Color Vision in Billfish

Previous studies have reported that many members of the billfish family, including marlins and swordfish, were monochromats. In contrast, recent studies have reveled that there are actually two distinct types of cones in billfish. Billfish are capable of altering their banding coloration by adrenergic stimulation of iridophores. This study investigates the possibility of UV vision in billfish and its overall importance to the family. Individual billfish were caught and immediately killed, the retina isolated from the eye, and the pigment removed. A custom spectrophotometer was constructed to determine the light transmission through the optical media of the fish. Resulting from this is the proof of three photoreceptors: rods, twin cones (was predominant cone type), and single cones. Many of the cones group to form row or near square mosaics. Through microspectrophotometry, researchers found a peak absorbance at 484 nm. Based on cone concentrations in the dorsal and ventral portion of the eye, it is believed that the cones in the ventral retina are evolved for looking into bright light whereas the cones in the dorsal retina are specialized for sensitivity as opposed to overall color discrimination. Also, since the lenses for billfish do not transmit light below 400nm, it is not believed that UV light plays a role in their color vision.

This article relates to class because it combines two different concepts. For one it describes the overall concentration and arrangement of rods and cones in the eye. Rods and cones have separate but similar functions. Based on the arrangement of rods, signal cones, and twin cones the billfish eye has evolved to optimize sensitivity to light. This article also involves signaling. It is not believed that billfish can see in the UV spectrum so their UV signaling on their skin is likely an adaptation evolved for the visual system of their prey species. Many of these fish do see in the UV spectrum, thus it is believed that the UV colored bars may help disguise the attack or possibly confuse the fish during an attack.