Ever wonder how marlin “light up?” If you were to look at marlin skin under a microscope, you would notice that right under the mucous layer is an area of tiny crystal structures punctuated by dark cells known as melanifores.

When a marlin is at rest, these dark melanifores are spread out and cover the crystal structures. When a marlin becomes agitated or excited, the melanifores close up, revealing the structures underneath.

To understand how a marlin lights up, let’s imagine a field of mirrors with an open gray umbrella held over each mirror. From the air, you would only see the tops of the gray umbrella, just as you only see the extended melanifores when a marlin is at rest. However, if the umbrellas were closed up, the mirror would catch the sunlight and produce dazzling color.

That’s pretty much what happens when a marlin lights up: the melanifores are contracted and the tiny crystals reflect light, making the skin seem to glow. The light reflected off the marlin is blue, because that happens to be the diffraction pattern of the crystals. These crystals create interference patterns between their layers and reflect the blue wavelengths seen during the lighting up response.

The colors presented by live marlin are one of their most spectacular features. Iridescent blues that flash when they “light up” have to be observed firsthand to appreciate their brilliance. Many anglers have observed that marlin are sometimes electric blue but may be deep blue or even brownish-red on rare occasions. The explanation for this lies in the fact that there are two separate chemical reactions at work during the lighting up process.

A marlin’s natural fight or flight reaction produces both norepinephrin and epinephrine, which affect the crystal structures in different ways. Because the norepinephrin is released into the autonomic nervous system, it reaches the skin quickly and begins to immediately contract the melanifores and to light the crystals to a bright blue. The epinephrine, on the other hand, takes much longer to reach the skin because it travels through the fish’s circulatory system. Once there, it turns the crystals from a light blue to a deep, royal blue, and if enough epinephrine is added, the crystals will become completely transparent.

That means that anglers who claim they have seen a marlin turn red during a long fight aren’t telling a fish tale. What they are actually seeing is the red blood of the tissues beneath the skin that become transparent after prolonged epinephrine stimulation. If you ever had a flash picture where everyone’s eyes came out red, you have seen a similar phenomenon.

In “red eye” photos, the flash is actually illuminating the red blood vessels behind the transparent eyeball.

Exactly why and when do marlin light up? Some think it might be a mating reaction, while most will agree that it also serves as an intimidation tactic used on prey and as a sign of agitation.

Nevertheless, no matter what the reason, most fishermen and scientists will agree that there are few sights in the ocean as exciting and beautiful than a “lit up” marlin.