Reflective communication between fish, with special reference to the greater sand eel, Hyperoplus lanceolatus

Abstract

The reflecting units in the surfaces of silvery fish are small ‘platelets’ which have various orientations with respect to the surfaces in which they lie. In the fields of light found in the sea, these orientations are often such as to enable fish to make themselves almost invisible from most points of view. Here we show that the platelets can also be useful in signalling information about the movements that the fish are making. Such signals are clearly graded. Some, e.g. those that accompany C-starts, will be strong and visible at distances many times the length of the fish. Others, e.g. those given by a spot of silveriness on the tail, will only be visible to near neighbours. We consider the movements of fish in terms of three components, rolling, pitching and yawing and show that, by virtue of the organization of the silvery layers, a given movement will produce, to the eye of a neighbour, a characteristic changing pattern of bright and dark surfaces. The changes in brightness accompanying quite small movements are large and, to us and almost certainly to the fish, much more visible than the changes in shape or position (the detection of which must also depend on differences in brightness). Diffusely reflecting surfaces can, to some degree, serve the same purpose but they are less good at doing so. They certainly cannot give a fish the facility of passing quickly from being almost invisible to presenting striking unequivocal signals to their neighbours, as silvery surfaces do. We give an account of the reflecting surfaces of the greater sand eel, Hyperoplus lanceolatus (Le Sauvage), in relation to this hypothesis.