I turned this rock with TONS of sea spiders writhing underneath. As far as I'm concerned, sea spiders are pretty close to being the creepiest-looking animals on the planet, and yet they are awesome (mostly due to their creepiness), so I felt an odd mixture of delight and revulsion. Sort of like laughing and throwing up at the same time (except not really).

I'm puzzling over the species here. I know it's not Pycnogonum, the only other genus I've ever seen, because it has palps. Given this list and the pics and descriptions in Light's, I think Ammothea hilgendorfi seems like a possibility, but my pics aren't really good enough to count segments. There definitely aren't chelae (though there might be chelifores), and it did have ovigers.

Found in the San Francisco Bay, Albany Bulb, Albany, CA, swarming with many others under a rock.