The next image shows the entire appendage:
Did you guess crab claw? Do you have any ideas about what type of crab? That's much harder, but once you get to know this species, it's quite distinctive.
Here it is hiding in a crevice in the rocky intertidal zone:
This is a Granular Claw Crab (Oedignathus inermis). Note that the two claws are dramatically different in size. I've often wondered what they do with each claw, but it's not easy to find information about this crab. The next picture isn't great, but it's the best I have right now for comparing the two claw types.
We don't find Granular Claw Crabs that often, but when we do, they're often nestled in narrow crevices or small depressions in the rock. Recently Eric found a few deep within a mussel bed. The individuals he discovered were gravid females — they were carrying embryos.
This female was quite protective of her developing embryos. She carries them on the underside of her abdomen until they're ready to hatch into swimming larvae.
Below is a close-up of the embryos under the crab's abdomen. If you look closely, you can see tiny lime green dots in some of the embryos. I think these are the eyespots of the larvae.
One more view of this unusual crab, showing the shape of its carapace and the very broad, soft abdomen.