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Mathieu Badets and Louis Du Preez

If you’ve ever been too full to move after a large meal, envy the leech. That darling of medieval physicians and terror of river bathers can gorge as much as it wants and still squirm away, even if it bloats up to 10 times its normal size. Scientists have now figured out what keeps these leeches mobile even when they’re swollen like a balloon: a hormone that’s been linked to happiness in humans.

Leech muscles, like those of worms and most soft-bodied animals, expand and contract as the creatures crawl. When leeches swell up, however, they stretch these muscles to their limit, which should inhibit the expansion and contraction needed to wriggle away. Yet leeches get around just fine, says Shannon Gerry, a biomechanist at Wellesley College in Massachusetts.

Gerry and biomechanist co-author David Ellerby, also of Wellesley, set out to discover why leeches have this extraordinary ability. The duo allowed leeches to feast on sheep’s blood until they were full, measuring both the change in the length of their segments as they fed and the rate at which they spread as they crawled.

Bloodsucker. Filling with blood doesn’t slow leeches down; serotonin acts on their muscles to allow them to swim before and after feeding.
Credit: David Ellerby

Next, the researchers chopped some leeches into segments and then soaked the chunks in serotonin, which stimulates feeding in leeches, or in saline. Then they stretched the segments to lengths that would simulate crawling or swimming after feeding and stimulated the muscles with electrical wires to force them to contract. Segments exposed to serotonin exerted more force than did the control segments, no matter how strained they were, suggesting that serotonin allows the taxed muscles to push past the strain and move even farther. In their paper published in Biology Letters today, the authors suggest that serotonin might relax the molecules that crosslink muscle fibers so that the muscles can move more easily.

Lunch buffet. Leeches feeding in the lab can reach their fully expanded size in about half an hour.
Credit: Quentin Gaudry

The range of motion seen in the muscles of swollen leeches surprised the researchers. When a human arm curls a weight, it goes from fully extended to fully contracted. The length of a human muscle changes by only about 20%, says neurobiologist William Kristan of the University of California, San Diego, who was not involved in the study, since it is constrained by a skeleton. But a bloated leech can sport 300% the muscle length of a hungry, contracted leech.

Kristan calls the research a "very nice piece of work." He says that the results aren't terribly surprising, however, given serotonin's important role in feeding. He's interested in finding out whether serotonin makes leeches hungry as it prepares their muscles to stretch.

For her part, Gerry looks forward to further elucidating the biomechanics of leech muscles, despite the squeamishness of some colleagues. Although people are interested in her experiment, she says they never want to come too close.