Lernaeocera branchialis, sometimes called cod worm, is a parasite of marine fish, found mainly in the North Atlantic. It is a marine copepod which starts life as a small pelagic crustacean larva. It is among the largest of copepods, ranging in size from 2 to 3 millimetres (3⁄32 to 1⁄8 inch) when it matures as a copepodid larva to more than 40 mm (1+1⁄2 in) as a sessile adult.

Lernaeocera branchialis is ectoparasitic, which means it is a parasite that lives primarily on the surface of its hosts. It has many life stages, some of which are motile and some of which are sessile. It goes through two parasitic stages, one where it parasitizes as a secondary host a flounder or lumpsucker, and another stage where it parasitizes as a primary host a cod or other fishes of the cod family (gadoids). It negatively impacts the commercial fishing and mariculture of cod-like fish.

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