Category Archives: Fish

Genetically-Modified Salmon are Safe to Eat: Here’s How We Know

by Joseph DeSisto

One of the biggest news stories of today was the approval by the Food and Drug Administration of a genetically modified salmon for human consumption. This particular fish goes by the trademarked name “AquAdvantage,” and was developed by the company AquaBounty Technologies.

The approval is a big deal because, although scientists have been genetically modifying animals for many years, the AquAdvantage salmon is the first such animal ever to be approved for sale as food in the United States. Not surprisingly, this has inspired quite the outcry from anti-GMO advocacy groups.

One of their concerns is what will happen should these fish escape into the wild. The FDA had that same concern, which is why all AquAdvantage fish are sterile females, incapable of breeding with each other or with wild Atlantic salmon.

Of more immediate concern, however, is whether AquAdvantage salmon are truly safe to eat. Although the FDA claims this is the case, I am not an especially trusting person. I studied the data behind their claim to see whether I would reach the same conclusion.

Is AquAdvantage salmon safe to eat?

The FDA consumer fact sheet claims:

“After an exhaustive and rigorous scientific review, FDA has arrived at the decision that AquAdvantage salmon is as safe to eat as any non-genetically engineered (GE) Atlantic salmon, and also as nutritious.”

Because most people don’t like graphs, tables, and any phrase starting with the word “statistical,” the fact sheet does not include the actual data. The data is, however, available under the Freedom of Information Act and you are free to study it by reading the FOI summary. It is long and tedious, so I will summarize.

The AquAdvantage salmon is the result of adding two genes into the DNA of a normal Atlantic salmon egg. One of these genes produces growth factors, hormones that cause the fish to grow twice as fast as a typical Atlantic salmon. The growth gene comes from a closely related species, the Chinook salmon, which is similar to Atlantic salmon but grows to be much larger — up to 130 pounds.


Chinook salmon can grow to 58 inches long and weigh up to 130 pounds. Photo by D. Ross Robertson, licensed under CC BY-NC-SA 3.0.

Another gene is needed to make sure the growth gene is kept turned on in AquAdvantage salmon — otherwise the salmon might fail to produce Chinook growth factors. That second gene comes from another fish, an eelpout, and it simultaneously promotes the growth gene while also producing anti-freeze proteins, which make the salmon more cold-tolerant.

When the FDA says that a food product is safe, they “mean that there is ‘a reasonable certainty in the minds of competent scientists that the substance is not harmful under the intended conditions of use.*’” In this case there are several substances: the DNA, plus the molecules and hormones the DNA is supposed to make. Fish DNA by itself is not dangerous to humans – you cannot absorb it into your own genome, and it isn’t toxic. DNA is just DNA. It’s in all living things, modified or otherwise.


An eelpout. Photo copyright: President and Fellows of Harvard College, licensed under CC BY-NC-SA 3.0.

So the new question is, are the gene products unsafe for humans to eat? The eelpout’s anti-freeze proteins are already used in many other foods, including ice cream, so we will focus on the growth factors. The factor in this case is simply growth hormone (GH).

Atlantic salmon produce their own GH, just like all fish. And you eat them, anytime you eat salmon. If you’ve eaten Chinook salmon, you’ve also eaten Chinook GH. Although Chinook salmon are endangered in the wild, they are farmed just like Atlantic salmon and sold in U.S. markets every day – approved by the FDA and safe to eat.

How do we know?

Still scientists, funded by AquaBounty, chemically analyzed the meat and skin of AquAdvantage salmon and compared their results with ordinary, farm-raised salmon. Because farm-raised salmon are already fed supplemental GH (without being genetically modified), they compared these two with farm-raised salmon that were purposefully not given any additional hormones.

The result: statistically, modified salmon do not have higher levels of GH than ordinary farm-raised fish. As an aside, both had higher levels than fish which were not fed additional GH.


Juvenile Atlantic salmon (Salmo salar). Around 2/3 of salmon consumed in the U.S. comes from this species. Photo by Perhols, licensed under CC BY-SA 3.0.

GH itself is safe, but it can trigger the production of another molecule, insulin-like growth factor (IGF1), which can be toxic at high levels. To determine if there were high enough levels of IGF1 in AquAdvantage fish to warrant concern, FDA (not AquaBounty) scientists conducted their own study, a margin of exposure (MOE) assessment. Basically they tested salmon (modified and farm-raised, Atlantic and Chinook) to determine their maximum IGF1 levels. Then they calculated how much of this stuff you would have to eat before you might suffer ill effects.

The biggest fish-lovers in the U.S. eat around 300 grams of fish every day. To be safe, the FDA assumed that all 300 grams consisted of salmon, 2/3 of which (200 grams) was expected to be Atlantic salmon. They also assumed that IGF1 was always present at its maximum known level in AquAdvantage fish.

Say you are one of these obsessive salmon-lovers, but you are wary of GMOs, so you eat 200 grams of unmodified Atlantic salmon per day. Given the chemical analysis of salmon meat and skin, you would consume roughly 2.4 micrograms (0.0000024 grams) of IGF1. If, on the other hand, you only ate AquAdvantage fish, you would find yourself taking in 3.7 micrograms (0.0000037 grams) of IGF1 every day. For comparison, IGF1 levels of 1120 grams or higher are considered unsafe.

In other words, you would be eating too much IGF1 if you ate 66 kilograms (146 pounds) of genetically modified Atlantic salmon in a single day – or 102 kilograms (225 pounds) of non-modified, farm-raised salmon. No one likes fish that much.

Why are you writing about this?

This article started as a letter to a friend who shared an ad (via Facebook) on the FDA approval. This particular ad was misleading. My friend cares very deeply about environmental ethics, food safety, and the truth. I know he did not intend to misrepresent facts, so I wanted to try and clarify the issue from a scientific perspective. As I waded through FDA reports, legal documents, and old petitions, my message to a friend grew and evolved into the article you have just read.


A misleading advertisement by GMO Free USA.

The ad above comes from GMO Free USA, an advocacy group that seeks to “harness independent science and agroecological concepts to advocate for sustainable food and ecological systems.” They also envision a world “fully protected from GMO contamination.”

Advocating caution in developing new technology is, by and large, the right thing to do. Twisting the facts to inspire fear, however, is not, and this case caution was duly exercised.

The AquAdvantage fish was developed more than two decades ago, and AquaBounty nearly went out of business while waiting for FDA approval. When the FDA finally did approve, they didn’t “think you’re too busy to notice.” Everything you just read came from the Freedom of Information report, publicly available online here. The FDA’s press release on this subject was covered by major U.S. news organizations (with varying levels of objectivity) – The New York Times, CNN, ABC News, and many others.

There is no mandatory labeling of GMOs, but non-genetically modified fish will very likely be labeled (at the discretion of the companies selling them) — and if you only want to eat those salmon, that’s a choice you are free to make. You can also choose not to eat farm-raised salmon, or not to eat Atlantic salmon, thus avoiding any contact with GMOs since they would all have to be farm-raised Atlantic salmon.

Caution and skepticism are good things, and you are free to avoid genetically modified foods for ethical, personal, philosophical or religious reasons. Yet having studied the data, I can at least tell you there are no scientific reasons to panic over the FDA’s approval of AquAdvantage salmon.

*Here the FOI report is quoting the definition of food safety by Guidance for Industry 187: Regulation of Genetically Engineered Animals with Heritable Recombinant DNA Constructs. A Guidance for Industry is sort of like a public fact sheet, but for businesses – it explains the law in a (slightly) more readable format.


The Bucktoothed Slopefish

by Joseph DeSisto

We all love tales of rare sharks and squid, hauled up from the depths in nets and traps. Just a few days ago, an extremely rare deep-sea shark (the false catshark) was found off the coast of Scotland. Yet it is important to remember that the ocean is a big place and, in a paradoxical sort of way, it’s quite common for fish to be rare.

Enter the slopefishes, a handsome if under-appreciated family of marine fishes. All of the family’s 12 species live in rocky reefs at moderate depths. Most are rare, some extremely so. One species is known only from two specimens which were removed from the stomach of a coelacanth (a much larger fish) near the Comoros Islands. The slopefish were partially digested, so even though they represent new species, scientists have been unable to formally describe and name them (Anderson and Springer 2005). After 36 years, those two fish remain the only known representatives of their kind.

The bucktoothed slopefish. Photo by M.V. Chesalin, licensed under CC BY 3.0.

The bucktoothed slopefish. Photo by M.V. Chesalin, licensed under CC BY 3.0.

The bucktoothed slopefish’s tale might have ended similarly. Scientists named the species in 1974 based on only one specimen, found near the Gulf of Aden between Yemen and Somalia. Later efforts to capture more were fruitless. Finally, earlier this year, a bucktoothed slopefish made its way into a deep-sea fish trap off the coast of Oman (Anderson et al. 2015).

Now that a new specimen is available, we can appreciate the species for what it is: a thing of beauty, scarlet red and stream-lined, with rigid spines along the back. This discovery serves to remind us that the best finds in nature come not merely from knowledge, or even luck, but from days, weeks, or even decades of patience, persistence, and hard work.


Anderson Jr. W.D. and V.G. Springer. 2005. Review of the perciform fish genus Symphysanodon Bleeker (Symphysanodontidae), with descriptions of three new species, S. mona, S. parini, and S. rhax. Zootaxa 996: 1-44.

Anderson Jr. W.D., M.V. Chesalin, L.A. Jawad, and S.R. Al Shajibi. 2015. Redescription of the percoid fish Symphysanodon andersoni Kotthaus (Symphysanodontidae) from the northwestern Indian Ocean, based on the holotype and the second known specimen. Zootaxa 4021(3): 475-481.

Life in Blackwater

by Joseph DeSisto

Just a few years before Darwin published his work on evolution by natural selection, his contemporary, Alfred Russel Wallace, finished a four-year-long tour of the Amazon Basin. During these travels he explored the Amazon River and its tributaries, met with indigenous tribes, and collected a shipload of biological specimens, which he planned to return to England to sell. Sadly the ship and all its contents, save a few notes and sketches, were lost in a fire at sea. From those notes was forged a book documenting Wallace’s travels and his observations on natural history in the Amazon (Wallace 1853).

When Wallace began to explore the Rio Negro or “Black River,” the Amazon’s largest tributary, he noticed that the water seemed darkly stained, like tea or coffee. Similar, smaller rivers could be found across the Amazon — such rivers were usually deep, slow-moving, and wound through forests or swamps. “Blackwater” (aside from being an episode of Game of Thrones) is the name Wallace (1853) used to describe these stained waterways. Where the blackwater of the Rio Negro meets the silt-laden, “whitewater” of the Amazon, the transition is sharp and visible from space.

The junction of the whitewater Amazon (left) and the blackwater Rio Negro (right) near Manaus, Brazil. Photo by Lecomte, licensed under CC BY-SA 3.0.

The junction of the whitewater Amazon (left) and the blackwater Rio Negro (right) near Manaus, Brazil. Photo by Lecomte, licensed under CC BY-SA 3.0.

Not only do blackwater rivers look like tea, they effectively are tea — the color comes from tannins, organic molecules that seep into the water as certain types of tannin-bearing plants die and decompose (Janzen 1974). Whether a river has blackwater or not depends entirely on the plant life growing at its banks. In life, certain plants use tannins as a protection against insects. In death, the tannins play a new role, altering the aquatic environment and the life therein.

Blackwater rivers have a very different chemistry than other water bodies. They are more acidic but lower in oxygen, nutrients, and the dissolved elements many animals need (Ribeiro and Darwich 1993). There are, therefore, fewer animals in blackwater than in clearwater or whitewater. Snails and some other invertebrates, for example, need calcium to build their shells, and these do not fare well in low-calcium blackwater rivers. With fewer invertebrates to eat, fish and other predators are relatively scarce. Yet there is life in blackwater, and although it is a bit harder to find, it is unique and, in its own way, amazing.

A bdelloid rotifer. Photo by Donald Hobern, licensed under CC BY 2.0.

A bdelloid rotifer, found in a wet clump of moss. Photo by Donald Hobern, licensed under CC BY 2.0.

The deformed-zucchini-shaped thing above is in fact an animal, smaller than a grain of sand, called a rotifer. Rotifers can be found almost anywhere with moisture, though you’d need a microscope to spot them. They feed on tiny particles of all kinds, from bits of detritus and algae to bacteria and other single-celled organisms. Despite being tiny, rotifers are relatively complex creatures with minute brains, feelers, and a large mouth surrounded by hair-like appendages called cilia. Some species even have simple eyes.

When a rotifer wishes to swim, it simply vibrates the cilia to pull its body forward. The cilia are also important in feeding — if the rotifer is anchored by its “tail” end, the vibrating cilia create a water current that draw particles towards the mouth. Rotifers eat pretty much the same way street-sweepers sweep. Below is a video of what this looks like:

[Video credit is to “NotFromUtrecht,” licensed under CC BY-SA 3.0.]

In the Amazon Basin, blackwater is dominated by rotifers which, unlike many planktonic invertebrates, do not need calcium or other dissolved minerals to construct cells. At the junction of the Rio Negro and the Amazon River, rotifer populations can be up to ten times higher in the blackwater than in whitewater (Ribeiro and Darwich 1993), even though the two extremes are separated by only a few feet of transition. The same pattern exists in Argentina, where a different “Rio Negro” (also blackwater) meets the whitewater Rio Salado (Frutas 1998).

As long as there are rotifers and other blackwater-tolerant plankton around, fish can also live in blackwater, but low nutrient and oxygen levels make it difficult for them to do so. Still, some very special fish have evolved to tolerate blackwater, and perhaps the most recognizable of these is the neon tetra, a fish made famous by its popularity in home aquariums.

The neon tetra (Paracheirodon innesi), a popular aquarium fish. Photo by Holger Krisp, licensed under CC BY 3.0.

The neon tetra (Paracheirodon innesi), a popular aquarium fish. Photo by Holger Krisp, licensed under CC BY 3.0.

In Rio Negro (Brazil, not Argentina), fish are not especially abundant, but many of the species that live there are endemic. Of the 700 or so fish known from the river, around 100 are found nowhere else on earth. Among these fish is the cardinal tetra, a close relative of the neon tetra with similarly vivid red and blue streaks. Another is the cururu, a freshwater stingray.

Freshwater stingrays are common in the Amazon Basin, where they are considered to be more dangerous even than piranhas. The greatest abundance and diversity of stingrays is found in the whitewater, but surveys have revealed there are several species that prefer blackwater, and at least two in the genus Pomatotrygon are found exclusively in the blackwater of the Rio Negro (Duncan and Fernandes 2010). One of these is the cururu ray, a unique species that has only been discovered in the last decade.

One of the cururu ray's closest relatives, the porcupine river stingray (Potamotrygon histrix). Photo by Jim Capaldi, licensed under CC BY 2.0.

One of the cururu ray’s closest relatives, the porcupine river stingray (Potamotrygon histrix). Photo by Jim Capaldi, licensed under CC BY 2.0.

Studying the cururu ray has helped us understand what is required for a fish to thrive in blackwater. First, the extremely low levels of sodium, chlorine, and other salts in blackwater presents a problem, since fish and all other animals require salts to keep their bodies running. The cururu, like many fish in Rio Negro, can survive with far less sodium and chlorine than most other fish, but it is also more efficient at extracting salts from the water, however scarce they may be (Wood et al. 2002).These rays also have gills with finger-like projections, adapted to be as efficient as possible in gathering both salts and oxygen from blackwater (Duncan et al. 2010).

Although scientists have known for some time that the cururu ray represents an undescribed species, it has yet to be given a Latin name. Many more new species may yet be discovered in the tannin-soaked waters of Rio Negro and other blackwater rivers. Unique places yield unique creatures, often with amazing stories.


Duncan W.P. and M.N. Fernandes. 2010. Physicochemical characterization of the white, black, and clearwater rivers of the Amazon Basin and its implications on the distribution of freshwater stingrays (Chondrichthyes, Potamotrygonidae). Pan-American Journal of Aquatic Sciences 5(3): 454-464.

Duncan W. P., O.T.F. Costa, M.M. Sakuragui, and M.N. Fernandes. 2010. Functional morphology of the gill in Amazonian freshwater stingrays (Chondrichthyes: Potamotrygonidae): implications for adaptation to freshwater. Physiological and Biochemical Zoology 83: 19-32.

Frutos S.M. 1998. Densidad y diversidad del zooplancton en los Rios Salado y Negro — planicie del Rio Parana — Argentina. Revista Brasileira de Biologia 58(3): 431-444.

Janzen D.H. 1974. Tropical blackwater riversm animals, and mast fruiting by the Dipterocarpaceae. Biotropica 6(2): 69-103.

Ribeiro J.S.B. and A.J. Darwich. 1993. Phytoplanktonic primary productivity of a fluvial island lake in the Central Amazon (Lago do Rei, Ilha do Careiro). Amazoniana 12(3-4): 365-383.

Wallace A.R. 1853. Narrative of travels on the Amazon and Rio Negro. Reeve, London.

Wood C.M., A.Y.O. Matsuo, R.J. Gonzalez, R.W. Wilson, M.L. Patrick, and A.L. Val. 2002. Mechanisms of ion transport in Potamotrygon, a stenohaline freshwater elasmobranch native to the ion‐poor blackwater of the Rio Negro. Journal of Experimental Biology 205: 3039–3054.

How Poisons Work: Tetrodotoxin

by Joseph DeSisto

This is the first of a series of short articles, each featuring a different type of poison or venom used by animals.

Poisons and venoms are some of the most complex substances in nature, often containing hundreds of different chemicals, each with a particular purpose. As technology advances, scientists have begun to look at some of these chemicals, usually proteins, and try to figure out what they do.

The deadly southern blue-ringed octopus (Hapalochlaena maculosa). Photo by Bernard Dupont, licensed under CC BY-SA 2.0.

The deadly southern blue-ringed octopus (Hapalochlaena maculosa). Photo by Bernard Dupont, licensed under CC BY-SA 2.0.

In some cases, the results give us a new perspective on an animal’s biology: rattlesnake venom, for example, contains a unique protein that allows the snake to track its prey after the initial strike. In other cases, we discover potentially useful surprises: giant centipede venom contains a single protein that inhibits pain in mice, using the same chemistry as morphine, but with greater efficiency. However, regardless of whether the results are useful to us, studying nature’s chemical weapons gives us a whole new appreciation and understanding of the creatures that wield them.

Tetrodotoxin is the poison of choice for a variety of animals, especially in the ocean. These include blue-ringed octopuses, cone snails, moon snails, certain angelfish, some ribbon worms, a handful of amphibians, and the puffer/triggerfish order Tetraodontiformes, for whom the toxin is named. Many of the animals that use tetrodotoxin are brightly colored, a warning to passers-by. Would-be predators, save the immune and the unlucky, heed this warning well.

An inflated pufferfish (Diodon holocanthus). Photo from Williams et al. (2010), licensed under CC BY 2.5.

An inflated pufferfish (Diodon holocanthus). Photo from Williams et al. (2010), licensed under CC BY 2.5.

Tetrodotoxin, sometimes abbreviated TTX, is a complex molecule that prevents its victims’ nerves from functioning properly. This eventually leads to paralysis, and death comes when the muscles that control breathing no longer receive signals from the nervous system. Basically, you suffocate. Humans can get TTX poisoning when they eat improperly-prepared pufferfish, but stings from blue-ringed octopuses and cone snails (both from the southwestern Pacific) are also a possibility if you are foolish enough to pick them up.

Although many animals use TTX, none of them actually manufacture the toxin themselves. Instead bacteria generate the toxin, to their host’s benefit. In exchange, the bacteria are allowed to live safely within their host’s body — the bacterium Pseudoalteromonas tetraodonis, for example, makes its home within the livers and skins of pufferfish.

A toxic moon snail (Naticarius orientalis) from East Timor. Photo by Nick Hobgood, licensed under CC BY-SA 3.0.

A toxic moon snail (Naticarius orientalis) from East Timor. Photo by Nick Hobgood, licensed under CC BY-SA 3.0.

The rough-skinned newt, from British Colombia and the western United States, has the bacteria needed to make TTX for itself — as a result, this newt has few natural predators. Two animals, however, have managed to work around and even benefit from the rough-skinned newt’s toxins, all without toxin-producing bacteria of their own.

The only animal capable of eating an adult rough-skinned newt is the garter snake. After ingesting the newt, any other predator would almost certainly die, but a few populations of garter snakes have evolved an immunity to TTX. What’s more, the snakes are able to sequester the toxins within their own bodies, so that the garter snakes themselves become poisonous (Williams et al. 2004).

The extremely toxic rough-skinned newt (Taricha granulosa). Photo by Rennett Stowe, licensed under CC BY 2.0.

The extremely toxic rough-skinned newt (Taricha granulosa). Photo by Rennett Stowe, licensed under CC BY 2.0.

Although many snakes inject toxins with their fangs, and so are venomous, garter snakes that eat rough-skinned newts are the only snakes that can truly be considered poisonous. The difference is that venomous animals have to inject their poisons into predators or prey, via fangs or stingers, while poisonous animals are laced with their chemical weapons and are dangerous to eat.

The other TTX-robber is, unexpectedly, a caddisfly. Caddisflies are insects whose larvae resemble caterpillars, but live underwater and surround themselves with cases made from pebbles, twigs, and other debris. Most are scavengers, eating decomposing plant matter and tiny invertebrates, while the flying adults are short-lived and do not feed.

A Limnephilus caddisfly larva, in its protective case made of twigs. Photo by Tom Murray, used with permission.

A Limnephilus caddisfly larva, in its protective case made of twigs. Photo by Tom Murray, used with permission.

A few predatory species in the genus Limnephilus, however, have developed an unusual appetite for rough-skinned newt eggs, which also develop in the water. These eggs are loaded with TTX, and the toxin-resistant Limnephilus larvae manage to eat so many that even the caddisfly adults are toxic (Gall et al. 2012).

Even though many marine invertebrates contain tetrodotoxin, we still don’t know how many contain the bacteria that make it, versus how many, like the caddisfly, “steal” the toxin from their TTX-laced prey. Certain sea slugs, for example, are often washed on shore where they are eaten by beach-combing scavengers such as dogs. In New Zealand, several dogs have died after eating sea slugs that contained TTX (McNabb et al. 2009), and in Argentina, a population of the same slugs appears to have become invasive (Farias et al. 2015). Yet we still do not know whether they have their own TTX-generating bacteria.

A toxic sea slug, Pleurobranchaea meckelii. Photo from Wägele and Klussmann-Kolb (2005), licensed under CC BY 2.0.

A toxic sea slug, Pleurobranchaea meckelii. Photo from Wägele and Klussmann-Kolb (2005), licensed under CC BY 2.0.

Experiments have shown that certain populations of sea slugs have TTX, while others do not (Khor et al. 2014) — just like the garter snakes of North America. Does this mean the sea slugs are obtaining TTX from some unknown, toxic prey item? The same researchers conducted a survey of all the marine invertebrates that these slugs might be eating, testing everything for TTX. The only positive result was a toxic species of sand dollar, but the sand dollars didn’t produce nearly enough TTX to explain the huge amounts found in sea slugs.

Ignorance has consequences, and there is still plenty of exploring left to do. The slugs may in fact make tetrodotoxin themselves (using bacteria). A more enticing possibility, and just as likely, is that there are still more toxic animals on the sea floor, waiting to be discovered.

One of the very first articles I wrote for this site featured the same sea slugs mentioned in this one, but that was when my only readers were a few sympathetic family members. If you’re curious about sea slugs, and/or you want to know the difference between a nudibranch and a pleurobranch, you can read that article here.

I also recently wrote about how rattlesnake’s use their venom to track once-bitten prey (mentioned in the second paragraph of this article). To learn more about that, click here. It’s amazing, I promise.


Farias, N.E., S. Obenat, and A.B. Goya. 2015. Outbreaks of a neurotoxic side-gilled sea slug (Pleurobranchaea sp.) in Argentinian coasts. New Zealand Journal of Zoology, published online: DOI: 10.1080/ 03014223.2014.990045.

Gall, B.G., A.N. Stokes, S.S. French, E.D. Brodie III, and E.D. Brodie Jr. 2012. Predatory caddisfly larvae sequester tetrodotoxin from their prey, eggs of the rough-skinned newt (Taricha granulosa). Journal of Chemical Ecology 38(11): 1351-1357.

Khor, S., S.A. Wood, L. Salvitti, D.I. Taylor, J. Adamson, P. McNabb, and S.C. Cary. 2014. Investigating diet as the source of tetrodotoxin in Pleurobranchaea maculata. Marine Drugs 12(1): 1-16.

McNabb, P., L. Mackenzie, A. Selwood, L. Rhodes, D. Taylor, and C. Cornelison. 2009. Review of tetrodotoxins in the sea slug Pleurobranchaea maculata and coincidence of dog deaths along Auckland Beaches. Prepared by Cawthron Institute for the Auckland Regional Council Technical Report 2009/108.

Williams, B.L., E.D. Brodie Jr., and E.D. Brodie III. 2004. A resistant predator and its toxic prey: persistence of newt toxin leads to poisonous (not venomous) snakes. Journal of Chemical Ecology 30(10): 1901-1919.

Tongue-Eating Parasites with Mayonnaise

by Joseph DeSisto

Tongue-eating parasites that are found in cans of tuna eat tuna, not human, tongues. But that’s a boring headline, so most news outlets left that part out when a woman in Britain found a “thing” in her can of tuna. The victim was one Zoe Butler, who opened her can of Princes tuna chunks this week to find an eyed, globular, undeniably cute invertebrate staring back at her.

Since the specimen was sent back with the tuna can to Princes, Stuart Hine of the Natural History Museum in London had to make do with a photograph in making his identification. He postulated that it might be a parasite related to Cymothoa exigua. Unfortunately, he also mentioned the common name, the tongue-eating louse, to reporters, who got right to work making sure the public knew that we were about to take part in a real-life version of The Bay.

What these reporters didn’t appreciate, sadly, is that the actual life history of Cymothoa is far more interesting and twisted than any sensationalist headline. These creatures belong to the crustacean order Isopoda, and are distantly related to woodlice. Unlike woodlice, however, Cymothoa are roly-poly fish nightmares.

Cymothoa exigua, a tongue-eating isopod. Photo by Marco Vinci.

Cymothoa exigua, a tongue-eating isopod. Photo by Marco Vinci.

As free-living larvae, Cymothoa are all males, and spend their time swimming about in search of a fish host. When several individuals “colonize” a fish, they begin feeding on its gills. If C. exigua is the louse, the host will be red snapper (not tuna). Once a fish has been parasitized, some males will change into females, and mating takes place on the gills or in the fish’s mouth.

The females are the tongue-eaters. Either before or after mating, depending on the species, a female invades the fish’s mouth and begins to suck blood. Extracting blood from the tongue eventually causes the tongue to wither away, after which the female persists on bits of mucus and blood remaining in the fish’s mouth. In C. exigua, this is when mating takes place: a few males will migrate from the host’s gills and mate with the female inside the fish’s mouth.

During all this time, the fish is still able to function, because the female Cymothoa functionally replaces the tongue. So with a new segmented, chitinous, leggy tongue, the fish can still eat and reproduce normally. Eventually, the female’s young emerge as free-living males, and disperse to find a new fish.

So, are tongue-eating parasites dangerous to humans? An adult female can bite in self-defense, and revealing a giant isopod while cleaning a snapper might be emotionally scarring. But rest assured, our tongues are safe. Enjoy your tuna.

For more information:

Driscoll, Brogan. “Mystery ‘Crab’ Found in Tuna Could Actually Be A Tongue-Eating Parasite, Claims Expert.” Huffington Post. 2 Feb 2015. Web. 6 Feb. 2015.

Creighton, Jolene. “Meet The Sex-Changing, Tongue-Eating Parasite.” From Quarks to Quasars. 3 Apr 2014. Web. 6 Feb 2015. <;.