Horrifying Images Reveal Bizarre Parasite That Replaces Tongue of Its Fish Host and Feeds on Blood

A biologist made an unexpected discovery while researching fish heads—a bizarre parasitic crustacean that replaces the tongue of its host.

Kory Evans from Rice University in Houston spotted the strange parasite in an image of the skull of a herring cale—a species of ray-finned fish endemic to the waters off Australia.

Evans was scanning the fish as part of an ongoing project to study the 3D skull shape of wrasses—a group of nearly 500 marine fish species—using a technique known as micro-computed tomography (micro-CT).

"Micro-CT scanning is basically a 3D X-ray that allows researchers to visualize the internal anatomy of organisms without the need for dissection," Evans told Newsweek. "So far we've scanned nearly 200 species.

"To compare shapes between specimens I placed digital landmarks at different points along the skull to allow for detailed comparison. I found the parasite while trying to place digital landmarks inside the mouth cavity of the fish."

The parasite is a type of isopod—an ancient order of crustaceans that includes woodlice and their relatives that live in the sea, fresh water and on land—belonging to a genus called Cymothoa.

Members of this genus are known for their bizarre habit of parasitizing the tongues of their fish host.

isopod parasite
An isopod parasite (purple) inside the skull of a herring cale fish. Kory Evans, Ph.D, Rice University

"This is my first time seeing this parasite 'in-person,' though I had heard about it before. These parasites can be found in a variety of fishes," Evans said.

According to the biologist, the parasite visible in the micro-CT images probably entered the body of the fish through the gills before lodging itself on the tongue.

From there, it would have severed the blood vessels on the tongue, feeding on the body fluid—although some members of the genus consume fish mucus—as the organ gradually wasted away.

Now, all that's left is the underlying bone where the parasite is now attached, having essentially replaced the tongue of its host.

isopod parasite
The parasite seen from another angle. Kory Evans, Ph.D, Rice University

Remarkably, these parasites appear to cause little additional harm to the fish after destroying its tongue.

"However, individuals with more than one parasite on their tongue are typically underweight, presumably due to difficulties related to feeding," Evans said.

One particular species of this genus, Cymothoa exigua, goes even further than its relatives by providing the fish with a new, fully functioning tongue. This is the only known case in the animal kingdom of a parasite functionally replacing a host organ.

Intriguingly, isopods of this genus also change sex, first maturing as males before eventually turning into females.