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Saturday, May 17, 2014

Adding on

In December 2013, I posted about an 8-armed Sunflower Star (Pycnopodia helianthoides) and a 10-armed individual.  To review those images, see the posts from 17 December 2013 and 31 December 2013, respectively.  

I mentioned that Sunflower Stars add two arms at a time, and may end up with a total of 20 or more arms.  So you might be able to guess what's coming next!

It depends a little bit on exactly how you count, but this juvenile Sunflower Star has at least 12 active arms.  It also has two developing arm buds, so some people may count 14 arms.  The entire sea star, from arm tip to arm tip, was approximately 4 cm (1.5 inches) across.  It was photographed on 17 May 2014.

Remember that Sunflower Stars add two arms at a time and they add them bilaterally, or one on either side.  In the picture above, arms 11 and 12 are distinctive because they're shorter than all of the other arms, but they still have active tubefeet.  In the image above, they're at the top and bottom of the sea star.

Arms (or arm buds) 13 and 14 are very small.  They are adjacent to arms 11 and 12.  They have very pale tips.  

Below are two extreme close-ups that I hope will help you see all three arm typesthe regular-sized arms; arm 11, which is a little shorter than those; and arm 13, which is a tiny bud just to the right of arm 11.  [If you're wondering about the white circular disc on top of the sea starit's the madreporite (or sieve plate) that drives the sea star's water vascular system.]

It's been a lot of fun to see these Sunflower Stars growing up over the last 6 monthsfrom about 14 mm with 8 arms in mid-December to 40 mm with 12 (or 14?) arms in mid-May.

Here's one more picture of this recent individual exploring a local tidepool:

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