Jhe inky depths of the twilight zone of the ocean are home to fist-sized shrimp-like crustaceans with ridiculously large eyes. The most of Cystisomehis head is occupied by his eyes – all the better for seeing in the dark. “The more you enlarge your eye, the more likely you are to catch the photons that are there,” says Karen Osborn, a researcher at the Smithsonian Institution in Washington DC.
A great challenge for animals living in deep water, in CystisomeThe case between 200 and 900 meters deep is to see while not being seen by predators. “It’s like playing hide and seek on a football pitch,” says Osborn. “There is nothing to hide behind.”
The eyes are particularly difficult to hide because the retinas must always contain dark photon-absorbing pigments, which predators can distinguish either in the low light of the twilight zone or in the beams of their own bioluminescent spotlights. Cystisome disguises its large eyes in a unique way. Instead of concentrating the pigments in a small area, Osborn says, they spread out their retina into a thin sheet of tiny reddish dots that are too small for most animals to see.
Cystisome hides most of the rest of its body by being completely transparent. When scientists catch them in trawls and empty them into a bucket of seawater, they appear as palm-sized gaps between other animals. “You really can’t see these things until you pull them out of the water,” says Osborn.
The most of CystisomeThe internal organs of appear crystal clear thanks to the very orderly and structured way in which their tissues are laid out, explains Osborn. “The one thing they don’t seem to do very well with is their instincts,” she says. The golden structure visible under the eyes is the digestive organ. Even that is stacked high and straight to cast as little shadow as possible while still Cystisome hangs in its usual horizontal position.
These crustaceans make themselves even harder to spot underwater by reducing the light reflecting off their transparent bodies, Osborn and colleagues found in 2016. Seen under an electron microscope, parts of CystisomeThe exoskeleton is covered in tiny bumps, which Osborn likens to a shag carpet. Other parts are covered with a single layer of spherical shapes, which scientists believe could be colonies of an unknown form of bacteria.
Nanoscopic shag carpet and spheres make light 100 times more likely to pass directly through Cystisome, rather than being reflected in the eye of a passing predator. “It works exactly the same as an anti-reflective coating on a camera lens,” Osborn explains.
CystisomeThe legs in particular benefit from the furry anti-reflective coating and the joints covered in spheres, because otherwise they would easily catch the light when they flicker and wiggle. “These guys are absolute masters of transparent mid-water camouflage.”
But what happens when the almost invisible Cystisome really wants to be found? These crustaceans need to mate to reproduce. A clue to how mates are found is in the male Cystisomelarge antennae covered with structures that detect chemicals in the surrounding water. “They really feel for each other,” says Osborn.