The host-parasite relationship between the rainbow trout (Oncorhynchus mykiss) and the ectoparasite Argulus foliaceus (Crustacea : Branchiura): epithelial mucous cell response, cortisol and factors which may influence parasite establishment.

The effects of short-term infection with the branchurian crustacean ectopar asite Argulus foliaceus, and the fish stress hormone cortisol (which is rep orted to stimulate mucus discharge), were studied on the mucous cell popula tion of the head skin of rainbow trout (Oncorhynchus mykiss). Argulus infec tion did not raise plasma cortisol significantly and had no effect on the n umber of epidermal mucous cells in the head skin. Cortisol was administered twice to groups of trout via the food, significantly elevating circulating plasma cortisol at 24 h post feeding without affecting numbers of mucous c ells, and increasing the numbers of vesicles in the upper cells of the epid ermis. Subsequent infection with the parasite (6 Argulus/fish) did not affe ct either plasma cortisol or total numbers of mucous cells at 48 h post-inf ection with the parasite, but led to a significantly lower parasite infesta tion per fish in the cortisol-administered groups. A 24 h culture system wa s used to expose pieces of trout skin to 50 ng/ml cortisol in vitro to inve stigate whether cortisol alone would stimulate reductions in mucous cell nu mbers. These were unaffected by the addition of cortisol. The in vivo and i n vitro results are discussed in relation to the current understanding of c rustacean host-parasite interactions.