One of our readers found a critter in her salt water pool. She wrote that the specimen is 6 inches long, black, and moves like a snake, but acts like a parasite. We aren’t sure what behaviors or characteristics this specimen encompasses that can be portrayed as “parasite like” to our reader, but she is on to something! Based off of the detailed clues she provided us and the included photograph, we believe this is a horsehair worm, which is indeed a parasite! Here is one of the pictures she sent us:
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We have written about horsehair worms on numerous occasions, and we are pleased to share our knowledge on these specimens with our reader. While commonly known as horsehair worms, Nematomorpha are actually a phylum of parasitoid animals. Horsehair worms are found in swimming pools, watering troughs, streams, puddles, and other damp areas. There are over 300 known species of freshwater horsehair worms. However, we reckon that our reader discovered a salt water horsehair worm because she mentioned that her pool has salt water in it.
Adult horsehair worms are free living specimens, but in their larval stage they are parasites. Their hosts are always arthropods, such as cockroaches, beetles, and crustaceans. Horsehair worm larvae enter an arthropod host and then eject themselves when they sense they are in or near water. This is why horsehair worms are so typically found in water.
Although these creatures can technically be classified as parasites when they are still immature worms, they are not harmful to humans. Our reader shouldn’t stress too much about finding one of these creatures in her pool.
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To summarize, a reader sent us a photo of a thread-like black organism that she found in her salt water pool. We believe that the specimen is a Nematomorpha, or a horsehair worm.