Elusive anemone

For about 7 years, an old abalone shell, purchased in a garage sale years before, has been a fixture in my saltwater aquarium. At first, it was a place for hermit crabs to stand tall and watch the world go by. Later, crabs found the inner surface to be a good hiding place. Now, the plumose anemone calls it home.

Over the years, it has lost its pearly sheen; it is gradually dissolving, and one side has a great hole, crumbling at the edges. The outer part of the shell is riddled with pores, each one housing a tiny, two-tentacled worm. Limpets browse among them, hermits pick their way across, on their way to the eelgrass bed and climbing ground.

One of the worms, a Siponid. The tentacles are, at most, 5 mm. long.

A couple of years ago, I noticed several miniature anemone-like critters, extremely small, almost invisible unless the light was just right, on the shell. They have a circle of a few tentacles, a dozen to twenty-something, with alternating clear and white patches. The column is never clearly visible.

I've watched them, always under bright light, with a lens; with the naked eye they're barely there. Tonight, with all the lights on and a bright flashlight, I spent 15 minutes examining their favourite spots before I found one.

They seem to move about; I see four or five in one spot, and watch them for a few days, and then can't find them. Later, they show up in another area, but always on the abalone shell. I've never seen one anywhere else. I've looked.

And I've tried, over and over, to get a photo; they're even more elusive under a flash. But the other day, with the abalone shell a mere inch from the glass, a couple showed up clearly.

Mini-anemones, with two-tentacled worm in background. Approximate size, maybe 1 cm. across, tentacle tip to tip. They never get any bigger than this. Contrast increased slightly.

I can't find these in my Encyclopedia, nor in any website; they're too small to be noticed, maybe. They should be easy to identify (more or less) by the size, the limited number of tentacles, and the white spots, so I've browsed hundreds of photos, but without seeing any.

(I found a similar one, here, towards the bottom of the page, also unidentified.)