Ma crab and the "hairy spine thingy".

The mystery is solved!

We've got a fairly close identification of the "Hairy spine thingy". (First post about it, second post.) A couple of days ago, among other things, various biologists suggested crustacean pleopods, but I was unable to find any to match, and no-one could be positively sure.

Then, this morning, Dr. Tony Irwin, of the Norfolk Museums and Archaeology Service (in the U.K.) posted a link to a previous ID made by the Natural History Museum (London). And there are good photos! I'm not sure about copyright issues here; so here are the links: Female crab abdominal carapace, the part usually visible outside. And F. crab abdominal plate, inside.

The pleopods are the end segments of the six hairy "legs", three on each side, folded towards the centre of the photo. Dr. Irwin says that in a large crab, they would be about 3 cm. long, the length of the piece that I found.

A crab is a decapod, which means that she has 10 legs.

As their name implies, all decapods have ten legs; these are the last five of the eight pairs of thoracic appendages characteristic of crustaceans. The front three pairs function as mouthparts and are generally referred to as maxillipeds, the remainder being pereiopods. In many decapods, however, one pair of legs has enlarged pincers; the claws are called chelae, so those legs may be called chelipeds. Further appendages are found on the abdomen, with each segment capable of carrying a pair of biramous pleopods, the last of which form part of the tail fan (together with the telson) and are called uropods. Wikipedia

Repeating myself from long ago: Simple and logical, isn't it?

Well, sort of. The legs and the mouthparts and pincers, which also count as legs for the name, are on the thorax. The abdomen, the part that crabs fold underneath so that they look as if they were all one piece, has its own set of "pods", but they aren't legs, so don't count for the "decapod" name, even if they are built more or less on the same pattern.

Males have two sets of pleopods, which they use to transfer sperm to the female. Females have several sets. These are used to hold the eggs under the abdomen while she is in berry, to fan them and oxygenate them.

I found a couple of photos of local crabs where the pleopods are recognizable:

Dungeness crab, a large crab, up to around 20 cm (8 inches) across the shell. 

"Hairy spine thingies", aka pleopods under her abdominal plate.

One of our small purple shore crabs, also showing off her pleopods. (Not very happy about it, though.)

Her abdominal plate, as it would normally be seen.

(All these photos are from Walla Walla University, Department of Biology.)

So there we have it. My "hairy spine thingy" is a pleopod of one of our larger crabs. I still want to find a dead one and confirm the species, if possible.