When you tempt it with a chopstick.
Let me explain: every day I tend to my salt-water beasties (now moved from the dishpan to a regular aquarium). I clean the filter, settle any seaweed that has come loose, and check to see if all the visible critters are alive. I keep a chopstick handy; with this, I gently tap any open clam or mussel; if they close, they're alive.
Yesterday, one of the clams had moved over beside a sand dollar shell tipped up against a rock. When the chopstick passed the gap underneath, something slashed out at it, much as a kitten under the bed attacks your toes. I waved the stick again; same result.
It turned out to be a crab. After a few attempts at the chopstick from shelter, he came out and tangled with it in the open, chasing it here and there, grabbing and pinching.
Laying in wait.
Later on, I watched him wander about, eating. He uses those pincers much like a knife and fork, and surprisingly quickly, picking up tiny morsels of food and bringing them to his mouth. The "jaws" chomp away, and the pincers go back for the next bite.
The two white strips (maxillipeds) at the mouth move like sideways lips and teeth.
It was interesting to watch him find a meal. For the appetizer, he scraped at the back of the clam, picking up invisible (to me) specks and eating them. (The clam ignored him, except when he got too near the lip. Then it closed down for a few seconds.)
Then the main dish: he moved out into the open and grabbed a piece of dead barnacle. This he broke in little pieces, chewed on them, and spit out the crumbs.
Occasionally, the current brought a fragment of seaweed past his face. He slashed out at these, the same way he had attacked my chopstick. When he caught one, he held it up to his mouth and chewed away.
Picking some salad greens.
And for dessert: more salad. He went to the back of the aquarium, where a forest of sea lettuce waved above him, raised the pincers and picked himself a few good-sized pieces. Yum!
On to the wandering anemones.
I had always thought that anemones were sessile: once they had settled onto a rock, they were there for life. I was mistaken.
Several of my anemones were anchored on pieces of kelp. With time, the kelp started to rot away.
Small brownish anemones on kelp. The edges are disintegrating.
Plumed anemone on disappearing kelp.
When the kelp was almost gone, the anemones moved to anything solid in the vicinity; the glass, a mussel, a rock. And moved again, and again. Almost every morning I find them in a different location.
And a baby white on another stone. Oregon pillbug (5 mm.) behind it, for size comparison.
Bonus: a small hermit crab, in an Amphissa shell.
Grainy-hand hermit, Pagurus granosimanus.