Leeches have evolved a strategy for hunting worms that is the ultimate death spiral.
California blackworms (Lumbriculus variegatus), which live in shallow waters of swamps and marshes, defend themselves from predators by swimming ultra-fast and forming tangled blobs made of dozens of worms. This usually protects them, but Helobdella leeches have devised a special trapping technique to feed on them anyway.
“What’s been known is that leeches will just pick off worms [from these worm balls] like spaghetti,” says at the Georgia Institute of Technology in Atlanta. But he and his colleagues observed them doing something that has never been seen before. “It was definitely a surprise.”
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The researchers put leeches and blackworms in a Petri dish filled with water and recorded the leeches hunting behaviour. Analysing the recordings one frame at a time revealed that of 36 attacks, leeches successfully captured the worm or broke off a part of it 18 times – in each of these, the leech used its whole body to execute what the team calls “spiral entombment”.
This is where the leech latches onto a single worm with a sucker, and then envelops the worm by folding its body into a spiral shape, like a cocoon. Once the thrashing worm was trapped within the contorted leech, the predator would insert an appendage and feed on the worm’s internal liquids.
Tuazon says that while larger leeches were the best at spiral entombment of worms swimming alone, all leeches struggled when faced with the worm blob. The hunting technique could still work, but they had to get lucky – most of the time, any worm they grabbed would thrash and tangle with the others and consequently bring the whole undulating blob with it, he says.
The team wants to further study the leech’s movements as inspiration for soft robots or even self-folding bandages, but questions remain about its behaviour as well. For instance, it is unclear how the leech breathes while it is so tightly wound around a worm.
“Nobody really sees what’s going on under the water that much, but when you watch closely, it’s intense what’s going on. There’s very aggressive movements, it’s quite the warfare,” says Tuazon.
Integrative and Comparative Biology
