We received a question through the All About Worms Facebook page a few days ago about “a tiny black skinny worm in my salt water pool,” to use the reader’s exact wording. The reader lives in Florida, and also reports that the worms are unsegmented and appear to have “diamond shaped scales,” but she concedes it is hard to tell if this is an accurate description of the worm’s exterior. The reader was having trouble figuring out what she found, so she asked us to identify the tiny, skinny, black worm that she found in her pool.
The reader submitted two pictures along with her question, and seems to imply that she didn’t actually take the pictures. (She mentions in her message that she found pictures online that looked like what she found, but then doesn’t indicate that the pictures she submitted were in fact the same ones that she found online. For all we know, the pictures are hers, and they depict a worm that is similar to a worm she saw in other online pictures.) In any case, here is the clearer of the two images:
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Again, this may or may not be the exact worm are reader found. It could merely look like what she found, so we don’t want to place too much emphasis on all the details of this picture. Clearly, we are dealing with incomplete information, so any answer we give will be necessarily speculative.
When we first read about the skinny black worm in the reader’s swimming pool, horsehair worms came to mind. Horsehair worms belong to the Nematomorpha phylum and are morphologically similar to nematodes (the phylum composed of roundworms). Indeed, horsehair worms were once classified as nematodes, but now have their own phylum. (Both are now said to belong to the the Nematoida clade.) We get into the taxonomy primarily to point out that these worms aren’t annelids, or segmented worms. Thus, they do tend to have smooth bodies, just like what our reader found (although we have never heard of horsehair worms having skin composed of anything resembling diamond-shaped scales). The main problem with this suggestion is that horsehair tend to be quite long relative to their extremely skinny bodies. They literally look like the hair of a horse’s tail, and hence their name. The reader described what she found as “skinny,” but she didn’t emphasize the creature’s length. Moreover, while horsehair worms gravitate toward water sources and are commonly found in swimming pools, all but one known species are fresh water worms. Thus, the fact that our reader has a salt water swimming pool complicates this possibility, although we suppose it is possible that one could end up in a salt water pool by mistake.
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We aren’t very confident in the horsehair worm hypothesis, but in truth we don’t know what else to suggest. When we answer a lot of questions, we can normally offer a few types of larvae as possibilities, but this isn’t possible in this case. Larvae come in many shapes and sizes, but none of them look like the creature pictured above, which does in fact appear to be an actual worm. Based on what our reader said about the worm she found, we suppose it is most likely she found some type of roundworm (because its body isn’t quite long and skinny enough for it to be a member of the Nematomorpha phylum). She didn’t find an annelid (segmented worm) and she gave us no reason to think she found a flatworm (of the phylum Platyhelminthes) or some type of exotic marine worm (like one of the various species of arrow worm). Saying she found a roundworm isn’t very precise – there could be as many as a million different species – but in the present case this is all we can offer.