Blue-mouthed flattops

Yesterday, I called them "blue-mouthed crabs". But they aren't crabs at all. At least not "true" crabs. They're more closely related to the hermit crabs, which also are not "true" crabs.

Decapods, certainly. But these porcelain "crabs" have only three pairs of walking legs;  the others, like the last two pairs of the hermits', are much reduced; porcelain crabs carry them tucked up at the back of the carapace.

And true crabs have short antennae; the porcelain's, like those of a hermit, are long.

And they have tail fins that they can use for swimming.

This one, the one with blue mouthparts, is a Flattop crab, Petrolisthes eriomerus. The porcelain crabs, in general, are flat, even the chelipeds; they live squeezed between and beneath rocks.

And they are filter feeders; which explains the constant waving of blue mouthparts.

Filter feeds (mostly diatoms) using long setae on its second and third maxillipeds, and also uses the setal tufts ... on its chelipeds to sweep up material from rock surfaces. (invertsWallaWalla) 

 And here's one I found on the shore 10 years ago; find the 4th pair of legs.

Out of water, the blue mouthparts are barely visible.

From that post: 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.

Also:A dropped claw may continue to move to distract a predator while the crab makes a getaway. (Texas  A&M U.) 

 ~~~~~~~~~~~~

Ayer, les di el nombre de  "cangrejos con la boca azul". Pero no son cangrejos. Por lo menos, no son cangrejos "verdaderos",  sino que son más bien primos de los cangrejos ermitaños. Que tampoco son cangrejos "verdaderos".

Sí, son crustáceos decápodos, como los cangrejos. Pero estos "cangrejos" de porcelana usan solamente tres pares de patas para caminar; las últimas son muy reducidas y las llevan dobladas contra el carapacho.

Y los cangrejos "verdaderos" tienen antenas cortas; las del c. de porcelana, al igual que las de los ermitaños, son largas.

Y tienen aletas caudales que les ayudan a nadar.

Este, el que tiene los maxilípedos azules, es un Cangrejo Plano. Los cangrejos de porcelana, generalmente son aplanados; lo que les sirve para esconderse bajo y entre las rocas.

Y se alimentan por filtración, lo que explica la agitación continua de los maxilípedos.

Video: un "cangrejo" de porcelana en el acuario.

Se alimenta por filtración (principalmente de diatomeas) usando setae largas en el segundo y tercero maxilípedos, y también usa los pelos ... en sus quelípodos para barrer las superficies de las rocas. (invertsWallaWalla) 

 Foto: Aquí está uno que encontré en la playa hace 10 años; busca las patas traseras.

Escribí entonces: Las pinzas grandes se usan para defenderse o para atacar, no para adquirir alimento,  como lo hacen los ermitaños y los cangrejos verdaderos. Aunque los cangrejos de porcelana frecuentemente mudan las extremidades para escapar de los predadores, la pinza perdida de un "Flattop" sigue peleando, deteniendo al predador mientras su dueno corre a esconderse.

También:  "Una pinza desechada puede seguir moviéndose al fin de distraer al  predador mientras que el cangrejo logra escapar. (Texas  A&M U.) 

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.