Here's a series of pictures to introduce this wonderful crab. I found this one on a small piece of driftwood washed up on the beach. I believe this is Planes major (formerly Planes cyaneus).
Flotsam crabs are relatively small — the length of this individual's carapace was less than 1 cm:
These crabs are good swimmers. Their legs are specialized for swimming — long setae ("natatory hairs") on their legs increase surface area for propulsion:
Flotsam crabs are often brown, but they can also be blue (and given time, they can change color). The color is dependent on chromatophores. In this picture, some of the chromatophores are sparkling blue:
Not only were the chromatophores spectacular, but check out the colors in their compound eyes!
Flotsam crabs live on floating objects and therefore are often associated with other organisms such as pelagic gooseneck barnacles (Lepas spp.). [Although large barnacles may provide shelter, small barnacles might be eaten by the crab.]:
Flotsam crabs are apparently unafraid of people (they probably don't encounter that many in their typical open ocean habitat). I'd read that they would do this, and here's an example of one crawling off its float and onto my thumb:
Can you imagine what it would be like for a flotsam crab to be holding on to a piece of driftwood, or a glass float, or a piece of kelp, floating far offshore in stormy seas?
The status of flotsam crabs is somewhat of a mystery in Northern California. They're not reported that often, but perhaps they're being overlooked? If you see driftwood (or other floating objects) washing up on the beach, especially with pelagic barnacles (a clue that it's been at sea for a while), keep an eye out for these charismatic crabs!
P.S. If I can get some footage of this crab swimming, I'll try to add a video clip, too.