The inky depths of the twilight zone of the ocean are home to fist-sized crab-like crustaceans with ridiculously large eyes. Most of the head of the cystisoma is removed from its eyes, clearly visible in the dark. “The more you widen your eye, the more likely you are to trap photons there,” says Karen Osborne, a researcher at the Smithsonian in Washington, DC.
The main problem for animals that live at great depths, in the matter of the cystisome between 200 and 900 meters deep, is to see without being detected by predators. “Basically, it’s like playing hide-and-seek on a football pitch,” Osborne says. “There’s nothing to hide here.”
The eyes are especially difficult to hide, since the retina must always contain dark photon-absorbing pigments, which predators can discern in low light at dusk or with the rays of their own bioluminescent spotlights. The cystisoma hides its huge eyes in a unique way. Instead of concentrating the pigments in a small area, they cover the retina with a thin layer of tiny reddish dots that are too small for most animals, Osborne says.
The cystisoma hides most of the rest of your body, being completely transparent. When scientists catch them with trawls and empty them into a bucket of salt water, they look like palm-sized empty spaces among other animals. “You can’t really see these things until you pull them out of the water,” Osborne says.
According to Osborne, most of the internal organs in the cystisome look crystalline due to the very orderly and structured arrangement of their tissues. “The only thing they don’t get along with is their intuition,” she says. The golden structure that you see under the eyes is the digestive organ. Even this model is placed high and straight to cast as little shadow as possible, while the cystisoma is suspended in its usual horizontal position.
Osborne and his colleagues discovered in 2016 that these crustaceans are even more difficult to detect underwater due to the decrease in light reflecting off their transparent bodies.under an electron microscope, parts of the cystisome exoskeleton are covered in tiny bumps that Osborne likens to a carpet of bristles. Other parts are covered with a single layer of spherical shapes that scientists believe could have been colonies of bacteria of not-known shape.
Thanks to the carpet and nanoscopic bristle balls, the probability of light passing directly through the cystisome is 100 times higher than when reflected in the eye of a passing predator. “It works the same way as the anti-reflective coating on the camera lens,” Osborne says.
In particular, the legs of the cystisoma benefit from an anti-reflective coating, and the joints are covered with balls, otherwise they would easily pick up light when twisted. “These guys are absolute masters of transparent camouflage in mid-water.”
But what if an almost invisible cystisoma really wants to be discovered? These crustaceans need to mate in order to reproduce. An indication of how the pairs meet can be found in the whiskers of the male cystisoma, which are covered by structures that detect chemicals in the surrounding water. “They really feel for each other,” Osborne says.