Tonguebiter Cymothoid Isopod

The tonguebiter (Cymothoa exigua) is a parasitic isopod that destroys a fish's tongue and then takes its place, living on in the mouth as a working replacement.

It lives in shallow coastal seas, usually inside the mouths of snappers and a few other fish, at depths of about 2 to 60m where hosts are plentiful. Its home is the eastern Pacific off California and Mexico, and it has also been recorded off Ecuador and in parts of the Atlantic.

Appearance

Built like other isopods, it has a flattened, segmented body and seven pairs of sharp legs that lock it in place on the host. All individuals start as males, so the females are the older and larger animals, with lengths up to 29mm against about 15mm for males.

Behaviour

Related cymothoids tend to stick to one part of a host, such as the gills, eyes, tail fin or even the flesh. This species heads for the mouth. Snappers are its usual target, and it has been recorded on eight host species across four fish families, among them a grunt, several drums and a grunion, with Colorado and Jordan's snappers added from Costa Rica.

A young isopod gets in through the gills and clings on with its front claws. Later, a female in position at the tongue uses those claws to cut the blood vessels, the tongue dies from lack of blood and falls away, and she anchors herself to the remaining stub. The fish can then use her much as it used its own tongue. The host does pay a price, though. Fish with two or more parasites are usually underweight, probably from trouble eating, and tissue damage and poor growth have also been reported.

Feeding

Blood is the main food from the start, taken at the gills by the newly arrived male. Once settled on the tongue stub, some feed on the fish's blood and mucus as well. The position gives the parasite two things at once: a meal on tap and a safe hiding place from its own predators.

Life cycle

Life begins with a short free-swimming stage, and every isopod enters its first host as a male. If a second male turns up on the same fish, the first can change into a female once it has grown to about 10mm. What triggers this is not clear. A female may give off pheromones that stop later males from changing too, since only females take over the mouth.

The male fertilises the eggs she lays, and these are held in a pouch beneath her body. The larvae fend for themselves as soon as they hatch and set off to find hosts of their own. Researchers are unsure about the life span. Some think there is a single brood, while the range of sizes seen hints at several and a life of up to three years, with the isopod growing along with its host. If the fish dies, the isopod lets go and leaves the mouth, though what happens next has not been studied in the wild and it seems unlikely to last long alone. Some fish outlive their parasites.

Dangers

It is not harmful to people, although it may bite if handled or pulled from its host. Infested snappers are shipped around the world, and a customer in Puerto Rico once sued after claiming to have been poisoned by one cooked in a fish. The case was dropped, as isopods are not poisonous to people.

Relationship with people

People mostly come across it by accident, through the fish trade. In 2005 a red snapper in the United Kingdom was found with what may have been this species inside it. That led to talk of its range spreading, but the isopod had probably just travelled from the Gulf of California in the mouth of the imported fish, and the find was a one-off.

It may also cost the trade money. It turns up on commercial fish such as snapper and salmon, and a fish carrying one can lose value because it looks so unsightly.