ODFW:
Flotsam crab - Planes spp.  The rare pelagic crab

In Oregon, there are not many crabs living anywhere but the bottom of the sea or estuary. Flotsam crabs are the exception.
Two species are found in Oregon: Planes major (formerly P. cyaneus) and Planes marinus. Both species is known for living on floating debris (flotsam), derelict fishing gear, and even upon live animals (e.g. sea turtles).
When found it is often by those looking for glass floats, which these crab often live on, needless to say, Planes crab are a rare find.

aka. Columbus crab - Planes spp.

The species Planes marinus is by far rarer of the two species found in the Pacific Northwest (G. Jensen, pers comm.) This genus of crab is also referred to as "Columbus crabs"
Columbus and his crew, during their journey to the new world “saw much more weed appearing, like herbs from rivers, in which they found a live crab, which the Admiral kept. He says these crabs are certain signs of land…” (Markham, 1893)