Eels (Anguilliformes)are characterized by an elongated and cylindrical body, reminiscent of a snake. They are characterized by not having pelvic fins, and in several species the pectoral fins are also absent. In those species that do have pectoral fins, these are placed in an approximately mid-lateral position. Both the dorsal and anal fins are confluent with the caudal fin, thus forming a single continuous fin, which runs most of the length of the animal. Their skin is generally devoid of scales, so they have a bare skin, although there are certain species that do have scales and these are usually embedded in the skin. The gill openings are generally narrow and are located in a slightly backward position.
Predatory and generally active at night, eels have a highly developed dentition, with teeth throughout the mouth. Many species specialize in burrowing into small openings, crevices and hollows, although there are other habitat preferences such as species that live in the blue (pelagic species) or burrow by burrowing into soft substrates. They swim by making undulating movements that travel along their body, and as a curiosity, they are able to swim in the opposite direction just by reversing the direction of the undulations. Another curious fact about this group of species is that many of the species perform migrations in which they travel very considerable distances, especially considering that they are not species that can be considered as fast swimmers.
Among the eels we find species known to all, such as the Eels, Moray Eels and Conger eels. There are approximately 940 species within the order eels, which are distributed in 19 families and 159 different genera, most of which are marine. The size of the eels ranges from 5 cm in the case of Monognathus ahistromi, to 4 meters in the case of Strophidon sathete and weights from 30 g to over 100 kg (Conger conger). Eels species can be found in all marine aquatic habitats of the earth.
Eels begin their life cycle as flattened and translucent larvae that later undergo a process of metamorphosis until they become adults.