Acorns in Sonoma County

  

During the past 6 months, Eric and I have occasionally observed a bright orange animal among stones/gravel in the intertidal zone.  At first we thought it might be a ribbon worm, but something didn't seem quite right.  Eventually we figured out that we were seeing our first acorn worm in Sonoma County.  Meet Saccoglossus pusillus!

Acorn worms are probably unfamiliar to many people — including me!  They're in the phylum Hemichordata and are really different from other animals you might encounter in local tidepools.

Here's a basic illustration of their body plan.  They're worm-like (vermiform), but note the three major body sections — the proboscis, the collar, and the trunk:

 

Modified from Biology of the Invertebrates (Pechenik 2015)

In the first photo (above), you're seeing the long, highly extensible proboscis.  Before seeing these individuals, I hadn't realized how far the proboscis could stretch.

Behind the proboscis is the much shorter collar.  The acorn worm's mouth is hidden inside the collar.  In this species, the collar is darker in color, so somewhat reddish.  In the next two photos, you can see just a hint of the collar where the proboscis meets the substrate.  [Acorn worms live in burrows below the surface, so the trunk isn't visible in these photos...but see the video clip at the bottom of this post.]

 

 

 

 

 

The proboscis is covered with active cilia (tiny hair-like structures).  This species of acorn worm is a deposit feeder — it uses mucous and cilia to gather sand grains and other materials from the bottom, swallows some of it, digests organic material from the surface of the sand, and then expels the undigested materials.

What's cool is that you can see these acorn worms gathering sand/shell fragments and moving them along a narrow groove on the proboscis that functions like a conveyor belt to bring food towards the collar/mouth:

 

 

 

 

 

 

 

Another remarkable acorn worm feature is visible in the short video clip below.  Acorn worms have rows of gill slits along their pharynx — this is behind the collar, in the upper portion of the digestive tract.

After traveling down the proboscis, the mucous food strands are moved through the mouth and excess water is ejected through the gill slits.  [While they play a role in feeding, the gill slits are also used for respiration.  Because of this, acorn worms are in a group of animals known as enteropneusts, "entero" = gut and "pneust" = breathing.]

 

 

P.S.  Another fascinating fact about acorn worms — believe it or not, their closest relatives are echinoderms (such as sea stars and sea urchins)!  

P.P.S.  Many thanks to Eric for an excellent video clip!