Carrkin
The carrkin has gone through physical adaptations that are very similar to carcinization on earth, having evolved very crab-like features from a non-crab-like form. The segmented tail has curled underneath it in order for the organism to hide in crevices more efficiently, as well as to help with breathing. The curled tail acts as a sort of pump in order to pull in oxygen-rich water more effectively. The hole where the carrkin breathes out points in an opposite direction to where it breathes in to prevent the carrkin from breathing in water it already breathed out. The curled tail has created other opportunities such as egg rearing. The carrkin will keep fertilized eggs curled inside its tail much like a crab. This care will go on after they hatch, the young getting their nourishment from eating the scraps of their mother's food. This behavior allows young carrkins to get a good start in life.
The antennae of the carrkin are even more derived from its ancestor, having moved completely to the bottom of the organism's body and having evolved claws on the end of them. These are used to grab food to move to the mouth as well as for defense. The location of the antennae are meant to help the carrkin reach toward the mouth when feeding with no troubles. The antennae have also evolved more joints for flexibility to aid it when doing these activities. The carrkin has abandoned the ability to swim nearly entirely in favor of a completely benthic lifestyle. The only swimming ability they have is using the fins on their back legs for a tiny burst of speed. They have a keen sense of smell in order to find dead food matter in which they feed on, sometimes even climbing onto land in order to get to the food that they desire. The mouth has developed sharp spikes in order to better tear off bits of food. They have good eyesight for spotting potential threats. If a predator is spotted, they will usually dart to the nearest viable hiding space. If there is none to be found they will bury themselves in the sand. If the predator pursues, the older individuals will attempt to scare it off with their claws, while younger carrkins will usually flee.