Visual ecology of the three-spined stickleback

For almost a century the three-spine stickleback, Gasterosteus aculeatus, has been a principle subject in the field of animal behaviour, and as a consequence, we know more about the behaviour and ecology of this fish than any other. Yet, despite our understanding, implicit in much of this study is the assumption that the three-spined stickleback views the world in much the same manner as humans do. Such an assumption is, however, critically flawed. An emerging body of work now suggests that shallow dwelling fish such as the three-spine are often found to be sensitive to ultraviolet light, and that this sensitivity is often conferred by the existence of a fourth, independent cone photoreceptor in the retina. Moreover, since colour has been found to be crucial in the mate choice decisions of the stickleback, with the potential for ultraviolet sensitivity arises the possibility that visual behaviour in this species may also have been misclassified. In fact, rather than being static, there is evidence that the visual system of the stickleback is plastic, and this could be responsible for the maintenance of secondary sexual characteristics in the stickleback. Using various spectrophotometric, behavioural, and electrophysio logical techniques, I have investigated the visual capabilities of the three-spined stickleback. Scans of the ocular media of the stickleback revealed a visual system compatible with ultraviolet photoreception, where the optics of the eye permitted wavelengths as low as 350nm to reach the retina. Electroretinograms obtained from the stickleback confirmed that the photoreceptors within the retina are capable of ultraviolet photoreception, and using chromatic isolation, it was found that this sensitivity was mediated by the presence of a fourth, independent cone photoreceptor. A survey of the habitat from which subjects were collected confirmed that functional levels of ultraviolet wavelengths were present in the photic environment. The presence of an ultraviolet photoreceptor in the retina does not automatically imply its input to the highest level of the visual system. A simple two choice test, run under conditions similar to ambient, confirmed that the three-spined stickleback is able to respond behaviourally to ultraviolet light. In view of this, mate choice in the sticklebacks was re-examined under conditions where the ultraviolet component of the male's nuptial signal could be altered. Females were able to recognise individuals not displaying in the ultraviolet region of the spectrum as conspecifics but were, nevertheless, found to prefer males displaying across the ultraviolet and human visible region of the spectrum during trials. The issue of seasonal plasticity in the visual sensitivity of the stickleback was reexamined using modern techniques. Contrary to a previous study where only females were found to undergo a change in sensitivity, behavioural techniques investigating seasonal plasticity in spectral sensitivity revealed that both males and females became increasingly sensitive to red light during summer. This shift was found to accompany a summer shift towards red light in the ambient spectra. The mechanisms that might cause such a shift are discussed.