The Whale louse is a parasitic crustacean of the family Cyamidae, the only family in the infraorder Cyamida. It is related to the better-known skeleton shrimp, most species of which are found in shallower waters.
Whale lice are external parasites, found in skin lesions, genital folds, nostrils, and eyes of marine mammals of the order Cetacea. These include not only whales but dolphins and porpoises as well.
In a comparison of Isocyamus kogiae from California with those of Australia, an article describes the dimensions as: "our specimen measured: total length 4.7 mm [4.75]; width of body 1.4 mm [1.5]; length of primary gill 1.0 mm [1.0]; length of propodus of first gnathopod 0.7 mm [0.5] .
Whale lice attach themselves to the host body in places where they are protected from water currents, so they can be found in all of the natural body openings and in wounds; with baleen whales they are found primarily on the head and in the ventral pleats. With slow swimming baleen whales, up to 100,000 whale lice parasites can occur per whale, while with toothed whales or fast swimming baleen whales, the individual parasite number is significantly lower.
With some species of whale lice, the infestation of the host with barnacles appears to play a great role. Species like Cyamus rhachianecti settle directly where the barnacles attach to the whale and scoop out the surrounding area so much that the barnacles fall off. On the right whale, the parasites live mainly on the raised callus-like patches of skin on the whales' heads, called callosities. The clusters of white-colored lice contrast greatly with the dark skin of the whale, and help researchers identify individual whales because of the unique shapes that the white lice clusters form.
The food of the whale lice is predominantly the algae that settle on the body of the host. Additionally they cause minor skin damage to the whale, but this does not lead to significant illness.
It is apparent that the development of the whale louse is closely connected with the life pattern of whales. The distribution of various lice species reflects the migratory patterns of the whales.