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!  :) 

Golden-edged and fine-lined

  

It's hard to believe that this striking flatworm remains undescribed.  Abby noticed it in the low intertidal zone last week and Eric was able to get some video for documentation (see video clip below).

This flatworm is listed in the Intertidal Invertebrates of California (Morris, Abbott, and Haderlie 1980) as Eurylepta sp.  They didn't include an actual photo, but instead represented this species with a painting.  The colors are a little different, but I think you'll see the resemblance:

 

Modified slightly from Morris, Abbott, and Haderlie (1980) 

 

There have been more recent discussions regarding whether this flatworm is in the genus Prostheceraeous, but at this time we can't locate a reference to cite for that, so we'll just say that it's in the Euryleptidae family.

 

There are only a few records of this species in Marin County, and none that we know of north of there.  Most observations are in the Monterey area and farther south. 

 

If you happen to see one, we'd love to see photos and to hear about other records. 

 

And now for the video!  Eric put together a few clips so you can appreciate the color pattern and the flowing movements of this flatworm.  [If you're reading this in an e-mail, click on the title of the post above to go directly to the NHBH website to see the video player.]

 

 

 

With many thanks to Abby for sharing her observation and to Eric for creating the video!

Seeing stripes

  

Abby spotted a couple of very nice invertebrates in Marin County last week.  

First, this beautiful flatworm (above), meet Eurylepta californica!  Note the maroon streaks on the white background, and the red patches at the base of the tentacles (the two tentacles are upright, but pointing downward, in the lower right corner).  This species is more often found from Monterey south.  There aren't many northern records, and observations at higher latitudes might be associated with warm-water years.

Similarly, the nudibranch Polycera atra is more common south of Point Reyes.  We've seen it in Bodega Harbor, and it's been documented as far north as British Columbia, but the northern records might be more frequent during El Niño years.

 

 

It's a good time to remain vigilant for marine invertebrates associated with warm water.  Many thanks to Abby for documenting these handsome species and for sharing her beautiful photos!

Peanut brittle?

  

Okay, I know I've shown a few peanut worms recently, but this was a nice view of several different peanut worms (Phascolosoma agassizii) extending their introverts from a crevice and using the short ring of tentacles at the end to feed.  You can also see several arms of a brittle star extending out of the crevice and even lying across one of the peanut worms.

P.S.  If you want to play the "phyla game" again, look around to see what you can find.  Below I'll list the phyla in this image.

1. Peanut worm (Phascolosoma agassizii), Phylum Sipuncula
2. Brittle star, Phylum Echinodermata
3. Barnacle, Phylum Arthropoda
4. Limpet (checkered, in lower left corner), Phylum Mollusca
5. Tube worm (orange tentacles, upper right corner), Phylum Annelida
6. Flatworm (gray/white, same species as last night), Phylum Platyhelminthes

Local diversity

  

Eric noticed a beautiful scene in the low intertidal zone a few days ago.  There are six (!) different phyla (major animal groups) represented in this image, all in an area that was only ~4 cm x 4 cm (or ~1.5" x 1.5").  

Feel free to look around and see what you can find.

In the next image, I've circled or pointed to representatives of each phylum.

 

 

Green arrow = sea anemone (Epiactis handi), Phylum Cnidaria
White circle = barnacle (Chthamalus dalli), Phylum Arthropoda
Blue circles = flatworms (unidentified), Phylum Platyhelminthes
Orange arrow = peanut worm (Themiste pyroides), Phylum Sipuncula
Purple oval = tubeworm tentacles (Phragmatopoma californica), Phylum Annelida
Pink arrow = chiton (Tonicella lokii), Phylum Mollusca

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.

Find the Flatworm

Recently, I was trying to photograph a bryozoan (Tubulipora sp.) under a microscope...when I noticed a tiny flatworm nestled among the tubes of the bryozoan!  

I'm not which species of flatworm this, but I thought you might like to see it.  The main body color is white, but there's a nice orange color on the dorsal surface, and you can also see the two black eyes:

 

Can you find the flatworm in the next photo?  It blends in amazingly well.

 

One more "Find the Flatworm" photo.  Can you spot it?  Click on the photo for a larger version. 

 

  

What's it like to live within a bryozoan colony?

Not sure if anyone out there knows the identity of this flatworm, but if you do, let me know!

 

What swims, and has ruffles and horns?

We're back from Sacramento, but it was a long weekend, so here's a picture from September that I haven't had a chance to share yet:

 

Meet Pseudoceros luteus, a beautiful, graceful flatworm.  This image is from Bodega Harbor, but I also received reports of them from Tomales Bay this year.  Jarrett was the first one to alert me to this species in Bodega Harbor — perhaps around 2007?  This year, Pseudoceros luteus seems much more common in the Harbor, perhaps due to warmer water conditions.

I was wondering about this flatworm's scientific name.  I don't have access to a book of Latin names right now, so I'm just guessing that "Pseudoceros" means something like, "false horns."  [Think about a different animal with a somewhat similar name = rhinoceros = "nose horn".]

But the next question would be — where are this flatworm's "false horns"?  Perhaps the name refers to the black tentacular flaps that stick up like small "horns" — they're more easily seen in the picture below.

 

Watch for these flatworms over the tidal flats or swimming by the docks.