Tiny settlers

A few days ago, Eric and I discovered some tiny crabs hiding among the branches of a clump of Green Pin-cushion Algae (Cladophora columbiana).  

 

Here's an even closer view of the crab.  Look at those beautiful bronzy eyes...and the wonderful red and yellow chromatophores.  My best guess — a juvenile porcelain crab (Petrolisthes sp.), but let me know if you think otherwise.

 

You might be familiar with Green Pin-cushion Algae from the high intertidal zone.  It looks like patches of dark green moss:

 

So there are a couple of questions about the crabs we found in these algae clumps.  Were the crabs already in the algae while it was growing on the rocks, and then they were swept out to sea with the algae when it was dislodged?  

Or, was the algae drifting offshore and the crabs found it there during their larval stage — i.e., the crabs settled out of the plankton onto the drifting algae, thinking it was suitable substrate?