What kind of animal is an adult elver?

An adult elver is a young eel, specifically the juvenile stage of the European eel (Anguilla anguilla) and the American eel (Anguilla rostrata). After hatching in the ocean, elvers migrate to freshwater or coastal habitats where they grow into adult eels. They are characterized by their elongated, slender bodies and are typically transparent or pale in color at this stage. As they mature, they develop a more distinctive coloration and physical features before migrating back to the ocean to spawn.