branchiurans

Branchiurans are a small group of crustaceans, commonly known as fish lice, belonging to the subclass Branchiura. They are exclusively ectoparasites that live on the skin and in the gill chambers of fish and amphibians. Despite their common name, they are not true lice, which are insects. Branchiurans are characterized by their dorsoventrally flattened bodies, which are often oval or shield-shaped, and their prominent compound eyes. A distinctive feature is their large, sucking mouthparts, which are modified into a proboscis used to pierce the host's skin and feed on blood and mucus. They possess two pairs of antennae, the first pair of which is modified into a prehensile organ used to hold onto the host. Their legs are adapted for swimming and clinging.

The life cycle of branchiurans typically involves a free-swimming larval stage followed by attachment to a host. After feeding and growing, they may detach from the host to molt and reproduce. Females typically lay their eggs on the substrate, although some species may lay them on the host. Branchiurans are found worldwide in both freshwater and marine environments. While generally not fatal to healthy adult fish, heavy infestations can cause stress, anemia, secondary infections, and even death, particularly in young or weakened individuals. They are of economic concern in aquaculture due to potential damage to farmed fish. Common genera include Argulus and Dolops.