; subclass Actinopterygii, order Atheriniformes)
A large family of small fresh- to brackish-water fish, that have a single dorsal fin opposite the anal fin, the slender body ending in a truncate or rounded tail fin. There is no lateral line. There are teeth in the jaws. Several species are known to enter brackish water or even sea water; these include Fundulus heteroclitus (killifish). There are about 300 species, found in most tropical freshwater systems of the world.
Subjects: Zoology and Animal Sciences.