Lernaeocera branchialis

The life-cycle of a cod worm involves a complex progression of life stages, including two successive hosts. It comprises "two free-swimming nauplius stages, one infective copepodid stage, four chalimus stages and the adult copepod, each separated by a moult". The female cod worm still resembles a copepod and is 2 to 3 mm long. Females undergo another pelagic quest, searching this time for a definitive or primary host. With her fertilised eggs, she looks for a cod or a fish belonging to the same family as cod, such as a haddock or whiting. The oral end of the female copepod penetrates the body of the cod until it enters the rear bulb of the host's heart. 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. In this way, while safely tucked beneath the cod's gill cover, the female's deeply embedded oral end can feed on blood while eggs develop and are released into the water column from the posterior end. ==Behaviour==

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. ==Effects on fisheries==

The most serious parasitic crustaceans among fish in general are sea lice. However, L. branchialis is probably the most serious parasitic crustacean among cod. Infestation reduces the efficiency with which food can be utilised, delaying the development of the gonads. Up to 30% loss in weight can occur, with increases in mortality because of open lesions with loss of blood, and possibly occlusion of vessels or aorta. Gadoids, particularly cod, are emerging marine aquaculture species in some North Atlantic countries. L. branchialis present potential problems for their successful mariculture. == See also ==