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I collected some bladder snails several months ago. While checking them out one day, I noticed that they were now infested with worm-like ectoparasites.

Appearance and behavior

Here's a picture of a snail with a bunch of the worms hanging off of it's head, where they tend to congregate. I've also circled one of the worms crawling along the glass.

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I sampled some of the worms and took a look at them under a microscope. Here's a video ("Video 1" below) I recorded of one of them crawling around. They have some notable characteristics.

First, they appear to have some sort of sucker mouth.

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Second, they have some interesting "legs" tipped with hairs (?) at the ends of their tails. Here's a short video ("Video 2" below) of a worm writhing around that shows them off a little better than pictures do. They appear to be in pairs of two.

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Animated clips from the linked videos:

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Video 1 (left) and Video 2 (right)

The length of the worms ranges from 1-5 mm (edit: they get larger; see below).

Update: After keeping infected snails for a while longer, I believe that these worms only parasitize bladder snails when the worms are young (within the aforementioned size range). Larger, adult worms appear to be free-living and growing up to 1-1.5cm, and will actually crawl away when snails disturb them. Here is a picture of an adult specimen (along with some smaller ones), no microscope required:

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Here's a much better picture of the hair-like legs, of which this adult specimen has numerous pairs:

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Lastly, here's a very interesting detail: These worms are highly specialized, and do not seem to affect ramshorn or pond snails, which I have numerous of in the same tank, in the slightest. It's only the bladder snails that end up infested.

Location

These snails were gathered in Canada, in a still-water area connected to the Oldman River. The river is accessible by cows and deer.

Reproduction

I have witnessed the snails shedding tons of cercariae into the water, which eventually disappear after a couple of hours (reinfect the snails?). I'm not sure if those are related to this worm, or if the snails are also infected with some other type of fluke.

Update: I now believe the aforementioned cercariae are unrelated to the worm in question. I managed to record an adult worm releasing numerous live young, which resemble tadpoles (trochophores?):

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I've recorded a video of the worm giving birth that can be viewed here (left image below). The young seem to be stored in chambers inside of the parent, and then are pumped through the body and out the back end. While the young appear to be still in the video, some of them are quite active (right image below).

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My theory at this point is that the young penetrate snails, grow inside of them, emerge and hang onto the host for a while, and then eventually dismount as free-living adults. Adult worms seem to be independent, while smaller worms hang off of the host the snail (feeding off of it?) until they're large enough to live freely.

Treatment

I would like to eliminate these worms if possible (I can't gather more snails any time soon, and they eventually kill off smaller snails). My first attempt was dosing with praziquantel, which is supposedly effective against flukes while being snail-friendly. Unfortunately, three days later, one of the snails died, and the worms appear to be unharmed.

Update: I gathered some more snails, and attempted praziquantel once again. While it did not harm the snails, it also continued to be ineffective against the worms. I have been experimenting with Seachem ParaGuard, and the results seem to be positive for minor infestations, but not against fully mature worms.

Conclusion

After some research, my best guess is that they're trematodes from the class Digenea-- They appear have sucker mouths, can parasitize snails exclusively, can have a free-swimming form, and can shed cercariae. That said, I'm not terribly confident, and would ideally like to be able to identify them more specifically.

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    $\begingroup$ Hi welcome to Bio.SE! This is a very well thought out and well written question that shows prior research effort on your part and provides useful pictures and specimen characteristics (including size and your location). These are all excellent qualities we hope to see in all of our questions! Thanks. Unfortunately, I find it unlikely that anyone here will know this answer. My hope is that my bounty (though little, given how unlikely I think a good response will be) will attract someone to dive in and do a little more research for a good answer. We'll see. Good luck! $\endgroup$ – theforestecologist Nov 30 '18 at 21:11
  • $\begingroup$ Thanks a lot! If no one here can provide an answer, I'm not sure where else to turn. Anything to narrow it down a bit would helpful. I've been trying to find any parasitic worms that feature these "legs" (or even what the proper term for them is if not that), and so far my search has been futile. $\endgroup$ – 560812508 Dec 1 '18 at 3:39
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    $\begingroup$ I would guess the pseudopod/protoleg looking structures and the mouth parts should be the most helpful in narrowing this down... $\endgroup$ – theforestecologist Dec 5 '18 at 22:04
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    $\begingroup$ The oral sucker and association with snails almost certainly makes this a trematode (aka fluke). The cercaria life stage usually has one or more tails, which I don't see on your specimen. However, I don't know of any adult flukes with the leg-like structures seen in your images (though this old drawing identified as Phyllodistomum americanum appears somewhat similar) $\endgroup$ – theforestecologist Mar 12 at 18:50
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    $\begingroup$ This is single handedly the best series of updates I've ever seen on a Stack Exchange post anywhere. Bravo, mate! your answer is much more possible now, I'm sure (though I admittedly don't know the answer). I am not an expert in these things so I'll have to do a bit of digging/thinking etc. -- which will take time when I can find some -- but I will try to see if I can help you with this. If you don't get an answer from me within the next few months, ping me. Boy, I hope you get a great answer for this post! $\endgroup$ – theforestecologist Jul 3 at 3:51

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