Gimme an F! Gimme a U!
Okay, so the thing up there that looks like Grumpy Cat’s ten-legged cousin is a pom-pom crab (genus Lybia). It is less-adorably known as the boxer crab or the get-that-thing-the-fuck-out-of-my-face crab.
Those cute little tufty things on its claws? Anemones. The stinging kind. Which is why the crab put them on the end of its claws. (Just sort of mentally insert a “hwa-CHA!” noise whenever it moves them around.) It uses them to ward off predators and may get an unfair advantage over bigger prey items with them. The actual claws, sans anemone, are completely fucking useless. They’re just these dainty little anemone-holders with a few specialized spines to help hold the anemones in place. When they molt, they don’t even wait until their shells are hard before they pick their anemones back up, because they’re well aware that they’re an appetizer on the half-shell without them.
The anemones, while they’re usually not asked if they want to shack up with a crab first, do get leftover food, companionship, and to see the world out of the deal. Since anemones generally are not real good at running the fuck away from things, and crabs are, we can probably also add “the ability to run the fuck away from things” to the “pros” side of the sheet. The particular sort of anemone favored by the crabs is primarily photosynthetic, and getting buried in shifting sand or stuck in murky or particularly turbid water for a while would fuck it up pretty badly, so this isn’t necessarily a negligible benefit.
They also kind of get assisted reproduction out of the deal, but it would be pretty fucking loaded to call it a good thing. See, anemones can reproduce either sexually, in the standard “spray genetic material everywhere” method favored by sea creatures, or asexually, by fission. Fission here means “the crab rips one the fuck in half and both halves live.”* So if a crab can only find one anemone or loses one of its anemones, the other anemone is probably not a happy camper for a few weeks afterwards.
*You may remember from basic science class that other non-vertebrates can do this, too. Most notably, starfish can be basically slasher-murdered, and it only makes them stronger. A few years back, Australians forgot this most important of lessons and accidentally turned their crown-of-thorns starfish problem into a crown-of-thorns starfish crisis. Like, the starfish went “If you strike me down, I shall become more powerful than you could possibly imagine.” and the Australians went “Oi, mates.” and chopped them up. Then all the pieces grew back as more starfish and ate all the coral ever.