Nest orientation patterns in Salaria fluviatilis

Abstract

The entrances of nests established under small stones by male river blennies Salaria fluviatilis in two rivers of the Ebro Basin in eastern Spain were randomly oriented in slow-flowing sites. In fast-flowing stretches or under large stones, however, nest entrances tended to open at an angle of c. 30° relative to current flow direction, i.e. near a south-east direction. As current velocity increased, males positioned their nest entrance closer and closer to the direction of flow. Selective nest entrance orientation reduced significantly the speed of current reaching the nest entrance such that current velocity was similar (5-7 cm s−1) for all nests, regardless of stone size, prevailing current speed, or study site. Male mating success, measured as egg clutch area, however, was not related to current speed at the nest entrance but instead, it increased with nest stone size and decreased with deviations from a south-east direction. The reasons for female river blenny preference for this specific nest orientation are unknown but may be related to patterns of water flow, and hence oxygenation of the eggs, in the nest.