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Nematodes are among the planet's most abundant animals, "free-living" in water, soil and the Earth's crust in addition to parasitizing a large collection of plant and animal species.

Photosynthesis takes place inside chloroplasts, small compartments within plant cells that contain their own genome, reflecting their past as free-living photosynthetic bacteria before they were engulfed and co-opted by plants.

The lifecycle of soil-transmitted helminths has two phases -- a free-living stage as eggs and larvae in the environment and an adult stage inside the host.

"It opens the door to experimentally manipulate the demographic or genetic composition of free-living populations to ask questions about how realistic natural social environments affect health, fitness and life outcomes for individuals," Sheehan said.

And most recently, some of those species again traded their ability to make silk for a free-living, clinging life in these faster waters.