Cassiopea

I know what you're probably thinking. You're thinking this is a sand anemone, right?

The sort of thing which hosts cute little clownfish in the middle of nowhere. If that is indeed what you were thinking, you are wrong for 'tis no anemone. It is in fact a jelly doing a rather good anemone inpersonation. This is a Cassiopea or upside-down jelly.

These are fairly common in shallow bays and mangrove areas around the tropics and are one of the few jellies that can be maintained in standard aquaria.