Rheotaxis by epaulette sharks, Hemiscyllium ocellatum (Chondrichthyes : Hemiscylliidae), on a coral reef flat

Abstract

Rheotaxis (orientation to water currents) is commonly observed in fishes. Facing upstream is thought to be an element of shark behaviour during prey search and station-holding, but quantitative studies of rheotaxis by sharks in the wild are lacking. In this study, rheotaxis by the epaulette shark, Hemiscyllium ocellatum, was investigated on a coral reef flat at Heron Island, Great Barrier Reef, Australia. The orientation of 78 individuals in open areas (on sand or on top of coral heads, directly exposed to the prevailing current) was analysed with respect to current direction and velocity. H. ocellatum showed a significant (P < 0.05) preference for facing upstream (mean angle 1°, where the current origin was taken to be 0/360°) while resting on the substrate (n = 23), but showed no evidence of a preferred direction while swimming (n = 50). Observations of foraging were few (n = 5) but there was no indication of a preferred direction by these sharks. Resting H. ocellatum faced significantly more upstream at faster current velocities (P < 0.05), suggesting that rheotaxis may function in station-holding. There was no apparent relationship between rheotaxis and current velocity for swimming H. ocellatum.