A female Pacific blackdragon
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.
The Pacific blackdragon
Credit: Karen Osborn/Smithsonian
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.
A crested bigscale
Credit: Karen Osborn/Smithsonian
"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.
Pacific viperfish
David Csepp, NMFS/AKFSC/ABL
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.
Distrustful dog.
Is humanity's best friend catching on to our shenanigans? Researchers at the University of Vienna discovered that dogs can in certain cases know when people are lying.
The scientists carried out a study with hundreds of dogs to determine to what extent dogs could spot deception. The team's new paper, published in Proceedings of the Royal Society B, outlined experiments that tested whether dogs, like humans, have some inner sense of how to assess truthfulness.
As the researchers wrote in their paper, "Among non-primates, dogs (Canis familiaris) constitute a particularly interesting case, as their social environment has been shared with humans for at least 14,000 years. For this reason, dogs have been considered as a model species for the comparative investigation of socio-cognitive abilities." The investigation focused specifically on understanding if dogs were "sensitive to some mental or psychological states of humans."
The experiment
The experiments involved 260 dogs, which were made to listen to advice from a human "communicator" whom they did not know. The human told them which one of two bowls had a treat hidden inside by touching it and saying, "Look, this is very good!" If the dogs took the person's advice, they would get the treat.
Once they established the trust of the dogs, the researchers then complicated the experience by letting dogs watch another human that they did not know transfer the treat from one bowl to another. In some cases, the original communicator would also be present to watch but not always.
The findings revealed that half of the dogs did not follow the advice of the communicator if that person was not present when the food was switched to a different bowl. The dogs had a sense that this human could not have known the true location of the treat. Furthermore, two-thirds of the dogs ignored the human's suggestion if she did see the food switch but pointed to the wrong bowl. The dogs figured out the human was lying to them.
Photos of experiments showing the dog, human communicator, and person hiding the treat. Credit: Lucrezia Lonardo et al / Proceedings of the Royal Society B.
"We thought dogs would behave like children under age five and apes, but now we speculate that perhaps dogs can understand when someone is being deceitful," co-author Ludwig Huber from the University of Vienna told New Scientist. "Maybe they think, 'This person has the same knowledge as me, and is nevertheless giving me the wrong [information].' It's possible they could see that as intentionally misleading, which is lying."
This is not the first time such experiments have been carried out. Previously, children under age five, macaques, and chimps were tested in a similar way. It turned out that children and other animals were more likely than dogs to listen to the advice of the liars. Notably, among the dogs, terriers were found to be more like children and apes, more eagerly following false suggestions.
