Chironomidae: Worms in the Swimming Pool

Chironomidae Larvae
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A reader wrote to us a while ago about worms in his swimming pool, or what are believed to be worms in his swimming pool. (The worms could be larvae, potentially of the Chironomidae family.) The worms or larvae are about an inch (2.5 centimeters) long, brown and yellow in color, and quite thin, about the width of pencil lead, according to the reader. The reader only wants to know what the brownish, yellowish worms (or larvae) in his swimming pool are, so we will limit our remarks to the matter of identification.

Unfortunately, the reader did not include a picture of the creatures in his swimming pool, which is filled with salt water. While an image would not guarantee a successful identification, it would at least allow us to rule out a few possibilities. An enormous number of worms or larvae might be found in a swimming pool, as anyone who has cleaned a swimming pool filter will attest. The fact that the creatures are about inch-long and a brownish/yellowish color isn’t too much of a limiting factor either.

However, this does indicate that our reader probably isn’t finding any sort of earthworm, which because of their dependence on moisture to survive often end up in pools, particularly after it rains. Earthworms are almost always longer than an inch, and it would be unusual to describe their relatively dark color as “brownish and yellowish.” (Rightly or wrongly, we are interpreting this color as essentially tan, or basically an off-white, because lots of larvae are this color.) We are also all but certain our reader didn’t find horsehair worms, another common creature found in pools, even though the reader said that what he found is about the width of pencil lead, which is a comparison we have used multiple times to describe these hairlike worms. Horsehair worms are much longer than an inch long, so we’re fairly confident he didn’t find these worms either. For the record, it is not clear if the fact that our reader’s pool is filled with salt water would deter either creature from entering it. Both worms would not do well in salt water and thus would avoid it (with the exception of a species of horsehair worm that parasitizes crabs), but if left the choice of either drying out or entering a pool of salt water, we suspect they might naturally gravitate toward the water source.

So, that’s what the reader (likely) didn’t find, but what about a positive identification? Without more information and a picture, we are firmly in the realm of speculation, but we think there is a possibility our reader found Chironomidae larvae, the larval form of what are commonly called “nonbiting midges” or simply “midges.” The Chironomidae family is very diverse – there are likely over 10,000 species in it – and its members are found all over the world in virtually any sort of aquatic habitat, including salt water. Chironomidae larvae, which are also somewhat incoherently called red worm larvae (because larvae are to be distinguished from worms), are also commonly found in swimming pools and are commonly a brownish, yellowish color. Although they tend to be about a centimeter (or about a half an inch) long, they could be a little bigger, so it’s not inconceivable our reader could describe Chironomidae larvae as “about an inch long.”

Although we aren’t sure what our reader found, we think Chironomidae larvae is a definite possibility. The reader’s description of the creature he found is consistent with this suggestion, and moreover Chironomidae larvae are often found in swimming pools. However, there are lots worms and larvae in this world that could make it into a swimming pool, so unfortunately our reader might always have some difficulty precisely identifying the things that end up in his pool.

Credit for thumbnail image: ORGF / CC-BY-SA-3.0


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1 thought on “Chironomidae: Worms in the Swimming Pool

  1. I have read this article with interest. I have a 45k litre saltwater pool which I vacuumed tonight and found many ~5-10mm long red ‘wrigglers’ which I also believe are chiromonids in the pool sock. The pool had gone slightly green which I guess would be great food for the chiromonids. The puzzling thing is that the previous evening I treated the algae in the pool with 20 l of liquid chlorine which, I would have thought, should have nuked everything in the pool. Does anyone have any comments?

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