This is a plate coral (Fungia sp.).
It’s not an anemone, though people sometimes mistake it for one. Plate corals live in the sand, and despite their sessile appearance they do have the ability to move. (Note: ability does not always equate to desire. This one is perfectly happy to remain where it is.)
Fungia expand and contract periodically, extending their tentacles when “happy” or to feed. The orange spot in the center surrounds its mouth (which opens much wider than I originally expected).
I’ve seen this one catch and eat a live ghost shrimp whole. It also eats frozen mysis, brine shrimp, and anything else it can get its tentacles around. It takes about a minute for the coral to pass food from the tentacles to the mouth (less when it’s really hungry) and it’s fascinating to watch.
For the most part, the mobile residents of the tank give the fungid a fairly wide berth. Phobos and Deimos, the skunk cleaners, steal food from it so I feed them separately to avoid arguments.
Johnny, the Starry Dragonet, considers the fungid his personal plaything. He studied it carefully upon its arrival and then began leaping back and forth across it, as though he considered it a game. Johnny eventually returned to his normal scooting around, but he still hops over the coral regularly and seems to derive no small amusement from it.
Johnny even slept in the sand beneath the fungid the first night it spent in the tank. I’m not sure whether he thought he was guarding it or vice versa. He’s also been known to “defend” it against other fish that come too close, though I suspect that’s less about the fungid and more about Johnny’s attitude.
Either way, it’s amusing to see.
Aquarium keepers – do you keep fungia? Non-fish-keepers – what do you think? Hop into the comments and let me know.