Effects of running water on lateral line responses to moving objects

We investigated in goldfish, Carassius auratus, and trout, Oncorhynchus mykiss, how running water affects the responses of afferent fibers in the posterior lateral line nerve and of lateral line units in the brainstem medial octavolateralis nucleus to an object that is moved from anterior to posterior or opposite along the side of the fish. In still water, nerve fibers in both species responded to the moving object with alternating periods of increased and decreased firing rate. Most fibers in goldfish but none in trout discharged bursts of spikes in response to the object's wake. Responses of brainstem units were more variable and less distinct than nerve fiber responses. Bursting activity in response to the object's wake was found in only one brainstem unit. In running water, responses of goldfish nerve fibers were weaker than in still water. This effect was independent of object motion direction. Responses of trout fibers were weaker when the object was moved with the flow but were slightly stronger when the object was moved against the flow. In general, running water affected the responses of goldfish nerve fibers more strongly than the responses of trout fibers. Compared to still water, brainstem units in both species responded more weakly when the object was moved with the flow. When the object was moved against the flow, brainstem responses were on average comparable to those in still water. Measurements of changes in pressure and water velocity caused by the moving object indicate that the observed effects can largely be explained by peripheral hydrodynamic effects. However, physiological differences between goldfish and trout units indicate that the lateral line systems in these two species are adapted to different hydrodynamic conditions.