American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Tradeoffs with growth limit host range in complex life cycle helminths”

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Daniel Benesh, Geoff Parker, James C. Chubb, and Kevin D. Lafferty (Feb 2021)

Host availability, tradeoffs, or both – why do complex life cycle worms infect more host species at some life stages?

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Proboscis of the acanthocephalan <i>Echinorhynchus borealis</i>, a parasite specific to burbot.<br />(Credit: Dan Benesh)
Proboscis of the acanthocephalan Echinorhynchus borealis, a parasite specific to burbot.
(Credit: Dan Benesh)

Is a worm of all trades a master of none? Some worms have mastered living in whales, but they are unable to live in other mammals. This is important because it means that a whale worm can’t mature in you after you eat its coiled larva hidden in your sashimi (though it might live long enough to make you sick). The coiled larva in your sashimi is a jack of all trades, being able to infect dozens of fish species. So the whale worm is a generalist in fish then a specialist in whales. Why? A new study finds that parasitic worms are generalists when they have the opportunity (there are lots of fish in the sea), but generalist worms don’t grow as much as their specialist relatives. And this is not just for whale worms, it seems to be the case for hundreds of parasitic roundworms, tapeworms, and spiny-headed worms.

A parasite that lives in a host eaten by different predators has many opportunities to be a generalist. By analyzing food webs with parasites in them, the authors discovered that ‘middle’ (e.g., second) stage parasitic larvae have more host opportunities, and this matched what parasites do – ‘middle’ life stages infect more host species than early or late life stages. Regardless, parasites usually infect fewer host species than they could, suggesting generalism also has costs.

In some stages, parasites grow a lot, like tapeworms that start out the size of this period “.”, but grow to be longer than an average human is tall. In contrast, less industrious stages simply encyst as they wait to be passed to their next host. The stages that grow a lot tend to infect a narrower range of hosts, hinting that worms must specialize to grow large. Overall, this study suggests that a worm of all trades is indeed a master of none.


Parasitic worms with complex life cycles have several developmental stages, with each stage creating opportunities to infect additional host species. Using a dataset for 973 species of trophically transmitted acanthocephalans, cestodes, and nematodes, we confirmed that worms with longer life cycles (i.e. more successive hosts) infect a greater diversity of host species and taxa (after controlling for study effort). Generalism at the stage level was highest for ‘middle’ life stages, the second and third intermediate hosts of long life cycles. By simulating life cycles in real food webs, we found that middle stages had more potential host species to infect, suggesting that opportunity constrains generalism. However, parasites usually infected fewer host species than expected from simulated cycles, suggesting generalism also has costs. There was no tradeoff in generalism from one stage to the next, but worms spent less time growing and developing in stages where they infected more taxonomically diverse hosts. Our results demonstrate that life cycle complexity favors high generalism, and host use across life stages is determined by both ecological opportunity and life history tradeoffs.