Keeping a low profile

 

A beautiful flatworm that we've encountered a couple of times this year.  Originally we thought it might be Acanthozoon lepidum, but we now think it's Pseudoceros canadensis.  [Longer story, but there appears to be an error in the flatworm key in The Light and Smith Manual.  See corrigendum at the end of this post.]

With a close-up view (below) you can see both the marginal tentacles (sometimes called pseudotentacles because they're simply upturned folds of the margin) and two clusters of eyes.  

The main cluster of eyes (cerebral eyes) appears as more than 20 black dots a bit behind the marginal tentacles (they're situated over the brain of the flatworm).  The second cluster (marginal eyes) is harder to see, but look closely and you'll see tiny black dots scattered between the marginal tentacles (click on the image for a larger version).

 

I'm a little puzzled by this flatworm.  It's the first time we've seen this species, and we found it twice within the last month in two different places.  Has it been around and we just haven't encountered it?  Is it more common this year?  And if so, why?

Eric and I took some nice video of one individual and put together the clips to show the wonderful body movements as well as some of the important characteristics.  Enjoy!  (If you can't see the video below, click on the title of the post to go directly to the web page.)

Pseudoceros canadensis from Jackie Sones on Vimeo.

CORRIGENDUM (15 August 2018): I've changed the name of this flatworm from the original post.  We were confused about why we couldn't find any other records for Acanthozoon lepidum.  So we continued to look around and Eric finally encountered photos of a similar flatworm identified as Pseudoceros canadensis.  Although the key in The Light and Smith Manual says that Pseudoceros canadensis has a smooth dorsal surface (as originally described), this species was later re-described with dorsal papillae.  The flatworm in these photographs has dorsal papillae (look for the small bumps in the video close-ups).  So we followed the key to Acanthozoon, but we now feel that the key is wrong.  I've corrected the name to reflect that we now believe this is Pseudoceros canadensis.