If you're an aquatic mammal, how do you hunt in the darkest seas, where light can't penetrate? Dolphins have it easy with their echolocation, but what about the other animals with much more limited senses? They chase after critters that provide their own light.
In a paper published in the journal PLOS One, researchers describe how they tracked four female southern elephant seals (Mirounga leonina) as they hunted, recording the time, depth, and light levels. In cases where the light was recorded as brighter than ambient levels, the seals were near sources of bioluminescence — and wouldn't you know it, that was when they foraged more.
The seals have eyes uniquely tuned to detect light at 485 nm, the same wavelength as the bioluminescence of the lantern fish that make up much of their prey. While it may seem self-evident that a deep sea diving predator would go after prey that glows in the dark, this is some of the first hard evidence showing that this does happen.
It also hints at a whole world of other possibilities — the sensors on the seals were only capable of registering light levels every two seconds, and could only detect the bright flashes that some animals create, rather than soft glows. There was also no way to directly tie the seals' predation to specific prey species or the many others that can bioluminesce. The researchers also noted that some species of shrimp and squid will eject clouds of bioluminescent materials to distract predators that focus on the glow.
The seals are also perfectly capable of hunting animals that don't glow, suggesting they have other tools at their disposal, too. It's a bizarre and glowing world in the depths, and this takes us one step closer to understanding how it functions.