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October 1, 2022

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See How a Parasite Travels from Tiny Crustacean to Fish to Bird

A tiny bug manipulates the behavior of its hosts to ensure its unlikely life cycle

By Madhusree Mukerjee

Detail of lifecycle drawing follows parasitic cestode Schistocephalus solidus through 3 hosts: a copepod, a fish and a bird.

Daisy Chung

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Parasites give new meaning to the cliché “eat or be eaten.” Often their life cycle can be completed only if they are ingested by a host—multiple times for some—making the odds of their survival seemingly minuscule. To improve their chances, certain parasites manipulate their hosts' behavior to make it more likely the eater will get eaten.

The parasitic cestode Schistocephalus solidus requires a much larger host—specifically, a three-spined stickleback fish—to grow in and then a bird to breed in. But the parasite's larvae, less than a millimeter long, are too small to be eaten by the fish.

Instead a larva must first be ingested by a copepod, a crustacean akin to a tiny shrimp. When ready for its next host, the larva makes the copepod twitch. If all goes well (for the parasite), a three-spined stickleback then eats the copepod. Inside the fish, the larva grows enormously, making the poor stickleback gasp at the water's surface, where it is likely to get snacked on by a bird. Inside the bird, the parasite matures and mates, sending its eggs back to the water through the bird's poop. And so the cycle begins again.

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Credit: Daisy Chung

Madhusree Mukerjee was a senior editor at Scientific American, where she covered psychology, anthropology, and diverse other topics. She has authored two nonfiction books: Churchill’s Secret War (Basic Books, 2010) and The Land of Naked People (Houghton-Mifflin, 2003). She has a Ph.D. in physics from the University of Chicago and received a Guggenheim fellowship to complete her first book. She has written numerous articles on Indigenous issues, development, and colonialism and is working on a third book.

More by Madhusree Mukerjee
Scientific American Magazine Vol 327 Issue 4This article was published with the title “An Improbable Life Cycle” in Scientific American Magazine Vol. 327 No. 4 (), p. 78
doi:10.1038/scientificamerican1022-78
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