Nekton Euryptile
The Nekton Euryptile has split from their ancestors, and has begun shifting towards a lifestyle in the open ocean. Their carapaces have thickened significantly, as a response to the predation of organisms like the carvebulls. It's not enough to prevent multiple bites from penetrating, but a single bite will not usually be enough to break through, allowing the nekton euryptiles enough of an opportunity to escape. Though not a guarantee (this method has a success rate of about 25%-35%) it allows some the chance to flee and reproduce. Their tail ends are still vulnerable, so they have developed long fiberous whiskers that act as a warning if a predator approaches from behind.
Their limbs have had to adapt to the thickened carapace, as their body forms are not the most adapted to the rigors of free swimming. They have made tradeoffs in mobility for the sake of developing protections against predation, and their limbs have lost articulation in the top joints. This isn't inherently much of a problem, as their middle limbs aren't of much use in the open water, and just act to hold purchase on the rare occasions they rest on the floor. To compensate somewhat, their three pairs of limbs have developed ball-and-socket joints to allow for increased mobility. They rely primarily on their tail for forward propulsion, while their hind fins provide some control for course correction. They have adapted a reduplicated pair of fins on the back limb, a product of developmental gene mirroring, which allow for further control. They are still somewhat awkward on the water, but successive generations show gradual improvements.
They live primarily in the midnight zones of the worlds oceans, feeding mainly on siluros, nautstars, roufos and relatives, although fully grown adult regularly can be found in the twilight zones when hunting is poor in the midnight zones. They are no longer ambush predators sticking to small swarms and instead rely on swarm tactics to simply catch prey. They rarely live independently, and instead live in simple schools of a few dozen to low hundreds, though the sizes of these swarm regularly change as schools break apart and merge. This schooling provides both a benefit in hunting, protection from predation, and an increase in mobility as they rely on the motion of others to increase hydrodynamic efficiency. They reproduce in a similar manner to their ancestors, but instead do so whenever a bounty comes along in the form of either a large carcass or a large collection of carcasses on the midnight floor. The males are now the only ones presenting with external gonopods, as the females no longer need them as they adapt further to external fertilization. Instead, females have adopted a more simplified ovipositor. The females will lay eggs in them and the males will fertilize them. The males will stay, and guard the eggs until the hatch, then go off in a smaller school. The young will then feed on the carcasses before swimming off themselves. Those that survive will join whatever school they can.