A hostile takeover
In the warm Pacific waters off Costa Rica, a unique biological event occurs, orchestrated by the parasitic isopod Cymothoa exigua. This crustacean, often called the tongue-eating louse, is the only parasite to functionally replace a host's organ. The process begins when a juvenile male isopod enters a fish through its gills. These isopods are protandric hermaphrodites; they all start life as males and can transform into females later.
If a host fish, such as a Colorado snapper (Lutjanus colorado) or Jordan's snapper (Lutjanus jordani), has no female parasite, a male will grow to about 10 millimeters and change sex. This new, larger female then migrates from the gills into the fish's mouth. Using its seven pairs of sharp, hook-like legs, the female latches onto the base of the fish's tongue. It pierces the main artery, severs the blood vessels, and begins to feed. Deprived of blood, the tongue tissue experiences necrosis, withers, and eventually disintegrates. The parasite then securely attaches its body to the remaining tongue stub, where it will live out its life.
Life as a new tongue
Once attached, the parasite's body is a functional, living replacement for the original tongue. The host fish adapts, using the rigid body of the isopod to push food against the roof of its mouth and down its throat, allowing it to eat normally. The female parasite, which can grow up to 29 millimeters long and 14 millimeters wide, sustains itself by feeding on the fish's blood and mucus.
While the female occupies the mouth, smaller males often remain in the gill chamber. Mating occurs within the host, and the female holds the fertilized eggs in a kangaroo-like pouch called a marsupium until they hatch. The host fish typically survives this occupation, although some specimens with parasites have been found to be underweight. This relationship is a specialized form of parasitism where the invader integrates itself completely into the host's anatomy, becoming a permanent, functional part of the fish's body.