Effect of current velocity and case adaptations on the distribution of caddisfly larvae (Glossosoma, Trichoptera)

Abstract

In aquatic systems, oxygen availability is often a limiting factor affecting animal distribution. We documented the distribution of case-bearing caddisfly larvae (Glossosoma) in a riffle of the Hirose River, Japan, and found that larval density increased with increasing current velocity. We also experimentally measured the effect of flow and case morphology on oxygen consumption and mortality. Larvae were divided into “perforated” and “closed” case treatment groups where gaps in the upper side of the dome-shaped case were left open or closed. They were placed in either lotic or lentic conditions. Oxygen consumption was higher in the lotic compared to the lentic environment for both groups, and was higher in individuals of the perforated-case group than in those belonging to the closed-case group in both environments. Larval mortality was lower under lotic conditions than lentic conditions for the perforated-case group, and higher in the closed-case group in both environments. Our results suggest that larvae prefer areas with high current velocity as they are otherwise unable to obtain sufficient oxygen because of their lack of undulatory behavior. Furthermore, gaps in the larval case appear to enhance passive gaseous exchange via water flow. We also discuss the functional significance of the glossosomatid case.