A short-wavelength photoreceptor class in a deep-sea shrimp

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Abstract
In the world of midwater, mesopelagic animals, downwelling sunlight is filtered by the overlying water to a limited waveband centered near 475 nm. Consequently, the visual pigments of most of these species absorb maximally between 450 and 500 nm. The only exceptions occur in some fishes, which have additional visual pigments absorbing at long wavelengths (550-580 nm) matched to their red bioluminescence. We now find that the mesopelagic decapod shrimp Systellaspis debilis has two visual pigments. One of these absorbs maximally in the expected range (lambda(max) = 498 nm), but the other is maximally sensitive at very short wavelengths, approaching the near-ultraviolet (lambda(max) = 410 nm). The discovery of a visual receptor class absorbing at such short wavelengths in a mesopelagic animal suggests that visual systems in the deep sea may be far more diverse, and potentially more complex, than previously suspected.