Mom and Alden and I spent some time checking it out, and then thought we really should help it get back down to the water, where we thought it really needed to be. Poking and prodding didn't do much, and eventually I picked it up and took it down to the water. Even that did not produce much reaction.
We continued to be worried.
Eventually we got it fully out and submerged in the water, and it did swim off, with some energy. But, as I said, we were worried, it just did not seem to be doing what a crab should do...
Later, Catharine told a story of seeing a sad crab, during her late afternoon swim, being eaten by a seagull. Mom and I had a pretty good idea of who that crab was. We were a little worried about telling Alden, but thought we needed to tell him the truth (and when we had been hanging out with the crab earlier, we had told him we were worried about it, so we hoped it would not be too much of a blow to hear the end of the story).
His reaction was fantastic - mature and philosophical:
"well, seagulls need to eat, too."