Piranha

 (ピラニア, Pirania) are omnivorous freshwater fish native to the Amazon regions of South America. They are well known for their sharp teeth, large appetites, and attack/eating in large schools.

Fine Eaters
Fine Eaters are a subspecies of the piranhas that appear in Chapter 37. These piranhas are characterized by the fact that they feed on thin, soft materials. In the wild they feed on the fins of other fishes, which is why they are also a danger to Mermaids and Fishfolks. While normally harmless to humans and most other Liminals, their attacks can be very disruptive as they also biting clothes.

Fine Eaters are a fictional piranha subspecies created for the manga series and as such they have no relation to piranhas from the real world.

Trivia

 * Although Fine Eaters do not exist in the real world, there are still some piranha subspecies, such as Serrasalmus elongatus, that mainly feed on the scales and fins of other fishes. In fact scale- and fine-eating widespread among juvenile and sub-adult piranhas.