Colour vision in billfish

Members of the billfish family are highly visual predatory teleosts inhabit ing the open ocean. Little is known about their visual abilities in detail, but past studies have indicated that these fishes were:ere likely to be mo nochromats. This study however, presents evidence of two anatomically disti nct cone types in billfish. The cells are arranged in a regular mosaic patt ern of single and twin cones as in many fishes, and this arrangement sugges ts that the different cone types also show different spectral sensitivity, which is the basis for colour vision. First measurements using microspectro photometry (MSP) revealed a peak absorption of the rod pigment at 484 nm, i ndicating that MSP, despite technical difficulties, will be a decisive tool in proving colour vision in these offshore fishes. When hunting, billfish such as the sailfish flash bright blue bars on their sides. This colour ref lects largely in ultraviolet (UV) light at 350 nm as revealed by spectropho tometric measurements. Billfish lenses block light of wavelengths below 400 nm, presumably rendering the animal blind to the UV component of its own b ody colour. Interestingly at least two prey species of billfish 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 sailfish. The possible biological significance of this finding is discussed.