Invertebrates
Tongue-eating Louse
Cymothoa exigua · also called tongue-eating louse, tongue-biter
The tongue-eating louse does something no other known parasite does — it destroys one of its host’s organs and then physically takes its place. Entering a fish through the gills, it severs the blood supply to the tongue until the tongue falls away, then anchors itself where the tongue used to be.
Overview
Tongue-eating lice live in coastal and shelf waters of the eastern Pacific, parasitising snappers, grunts, and other fish. Females are about 2–3 cm, pale and woodlouse-like with seven pairs of hooked legs, feeding on the host’s blood and mucus.
Replacing an organ
A juvenile enters through the gills, clamps onto the tongue’s base, and drains its blood vessels until the tongue withers and drops off — then stays in its place, and the fish presses food against the louse to feed.
Rarely fatal
Most infested fish continue to feed and survive, though heavy infestation or gill damage can weaken or kill a host.
Status
The tongue-eating louse is Not Evaluated, widespread wherever its host fish occur, with no conservation concern and no danger to people.
Tongue-eating Louse: common questions
How does the tongue-eating louse actually replace a fish's tongue?
A juvenile louse enters a host fish's mouth by swimming in through the gill openings, then moves forward and attaches itself to the base of the fish's tongue using its hooked legs. It begins feeding on the blood flowing through the arteries that supply the tongue, and over time this steady blood loss causes the tongue tissue to waste away and eventually detach entirely. The louse then remains permanently anchored in the same spot where the tongue used to be, and because it is roughly the right size and position, the fish is able to press food against the louse's hard body in much the same way it previously used its own tongue, so the fish continues feeding relatively normally.
Does the tongue-eating louse kill the fish it infests?
In most cases, no. The relationship is parasitic rather than immediately lethal, and a fish that has lost its tongue to a single louse can generally continue to feed, grow, and reproduce, since the louse effectively takes over the tongue's mechanical function. However, the parasite does impose real costs, as heavily infested fish often show slower growth and poorer body condition from the ongoing blood loss and the energy diverted to the parasite, and a fish carrying multiple lice, or one whose gills were damaged during the parasite's entry, can be seriously weakened or killed.
Is the tongue-eating louse dangerous to humans?
It poses essentially no danger to people. It is a specialist parasite of certain marine fish and has no interest in or ability to parasitise humans, and it cannot survive outside its aquatic host environment. The main way people encounter one is by finding a live or dead louse inside the mouth of a fish they have caught or bought, which is startling but harmless, and there have been occasional minor reports of a louse from a very fresh fish pinching a finger if handled carelessly, but it cannot attach to or harm a person in any meaningful way.
Is the tongue-eating louse unique among parasites?
It is widely described as the only known parasite that functionally replaces an entire organ of its host rather than simply feeding on the host or living within it. Many parasites damage, consume, or manipulate host tissues, but the tongue-eating louse is distinctive in that the fish actually continues to use the parasite in place of the lost body part, pressing food against the louse's body to help manipulate and swallow it. This unusual outcome, where the parasite becomes a working substitute for the organ it destroyed, is what has made the species one of the most frequently cited examples of bizarre parasitic strategies in the animal kingdom.