Large leach
Photo by Marin Anderson

Is this type of leach common in Ontario?

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We found this huge leech in our lake. Is this type of leech common in Ontario lakes?
—Marin Anderson, Head Lake, Ont.

We assume you had the same reaction that we did upon seeing this sucker: horror, revulsion, and then, finally, 
curiosity. Unfortunately, it’s tough to definitively i.d. leeches from photos because they’re able to expand and contract, explains Jacqueline Madill, a senior research assistant at the Canadian Museum of Nature, who studied leeches for 15 years. “It’s really difficult 
to get a good measure.”

That said, Tom Mason, the curator of invertebrates and birds at the Toronto Zoo, believes that this one is native, and probably from the genus Haemopis, a.k.a. a horse or ribbon leech.

Of the 30 or so kinds of leeches found in Ontario, most are less than a few 
centimetres long. But some are huge. The largest in Ontario, the giant horse leech, can reach 36 cm. (Think: the length of your laptop screen…or a small, legless dachshund.) You may have never seen the large ones in your lake before because they’ve been avoiding you. Most large species don’t feed on people. They prefer to hide from fish predators in weedy habitats or look for their own sources of food: dead fish, snails, worms, and aquatic insects. “They’re generally harmless,” says Mason. “They’re scary to look at, but they will not eat children.”