Red bioluminescence in fishes: on the suborbital photophores of Malacosteus, Pachystomias and Aristostomias

Abstract

Many deep-sea animals produce blue bioluminescence, but species of three genera of midwater dragonfishes also produce red light. In addition to numerous small body photophores, species of these genera (Malacosteus, Pachystomias and Aristostomias) have large suborbital photophores that emit red light and postorbital ones that emit blue light. Pachystomias microdon additionally has a red-emitting preorbital photophore. Fluorescence measurements from the intact suborbital photophores, and from their exposed cores, confirm the previous hypothesis that the red light emitted by Malacosteus is spectrally altered by a superficial shortwave cutoff brown filter. No such filter is present in the other two genera. Studies of the anatomy of the photophores show that the suborbital photophore of Malacosteus is similar in general organisation to other photophores, including the postorbital photophore. The red-emitting photophores of Pachystomias and Aristostomias, however, have a unique organisation, in which the bulk of the photophore comprises a gland that lies outside the thick reflector. Strands of tissue run from the gland into the photogenic core of the photophore through numerous pores in the reflector. The production and use of red light by these fishes is discussed in the context of these results.