Preparing for escape: anti-predator posture and fast-start performance in gobies.

The adoption of postures as a response to threats is often interpreted in terms of predator detection or signalling (e.g. vigilance and defence display). The possibility that an alternative or additional function of anti-predator postures might be to enhance the subsequent escape has been largely unexplored. Here, we use black goby (Gobius niger) to test the hypothesis that a postural curvature caused by a bending response (i.e. a slow muscle contraction which bends the body with no forward displacement) induced by a weak stimulus (WS) may affect escape responses. Three experiments were carried out. (1) Control and WS-stimulated fish were startled using lateral mechanical stimuli, to test whether the orientation of the postural C-bend affected escape direction and performance. Postural curvature was defined as positive when escapes were towards the convex side of the postural C-shape, and negative when they were towards the concave side. Locomotor performance increased with postural curvature, although fish showed a preference for escaping away from the stimulus regardless of postural curvature. (2) Control and WS-stimulated fish were startled from above, hence minimising the directionality of the threat on the horizontal plane. WS-stimulated fish showed a bias towards escaping from a positive curvature, thereby enhancing their locomotor performance. (3) Field observations with stimuli coming from above showed that gobies escape most often towards the convex side of the postural C-shape. By escaping from positively curved postures, most of the initial tailsweep is directed backwards and may provide more thrust than when starting from straight or negatively curved postures. Hence, the anti-predator posture adopted by alerted benthic fishes may ;prepare' them for their subsequent escape response because it conveys an advantage when they are attacked from above (a likely occurrence), although when gobies are stimulated horizontally, escape direction may be favoured over high locomotor performance when the two trade off.