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Partridge, Julian C.
School of Biological Sciences, University of Bristol, Woodland Road, Bristol, BS8 1UG, UK (email: j.c.partridge@bristol.ac.uk)
Most deep-sea fishes well developed eyes, suggesting that vision is an important sensory-modality for fish that live in the deep oceans. Nevertheless, the deep-sea is essentially a photon limited environment and numerous visual adaptations have been identified that serve to increase photon catch, including modifications to gross ocular anatomy, photoreceptor morphology and retinal spectral sensitivities. Almost all deep-sea fishes have a single type of rod photoreceptor in their retinas and a single visual pigment which is more short-wave sensitive than that found in shallow living fishes. It is generally accepted that the evolution of such short-wave sensitive pigments is correlated with the spectral transmission properties of oceanic waters, and a link has often been suggested between the spectral distribution of the dim, blue downwelling irradiance found at depth and the spectral sensitivities of deep-sea fish visual pigments. Indeed, the match between the ambient daylight at depth and visual pigment spectral absorption has become a much cited example of how a sensory system can be tuned to some feature of the environment in which it operates; in this case to maximise sensitivity.
However, such conclusions may be based on a misconception about the nature of the light environment at depth and do not adequately consider the visual tasks of deep-sea animals. This paper presents computer models that optimise visual pigment (max for maximum sensitivity. These models indicate that spectral tuning to downwelling light may not, in fact, be the main selection pressure operating on the visual pigments of deep-sea fishes. Instead the visualisation of bioluminescence may be more significant, with eye design and visual task interacting to determine the optimal (i.e. most sensitive) visual pigment. This conclusion may not only provide a new understanding of the selection pressures acting on deep-sea fish visual systems but, by stressing the importance of interaction distances in visualisation tasks, may also present new insights into communication and community structure in the deep-sea.
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