Catadromous

Part of speech: adjective

Definitions

1. Relating to fish that live in the ocean but migrate to freshwater rivers to reproduce
2. (of fish) migrating from ocean habitats to freshwater environments for spawning and reproduction
3. describing aquatic organisms that spend most of their lives in saltwater but travel upstream into freshwater to breed

Etymology: "Catadromous" is a modern scientific coinage built from two Greek elements: "kata", meaning "down", and "dromos", meaning "running" or "course". The "-dromous" ending is the same element found in words like "palindromic" and "hippodrome," which points to a racecourse or path. Put together, the root describes movement downward, which is exactly what fish like eels do when they head back out to sea to spawn. The word was built by analogy with "anadromous", which describes fish such as salmon that travel upstream from the sea into rivers to breed. Its "ana-" element means "up". Naturalists created the pair to contrast the two migration patterns, so the words mirror each other: one is a journey up toward fresh water, the other a journey down toward the ocean.