Fire coral (genus Millepora): secretly a jellyfish.
Well, secretly a cnidarian, anyway.
These bastards aren’t real corals at all, but they do lead a coral kind of life. Like, you know how cnidarians are usually less a single, whole animal and more a colony of polyps working together for a common cause? I mean, they’re almost more like the HMS Stingyourass than a unified whole. Well, fire corals got the bright idea to do all that within the framework of a calcified external skeleton.
So they basically hide in their little skeleton, hang their stinging cells out to catch shit, drag it inside, and have other, specialized polyps digest it. Much like real coral, fire coral comes in three basic sorts: branching, sheeting, and “we grew on something convenient, fuck whoever was trying to use it first.” Also like real coral, these are far from the only configurations you’ll find them in. Also also like real coral, they don’t feel the slightest bit of remorse when it comes to encrusting other things.
Aside from the skeleton, they’re not that different from other sorts of cnidarians. To reproduce, they release medusae, which then release eggs and sperm into the water column. Free-swimming larvae that develop from fertilized eggs poke around until they find a suitable spot to settle down, at which point they morph into their adult form and park their asses to develop into a new colony.
If you get stung by one, you’re in for a few days to a few weeks of discomfort. Since they look more or less like regular coral, it can be difficult to tell whether or not any given coral is going to sting you. Since touching coral is bad for the coral anyway and is considered a pretty dick move given the rate at which coral grows, everybody wins if you just refrain from touching any coral at all.