A study of the visual pigments from three families of teleost that inhabit atypical light environments

Over evolutionary time the planet has become populated by a multitude of organisms highly adapted to their own ecological niche. Each individual species relies on a range of modalities that have become fine-tuned to provide accurate and reliable information. Vision is one such sense and shows interspecific variation in both acuity and chromatic sensitivity. One of the prime examples in recent years has been the short-wave shift in sensitivity found in the visual pigments of deep-sea fish (Lythgoe, 1972; Douglas et al., 1998, 2003). In the majority of species a single rod opsin is expressed, which is maximally sensitive to the wavelengths of bioluminescence and down-welling sunlight (λmax = 470 - 490 nm). This study examines two deep-sea species that have further adapted their visual sense to aid prey detection, and investigates the notothenioid family of ice fish that live beneath pack ice and inhabit a variety of depths. The three stomiid genera, Malacosteus, Pachystomias and Aristostomias are unique in emitting far-red light as well as the typical blue bioluminescence. All three utilize a rhodopsin/porphyropsin pigment pair with λmax values around 520/550 nm, approximately 40 and 70 nm long-wave shifted when compared to other deep-sea pigments, to increase sensitivity to long wavelengths (O'Day and Fernandez, 1974; Dartnall, 1975; Bowmaker et al., 1988; Partridge and Douglas, 1995). There is also evidence that Pachystomias and Aristostomias express a second visual opsin, which has a rhodopsin λmax at around 580 nm (Partridge and Douglas, 1995; Douglas et al., 1998). The aim of this study was to identify these two opsins. A single rod opsin has been identified from both Pachystomias and Aristostomias, and there has been no evidence for a second opsin. Histology shows a single class of rod photoreceptor in the Pachystomias retina. Another deep-sea species, Scopelarchus analis, has highly adapted eyes with seven different retinal specializations. Previously MSP identified three rhodopsins with λmax values at 444, 479 and 505 nm, which show different expression patterns in the main and accessory retinae. It was also demonstrated that there might be a switch in expression from the 505 nm to the 444 nm pigment during maturation (Partridge et al., 1992). This study has identified four retinally expressed opsin genes, an RH2, an SWS2 and two RH1 opsins. The two rod opsins regenerate in vitro with λmax at 480 and 490 nm. Both are expressed in adult mRNA, but the 480 nm rod is absent from juvenile mRNA. Further experiments linking the identified genes to the MSP values and examining expression patterns are also shown. Work has also been undertaken on members of the notothenioid family of ice fish. These live at various depths under thick pack ice in the Antarctic. The light that reaches the water below has been filtered, with long and short wavelengths restricted. This study shows that a number of species express SWS1, SWS2 and RH2 opsins, though the level of SWS1 expression may be low in the majority of species. In situ hybridization and histology have helped to localize expression of these genes to different classes of photoreceptor. MSP and in vitro regeneration have identified the λmax of these pigments