New to the north?

  

Abby wrote to let us know she had observed an unusual flatworm on 17 June 2026.  Amazingly, we encountered what looks like the same species in a different location on 20 June 2026.  Both were found along the rocky shore in the low intertidal zone.

We think this is Stylochus franciscanus, although it's a new species for us, so we're open to feedback.

There are two flatworm species that apparently look similar but differ in subtle ways.  For example, Stylochus franciscanus has marginal eyes that only extend ~1/4 of the way along the perimeter of the body, while Stylochus californicus has marginal eyes that extend along the entire perimeter.  And the tentacles of Stylochus franciscanus are closer to the anterior margin vs. farther from the margin in Stylochus californicus.  See illustrations below (modified from Libbie Hyman's 1953 account of flatworms in California -- see a photo of Libbie in this post):

 

 

 

Because both of these flatworms are unusual in northern California, we documented this individual with a microscope video (below).

 

In the video clip, watch for several features: 

- an overall pattern of lighter and darker brown spots

- at 18-20 seconds, you can see the marginal eyes (tiny black dots) along the margins, but only at the anterior/head end

- at ~30 seconds, note the pair of transparent nuchal tentacles that are relatively close to the anterior margin

- at ~38-40 seconds, a head-on view shows the clusters of cerebral eyes (black dots) *within* the nuchal tentacles (!)

 

 

Stylochus franciscanus records primarily come from San Francisco Bay and the central and southern California coasts.  From what we can tell so far, these are the first records north of San Francisco (although we need to do additional research to confirm the distribution records).  

 

With many thanks to Abby for alerting us to the presence of this species in Marin County and to Eric for capturing and editing the video footage. 

P.S.  If you can't see the video player in the e-mail message, click on the title of the blog post to go to the NHBH website to see the video.

P.P.S.  Fun fact -- "franciscanus" in the species name likely refers to San Francisco Bay, as the first specimens used to describe the species were collected from locations within the Bay.

P.P.P.S.  If you happen to see this flatworm north of Bodega Bay, we'd love to hear about it/see photos!

P.P.P.P.S.  Happy summer!  :)