Risky business

Tonight I was simply going to share a nice photo of a juvenile Short-spined Sea Star (Pisaster brevispinus):

 

And a photo of an adult for comparison — this one was taken on a piling at the Spud Point Marina in October:

 

But when I was evaluating the pictures to choose the best one to share, something caught my eye that raised a question.

I noticed that one of the barnacles next to the sea star appeared to have its cirral fan (feeding structure) extended.

Do you want to try to spot it in the picture below?  It's subtle, but you can see a delicate fan extending upward from one of the barnacles (it's a side view).

 

I've highlighted the feeding barnacle in the next picture.  The arrow points to the extended cirral fan: 

 

Many species of sea stars eat barnacles. So this made me wonder: Can barnacles sense the presence of sea stars?  If so, why would one have its appendages out if a sea star was so close?  Why wouldn't it withdraw its cirral net and close its plates to discourage the sea star? 

Has someone studied this?  I did a quick search on the Internet and found that someone has.  In 1982, Richard Palmer et al. published a paper where they discovered that barnacles (Balanus glandula, the same species shown in these photographs) withdrew and stayed closed longer in response to predators compared to non-predators.

But Short-spined Sea Stars do eat barnacles.  So why was the barnacle in my picture still out feeding?

I don't know the answer, but it was fun to wonder about.