In brief, it still has me (us) stumped. History, to date:
The post with the photos and description (here) was cross-posted to my Facebook timeline and my Facebook page. A friend there (JG) suggested that I try the burn test for fabric. Was it vegetable or animal? I broke off a tiny piece and burned it; it smelled of hair or feathers. Animal, then.
Another friend (Fred Schueler from the U. of T.) asked the TAXACOM list of experts for help. They came up with various suggestions, from feathers to the tip of a whale's baleen plates, to an abdominal appendage of a crab.
"Crustaceans are jam-packed with wacky appendages:" - Doug Yanega (Entomology).They sent me off to search crustacean pleopods. Some of the images look sort of, almost, but not quite,* right.
JG then volunteered to take the photos to the Marine Biology meeting of Nature Vancouver. I took more photos for her, destroying another tiny piece of the sample to look at its "innards".
|I de-haired two segments as well as I could.|
|It turned out to be a semi-hollow tube. Here's the inner lining.|
|One side has a rough section, that on closer examination looks like a groove lined with some sort of fibers.|
|The inside of a broken segment. This was very fragile, and snapped like a too-thin eggshell. From the inside, what looked like a dark line turns out to be transparent; the dark colour came from the inner stuff.|
The Nature Vancouver marine biologists suggested e-mailing Andy Lamb, one of the authors of the Encyclopedia I use, Marine Life of the Pacific Northwest. I just now got an answer from him: he's stumped, too, and is forwarding the lot to several other people.
So that's where we are now. My next step is to go to the beach at a lowish tide and find a large, dead crab, with all his appendages, and examine them. I'll report then, or as soon as anyone else provides an identification.
A big "Thank you!" to all who have helped so far!
*Pleopod of female blue crab. Not on this coast.