|Flattop crab, aka blue-mouth crab, Petrolisthes eriomerus, almost 2 cm. across carapace. Count the legs!|
"Blue mouthparts and blue spots at the thumb joints distinguish this species from the flat porcelain crab. ... However, mouth and claws must be open to show coloration." (Marine Life of the Pacific Northwest)
The same wording, except with the substitution of red for blue, describes the flat porcelain crab, Petrolisthes cinctipes. And they're about the same size, with the same habits, living in more or less the same areas. So I'm glad this one opened her mouth for me.
|Flattop, showing her blue mouthparts, and a hint of blue at the thumb joints.|
These crabs, though they look like ordinary crabs, are not true crabs, but are related to the hermit crabs. Like the "true" crabs and hermit crabs, they are decapods, having 5 pairs of legs, but like the hermits, the last pair of legs is greatly reduced. The hermit crabs use them to hold onto the shell they carry; I don't know if the porcelain crabs have found a use for them. This one has her 5th legs folded up against the top of the carapace.
The flattop is a filter feeder and a street sweeper. She filters diatoms and other food from the water with the hairs on her blue mouthparts, and uses hairy brushes on her pincers (visible only underwater) to sweep edible material from the rocks.
The huge pincers are used for defense or attack, rather than for gathering food, as the hermits and true crabs do. Athough the porcelain crabs discard their limbs easily to escape from predators, the flattop's dropped pincer continues to fight, holding onto the attacker while it's owner scuttles off to safety.