* Japanese name: Ikarimushi
* Scientific name: Lernaea cyprinacea
* Description: The anchor worm is a small parasite (1-1.5 cm long) that lives in the muscles of freshwater fishes. The female anchor worm has a tubular body divided into a cephalothorax, thoracic region and abdomen. However, you’re unlikely to see all these areas, as the front part of the parasite bears hooks that anchor it in the muscle of the host. The anchors are cylindrical, branched outgrowths on the cephalothorax. The remainder of the animal hangs like a string outside the host’s body. This part forms a Y shape, made up of two egg sacs. Anchor worms are not true worms, they are free-living copepods, a type of crustacean.
* Where to find them: Attached to the body of host fish, most commonly at the base of the fins. The attachment process can cause acute hemorrhage and ulcers at the site of penetration (ulcers are the result of bacterial infection). While the host may sometimes die from blood loss and secondary infections, the anchor worm would rather the fish stay alive, as this allows it to produce several hundred eggs every two weeks.
* Food: Fish blood. For parasites, at least, there is such a thing as a free lunch.
* Special features: Only female anchor worms are parasitic. The males are free-living but live for only one thing: to have sex. They die after mating. The larvae hatch and live off yolk material in their bodies and grow through 3 molts without feeding, reaching the first parasitic larval stage in as little as 4 to 8 days.
Like many other crustaceans, anchor worms have a shell, which protects the parasite when the fish, irritated by its presence, tries to scrape it off by rubbing against rocks (or, in the case of pet fish, on the sides of the tank). Anchor worms are unusual parasites in that they have little host specificity, infecting a wide range of fishes, including trout, salmon, goldfish and koi, first bred in Japan 160 years ago. Even tadpoles of some frogs may be infected with anchor worms.
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