Insidiator
The insidiator derives from twilight nippers that continued the drive into the sunlight. They found descendants of the twilight gill in much of their new range, along with many additional soft-bodied creatures that were ideal for their diet. They'll even feed on the nucleus-sharing shev; although its shell is far too tough and poisonous to attack it there, the pseudopods are easy enough for them to rip off. The shrank even further, too, so they could sustain themselves for even longer when feeding on the same prey.
The insidiator has a similar feeding strategy to its ancestors. It darts out and rapidly swims at its prey, tears off a chunk of flesh, and flees to consume it later. As before, their feeding tentacles are barbed, ideal for ripping and tearing, and the barbs can retract and lock into grooves in their opposite counterparts, allowing them to form a tapered shape when not in use. They must swallow their food whole. Therefore, they shred it as much as possible before swallowing, gripping it with their tentacles and ripping. And they often swarm together to bring down large prey, though their cooperation ends once their attack ends, and they will fight each other over remnants of their feast.
Now that they've made their way into shallower environments, they often lie in wait among whatever foliage is handy, be it wormstems, violetweed, krelp, or other available flora. They've adopted a generally murky bluish coloration, with countershading, as a compromise to keep as hidden as possible despite these diverse conditions. They'll rest on the sea floor or on a bit of flora, nearly dormant, their metabolism slowed, but keeping an eye out for prey. Then they'll swim out quickly and ambush their victim. With this successful strategy, they prefer sticking to shallow coasts, though they can cross the sunlight zone of the Vailnoff Ocean too, where there are plenty of sunlight gills and carpolantaians. This strategy has also led to a shift in their respiratory system, as they must breathe while motionless. Their gills are more efficient when they swim, and they store oxygen in their blood to make their ambushes possible after periods of inactivity. But they now can pump water in and out of their gills using internal musculature, and they have several additional intake holes, arranged all along their upper body. Their ability to store oxygen in their blood remains extremely useful for survival in low-oxygen water, if the water's too muddy or the like.
The three pairs of fins are sturdy, and they may rest on them and their lower body when lying in wait. They cannot, however, walk with their fins or use them to pull themselves along, and they cannot fully support their weight. The fins have become more flexible for greater maneuverability, however. No longer mere extensions of their armor plates, they have a joint segment at the base for independent motion, which connects to two main large sections that cover the main body of the fin. Beyond these are many much smaller plates, which it can manipulate to affect the shape of the fin.
They otherwise have much the same form as the twilight nipper: a long sleek body containing three notochords, with three chitin rings around the outside to provide support and protection. These rings have become even larger, though jointed in between, and the tail and head remain largely exposed. They also retain the internally chitinous eyes and the UV bulbs. They tend to rely on eyesight rather than the electroreceptors in their tentacles, so those have become rather less sensitive, but they sometimes use them when in murky water.