Riccardo Liguori The longest animal in the world is not the blue whale but a parasite that lives in its intestine. This is Tetragonoporus calyptocephalus, a tapeworm that can reach 40 meters in length
When we think of the biggest animal on Earth, our minds often race to the majestic blue whale, the titan of the oceans. But the true record for length is far less glamorous-a parasite that lives in the intestines of whales.
Meet Tetragonoporus calyptocephalus, a tapeworm of the class Cestoda, a class of flatworms infecting all vertebrates. This solitary worm can reach an astonishing length of 131 feet, way out of proportion to the size of its host.
A giant among giants
Tetragonoporus calyptocephalus is parasitic mainly in the intestines of whales and sperm whales, including the blue whale—the largest animal that has ever lived. In the Arctic, it prefers the waters in which the sperm whale, the world’s largest toothed animal, is its main host.
Inside these oceanic giants, the tapeworm reproduces, releasing its larvae through the host’s feces and continuing its life cycle. But the whale tapeworm isn’t the only massive parasite. Many tapeworm species grow to impressive sizes, particularly those infecting pigs, cows, and salmon. This highlights the importance of properly cooking meat to prevent these giant worms from making a home in human intestines.
An evolutionary history 100 million years long
Tapeworms have been part of our planet’s history for much longer than previously thought. In March 2024, a study published in Geology revealed the discovery of a tapeworm tentacle preserved in amber dating back to the Cretaceous period, about 100 million years ago.
Since then, tapeworms have thrived and diversified, colonizing virtually every ecosystem. Today, they are among the most widespread parasites, infecting a wide variety of animals, including humans.
Parasites in humans
Humans, too, are vulnerable to these intestinal parasites. A notable example is exhibited at the Meguro Parasitological Museum in Tokyo: a 28.9-foot specimen of Dibothriocephalus nihonkaiensis. This tapeworm was removed whole from a man’s intestine in the 1980s. There have been reports of as long as 82 feet!
Human infection usually occurs by ingestion of raw or undercooked meat contaminated with the adult parasite or its larvae. One of the most common parasites to infect humans is Taenia solium, the “pork tapeworm,” acquired by eating undercooked pork. Anisakis, a nematode worm, is found in the stomachs of marine mammals and crustaceans and can be transmitted to humans.
Parasites are integral components of marine ecosystems, just like jellyfish, corals, fish, whales, or any other marine organisms: they all have their parasites. The organisms are an essential modulus of population control and maintenance of ecological balance.
Some parasites have evolved ingenious ways of survival. For example, the tongue-eating isopod attaches itself to a fish’s tongue, eventually replacing the tongue while feeding on the host’s blood, whereas the barnacle Sacculina infects crabs and controls their behavior in order to do its bidding.
