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Tongue-eating parasites inspire new horror movie

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The symbiotic relationship between this parasitic isopod and its host is unique, but parasites are more common than you might think. (Image: Matthew R. Gilligan, Savannah State University)

The movie The Bay, hitting theaters on November 2, features mutant parasites that devour people's tongues and other organs and eventually take control of their minds. It sounds like a suitably terrifying premise for a science-fiction horror movie. But the inspiration appears to be science fact: a real-life parasite nicknamed the "tongue-eating louse."

These rare critters are a type of crustacean known to parasitize some species of fish. I spoke with Stefanie Kaiser, a postdoctoral fellow at the National Institute of Water and Atmospheric Research in Wellington, New Zealand about these strange parasites and learned that the truth is indeed stranger than fiction.

AAAS MemberCentral: Cymothoa exigua is a type of parasitic isopod also known as the tongue-eating louse. How did it get this name?
Dr. Stefanie Kaiser, Postdoctoral Fellow, National Institute of Water and Atmospheric Research:
Isopods (or pill bugs, woodlice, or slaters) are a group of highly diverse crustaceans that are known from a great variety of marine and terrestrial habitats. Many species inhabit groundwater and caves, and some have even been recorded in deserts. Most species, however, live in the sea, with more than 6,000 marine species being described globally. Cymothoa exigua belongs to the cymothoids, an isopod family, which often parasitizes bony (teleost) fish. Specimens of Cymothoa exigua enter the fish via the gills and become attached to the base of the tongue of the host fish with their legs. The isopod does not actually eat the fish's tongue, but sucks blood from the tissue, so that the tongue eventually withers and degenerates. The isopod remains attached to the tongue base and in that way becomes a living substitute.

AAAS MC: How does this relationship between the C. exigua and its fish host work? How do both organisms survive in this relationship?
Kaiser:
The isopod replaces the tongue and feeds on the host' blood or mucus, while the fish can stick to its usual diet. It has been argued that the isopod acts as a true "functional" replacement for the fish's tongue. However, there are some studies indicating negative effects of parasitic infestations on their hosts, for example on the growth, weight, or causing tissue damage. Nevertheless, the parasites can remain attached to the fish for several years, grow as the fish grows and then become detached. There are, in fact, many examples of fishes outliving their isopod parasites.

AAAS MC: Is this relationship, where a parasite replaces an organ, unique?
Kaiser:
This relationship is unique, in that only cymothoid isopods have been found to replace hosts' organs. However, there are several other isopod species that infest fish in the same way (e.g., Cymothoa borbonica, Ceratothoa imbricata, Glossogobius sp.).

AAAS MC: What do we know about the life cycle of the C. exigua?
Kaiser:
Specimens of Cymothoa exigua have a short free-living (pelagial) juvenile phase and a "stationary" adult parasitic phase. First, juveniles enter the fish's gill and become males. Cymothoids, like Cymothoa exigua, are protandric hermaphrodites; that is, adult males switch to female. The first male, which enters the gills, typically develops into a female, and the other males remain males. It is not quite clear what impedes this shift from one sex to another, but possibly pheromones released from the female prevent the other males from changing their sex. Only the females inhabit the fish's buccal cavity and become a tongue replacement. There are some doubts about the life span of tongue-eating isopods. Some consider that they produce only one brood. This, however, can contain more than 400 eggs, which they carry in a ventral brooding pouch (like all isopods). Given the hugely different size ranges across Cymothoa specimens, it is suggested that they potentially have several broods and can survive up to three years.

AAAS MC: Are these creatures harmful to humans in any way?
Kaiser:
Although they are certainly not pretty, parasitic isopods such as Cymothoa exigua are not physically harmful to humans. However, they can affect fishes' health, and as this species is also found on commercially used fishes such as snapper and salmon, it can cause economic impacts (e.g., for aquaculture industries).

AAAS MC: Is there anything else you think our readers would be interested in learning about C. exigua?
Kaiser:
Due to their adverse impacts on the host's fitness, parasites typically have a bad reputation. However, they play an important role in the functioning of ecosystems by, for example, controlling the abundance and stability of the host population. Remarkably, a huge proportion of the total number of global species recorded follow a parasitic life-style; more than 50% of described species (both on land and in the sea) are parasites. However, these numbers are probably underestimated still, as parasites are often highly modified and thus hard to identify. "Tongue-eating" or rather mouth-infesting isopods are probably among the most peculiar parasites, and among the largest isopods recorded, some of which reach 7 cm in length. To date, about 400 cymothoid species are described—about 50 in the genus Cymothoa alone (not all of them mouth-infesting though). Yet, as differences between the various species can be subtle, species numbers will certainly increase with more research being undertaken.

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The symbiotic relationship between this parasitic isopod and its host is unique, but parasites are more common than you might think. (Image: Matthew R. Gilligan, Savannah State University)
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Mary Bates, Ph.D.