Idiomysis is a genus of minuscule crustaceans that reach just a few millimetres in length. They belong to an order called Mysida.
Most Mysids look a lot like shrimp. Their head and thorax is fused into a more or less slender cephalothorax with a long abdomen trailing off behind. They also have two pairs of antennae and large eyes on stalks.
Idiomysis simply alter some of the proportions.
Their cephalothorax is exceptionally rotund and they keep their abdomen curled up behind them like a cat's tail.
Also they have enormous eyes, rather than merely "large" ones.
With their extremities carefully tucked in and gigantic goggles on their face, they look like adorable, little space-farers. Or sinister, maybe. They'd look scary if they weren't smaller than most grains of rice.
Hordes of Idiomysis can be seen buzzing around various reef furniture like flies around exceptionally attractive turds.
Idiomysis are already incredibly tiny, but they start off life even smaller! Which I guess is pretty obvious but still.
Mysids are also known as Opossum Shrimp because the females keep their eggs in a pouch called a marsupium (which comes from the Greek for "bag" or "pouch"). The male puts his sperm in there and the eggs eventually hatch directly into miniature adults, skipping over any larval period.
Giant eyes and a curly tail... If there's such a thing as space-kittens, these are they.