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Douglas, R.H.* and Partridge, J.C.+
* Department of Optometry & Visual Science, City University, 311-321 Goswell Rd, London EC1V 7DD, UK
+ Department of Zoology, University of Bristol, Woodlands Road, Bristol BS8 1UG, UK
The visual pigments of most deep-sea fish absorb radiation most readily at wavelengths around 470-490 nm, matching both the wavelengths of solar radiation that most easily penetrate clear oceanic waters and the bioluminescent emissions of most deep-sea organisms.
However, three genera of stomiid fish (Pachystomias, Aristostomias & Malacosteus), which have suborbital light organs emitting much longer wavelengths (above 700 nm), have been shown to posses two visual pigments which are ‘long-wave shifted’ in comparison to those of other deep-sea fish, with (maxs around 520 nm and 550 nm. Although such pigments give these species an enhanced ability to see the light generated by their suborbital photophores, they still provide a far from perfect match between the visual pigment absorption spectra and these long-wave bioluminescent emissions.
We have shown, using a novel retinal wholemount technique on fresh material, that the Aristostomias retina contains an additional visual pigment with (max around 590 nm. Similarly, Pachystomias also has at least three visual pigments within its retina, one of which absorbs maximally around 590-600 nm.
Using fresh outer segment suspensions, we were unable to find a similar additional long-wave sensitive pigment in the retina of Malacosteus niger. The photoreceptor outer segments of this species have, however, been shown to contain a photostable pigment with an absorption maximum at 672 nm. We have shown that this pigment acts as a photosensitiser, resulting in Malacosteus visual pigments being bleached more by a 670 nm light than by some shorter wavelengths. Preliminary evidence indicates that this photosensitising pigment is a derivative of chlorophyll a.
Thus, different strategies have been adopted by the three genera of stomiid fish to visualise their own far red bioluminescence. While Pachystomias and Aristostomias employ at least three long-wave shifted visual pigments, Malacosteus has only two such pigments coupled to a long-wave, chlorophyll-derived, photosensitiser. These species therefore have a ‘private’ region of the spectrum which might be used for intraspecific communication undetectable by other species and/or for covert illumination of prey.
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