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.

A confusion of pointy bits

"All decapods have ten legs..." (Wikipedia) Sounds so simple. And when the decapod in question is a crab, it is: five legs on either side, and a bunch of appendages around the face for eating, seeing, and smelling.

When it's a shrimp, however, things get confusing. The "walking legs" include the chelipeds, or pincers (two sets), which are right next to the maxillipeds, which Wikipedia counts among the five, but which would make up six on the shrimp. They also count as mouthparts. Then there are the 5 pairs of swimmerets; in diagrams, they look like legs. Luckily, in real life they're small, and mostly hidden under the tail segments.

I've been trying to count the legs on the largest of my shrimp, now that he's got a bit of colour. It's difficult. Maybe if he would just stop moving for a moment ...

Counting from the rear; 1 2 3 4 ... Aaargh!

Some things are easy; he has two sets of antennae, one long and one short. The long ones are very fine, transparent, always flicking about, and maybe twice his length. Can you see them here?

Photo cropped at tips of antennae.

5 legs countable. And with the chin up, separated from all those sharps on the head.

Let's name them.

Diagram from Louisiana Fisheries

So we've got 5 walking legs, the front two of which are also chelipeds, because they have pincers on the tips. Small ones, only visible in a few photos. If you look closely at the centre of the first clump of feet on the photo above, you can see one in the background.

Then there are the maxillipeds, which would correspond to our jaws (Maxilla): they would be the two straight blades sticking forward at the mouth.

Along the top, toothed like a saw blade, sharp as a spear, is the rostrum.  On either side of it are the two stalked eyes (another set of appendages), and the shorter antennae. (So far, so good.)

But then, what are the two flags he waves above his head? And what do they do?

Top view. Look at it full size (left click - open new tab) to see the various tools he's carrying.

The little shrimp, half the black one's size, are just beginning to show their stripes, and pick up a bit of colour. They wave tiny flags, too.

1/2 inch long shrimp. His innards are easier to see than his head accroutements.

Stretch and grab

Hermit crabs, like "true" crabs, are decapods; they have ten feet. Usually we see only 6, counting the chelipeds (pincers) as feet. The two pairs of back legs stay deep inside the chosen shell, holding it in place.

Grainy hand hermit, Pagurus granosimanus.

A couple of days ago, I caught a small hermit half-way out of his shell, stretching out to snatch another hermit's meal. In one of the photos I took, I got a clear shot of two of the back legs.

"Gimme!"

They are quite a bit shorter than the front legs, barely the length of one segment. When the hermit is wandering naked, looking for a new shell, he holds his abdomen tightly coiled, with the 4 tiny legs barely visible. Here, they're stretched out to keep a grip on the shell; he'll retreat 'way back into it, out of reach, once he gets that food.

Green nudists

I missed it again. I've been hoping to catch one of my hermit crabs in the process of molting, but no luck. I wish they'd notify me when they start feeling itchy!

This afternoon, two hermits molted. I checked on the tank in the evening, and there were the two sets of legs and two carapaces abandoned on the sand. A bit of searching turned up a small one high up on the sea lettuce, and a larger hermit hanging on an eelgrass leaf towards the floor. And both of them were naked; they hadn't got around to finding new shells yet. I ran for the camera.

Here's nudist # 1.

The smaller hermit was well camouflaged, but this little guy was in the open, right by the glass. And he was showing off his shell-clutching legs:

Hermits are decapods; they have ten "legs". Of these, only 6 are walking legs. The first pair is tipped with pincers, one large and one small; the second are clawed legs; the third pair has no claw. The last two pairs are reduced to hooks that hold the borrowed shell. Three of these legs are visible here, sticking out at almost right angles from the abdomen.

The abdomen is soft and unprotected. While the hermit rests after the molt, he is vulnerable to predation or accident. When a second hermit came along, this one left the eelgrass, scuttled across a sand dollar test, and hid underneath. When I checked a quarter of an hour later, he was safely settled into a nice shell, bigger than the one he'd left behind.

Now, four hours later, the little one high in the sea lettuce is still resting without a shell. He must feel safe there, a green coiled youngster wrapped in silky green seaweed.