This parasite replaces a fish's tongue with its own body, and the fish can keep using it

Photo of Cymothoa exigua in the mouth of a fish. Original photographer unknown.

Just imagine opening a fish’s mouth and finding that its tongue has been replaced by a parasite.

This is an interesting example of marine creatures: Cymothoa exigua (the tongue-eating parasite).

So what does the parasite do?

It attaches itself to the tongue of the fish, then it starts consuming its blood, which eventually causes the tongue to either decay or die. Then this parasite keeps itself attached to the surface of the tongue base, where it uses the fish's mouth as its home.

Then the fun begins.

It takes over the function of the tongue and enables the fish to continue feeding even in the absence of its tongue. One of the most amazing facts about this parasite is that it can also consume the fluids of its host.

Interestingly enough, the tongue-eating louse does not become part of the fish. Instead, it retains its independent existence and continues to perform some of the functions of the tongue.

Moreover, not all fish infected by tongue-eating isopods are affected by the parasites in the same manner.

Normally, parasites feed on the nutrients found in the body of the host. However, the tongue-eating louse is an exception, as it can replace a lost body part and enable the fish to continue feeding. This entire event shows how well these parasites can adapt in order to live inside their hosts.

References:

Brusca, R. C. & Gilligan, M. R. (1983). “Tongue Replacement in a Marine Fish (Lutjanus guttatus) by a Parasitic Isopod (Crustacea: Isopoda).”
https://doi.org/10.2307/1444352

“Hooked on you: shape of attachment structures in cymothoid isopods reflects parasitic strategy.” A peer-reviewed study examining the attachment adaptations of fish-parasitizing isopods.
https://pmc.ncbi.nlm.nih.gov/articles/PMC6839256/