Colour vision in bill sh

Members of the bill¢sh family are highly visual predatory teleosts inhabiting the open ocean. Little is known about their visual abilities in detail, but past studies have indicated that these ¢shes were likely to be monochromats. This study, however, presents evidence of two anatomically distinct cone types in bill¢sh. The cells are arranged in a regular mosaic pattern of single and twin cones as in many ¢shes, and this arrangement suggests that the di¡erent cone types also show di¡erent spectral sensitivity, which is the basis for colour vision. First measurements using microspectrophotometry (MSP) revealed a peak absorption of the rod pigment at 484 nm, indicating that MSP, despite technical di¤culties, will be a decisive tool in proving colour vision in these o¡shore ¢shes. When hunting, bill¢sh such as the sail¢sh £ash bright blue bars on their sides. This colour re£ects largely in ultraviolet (UV) light at 350 nm as revealed by spectrophotometric measurements. Bill¢sh lenses block light of wavelengths below 400 nm, presumably rendering the animal blind to the UV component of its own body colour. Interestingly, at least two prey species of bill¢sh have lenses transmitting light in the UV waveband and are therefore likely to perceive a large fraction of the UV peak found in the blue bar of the sail¢sh. The possible biological signi¢cance of this ¢nding is discussed.