“When I came out of our northern Minnesota lake in early October (cooling off on a warm fall day), my legs and swim trunks were covered in over 100 small dark brown to black larvae/worms”, writes this reader to us in his submission. “My guess is that they detached from the weeds that I had walked through and floated onto my legs. It is a deep (50 ft) soft bottom lake. Unfortunately I didn’t have a camera to take any photos. I saw an article on midge fly larvae but none of these were lighter-colored (all very dark) and they were all about 3 mm (0.1-inches) in length and 1mm (0.04-inches) in diameter? I would not call them slender. They didn’t fall off easily but I was able to brush them off. My neighbors say they have never seen this before and we had multiple new (to our lake) watercraft put in at our shore this summer so my main interest/concern would be whether this might be some sort of invasive species.”

According to SailorsForTheSea’s “page on invasive species in aquatic habitats“, invasive species can be transported to a new body of water in a number of ways, including on boats. However, it is usually only mussels, seaweeds and barnacles that can travel on any old boat. To carry other sorts of species would require a big vessel that contains ballast water tanks (in which the new species would reside), and we doubt there are any cargo ships on a lake in Minnesota.

Of course, it is still not impossible for these to be a new species, but whether or not they are an invasive species is another question. To be an invasive species, the new species needs to have a negative impact on the ecosystem, either through degrading the quality of the water, or through threatening populations of pre-existing species. It is difficult to say without doing proper tests if these will prove to be an invasive species. If our reader wants a professional to take a look and confirm if these are an invasive species, the USDA’s has “a page dedicated to reporting invasive species in Minnesota“. There our reader will find information on how to report the location of any invasive species he finds.

To conclude, we cannot say for certain if the organisms our reader found are an invasive species, though we hope that the information we did provide will prove useful. We wish him and his neighbors the very best and hope for the sake of the lake that these worms are a friendly, non-invasive bunch!

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