Epidermal responses of chum salmon (Oncorhynchus keta) fry to the ectoparasitic flagellate Ichthyobodo necator

Abstract

The numbers of the ectoparasitic flagellate Ichthyobodo necator on chum salmon fry (Oncorhynchus keta) and the host's epidermal responses to infection were monitored for 15 weeks with light and scanning electron microscopy. Parasite density on the skin peaked at 3 weeks postinfection, at an average of 625 parasites/mm2, and then decreased to near zero by week 10. The staining procedure with Alcian blue (AB, pH 2.5) and periodic acid – Schiffs reagent (PAS) revealed dramatic changes in both the degree and histochemistry of epidermal mucus secretion during the course of the parasite infections. The numbers of mucous cells declined significantly during the first 4 weeks as parasite density increased, but thereafter rose to 2000 cells/mm2 by week 8, when mucous cells stained with PAS alone predominated. This increase occurred concomitantly with a reduction in parasite density. In the control fish, the concentration of epidermal mucous cells was stable at 400 – 800 cells/mm2 during the same period and acid mucopolysaccharides (PAS-negative, AB-positive) were a major constituent of the mucous cell secretion. It is suspected that the PAS-positive mucous cells may play an important role in the defence mechanism against the parasite infection.