The influence of density and behavioural type on the active entry of two mayfly species (Ephemeroptera) into the water column

Abstract

Laboratory experiments were conducted in still water and under lighted conditions to induce maximal departure rates of two mayfly species from a substrate. A larger proportion of Baetis vagans McDunnough nymphs left the substrate than of Paraleptophlebia mollis (Eaton) at densities of up to 750 nymphs per 330 cm². Differences in the tendency of nymphs of each species to leave the substrate were related to behavioural traits rather than density. Overt aggression between interacting P. mollis nymphs served as a spacing mechanism on substrate surfaces. Interactions among B. vagans nymphs were never observed. Rather than developing a spacing mechanism on the substrate, these nymphs are adapted to swim into the water column. A comparison of drift–density relationships for mayflies used in this and other studies revealed a gradation among behavioural groups within the Ephemeroptera in their tendency to leave the substrate and enter the water column.