Tetra a tall dorsal fin characterized by a short connection to the fish’s body. Additionally, tetras possess a long anal fin stretching from a position just posterior of the dorsal fin and ending on the ventral caudal peduncle, and a small, fleshy adipose fin located dorsally between the dorsal and caudal fins. This adipose fin represents the fourth unpaired fin on the fish (the four unpaired fins are the caudal fin, dorsal fin, anal fin, and adipose fin), lending to the name tetra, which is Greek for four. While this adipose fin is generally considered the distinguishing feature, some tetras (such
Largetooth cookiecutter shark and sparsely scattered light-emitting photophores on the belly. Most specimens have lacked the dark "collar" found on the throat of "I. brasiliensis". However, a specimen caught in 2004 off the Azores did possess the collar. The maximum recorded length is . Based on its smaller dorsal and caudal fins, the largetooth cookiecutter shark is believed to be less active than "I. brasiliensis" and an overall weak swimmer. Much of its body cavity is occupied by an enormous oil-filled liver, which allows it to maintain neutral buoyancy in the water column with little effort. Unlike "I. brasiliensis", this shark possesses binocular