In warm Pacific waters from the Gulf of California to the Gulf of Guayaquil thrives a parasite that has a very twisted relationship with its host.

The Tongue-Eating Louse

A member of the family Cymothoidae, the tiny Cymothoa exigua, or tongue eating louse, enters its victim through the gills. Once inside, the female latches onto the base of the fish’s tongue, while the male attaches behind her or on the gills. Through her front claws, she sucks the fish’s blood, which causes the tongue to die and drop off. She will then re-attach, this time to the tongue stub, effectively becoming its replacement. With an eww factor of 10, the fish can and will use the parasite as if it were its natural tongue.

Each infested fish almost always has more than one mating pair in its mouth. In fact, because these lice are such loving couples (called isopods, where the male is attached to the female’s back), it is more rare to see three lice in one fish mouth than four or six.

Amazingly, other than losing a tongue, infected fish seem to suffer little from the louse infestation. As one researcher noted, although “C. exigua appears to be a benign parasite whose influence could modify the behavior of heavily parasitized fishes . . . the effect can be considered insignificant at the present time.”

Tongue-Eating Louse Reproduction

Most louse females found attached to a fish tongue are seriously in the family way. It is estimated that each female can have between 480 and 720 eggs, of which an average of over 200 typically reach maturity.

If there are two males in a fish without females to mate with, one of the C. exigua will change sexes from male to female, not unlike happens with clownfish if the female of a group dies.  Of course, in clownfish it gets a little more interesting, as clownfish are all born male.

One bit of good news for the species of fish that will have their tongues eaten off: research indicates that each female is only capable of producing a single brood.

The Tongue-Eating Louse Grosses Out, But Otherwise is Harmless to, Humans

If there is one bright spot in this story, it is that C. exigua does not eat human tongues, is not poisonous, and can only hurt you a little bit – and then only if you pick one up (like you would), and it bites you.

Despite this fact, a person in Puerto Rico who allegedly accidentally ate a bit of C. exigua filed a lawsuit against the supermarket chain where she purchased its host fish. It appears as if the suit was dropped when testimony was presented that “isopods are routinely consumed as food.” Yuck!

Tongue-Eating Louse on the Move

Until recently, it was believed that C. exigua had a range limited to the Eastern Pacific from California to Peru and preyed on only eight species of fish including three types of snapper, three of drum, 1 grunion and 1 grunt. However recent reports have led some to ask if it has expanded its range.

In 2005, some poor Brit found one in the mouth of a red snapper purchased from a fishmonger in London. When asked at the time, an expert who was consulted opined: “I suspect the tongue louse was either imported here in the mouth of the red snapper or perhaps it has started to breed in European seas.”

More recent developments are making the latter seem more likely. In 2009, in the waters off the Channel Islands near the coast of Normandy, France, fishermen discovered a tongue-eating louse in the mouth of a Weaver fish, and in April 2013, a Belfast man found one in his sea bass. Neither species was previously known to be susceptible to the parasite.

To date, C. exigua is the only known parasite that is capable of replacing an organ or other structure that it has removed from its host.

If you liked this article, you might also enjoy our new popular podcast, The BrainFood Show (iTunes, Spotify, Google Play Music, Feed), as well as:

• Sushi is Not Raw Fish
• How Fish Gills Work
• Goldfish Do Not Have a Three Second Memory
• The Species of Jellyfish That Can Age Backwards
• The Parasitic Wasp that Can Control the Mind of Its Host

Bonus Parasitic Facts:

Unlike C. exiguI, a wide variety of horrific parasites love to prey on humans:

• Although on the decline and slated for eradication by the World Health Organization (WHO), even today, the guinea worm continues to cause pain and discomfort to people around the world. Entering a person in the belly of a water flea, the worm grows inside the human body to a length of two to three feet. When it’s had enough, the guinea worm creates a blister (usually on the hands or feet) that encourages the host human to place the affected area in water. The worm leaves its host through the blister (all two to three feet of it) and releases larvae into the water to continue its life cycle. Without proper medical attention, the affected host can develop a debilitating secondary bacterial infection.
• Equally gross, elephantitis is caused when filial parasites, which enter the human body through a mosquito bite, infect the vessels of the human lymphatic system. Not everyone infected shows symptoms, but those that do develop lymphedema, a swelling caused by a backup of fluid. Most typically seen in the legs, it can also appear in the arms, breasts and even genitalia. When the latter occurs in men, it is known as hydrocele. According to WHO, over 40 million infected people are currently incapacitated or disfigured with lymphatic filariasis.
• Another parasitic infection suffered by humans, this one particularly painful, comes from Vandellia cirrhosa. A tiny (only an inch or so long) species of Amazonian catfish, V. cirrhosa typically preys on fish when it enters through and attaches to the gills where it feeds on the host’s blood. On occasion, a fish mistakes an active human urethra (read: peeing) for water being pushed from gills and enters the swimmer . . . down there! Unfortunately, the fish is designed with sharp fins that prevent it from leaving the skinny urethra; rather, V. cirrhosa dies in place and rots, causing swelling and obstruction in the human urethra. The only treatment for the condition is surgery. Prevention being 16 times more valuable than any cure, many westerners sent to the Amazon have taken to wearing protective shields prior to getting in the water (and, of course, getting out to whiz).