The carp louse Argulus coregoni is an obligate ectoparasitic branciuran species on fish.

[3] From the eggs, metanauplii emerge which actively search for a fish host to feed on.

[3] This means that infection rates of host typically reach their peak during the summer months.

Although some differences are seen in the development between males and females, the adults generally have the same size 9–12 mm (0.4–0.5 in).

This is in contrast with other dioecious crustacean species[6] The sex ratio — the ratio between males and females in the population — is generally close to 1:1,[3][6] as expected for sexually reproducing species under Fisher's principle.

[7][8] This is supported by evidence that larvae can successfully develop on both salmonids and cyprinids, but developmental rate and reproductive output on the latter host group is reduced.

The symptoms include scales peeling off, red spots at the site of infection, and reduced growth rates.

[16] Argulus coregoni infections do not lead to threats to natural fish populations.