; vacuolar and nonvacuolar. These cells probably were different developmental and functional stages of a single kind of columnar epithelial cell. The nonvacuolar cells possessed basally located nuclei, rough endoplasmic reticulum, and long microvilli indicative of an absorptive function. The vacuolar cells were characterized by short microvilli, endocytic vesicles, primary and secondary lysosomes, well-developed rough endoplasmic reticulum, and Golgi apparatus, suggesting that they were active in endocytosis as well as the synthesis of digestive and lysosomal enzymes. Distension of the apical surface of vacuolar cells with enzyme-containing vesicles resulted in the liberation of the distal part of the cell into the gut lumen. Subsequent lysis of the cell membrane released enzyme-containing vesicles, suggesting that digestive enzymes are discharged by apocrine secretion of the vacuolar cells. Both vacuolar and nonvacuolar cells absorb nutrients resulting from the extracellular digestion of host blood.