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, although their dangerousness and aggressiveness are often greatly exaggerated.

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 unpleasant, since they also chew up swimsuits.

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 don' t 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.
 * In their appearance in Chapter 37, the Fine Eaters have no pupils. This is a stylistic device often used in movies to portray animals as mindless and ferocious, based on piranha's negative reputation for bloodthirsty and evil.
 * Although piranhas are often portrayed as cruel and bloodthirsty, they actually fulfill an important role in the Amazonian ecosystem, preventing the spread of epidermicy by eating sick and dead animals. There are also herbivorous piranhas, despite their bad reputation.
 * While piranhas are often portrayed as extremely dangerous, most of the time, they don't actually pose a huge risk. Attacks on humans are very rare and mostly result on only minor injuries. However, some caution should be exercised because while piranha attacks on humans are greatly exaggerated, a swarm of them can actually kill a human.
 * Although piranhas are often portrayed as swarm hunters, they don't actually live in groups to hunt larger animals, but because it affords them greater protection from their predators. Some species like the black piranha are also solitary.