Shimmerworms

Shimmerworms are a species of carnivorous nematodes native to the Kytaran Sea, which has a distinct taste for drowning sailors, and can often be spotted harassing shipwrecks in the Bay of Bone
Appearance
Shimmerworms reproduce through a cloud of spores on the sandy slopes that make the borders between shallow seas and the abyss of deeper oceans. When the microscopic eggs hatch, the infant shimmerworms grow rapidly, feasting on plankton and other microorganisms, often including their own brothers and sisters, and will reach their adult size in a matter of weeks. Most shimmerworms are between five and seven centimeters long, and have a flat, ribbon like body, with a pure white belly and a reflective blue shine on their backs. This gives the worms excellent camouflage in their natural habitats, whether they are being seen from below or from above. Most notably, the shimmerworm has a single perfectly circular jawbone, ringed with hundreds of sharp, serrated teeth.
Swarm
After reaching their full size, shimmerworms are no longer hostile towards each other, and will instead form large swarms, moving in intricate patterns just below the surface. The swarm will attack any target that it's mass could cover twice over, covering the poor sea creature as one half of the swarm attacks first, then retreats for the second half. Carrying on in this pattern, a large enough swarm could strip the flesh from a dolphin in roughly two minutes. The average size for a shimmerworm swarm is more than enough to take on a drowning humanoid, and shimmerworms have learned to recognize a meal that can't do very much to fight back. There have been cases of ships sinking due to very ordinary causes, with a rescue vessel very nearby, only for the swarms, who will have been attracted by the commotion of a ship sinking, to make quick work of every member of the crew who thought to find safety by swimming as far from the sinking ship as possible.
Extermination
To the dismay of pest control businesses throughout the sea, shimmerworms do not seem to be affected by the go-to method of carnivorous swarm extermination, which is to tempt them with poisoned meat. Shimmerworms have very rudimentary digestive systems, and seem to simply pass anything they can't digest, and the only thing they can digest is protein, meaning any non-protein based poisons will not be metabolized. Area of effect spells and, strangely, fire seem to be the only sure methods of destroying entire swarms in one go, or at least destroying enough individual worms to scatter the survivors. For this reason, the duty of rescuing crews from this threat usually falls to any shipboard mages. If the ship has no mages, explosives thrown directly into the swarm have proved mostly affective in the past, if not to kill the shimmerworms, at least the concussion from multiple detonations will make them lose interest and swim away of their own accord. This method is not going to be any crews plan A, however, due to the inherent risks of throwing grenades into the water, right next to a wooden ship, while surrounded by blood thirsty prehistoric worms.