Long-wave sensitivity in deep-sea stomiid dragonfish with far-red bioluminescence: evidence for a dietary origin of the chlorophyll-derived retinal photosensitizer of Malacosteus niger

Both residual downwelling sunlight and bioluminescence, which are the two m ain sources of illumination available in the deep sea, have limited waveban ds concentrated around 450 500 nm. Consequently, the wavelengths of maximum absorption (lambda(max)) of the vast majority of deep-sea fish visual pigm ents also clustr in this part of the spectrum. Three genera of deep-sea loo se-jawed dragonfish (Arisbostomias, Pachystomias and Malacosteus), however, in addition to the blue bioluminescence typical of most deep-sea animals, also produce far-red light (maximum emission >700nm) from suborbital photop hores. All three genera are sensitive in this part of the spectrum, to whic h all other animals of the deep sea are blind, potentially affording them a private waveband for illuminating prey and for interspecific communication that is immune from detection by predators and prey. Aristostomias and Pac hystomias enhance their long-wave visual sensitivity by the possession of a t least three visual pigments that are long-wave shifted (lambda(max) value s ca. 515, 550 and 590 nm) compared with those of other deep-sea fishes. Ma lacosteus, on the other hand, although it does possess two of these red-shi fted pigments (lambda(max) values ca. 520 and 540 nm), lacks the most long- wave-sensitive pigments found in the other two genera. However, it further enhances its long-wave sensitivity with a chlorophyll-derived photosensitiz er within its outer segments. The fluorescence emission and excitation spec tra of this pigment are very similar to spectra obtained from mesopelagic c opepods, which are all important component of diet of Malacosteus, suggesti ng a dietary origin for this pigment.