Left: dorsal view of branchiuran Argulus japonicus
Right: ventral view of branchiuran Argulus japonicus, courtesy of David Allard (TAMU)
The Branchiura are a parasitic crustacean group of uncertain relationship within the Maxillopoda. They are thought to be an ancient group but there are no known fossils. There are approximately 130 species described currently, all of which are ectoparasites (attach to the outside of the host) on fish. A few exceptions have been found attached to amphibians.
The branchiurans' very flat, oval body is covered almost completely by their broad carapace with unusual, prominent compound eyes. Branchiuran mouthparts and antennae are modified for their parasitic lifestyle and form a proboscis with hooks, spines, and suckers. They are capable of leaving their host for up to three weeks. Branchiurans leave their hosts for several reasons: to find a different host, to mate, and to lay eggs. When they locate a new host they grab a hold, scurry toward the head, and attach behind the operculum (the flap that covers a fish's gills) or a fin. Attaching in these slightly protected areas reduces the likelihood they will be brushed or washed off of the host. Once attached to the host they either pierce the hosts skin and suck blood and other internal fluids, or they feed on mucus and skin sloughed off by the host. Sever infestations can devastate and aquarium. They can move very fast which is rather disconcerting when watching a swarm in your aquarium.