The wave-fin plagu has replaced its ancestor. It has adapted to the competition of the Jujubee sea floor. As the area has become more crowded with better predators and scavengers the plagueater was being out competed. It adapted by becoming an opportunist.
The largest change is its form of locomotion. Its fin now runs from the top of its head to the tip of its tail. Strong muscles at the base of the fin help the plague to move it in a wave motion that runs down the body. Plagus can now control their speed by reducing or increasing the speed of their wave fin. This helps chase after small prey such as jumping shellpede and swim away from larger predators such as the featherback snark.
Another opportunistic adaptation is the crop that the wave-fin has evolved. When it finds something large to eat such as carrion of dead animals it will eat as quickly as it can. It has evolved a crop at the base of its mouth which can expand to four times its normal size. This means that the wave-fin can fill up when food is plentiful and not need to find food for a while. It also means if it gets disturbed by a predator whilst feeding it can swim away whilst carrying food for later.
Reproduction and general behavior are the same as its ancestor.