A team of
marine biologists has discovered 16 species of "ultra-black" fish
that absorb more than 99 percent of the light that hits their skin, making them
virtually invisible to other deep-sea fish.
The
researchers, who published their findings Thursday in Current Biology, caught
the species after dropping nets more than 200 meters deep near California's
Monterey Bay. At those depths, sunlight fizzles out. That's one reason why many
deep-sea species have evolved the ability to illuminate the dark waters through
bioluminescence.
But what if
deep-sea fish don't want to be spotted? To counter bioluminescence, some
species have evolved ultra-black skin that's exceptionally good at absorbing
light. Only a few other species are known to possess this strange trait,
including birds of paradise and some spiders and butterflies.
When
researchers first saw the deep-sea species, it wasn't immediately obvious that
their skin was ultra-black. Then, marine biologist Karen Osborn, a co-author on
the new paper, noticed something strange about the photos she took of the fish.
"I had tried to take pictures of deep-sea fish before and got nothing but these really horrible pictures, where you can't see any detail," Osborn told Wired. "How is it that I can shine two strobe lights at them and all that light just disappears?"
After
examining samples of fish skin under the microscope, the researchers discovered
that the fish skin contains a layer of organelles called melanosomes, which
contain melanin, the same pigment that gives color to human skin and hair. This
layer of melanosomes absorbs most of the light that hits them.
"But what isn't absorbed side-scatters into the layer, and it's absorbed by the neighboring pigments that are all packed right up close to it," Osborn told Wired. "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."
The result?
Strange and terrifying deep-sea species, like the crested bigscale, fangtooth,
and Pacific blackdragon, all of which appear in the deep sea as barely more
than faint silhouettes.
But interestingly,
this unique disappearing trick wasn't passed on to these species by a common
ancestor. Rather, they each developed it independently. As such, the different
species use their ultra-blackness for different purposes. For example, the
threadfin dragonfish only has ultra-black skin during its adolescent years,
when it's rather defenseless, as Wired notes.
Other
fish—like the oneirodes species, which use bioluminescent lures to bait
prey—probably evolved ultra-black skin to avoid reflecting the light their own
bodies produce. Meanwhile, species like C. acclinidens only have ultra-black
skin around their gut, possibly to hide light of bioluminescent fish they've
eaten.
Given that
these newly described species are just ones that this team found off the coast
of California, there are likely many more, and possibly much darker,
ultra-black fish swimming in the deep ocean.