When Robert Heathcote dips his head into the rivers of Trinidad, he’ll often see little black dots moving through the water, even when it’s murky. Those dots are the eyes of small, inch-long fish called guppies. And those guppies are angry.
Guppies are among the most popular aquarium fish in the world, but they originally hail from the Caribbean. In Trinidad, some populations live upstream of waterfalls, which separate them from predators. In these safe havens, guppies flourish in such numbers that they compete intensely for food. They’ll guard any fruit that drops from overhanging trees with a ferocity that belies their puny size. Tail smacks are common, as are biting and ramming.
“You’ll see these little angry fish swimming over food patches, attacking anything coming over them,” says Heathcote. “We study guppies because they’re the archetypal rubbish animal that gets eaten by everything else, so seeing them display aggressive behavior is really cool.”
Heathcote and his colleagues, led by Darren Croft from the University of Exeter, noticed that when the guppies get aggressive, their eyes go black. Specifically, their irises—the colored rings around their pupils—change from a silvery color to a jet black. The change is extremely fast. “Sometimes, I’ll be watching a fish and I’ll blink and it’s got a completely different eye color,” says Heathcote.
It’s not clear exactly how the guppies darken their eyes. There are cells in their irises that contain dark pigment; it’s likely that their nervous systems can issue commands that change the size of those cells.“We have embarrassingly little idea about the mechanism,” says Heathcote. But here’s a twist: Even though black is the rarer color, it’s the default one. When Heathcote anesthetizes guppies, their irises always go dark. It seems that they’re constantly suppressing that darkness, and then releasing it when it’s time for a fight.