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Isopod Facts, Taxonomy, and the Tongue-Eating Louse in Fish

Linda Crampton is a writer and teacher with an honors degree in biology. She loves to study nature and write about living things.

Cymothoa exigua or the tongue-eating louse outside the body of the sand steenbras that it parasitized

Cymothoa exigua or the tongue-eating louse outside the body of the sand steenbras that it parasitized

Unusual and Parasitic Animals

The tongue-eating louse is a parasite that enters a fish through its gills and then attaches to its tongue. The parasite eats the blood from the tongue, causing the organ to shrivel. It then lives inside the fish's mouth. It survives by eating blood or mucus from the body of its host, which often survives the invasion. The invader has the scientific name Cymothoa exigua.

The parasite is actually an isopod and not a louse, despite its name. Common names of organisms can be deceiving. Some people prefer to use the term tongue-eating isopod or tongue-eating parasite for the animal because it's more accurate. Isopods aren't insects, but lice are. In this article, I discuss the biological classification or taxonomy of isopods as well as major features of the animals. I also describe the life of the fascinating tongue-eating louse.

Biological classification of the red fox

Biological classification of the red fox

Biological Classification or Taxonomy

The brief overview of biological classification given below should help someone understand the classification of isopods and lice, which I discuss in the next section of the article.

Major Categories in Taxonomy

The illustration above shows major categories used in the biological classification of organisms, or taxonomy: domain, kingdom, phylum, class, order, family, genus, and species. Multiple versions of each category exist. Three domains exist, for example, and five or six kingdoms, based on a specific biologist's point of view. The illustration shows the version of each category used for the classification of the red fox.

How Taxonomy Works

Biological classification is based on structure and presumed evolutionary history. The more similar the category versions for two species, the more similar in structure and the more closely related they are.

The boxes in the illustration above get smaller as we move down the list because there are fewer organisms in each group. For example, earthworms are classified in the kingdom Animalia with the red fox, but after that they branch away from the fox's classification because they belong to the phylum Annelida instead of the phylum Chordata. Their different classification reflects their different anatomy and physiology.

Another Classification Example

A wolf is quite similar in structure to a red fox. It's classified in the same way as the fox, except with respect to the genus and species at the bottom of the list in the illustration above. A human is classified in the same way as the red fox up to and including the class Mammalia, reflecting some similarities in our internal anatomy to that of the fox. Since our body has a considerable number of differences from the body of the red fox, however, the rest of our classification is different.

Nerocila armata is a parasitic isopod from the family Cymothoidae. In this photo, it's living on a diamond lizardfish (Synodus synodus)

Nerocila armata is a parasitic isopod from the family Cymothoidae. In this photo, it's living on a diamond lizardfish (Synodus synodus)

Isopod and Lice Classification

Phylum Arthropoda

Isopods and lice belong to the same phylum, but they aren't closely related. Their different biological classification (shown below) reflects this fact. They are both classified in the phylum Arthropoda, along with other insects, spiders, scorpions, crabs, lobsters, shrimp, and additional animals. The rest of the classification for isopods and lice is different.

Classification

The classification of isopods and lice can be summarized as follows. As the classification shows, subcategories exist in some of the categories, such as the subphylum. Though the names and general ideas presented below are not in dispute, there are some disagreements about category labels. Some scientists use an older system in which Crustacea is a class and Malacostra an order, for example.

  • Isopods: Phylum Arthropoda, subphylum Crustacea, class Malacostra, order Isopoda
  • Lice: Phylum Arthropoda, subphylum Hexapoda, class Insecta, order Phthiraptera

Order Isopoda

Around 10,000 species are thought to exist in the order Isopoda. The parasitic isopods shown in this article belong to the family Cymothoidae within the order Isopoda. The members of the family have some interesting characteristics in addition to being parasitic.

Pentidotea stenops is an isopod that lives in seaweed. This one is well camouflaged. The animal lives on the west coast of North America.

Pentidotea stenops is an isopod that lives in seaweed. This one is well camouflaged. The animal lives on the west coast of North America.

External Features of Isopods

Isopods are a diverse group. The range in size from a few micrometers in length to as long as half a meter. They live in many types of habitat (both terrestrial and aquatic) and are found around the world.

Though isopods are quite variable in structure, they have some features in common. They have an elongated and segmented body that is fairly flat but slightly arched. It bears noticeable plates.

Head

The head has unstalked compound eyes, jaws, and a pair of antennae. One pair of antennae is usually vestigial (existing as a small remnant of its earlier form in evolution).

Thorax

The section behind the head is called the thorax or the pereon. It has seven segments. The animal usually has seven pairs of legs attached to the thorax.

Abdomen

The last section of the body is the abdomen or pleon, which consists of six segments. The abdomen has a tail-like structure at the end that is composed of one or more of the abdominal segments.

Abdominal Appendages

The abdomen bears pairs of leg-like appendages called pleopods, which function in respiration and swimming. Each pleopod is branched. In water, the appendages carry out gas exchange directly. Oxygen goes into the body through the pleopods and carbon dioxide leaves. Terrestrial isopods have structures called pseudotracheae in their pleopods to enable gas exchange to occur. The structures somewhat resemble our trachea, or windpipe.

The Family Cymothoidae

All of the members of the isopod family known as the Cymothoidae are parasites. Some species in the family live in the mouth of their host (the buccal cavity), some live on the surface of the fish, and some burrow into the host's flesh. The parasites infect both marine and freshwater fish.

Females are bigger than males, but both genders of some species are surprisingly large. The body of the parasites is covered with a thick cuticle for protection. It also has hooks at the end of the pleopods, which enables the animal to grasp hold of their host.

The effects of the parasites varies considerably. In some fish, a parasite in the family Cymothoidae appears to cause no problems. In others, serious effects result from the parasite's presence. There is still a lot to learn about the family, including information about their reproduction.

A tongue-eating louse inside the mouth of a sand steenbras

A tongue-eating louse inside the mouth of a sand steenbras

Behavior of the Tongue-Eating Isopod

A parasite that eats the tongue of its host sounds impressive. Even some of the scientists that have discovered a fish with one of the parasites living in its mouth have sound excited. The distribution of the parasite is unknown. Though not all of the aspects in its life are known, some points about its life cycle have been discovered.

Parasite Entry into the Host

The juvenile parasite enters the host through the operculum, or the gill covering. The operculum has an opening at the back. During respiration, water enters the mouth of a fish and travels over the gills, The gills extract oxygen from the water, which enters the blood vessels in the gills. Carbon dioxide is sent from the gills into the outgoing water and leaves the body via the opening at the back of the operculum.

A Possible Gender Change

Like the other members of the Cymothoidae family, the tongue-eating louse is a protandric hermaphrodite. The term means that the fish is male at first but has the ability to change sex. If it doesn't encounter a female for reproduction, it may become a female itself.

Once the parasite enters its host through the operculum, it travels to the mouth and matures into a male. If there is no other parasite present when it reaches the mouth, it becomes a female. The female attaches her body to the muscles at the base of the tongue and begins the destruction of the organ by eating its blood supply. Since it's attached to the tongue muscles, the parasite is sometimes rather bizarrely referred to as a "tongue substitute." The real tongue withers without its blood supply.

Meeting a Male

If a male later reaches the female, reproduction will occur. Only the female attaches to the muscles of the tongue. The male that fertilizes her attaches to one of the arches supporting the gills, at least for some of his time in the host. The exact location of the mating process is unknown.

It's unclear how a male is prevented from turning into a female when a female is already present in the mouth of the fish. One researcher says that the female "possibly" releases chemicals that prevent the change when they enter the male's body.

Reproduction and Larval Development

Fertilization in isopods is internal. The male inserts sperm into the female's reproductive tract by means of a modified pleopod. The sperm joins with the eggs in her body.

The female tongue-eating louse (and the females of the other species in the family Cymothoidae) carries her youngsters in a marsupium, or a brood pouch. The eggs hatch in the pouch and form the first pullus stage, which is neither male nor female. After it has matured, it molts into the second pullus stage, which looks more like an adult. Researchers say that this stage doesn't become sexually mature until it has left the brood pouch, at which point it is called a manca.

The manca leaves its original host and swims in search of a new one. Once it finds a host, it develops pre-adult features and is known as a juvenile. It functions as a male but will turn into a female if necessary. The male organs regress and the immature female ones develop as the gender changes.

Ceratothoa oestroides in a European sea bass

Ceratothoa oestroides in a European sea bass

Intriguing and Impressive Parasites

The parasites described and shown in this article have intriguing features. Understanding their behavior and effects is interesting biologically and in some cases is important with respect to fish health. In some parts of the world, species in the family Cymothoidae are serious problems in fish farms.

There's a lot to learn about tongue-eating isopods and their relatives. At least one researcher says that the parasite may leave its host after living in it for several years. If this is the case, it would be interesting to know why the departure happens. The parasite can be hard to find in the wild, but the study of its life cycle, behavior, and effects could be very worthwhile.

References

  • Introduction to isopods from NOAA Ocean Exploration and Research
  • Isopod information from the Encyclopedia Britannica
  • Global diversity of fish parasitic isopod crustaceans of the family Cymothoidae from ScienceDirect
  • Discovery of a tongue-eating louse by Rice University from USA today
  • The tongue-eating isopod: a video and transcript from NOVA and PBS
  • Tongue-eating parasites from the AAAS (American Association for the Advancement of Science)

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2020 Linda Crampton

Comments

Linda Crampton (author) from British Columbia, Canada on December 09, 2020:

Hi, Eric. The parasite is known to infect multiple species of fish and has quite a wide distribution. One report said that it infects eight species of fish, but the report is quite old. The animal is still being studied. It’s possible that it infects more species than researchers realize.

Eric Caunca from Laguna, Philippines on December 09, 2020:

Hi, Alicia.

Does the tongue-eating louse target a specific species of fish or any kind of fishe?

Linda Crampton (author) from British Columbia, Canada on December 08, 2020:

Thank you so much, Liza. I hope the rest of the week is enjoyable for you.

Liza from USA on December 08, 2020:

Absolutely astonishing! I admit I do not know about the tongue-eating louse in fish. It is uncanny but, now I know about I am thrilled to learn so much from the article, Linda.

Linda Crampton (author) from British Columbia, Canada on December 08, 2020:

Hi, Flourish. It does sound like a horrible process for the fish. I have heard of parasites that damage the tongue of other species besides fish, though not to the extent of destroying it.

FlourishAnyway from USA on December 08, 2020:

It's mind-boggling what adaptations living beings have developed in order to survive. I do, however, feel terrible for the fish as basically it's having part of its body eaten alive. It does make you wonder that if it could happen to a fish, why not other animals?

Linda Crampton (author) from British Columbia, Canada on December 08, 2020:

Hi, Mary. Yes, there is so much to learn about living things. I appreciate your comment.

Mary Norton from Ontario, Canada on December 08, 2020:

There are so many things we don't know about all the living organisms. I haven't heard of this one before. It's good you write about them here. I get the education on some of these unique organisms.

Linda Crampton (author) from British Columbia, Canada on December 08, 2020:

Hi, Devika. Thanks for the visit and the comment. Nature can be very interesting to explore. I wish I could see some of the plants and animals where you live.

Devika Primić from Dubrovnik, Croatia on December 08, 2020:

AliciaC An informative write here about nature as I am a nature lover I have noticed many different behaviors from my beautiful surroundings. The tongue eating louse in fish is weird something I had no idea of. Research is through and so much to learn from your hubs about nature.

Linda Crampton (author) from British Columbia, Canada on December 08, 2020:

Hi, Chitrangada. I appreciate your comment. Parasites can be annoying and even dangerous, but they are an interesting part of nature.

Chitrangada Sharan from New Delhi, India on December 08, 2020:

An interesting article with information about Isopods. Nature is full of mysteries and surprises.

It’s the characteristic of the parasites to survive on it’s host, in this case the poor fish. Sounds scary, but these are facts.

Thank you for sharing another wonderful and educational article.

Linda Crampton (author) from British Columbia, Canada on December 08, 2020:

Thank you, Eman. I appreciate your visit and comment.

Eman Abdallah Kamel from Egypt on December 08, 2020:

A very useful and detailed article about Isopods. I enjoyed reading it. Thank you, Linda, for sharing this informative article.

Linda Crampton (author) from British Columbia, Canada on December 08, 2020:

Yes, the louse does have a scary aspect! I hope you have a great day, too, Manatita.

Linda Crampton (author) from British Columbia, Canada on December 08, 2020:

I appreciate your comment, as always, Bill. I think the louse is an interesting animal.

Linda Crampton (author) from British Columbia, Canada on December 08, 2020:

Thanks, Ankita. I think the behavior of the parasite is interesting to explore.

manatita44 from london on December 08, 2020:

I read new things from you all the time, Linda. This world is full of strange entities.

"A parasite that eats the tongue of its host sounds impressive." Are you sure you don't mean scary? Lol. have a great day!!

Bill Holland from Olympia, WA on December 08, 2020:

Great information! Never trust a louse if it wants to be your friend. :) Seriously, I always learn from you, and for that I say thank you!

Ankita B on December 08, 2020:

It was really interesting to know about the tongue-eating isopods and the problem it causes in fish. Thank you for sharing this informative and well-explained article.

Linda Crampton (author) from British Columbia, Canada on December 07, 2020:

Thank you for the comment and for sharing your experience, Miebakagh.

Miebakagh Fiberesima from Port Harcourt, Rivers State, NIGERIA. on December 07, 2020:

Linda, I learnt much of this as a fisherman. Thanks for sharing.

Linda Crampton (author) from British Columbia, Canada on December 07, 2020:

Thank you, Heidi. The parasite is certainly an intriguing animal!

Heidi Thorne from Chicago Area on December 07, 2020:

Nowhere in my imagination would I ever think I'd be commenting on a post on this topic. :) What an amazing world. Thanks for sharing the most intriguing info from nature!

Linda Crampton (author) from British Columbia, Canada on December 07, 2020:

Thank you very much for the comment, Pamela. I hope you have a wonderful week as well.

Pamela Oglesby from Sunny Florida on December 07, 2020:

Linda, tongue-eating isopods sound kind of disgusting, yet nature is fascinating. This is a very well-written, interesting article that is also well-researched. I thought the videos were very interesting too.

Have a wonderful week.

Linda Crampton (author) from British Columbia, Canada on December 07, 2020:

Yes, I feel sorry for the fish as well. Losing a tongue to the parasite must be a painful experience. Thank you for commenting, Peggy.

Peggy Woods from Houston, Texas on December 07, 2020:

I cannot help but feel sorry for the fish that has this tongue-eating parasite in its mouth. Understandably, fish farms would not wish to have this parasite spreading throughout the farm.

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