Here’s a bit of reef irony, the porcelain crab (Neopetrolisthes maculatus) lives on carpet anemones where they can hide from potential predators. Their carapace protects them from the harmful nematocysts that might otherwise deliver a painful sting to the crab. Unlike most crabs, they do not scavenge for food, rather they have bristle-like hairs on modified appendages around the mouth that allows them to filter feed plankton. Their name derives from their desperate attempt to escape predators; they can detach their claws and leave them behind as decoys while they quickly retreat. Ultimately, they will regrow new claws. The crab, though is not one to run at the first sign of trouble. Along with any anemonefish that may share their host anemone, they will aid in protecting the anemone from predators such as butterflyfishes.
A boxer crab
The porcelain crab, however, is not the only type of crab that has a relationship with sea anemones. The boxer crab (Lybia tessellata), sometimes referred to as the pom-pom crab, enjoys a symbiotic relationship as well, but quite opposite from that of the porcelain crab. Instead of residing inside the disc of a larger anemone, the tiny crab (a shell of just 2.5cm across) finds and uses even smaller anemones for his defense. By picking a single anemone in each claw, they effectively have a painful punch. Plus, they often strip the anemone for food if nothing is immediately available. It may sound like this is not the balanced relationship that the porcelain crab has with the anemone, but the anemones, being as small as they are, are easy prey for a lot of reef creatures such as fishes, shrimps, other crabs, and nudibranchs. If they remain sessile, they must find a spot that allows them to be exposed to good water flow so they can capture food and maximize gas exchange.