Fish Lice: Branchiura

Most fish lice are external parasites of freshwater fishes. They live on the outside of the bodies of their hosts and feed on blood and other body fluids. Fish lice are flat, egg-shaped crustaceans and have bodies divided into three regions: head, thorax, and abdomen. A well-developed, shieldlike carapace (CARE-eh-pes) covers the head. In some species the carapace covers the sides of the body and legs, and sometimes covers part of the abdomen. The carapace has special organs inside that help the fish louse to digest its food and control the quality of fluids inside the body. The compound eyes are distinct and are not set on stalks. Each compound eye has multiple lenses. Both pairs of antennae are very short and have claws. The claws are used to help them attach to their host. The tubelike mouthparts have jaws, or mandibles, at the tip. The mandibles are used to scrape loose tiny chunks of skin. These bits of tissues, along with body fluids, are sucked into the mouth. A second pair of mandibles, or maxillae (mack-SIH-lee), has spines and claws. There are no maxillipeds (mack-SIH-leh-pehds), or thoracic limbs, that work together with the mouthparts. Instead, all four pairs of branched, or biramous (BY-ray-mus), limbs on the thorax are used for swimming. In males, the third and fourth pairs of limbs are used to transfer sperm to the female during mating. The unsegmented abdomen does not have any appendages underneath. It ends in two, rounded projections, or lobes, separated by a notch.