A farewell to arms

We were at a rocky intertidal zone site in northern California this morning and started noticing Six-armed Sea Stars (Leptasterias sp.) with fewer than six arms.  Here's one with three arms:

 

We also observed many Six-armed Sea Stars that were regrowing lost arms.  Below is an example of a "comet"— an individual that has one remaining original arm (the longest), with the rest of the arms regenerating:

 

We were curious about the cause of arm loss, when Eric discovered some possible evidence:

 

This Six-armed Sea Star was being ingested by another sea star.  

And the responsible sea star was...

 

...a Leather Star (Dermasterias imbricata)! 

We found at least three examples of Leather Stars eating Six-armed Sea Stars.  In one case, the Six-armed Sea Star had been completely digested.  But in two cases, some of the Six-armed Sea Star's arms had yet to be ingested.  We're wondering if it's possible that sometimes enough of the Six-armed Sea Star survives the predation attempt and escapes to regrow the arms it has lost.

If you're not familiar with Leather Stars, here's a photo from above:

 

Although it doesn't sound like this is the first time Leather Stars have been observed eating Six-armed Sea Stars, we're not sure how common it is.  (Leather Stars often eat sea anemones as well as a variety of other prey.)