Humans generally believe that the world is coloured the way they see it. They have named the spectral range that the human visual system is sensitive to the "visible spectrum' rarely considering that other animals might see the world in a different range of colours. In recent years, however, our understanding has improved and it has become clear that many animals have different photoreceptor sensitivities both to us, and amongst themselves, and that many different taxa see wavelengths that humans are blind to.
The human visual system has evolved to meet the requirements specific to the survival of primates, and the visual systems of other animals are presumably adapted to their respective environments. In the underwater world of fish, the spectrum of light is different from that on the surface due to the effects of scattering and absorption in water. How has the evolution of the visual system of fish been influenced by the available colour spectrum? Many fish have ultraviolet colour patterns. These are commonly found on their faces and on their fins, body regions that are frequently presented to other fish during interactions. Is it possible that these fish communicate in the ultraviolet? Or do they use UV vision for other aspects of their lives, such as feeding? Before the adaptive significance can be investigated more general questions have to be solved first: are coral reef fish sensitive to ultraviolet light, and if so, is it a widespread ability, or one restricted only to fish with ultraviolet patterns?
The experimental work of this thesis starts by establishing the potential for ultraviolet sensitivity amongst reef fishes. Important prerequisites for ultraviolet vision are that the ultraviolet signal is transmitted through the ocular media to the retina and that the retina contains photoreceptors sensitive to those wavelengths. If the ocular media prevent ultraviolet light from entering the eye (as is the case for humans) ultraviolet vision is not possible. A survey of the ocular media transmission properties of 360 coral reef fish species revealed that 53% of the species had ocular media transparent in one region of the ultraviolet spectrum. Thirty-three percent absorbed ultraviolet light and 14% were found to have yellow pigmented corneas that also absorb substantial proportions of blue light. Two types of yellow corneas were found: one with fixed patterns of patchy pigmentation, and the other with pigmentation that can be retracted and extended in response to the illumination intensity. Analyses were made of the pigmentation patterns, the dynamic properties of the occlusable corneas, the ultrastructure of the cornea and the photoreceptor distribution in the retina.
The results of the ocular media survey also revealed that the possession of ultraviolet reflecting colours is not an indicator of ultraviolet transmission to the retina. So why do fish that do not possess UV colours have UV transparent ocular media? A correlation was found between life style and the degree of ocular media transmittance. The ocular media of planktivorous fish, for example, transmit significantly shorter wavelengths than those of herbivorous and carnivorous fish. As a test case for the importance of ultraviolet sensitivity for the planktivorous lifestyle the hypothesis was tested that larval fish that exclusively feed on plankton should have ultraviolet transmitting ocular media. The ocular media analysis of 88 species revealed that while larvae generally transmit shorter wavelengths than adults, they do not always have ultraviolet transparent ocular media.
Beyond anatomy, the thesis continues with behavioural experiments to investigate the possibility that ultraviolet colours are used as a 'secret communication channel' amongst some reef fishes. Pomacentrus amboinensis were selected because of their territorial behaviour, transparent ocular media and ultraviolet reflective facial pattern. When presented with a choice of two conspecific intruders, one seen through an ultraviolet absorbing and the other one through an ultraviolet transparent filter, the resident fish preferentially attacked the conspecific with visible ultraviolet patterns, even during brightness controls.
Further evidence for the communication role of ultraviolet was revealed by an analysis of the components of the facial ultraviolet pattern. Results show that the patterns contain features that make the recognition of species, individuals and possibly even mates possible. Also, comparisons of the calculated colour contrast signals received by a damselfish looking at a conspecific with those received by humans and a predator, showed that when viewed in the natural habitat of the fish, the facial patterns are only conspicuous to the damselfish.
The thesis concludes with a summary and discussion of the findings described, and goes on to outline directions for further neurophysiological, anatomical and behavioural studies.