Diplostomum spathaceum metacercarial infection and colour change in salmonid fish

Colour changes in two salmonid fish, the salmon (Salmo salar) and sea trout (S. trutta), were examined in relation to infection with the trematode Diplostomum spathaceum. This parasite had no effect on the rate of colour change in these fish, although species specific differences in colour adjustment times were observed. Increasing asymmetry in parasite numbers between the right and left eye, which could lead to the retention of vision in one eye, nevertheless tended to reduce the colour change time in salmon with moderate infection (P=0.08). This first experimental attempt to examine colour changes in fish in relation to eye fluke infections provides grounds for future investigations. The darker appearance of the heavily infected fish described in the literature suggests that a high parasite burden actually causes colour changes. We emphasise that detailed quantitative studies using fish with higher parasite loads, especially from the tail of the aggregated parasite distribution, are needed to describe these relationships in detail.