Mystery below the sand

A few days ago, Eric and I were walking on Salmon Creek Beach and noticed an unusual pattern in the sand.  Most of the sand we'd been walking by had been very smooth, but this patch was marked with lots of nooks and crannies:

 

We decided to take a closer look to try to identify what was causing the pattern.  We started by swiping away the top layer of sand to see what was below.

This revealed a very high density of small animals!  Can you tell what they are?

 

To confirm their identity, Eric scooped up a small handful of sand and poured a little water over it for a better view of the animals:

 

Ah ha!  What an amazing number of juvenile Mole Crabs (Emerita analoga)!

I can't resist showing a few more pictures:

 

They burrowed into the sand very quickly.  Look for the paired dark eyes among the sand grains:

 

The next image is a little blurry, but I like it because Eric's fingers provide a sense of scale.  Each Mole Crab was ~5 mm long.

 

The entire roughened patch in the sand was about 6 meters long by 2 meters wide.  If the density of juvenile Mole Crabs was consistent across the patch, there were a lot of Mole Crabs in that area!

This made me think about the Sanderlings we observed feeding so intensively — see post from 24 March 2014.  Peter mentioned the possibility of them feeding on small Mole Crabs.  There are other options (e.g., amphipods or isopods), but it's intriguing to discover such a high density of juvenile Mole Crabs and to wonder about their vulnerability to shorebirds.