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.