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Wednesday, September 10, 2014

A star from the depths


This was another highlight from our pelagic trip to Cordell Bank on 7 September 2014.  On our way back, about 3 miles from Bodega Head, we pulled up some derelict fishing gear, an abandoned crab trap.  This sea star was inside the trap, so I asked to take a closer look.

Here's a view of the central disk, the prominent spines, and the overall coloration:


I found a good description of the color in a publication on the Asteroidea of the North Pacific and Adjacent Waters by W.K. Fisher in 1928: 

"General tint of dorsal surface olivaceous sepia; spines whitish.  Integument between the thick tufts of pedicellariae deep brown, especially on disk, but little of this brown is visible on the rays on account of the masses of pedicellariae."

So what are these "pedicallariae" he mentions?  I've talked about them before, but let's review.  Pedicellariae are small pincer-like structures found in some sea stars and sea urchins. 

This sea star has so many pedicellariae, and they are arranged in such an unusual way, that I was confused by them when looking at the live animal on the boat.

I'll explain with some pictures.  We'll start zooming in on the rays, or arms.  Can you see how it looks like there are little cushions surrounding each spine?


When I looked at the bases of the spines with my hand lens, I saw a "squiggly" or swirling pattern that I didn't recognize.



Later when I got home and we were working on the identification of this sea star, I realized that all of the "squiggles" at the bases of the spines were large pedicellariae!

This is Stylasterias forreri, an uncommon sea star sometimes called a Long-rayed Sea Star or a Velcro Star.  In most sea stars, pedicellariae are known to have a protective or defensive function.  But in Stylasterias, the pedicellariae can also capture prey, including fish!

Remarkably, the cushions or rosettes of pedicellariae can be elevated along the spines, exposing the dramatic jaws and teeth of the pedicellariae.  Each rosette contains ~35-40 pedicellariae, and there are ~210-245 rosettes per arm, resulting in a total of ~49,000 pedicellariae available for prey capture.

Chia and Amerongen (1975) provided wonderful images of this phenomenon and the pedicellariae (as well as the statistics above).

Here are the pedicellariae in the resting state (top) and elevated state (bottom): 


Modified from Chia, F-S. and H. Amerongen.  1975.  On the prey-catching pedicellariae of a starfish, Stylasterias forreri (de Loriol).  Can. J. Zool. 53: 748-755.


Now you get to see an actual pedicellaria.  (This next image was taken with a scanning electron microscope.)  The pedicellaria is crossed at the base and interlocked at the tip.  Pedicellaria are made of calcareous ossicles and muscles are attached at the base, so they open and close somewhat like a pair of scissors.



And here's a magnified view of the tip, which emphasizes the incredible teeth some for grasping and others for retaining prey.


Both SEM figures above were modified from Chia, F-S. and H. Amerongen.  1975.  On the prey-catching pedicellariae of a starfish, Stylasterias forreri (de Loriol).  Can. J. Zool. 53: 748-755.


Now that you've seen the shapes of these crossed pedicellariae (especially the first scanning electron micrograph), you can look for them on the live sea star (next image).  [Click on the image for a slightly larger view.]


Below I've circled a couple of the pedicellariae where the crossed shape is easier to see.

 
I searched the California Academy of Sciences invertebrate collection online for records of Stylasterias.  There's one record from Drakes Bay from 1950, but otherwise most of the records for California are from the Monterey area.  I'm not sure how often this species has been seen in Sonoma County, but it was lots of fun to learn about it.


P.S.  Thanks to Dana for opening the crab trap and letting us look at the sea star, to Lisa for holding Stylasterias while I took pictures, and to Alan for providing its location.

2 comments:

Anonymous said...

can't help but wonder what they might prey upon ???? c

Jackie Sones said...

Thanks for asking!

I've read that Stylasterias will eat snails and chitons in the "normal" way...and that it has captured sculpins and scallops with its pedicellariae (on the upper surface) and transferred them to its mouth (on the lower surface).

Astrometis sertulifera, a southern sea star with a similar arrangement of pedicellariae, has also been known to capture crustaceans (including small crabs) with its pedicellariae!