A reader sent us a picture of two smooth, black, worm-like creatures that he found by a swimming pool, which he thought might be leeches. The leeches, if they are leeches, were on a ledge right on the side of the pool, a little above the water line. They reader said they moved very slow by expanding and contracting, and that they are black on top, but a lighter, tan or brownish color on the bottom. The reader was only wondering if he found leeches, so we’ll limit ourselves to the matter of identification. Did our reader find leeches by the pool?

First, here is the picture our reader sent in:

The picture is a little small, but it clearly depicts something that looks like leeches, and indeed we think this is the most likely thing he found because nothing he said contradicts this possibility. Leeches are commonly black, like the creatures pictured above, and they often have lighter undersides (even though this part of their body isn’t seen unless you pick up the leech and examine it). They also move by expanding and contracting, in a way that is somewhat reminiscent of the way inchworms “inch” along the ground. The expansion and contraction of leeches can be fairly dramatic, as what appears to be a plump, inch-long worm can expand quickly into a skinny, elongated creature that is several inches long. (Leeches are, for the record, annelids and thus are worms according to the way we define the word “worm.”) Finally, the majority of leeches live in aquatic environments – generally freshwater, but some live in marine water. Obviously, none of the aquatic environments they naturally live in are swimming pools, but a leech could mistakenly crawl into a pool if it was searching for a water source, and an aquatic leech would always be searching for water if it were removed from its normal habitat. So, it does seem reasonably likely our reader found leeches.

If our reader wanted to make sure he found leeches, he would have to examine its body in slightly greater detail, which we are unable to do by simply looking at the picture. In particular, he would want to look for suckers on both ends of its body. On the front end of the leech is the oral sucker, which is what leeches use to suck blood out a host, and on the back end there is another sucker, one that is bigger than the oral sucker, that is used for movement. (This is why leeches look like inchworms when they move. They plant their posterior sucker on a surface and then extend forward to plant their oral sucker on a surface. Once attached, the posterior sucker lets go of the surface, allowing the rest of leech’s body to “catch up” by contracting. The posterior sucker is then replanted and the process begins anew.) If the reader does not find these suckers (although we suspect he will), he might have found some type of slug and not a leech. In general, we don’t think the slug hypothesis is very promising because the creature pictured above doesn’t appear to have tentacles – the things on slug heads that are commonly mistaken for antennae – but these aren’t always visible on slugs, so it’s a possibility to keep in mind.

So, the reader suspected he found leeches, and we think he did, as essentially all the information we were given, including the picture, aligns with this suggestion. To confirm he found leeches, he should check to see if the creatures he found have suckers. If for some reason they don’t, he might have found slugs. Overall, though, it seems likely he found leeches.

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