Microhabitats and Mode of Attachment of Neoheterobothrium hirame, a Monogenean Parasite of Japanese Flounder.

Abstract

A survey of Neoheterobothrium hirame infection of Japanese flounder Paralichthys olivaceus in several places in Japan (Kyoto, Fukui and Ibaraki) was conducted to examine the relationship between the development of the parasite and its microhabitats. The parasite first attached to the gill filaments, then migrated to the gill arches/rakers and finally moved to the buccal cavity wall. This migration was associated with growth and maturity of the parasite. Some worms already reached maturity on the gill rakers, but produced eggs only on the buccal cavity wall. The histology of infected tissues was examined to determine the mode of attachment and pathological effects of the parasite. On the gill filaments, the parasite used, at first, marginal hooks and hamuli to attach to the proximal region of the secondary gill lamellae and later, clamps to grasp these lamellae. On the gill arches/rakers, clamps were used to attach to the epithelium. On the buccal cavity wall, clamps were used initially for attachment to the tissue surface but later the haptor penetrated the underlying connective and muscle tissues until the haptor and almost the entire isthmus region were embedded deeply in the host tissue. Only a mild inflammatory response was observed on the gill filaments and in the epithelium of the gill arches/rakers, respectively, whereas a strong host response and necrosis were associated with the prolonged attachment of the parasite to the buccal cavity wall.