What do billfish see? Part II
Other handy visual adaptations also help billfish to catch and eat food. For example, the area of best vision in both eyes overlaps just ahead of the fish, right in front of its bill and mouth. Two eyes see more than one; hence, a fish directly in the marlin’s line of attack will be seen at maximum resolution.
Underwater footage of hunting marlin shows the animals moving their eyes constantly and quickly. This behavior moves the small area of best vision around in order to cover a maximum area while scanning around for prey fish or predators.
If this adaptation is so successful, why not have maximum resolution everywhere in the eye? This is partially because the high-tech abilities of the retina come at a price, causing this small piece of tissue to be one of the top oxygen consumers in the fish’s body, even when lazily scanning its surroundings. Presumably. to keep this price at a minimum, areas of best vision tend to be restricted to a small area of the retina in most animals. Billfish are no exception to this rule.
Also, high resolution of an image requires a lot of light, which is not always present in a billfish’s environment. Thus, the billfish eye is split into several different areas, with some designed for tasks such as working at low light levels, providing night vision, picking up fast movements or possibly distinguishing colors. How the billfish eye copes with all these different job requirements is one of the many questions scientists are currently working to answer.
Research has recently shown that UV light, which is invisible to humans, is actually used by many birds, fish and other animals, possibly as a “secret” communication channel or to detect food. They conducted a number of experiments to see whether billfish might also have this ability but found that the corneas of marlin and sailfish actually filter out UV light.
UV light is damaging to the retina in the same way as for skin, and such a UV filter sensibly protects the eye from light damage. While billfish can’t see UV, this is not the end of the story.
Scientists found that the brilliant blue stripes of a sailfish or striped marlin contain a UV color that is invisible to humans, to marlin and many other blue water fish. In fact, the UV color, measured using a spectrometer, is even brighter than the electric blue of a billfish stripe.
So why would marlin or sailfish display a color on their body which they, and many other fish, can’t see? It could be that the UV part of the billfish’s color has no meaning at all, simply being part of the make-up of the electric blue stripes that fishermen so admire. Billfish are capable of extremely rapid color change, and when hunting often show this striking blue coloration.
By the way, the stripes’ extraordinary color is created by a crystalline material deposited in cells called iridophores. These crystals reflect blue and UV light, which is why the stripes look blue to us. The iridophores can be hidden by cells containing dark pigment, giving the marlin a dull, blackish or grey color. Controlled by chemicals circulating through the marlin’s body, when it becomes excited, the dark covering layer is “moved” away, like curtain blinds, to allow the fish to light up.
The fact that the UV color of the stripes is actually brighter than the blue color that we see makes the stripes even more visible to any UV-receptive animal in the vicinity. There are some smaller fish that scientists believe are likely to see UV light – such as mackerel, a favored bait for marlin – which tends to suggest that the stripes may help in hunting.
Indeed, this is a lot of speculation about the “hunting” stripes of some marlin, tuna and mahi mahi. A means of communication between the hunters? A way of breaking up the silhouette of the approaching hunter to make it invisible to the baitfish or even to confuse baitfish schools? These are just some of the many questions.