Beachcombing

After four intensive days of surveying the island of Nantucket for galls and leaf mines last September, Noah and Sydne and I headed to the beach to bask and unwind.  I am mostly a forest creature, and I am ignorant of most things I see on the seashore, but after consulting some field guides (and with a correction from Ben Coulter) I can report that the birds busily scurrying around before us at the water’s edge were semipalmated sandpipers, with a few ruddy turnstones mixed in.

Semipalmated Sandpiper (Calidris pusilla)

Ruddy Turnstone (Arenaria interpres)

Well, maybe a bunch of them were sanderlings, actually–I think that’s what the ones in the last photo are.  Anyway, whatever they were, they were evidently finding lots to eat at the water’s edge, but I sat and stared right where they were feeding and could see no signs of life.  Finally, I tried blindly scooping up handfuls of sand.  More often than not, a handful of sand contained one of these:

This is a young mole crab (Hippidae: Emerita talpoida).  It’s pretty darn well camouflaged even when placed on the surface of the sand, which is not where they usually are.  I had seen these once before, three years earlier on Cape Cod.  Standing at the water’s edge, Noah had showed me these creatures (which he called “sand digglers,” and we still prefer to call them that even after learning their “real” name) that were only visible right after a wave had crashed down: in the receding water they could be seen quickly reburying themselves, digging in with their hind legs.  (They were easier to spot, being much larger–adults, I imagine–than the ones I found on Nantucket.)  For a few minutes we had been under the impression that bubbles in the sand indicated the presence of a buried mole crab, but after further investigation concluded that we could just as reliably produce one by digging where there was no bubble.  Both of these experiences with mole crabs suggest that there are lots and lots of them lurking just below the sand surface, enough to keep the sandpipers occupied indefinitely.

A mole crab disappearing into the sand on Cape Cod.

Satisfied that I had solved the mystery of what the shorebirds were feasting on, I was returning to my station on the sand, when I stumbled on another probable menu item:

This little crustacean is known as a sand flea or sandhopper, and it belongs to the order Amphipoda.  Its very similar-looking relatives in freshwater are called scuds.  I was suddenly in the middle of a big group of amphipods, bouncing around like popcorn.  And, just as suddenly, there were none.  The amphipods do higher up on the beach what the mole crabs do at the water’s edge: very quickly make themselves scarce, before the next wave of sandpipers comes sauntering by.

An amphipod disappearing into its burrow.