Dining out on the Innkeeper

I received a few inquiries about mystery animals washing up on Doran Beach during the last few days.  Liz thought they could be innkeeper worms, and Phil sent a photo that made it look like a possibility.  Both of them mentioned that gulls were eating the worm-like animals.  

It seemed worthwhile to confirm the identity of these animals, so I took a quick look today.  Sure enough, quite a few innkeeper worms (Urechis caupo), also known as spoon worms (echiurans), were washing up on the outside of Doran Beach (the bay side, not the harbor side), fairly close to the North Jetty.

Here are a few documentary photos of the gulls (primarily Western Gulls) taking advantage of a food resource not often available to them: 

 

 

 

 

And here's a close-up of an innkeeper worm washed up on the sand, before a gull encountered it:

 

This innkeeper worm was not in its best form — that is, they normally live in U-shaped burrows in the mud, and I'm guessing it had been out of its burrow for while and that its condition had deteriorated while washing around in the surf.  But you can still see a few important characteristics — the pink coloration, the sausage-like shape, and the golden setae (bristles) at both ends.  The ring of bristles at the posterior (back) end are quite prominent in good light, while the two bristles at the anterior (front) end are harder to see.

Perhaps you're wondering about why they're called innkeeper worms?  I mentioned they live in U-shaped burrows.  A few other animals are resident alongside the innkeeper worms in their burrows — see diagram below:

 

From Fisher, W.K. and G.E. MacGinitie.  1928.  The natural history of an echiuroid worm (Urechis).  Ann. Mag. Nat. Hist. 10: 204-213.  (a) innkeeper worm, (b) scale worm, (c) pea crab, (d) goby.

So the remaining question with this current observation is: Why are the innkeeper worms washing up on the outside of Doran Beach?

Have you seen this phenomenon before?  Do you have ideas about what could be happening?

As far as I know, most of the innkeeper worms live on the mudflats in Bodega Harbor.  Do some of them live on the outer coast?  Or, were they somehow flushed from their burrows in the harbor and then washed around to the bay side and concentrated near the jetty?  If so, how did this happen?  Could it have been related to the storms last weekend?  Did the heavy rain cause channel scouring that removed some innkeeper worms from their burrows?  It is possible they were impacted by fresh water?

Lots to wonder about.  Let me know if you can think of other scenarios that would concentrate innkeeper worms in this way.

Shake it, shake it

Tonight I feel very fortunate that I get to introduce you to such a charismatic invertebrate.  Some of you are probably familiar with spoon worms, a group that includes innkeeper worms (Urechis caupo).  If you studied marine biology on the West Coast, it's likely you learned about them in class or saw them in their U-shaped burrows on a tidal flat.  If you're a fisherman, you might harvest them for bait.  If you aren't yet familiar with spoon worms, don't worry!  I'll write about the adults in the future.  

But a couple of weeks ago, we encountered our very first larval spoon worm!  And I'm guessing many of you, even if you're familiar with the adults, have never seen the larval form of these fascinating marine worms.

So here you go — a spoon worm trochophore!  (It's ~1.6 mm long.)

 

The diagram below illustrates some of its major features: 

 

Modified from A Guide to Marine Coastal Plankton and Marine Invertebrate Larvae (Second Edition) by Smith and Johnson (1996)

• The prototroch and telotroch are ciliary bands — they are very active and aid in swimming.  
• We're pointing out the location of the mouth, but it's not easy to see in either the images or the video (now you know what's coming!).  
• The epidermal rings are quite noticeable, but note that they are not true body segments.  Unlike polychaete worms, spoon worms are unsegmented.
• The bonellin pigment spots are special.  Their green color is distinctive and they're unique to spoon worms (although not all spoon worms have them)!  The mysteries of bonellin are still being studied, but it's thought that it might act as a toxin, an antibiotic, and that in some cases it's involved with sex determination.

Spoon worms have an impressive hydrostatic skeleton — muscles in the body wall work against internal fluid — which means the whole body is involved in dramatic waves of peristaltic contractions.

You can see this in the amazing diversity of shapes in pictures of the larva taken only seconds apart — see sequence below:

 

 

 

You can appreciate this even more by observing the spoon worm larva in action!  Watch for a few things in this video: The whirling ciliary bands, the waves of contraction, the green bonellin pigment spots...and then after the larva undergoes metamorphosis into a juvenile worm, look for 2 setae (bristles) in the mid-section and the formation of the proboscis (the spoon!) at the tip.  (Then keep reading for one more highlight.)

Echiuran larva and juvenile from Jackie Sones on Vimeo.

Okay,  I know that video was fun, but this one is even better.  I'm guessing many of you have been waiting for the latest music video from Spineless Studios.  Don't miss this one!  You'll never see spoon worms the same way again!

http://www.youtube.com/watch?v=gv4RQQkhRaA 


P.S.  Many thanks to Carol for sharing her plankton sample that included this wonderful larval spoon worm!

P.P.S.  For anyone who's wondering, we think this is Listriolobus pelodes, but we're still contacting experts to help confirm the species identification.