Lernaeocera branchialis, sometimes called cod worm, is a parasite of marine fish, found mainly in the North Atlantic.

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.

[3] The cycle begins with the females laying eggs which hatch into a nauplius, the usual early larval stage of crustaceans.

[3] The next stage is finding a secondary or intermediate host, a demersal fish like a flounder or lumpfish which is often stationary and therefore easy to catch.

There, while attached to a gill, the female develops a plump, sinusoidal, worm-like body, with a coiled mass of egg strings at the posterior end.

There, firmly rooted in the cod's circulatory system, the front part of the parasite develops in the shape of antlers or branches on a tree, reaching into the main artery.

[4][5] It is not known how L. branchialis searches for its fish hosts, but it probably uses chemoreceptors and mechanoreceptors, and follows physical clues in the water column such as those provided by haloclines and thermoclines.