The ballast stones in Silo nigricomis cases (Insecta: Trichoptera): drift resistance and ecological benefits, investigated by acoustic Doppler velocimetry

Abstract

This study examined the ecological role of the lateral ballast stones of Silo nigricornis cases (Trichoptera; fourth and fifth instar larvae) by using acoustic Doppler velocimetry (ADV) in an artificial stream channel in the laboratory. Dead larvae in their original cases were exposed to increasing current speeds (= passive resistance to drift; PRD). In frontal position (= heads of larvae directed towards the water flow), the arithmetic means of current speed necessary to dislodge the larvae were 0.17 ms -1 (fourth instars) and 0.25 ms -1 in fifth instars. When the larvae were at right angles to the current (lateral position), these speeds were 0.13-0.14 ms -1 in both instars. In order to evaluate the ecological role of the ballast stones, we adopted cases from another caddisfly species fitting in size but without ballast stones. Two approaches were used: the first group retained its original weight, the second group was brought to the same weight as the original S. nigricornis cases by partially filling the cases with tungsten. In both experimental groups and in frontal position, mean current speeds at dislodgement were 0.05 and 0.22 ms -1 , respectively (fourth instars) and 0.10 and 0.33 ms -1 , respectively, in fifth instar larvae. Total resistance to (TRD) was studied by exposing living larvae to increasing current speeds. The speed just sufficient for dislodgement was 1.26 ms -1 in four last instar larvae; three larvae even remained stationary at this threshold speed, which was the upper limit for the stream channel used. The difference between total and passive resistance to was defined as active resistance to drift (ARD) and attributed to the effectiveness of a larva's attachment to the substrate. It was equivalent to 1.06 ms -1 . When combining laboratory with field data of previous studies, the ecological benefits of the lateral ballast stones are significantly lower entry frequencies of S. nigricornis compared with other scraping caddisfly species in its habitat. The ballast stones greatly expand PRD areas at the sediment surface towards midstream, where the highest epilithic algal densities are found; this enables the larvae to remain stationary by submerged weight alone and without the need to actively attach to the substrate.