“Imagine yourself in a bunker with no light whatsoever, and you have to find your mate. You hope you have a flashlight, right?” asks Scripps Institution of Oceanography’s Dimitri Deheyn, who studies bioluminescence but wasn’t involved in the new study. “Light is exploding there, but it's biological light. And most of the time it is mainly blue light, because it’s what propagates the farthest in these waters.”
There’s a problem, though: All this light attracts attention, both for prey and predators. If the prey can see an anglerfish’s face lit up by its dangling lure, the jig is up. Same thing if your face has headlights that help you spot your food. “Obviously, you don't want your prey to be seeing that light casting all over your face,” says Davis. “So if your head is now ultra-black, none of that light goes back to the prey, and you’re more likely to get within range.”
So how do these 16 species of ultra-black fish camouflage themselves so well? It’s all about melanin, the same stuff that gives us humans our skin coloration. The skin of these fishes is positively packed with a layer of organelles called melanosomes, themselves loaded with melanin.
Tons of melanin alone, though, couldn’t make these fish rival the darkness of Vantablack. When you shine a light on a fangtooth or its peers, the melanosomes absorb most of the photons right away. “But what isn't absorbed side-scatters into the layer, and it's absorbed by the neighbouring pigments that are all packed right up close to it,” says Osborn. “And so what they've done is create this super-efficient, very-little-material system where they can basically build a light trap with just the pigment particles and nothing else.”
“It's kind of like a gumball machine, where all of the gumballs inside are the right size and shape to trap light within the machine,” adds Davis. “Once it comes in, it bounces around between all of those little balls and doesn't come back out.”
Incredibly, there wasn’t one common ancestor of these 16 Vantafish species – or at least the 16 Osborn and Davis have discovered so far – that evolved this trick and passed it along to its evolutionary descendants. The species evolved it independently, so they have slightly different ways of going about absorbing light; their melanosomes are arranged differently in the skin.
“Some of them have just a big jumble,” says Osborn. “Some of them have three layers, some of them have two layers. Some of them have really thick layers, some of them have thinner layers. So there's these little, little differences in how they've actually accomplished this between the different species.”
The skin of all of these fishes is specifically tuned to absorb blue and green light, which are the colours of the majority of bioluminescence in the deep. But the species are using their ultra-blackness for diverse purposes, either as the prey hiding themselves from hunters, or as the hunters hiding themselves from prey. Interestingly, one of the species, the threadfin dragonfish, is only ultra-black as a juvenile. Why? Because tiny, defenceless babies need to hide from predators. As adults, once the threadfin dragonfish grow to over a foot long, they don’t have so much to hide from – they’re higher up the food chain.
