The scourge of the waterways of Skull Island was the swarming Killer-eel. Appropriately named, the bug-eyed fish were lethal in numbers. When attacking, their razor sharp teeth shredded skin and flesh to ribbons in no time.
Not true eels, but a unique species related to lampreys, Killer-eels staked claim to sections of river they patrolled and guarded jealously. Schools of up to a hundred hunted together. Injured or sick prey, fish or reptile, were the preferred variety. Their tactics involved overwhelming an animal with multiple attacks, each member of the school surging in to rasp bite-sized chunks of flesh out of the prey.
Slow prey was the mainstay of their diet, as the Killers could not sustain high-energy activity for long periods without resting. For the majority of the time the schools swam slowly, expending as little energy as possible, until one eel would come upon a potential food source. Then, spurred into activity by the scent of blood in the water, the entire school would attack, overwhelming the prey and often stripping it to the bone with their many-toothed, circular mouths within minutes like piranha's do.
Killer-eels' eggs were laid in sticky masses in the water reeds, but suffered dreadful attrition, being food to many small fish and invertebrate species. Coupled with short life spans, this helped keep their numbers in check.
The species was also very vulnerable to changes in water temperature or acidity. Even small charges, if too sudden, could have disastrous consequences for a group.