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Sunday, November 17, 2013

Fastest digger in the West

Think of the fastest digging animal you know of.  Now let me introduce you to an animal that might be faster. This is Amphiodia occidentalis, also known as a Long-armed Brittle Star.  

Eric found it under a rock in the low intertidal zone on Bodega Head.  After we identified it as Amphiodia occidentalis, we read this about it:

"...burrowing behavior is spectacular and is easily seen if an animal is placed on fine sand under water." 

Who could resist that?

Because this behavior is better appreciated through video, we're providing an action clip below.  Before you press play, keep two things in mind.  (1) It's tempting to focus on the central disk, but most of the action is happening along the arms.  So it's worth trying to watch the arms and to process how fast they are disappearing below the sand.  (2) We've actually done something a little unusual with this videoit plays forward and then in reverse (all in actual time, not sped up!).  We found that the brittle star disappears so quickly it's hard to understand what happened; seeing it in reverse somehow reminds you just how fast the arms sink below the sand.  [If you're reading this in an e-mail, you may need to click on the title of the post above to access the video on the web page.]

Amphiodia Blog from Jackie Sones on Vimeo.

Once buried, only the tips of the brittle star's arms remain above the surface for suspension feeding.

Here's a close-up of Amphiodia under the microscope:

It's hard to see them, but the amazing burrowing behavior is made possible by tubefeet located on the underside of the arms.  The tubefeet are long and flexible, and when they bend to kick up sand they fit neatly between the arm spines.

Because the tubefeet are transparent it was difficult to photograph them, but I've highlighted two of them with red arrows in the image below.  The upper arrow is pointing to a tubefoot that is bent and in the process of flicking upward.  The lower arrow is highlighting an extended tubefoot; note the smooth texture at the base (close to the arm) and the rough texture at the tip.

According to Intertidal Invertebrates of California, Amphiodia occidentalis may be found in a variety of habitats: under rocks in sand, in protected tidepools, near the roots of eelgrass, or in kelp holdfasts.  If you're lucky enough to find this brittle star, be sure to watch its magical disappearing act!

1 comment:

Jeremy Long said...

Super cool video! Reminds me of the blood brittle star that I studied in grad school (no pubs). They burrow into mud and extend arm tips above sediment surface. They commonly associate with Diopatra tube worms so I tried measuring growth of stars with and without worms. Unfortunately, I saw no growth in either treatment.