Can leeches survive fast-flowing water?


My daughters and I were swimming near a waterfall. The rocks below were covered with tiny, black, leech-like creatures (5-10 mm) that attached to our feet and hands. What were they? How do they survive the fast-flowing water? —Steve Power, Upper Stony Lake, Ont.

Without a specimen, or at least a photo, it’s hard to know. But it’s possible that those leech-like creatures were—wait for it—leeches.

Leeches are adaptable, and some can survive just fine in fast-flowing water, says Jacqueline Madill, a senior research assistant at the Canadian Museum of Nature in Ottawa, who studied leeches for 15 years. However, they may be more likely to seek you out in calm water, when your movement and splashing indicate that you’re there. Just because they attached to your skin doesn’t mean that they were after you. Madill suspects, based on the size, that they might have been bait leeches or turtle leeches, which generally don’t feed on people. And from time to time, some water-dwelling creatures—worms or the larvae of aquatic bugs—attach to you by mistake too.

Claire Healy, associate curator of invertebrate zoology at the Royal Ontario Museum, thinks your hangers-on were blackfly larvae, which are common in fast-flowing water. “We’re just a substrate to them,” she says. “Like a rock. We’re just there.”