Open up!

Down at the very bottom of the intertidal zone, where the water never quite disappears, where the sun never has a chance to heat up their hiding places, where the next infusion of cold water comes in strong and fast, porcelain crabs wait for the turning of the tide. I found a fair-sized one under a rock.

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.

Eating sand

Sammy wears his initials, S.L.G., tattooed on his leg; this is my leg, he says, and I don't want to lose it. He`s already lost three; enough's enough.

Poor. lopsided Sammy. Only one back leg on the far side. The rest were stolen.

See his initials here?

Eating is serious business when you've got three legs and a pincer to build from scratch. And it's difficult when half your eating utensils are gone. Sammy keeps very busy these days, eating, eating, eating; no time for play. But the goodies I feed him never do him much good: the hermits steal them right out of his solitary pincer.

So he eats sand.

Bluish sand grain in Sammy's mouth. Yum, yum!

All crabs do this; so do the hermits. They pick up grains of sand, or pieces of shell, and roll them around with their mouthparts to clean off any edible algae or protein. Then they spit out the sand and grab another mouthful. It's usually a relaxed, contemplative activity, a snack between meals. Sammy makes it his meals, too.

Zooming in. Here he's chewing on bits of broken shell.

It's a good thing that crabs have such a varied assortment of mouthparts. There are brushes and holders and chewers; whatever presents itself, the crab can figure out how to eat it.

Crabs employ 6 pairs of appendages to catch, crush, manipulate, and chew food.  First in order of use are the large claws that catch, crush, and tear apart prey.  From the claws the food bits are passed to 3 pairs of outer mouth appendages, the maxillipeds.  These render and sort the bits, and then pass them to 2 pairs of inner appendages, the maxillae.  From the maxillae the food is moved to a single pair of mandibles where, after a final maceration, it is swallowed. (From A Snail's Odyssey)