Spectral Sensitivity Tuning in the Deep-Sea

Abstract

Deep-sea fishes can be exposed to both dim residual sunlight and bioluminescence, both of which are usually most intense in the blue/green part of the spectrum. Not surprisingly, therefore, the vast majority of deep-sea fishes possess only one visual pigment with maximal absorbance (λmax) between 475 and 490 nm. While a single visual pigment with maximal sensitivity approximately “matched” to both the downwelling sunlight and bioluminescence is clearly the norm within the deep-sea, 21 of the 200 species examined to date have two visual pigments within their retina, while 3 have at least three. The λmax values of such multiple visual pigments cover a wider range of the spectrum than those found in single pigment species, allowing enhanced perception of more unusual signals. Although sunlight penetrating the water column is most intense around a relatively narrow, blue/green, part of the spectrum, it can contain significant amounts of shorterwave radiation. Similarly, bioluminescent emissions often extend beyond 500 nm. In three genera of stomiid dragon fishes with more than one visual pigment the link between visual pigment absorption and photic stimulus is very clear. These animals, in addition to normal blue/green bioluminescence, also produce extreme-longwave light. Two genera, Aristostomias and Pachystomias, have evolved very redsensitive visual pigments, while Malacosteus possesses a dietary-derived chlorophyll-based photosensitizing pigment, allowing perception of this “secret” waveband hitherto thought undetectable by other inhabitants of the deep-sea. Preliminary evidence, however, indicates that some myctophids, a group that are preyed upon by the stomiids, may also have evolved a photosensitizing mechanism to allow perception of the longwave bioluminescence produced by the dragon fish. The sequences of rod opsin genes from 28 species show that only nine sites are involved in the tuning of rod pigments in deep-sea fishes. The phylogenetic distribution of amino acid substitutions in the different species indicates that animals with shortwave-shifted rod visual pigments were present at or near the base of the Euteleost lineage, indicating that they were adapted to the same visual environment as present-day deep-sea fishes.