Many instances of this worm-like creature were found in a water system for a mountain cabin in the Sierra Nevada mountains of the US, in late June / early July. The water system collects water from an enclosed spring. Can you help identify this creature?


  1. The worms have two "points" on their heads.
  2. They can stretch out and be long and thin, or when poked (or are resting) they contract into a small blob.
  3. Here is a picture of a single worm with markings shown. enter image description here
  4. Here is a picture of a few worms, somewhat contracted. enter image description here
  5. Here is a movie of them moving, in natural light.
  6. Here is a movie of them moving, with backlight.
  7. Here are pictures of their undersides and their internal structures. enter image description here enter image description here enter image description here enter image description here
  8. Whenever I pull some out of the water and put them into a tupperware with the same water, they only survive for around 24 hours, after which they seem to disintegrate. Here is a picture of about 20 of the worms after about 24 hours, the bulk of them have halfway-decomposed and form a heap in the upper right. After another day it'll just look like debris in the water, you wouldn't even recognize that there were worms in it. There is a piece of bacon in there because some suggested putting it there to test if they were leeches, but this disintegration happens repeatably and regardless of the bacon; please ignore the bacon.
    enter image description here
  9. Someone suggested that they are trematodes, but there are no snails, frogs, or other creatures in the storage and collection sections of the water system; we have looked very closely. There may be something within a 30-foot section of pipe, but we doubt it.

Can you help us identify this creature? Even just possible families of creatures or general categories would be helpful for us.

  • 1
    $\begingroup$ My first thought, based on the two "points", is that they're slugs, but I think most water slugs live in salt water. This page seems to suggest that freshwater slugs are all tropical, but might have a useful lead. $\endgroup$
    – Ben Kovitz
    Jul 6, 2015 at 6:48
  • 2
    $\begingroup$ Are leeches supposed to like bacon ? I thought they needed fresh blood. $\endgroup$
    – bli
    Jul 6, 2015 at 7:39
  • 1
    $\begingroup$ @bli - leeches are attracted to odors as well. In swampy areas, it's not uncommon to find a leech or two on a carcass that's been in the water. $\endgroup$
    – Omegacron
    Jul 6, 2015 at 15:47
  • $\begingroup$ @JeffAmelang Thanks for good photography and description effort. $\endgroup$
    – user25568
    Sep 7, 2016 at 10:53

3 Answers 3


I am not too sure but this looks like Polycelis sierrensis.

Though it is apparent that the worm in your pictures is a tricladid planarian, I was not too sure about the species and the geographical distribution.

However, from the Smithsonian list of freshwater planarians in North America, I deduced that this should be Polycelis sierrensis which is found in California and Nevada (Kenk ,1973).

The EOL page on this species also concurs with the geographical distribution. Moreover, the shape and dimensions of the worm, as described in EOL, matches with that of the ones you have got.

I could not find a picture of P. sierrensis but this is how a general Polycelis looks like:

enter image description here

  • $\begingroup$ Wow, a google image search for Polycelis sierrensis shows this image as the first result, at least for me. $\endgroup$
    – user151841
    Jul 6, 2015 at 17:45
  • $\begingroup$ @user151841 Same here. But the caption just says Polycelis sp. but this one does look like the worms in the question. $\endgroup$
    Jul 6, 2015 at 18:05
  • $\begingroup$ Thanks so much for taking the time to answer, WYSIWYG. We really appreciate it. $\endgroup$ Jul 9, 2015 at 5:40

The overall shape appears to match that of a flatworm or planarian, but those don't have eyestalks. Also, in the images you posted, none of the worms appear to have the characteristic eyespots of a flatworm, although they DO look planarian-like in some photos:

enter image description here

If they didn't look so thin, I'd say they were common garden slugs. We get them all the time down in Southeast Texas, and they're attracted to humid areas, especially wetness. They usually have thicker bodies, though:

enter image description here

I've never put salt on a planarian before, but the behavior you describe is exactly what happens when a common slug hits salt. They curl up into what looks like a wad of gum, then shrivel up into a blackened husk. On the other hand, slugs are definitely NOT mountainous creatures. And they're definitely NOT leeches, which we also get down in Southeast Texas.

Based on your location and the images, you're probably looking at some sort of freshwater planarian. They're most likely in the source spring itself.


Their capacity to elongate / contract and their two head lobes make me think they are Planarians.

The two points on their heads must be their eyes: https://en.wikipedia.org/wiki/Simple_eye_in_invertebrates#Ocelli_or_eye_spots


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