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
Notoscopelus elongatus the anal fin 18 to 19, and in the pectoral fins, 12 to 13. There is a distinctive arrangement of photophores (light-emitting organs) on the head and body. At a length of about , males develop luminous, glandular patches on the dorsal surface of the caudal peduncle. It is very similar in morphology to "Notoscopelus kroyeri" but differs in having 23 to 25 gill rakers whereas "N. kroyeri" has 26 to 29. "N. elongatus" is endemic to the Mediterranean Sea where it is found in greatest abundance in the Western Basin and the Ligurian Sea and Tyrrhenian Sea. During the