The relationship between predators and prey is often described as an arms race, with each side looking for new ways to better their opponent. Predators that rely on stealth look for new ways to blend into the background, while their prey adopt strategies to detect dangers lurking in the shadows. If you were a tasty little morsel, wouldn't it be great to be able to shine a light into these dark and dangerous places? Some fishes might be able to do just that. The eyes of triplefin Tripterygion delaisi have a mirror on the lower part of the iris, which they use to redirect sunlight into a beam that can be pointed sideways towards nearby objects. This so-called ‘ocular spark’ causes reflective eye-shine in the eyes of nearby animals, allowing them to be easily spotted just like the glowing eyes of a deer caught in the headlights. Triplefin use this trick to detect when their tiny plankton prey is nearby, and Matteo Santon at the University of Tübingen, Germany, wondered if triplefin could use the same technique to detect reflective eye-shine in their otherwise well-camouflaged predators.
The team's approach was to use tiny temporary baseball caps to prevent triplefins from using their ocular spark and then measure how close they would get to a predatory scorpionfish. Fish given clear caps could redirect sunlight from their eyes as usual, but fish that had their eyes shaded by opaque black caps could not.
Under laboratory conditions, triplefins fitted with opaque caps that shaded their ocular spark tended to approach their predators more closely than fish with clear caps. This exciting finding suggested that ocular sparks improved the ability of triplefins with transparent caps to avoid a nearby threat. However, the team performed these tests on captive fish under artificial conditions, so next, they strapped on SCUBA gear and sank aquariums to the sea floor to repeat the experiments under the most natural conditions possible. Just like in the lab, triplefins that were unable to use ocular sparks tended to approach predators more closely. Thus, lighting up the eyes of their predators seems to be an important triplefin safety mechanism.
Next, the team wanted to know exactly how much light can be redirected by an ocular spark and the farthest a predator could be before detection. Using a complicated mathematical model that included real-world measurements of how much sunlight is available, eye size and scorpionfish eye shininess, the team estimated that the ocular spark of triplefins could produce detectable eyeshine in scorpionfish that are about 6–8 cm away. This may not seem like a great distance but considering that triplefins are only 3–5 cm long, this amount of extra space could make the difference between life and death.
The arms race between triplefins and their scorpionfish predators marches on and it is hard to predict what the next steps will be. Perhaps scorpionfish eyes will become duller, or they will learn to keep them closed as prey approaches. But for now, triplefins seem to have gained the upper hand with their bright-eyed tactics.