he Emerald Crab (Mithraculus sculptus) is a workhorse invertebrate that is often introduced into a reef tank to dispatch any bubble algae that is in the tank. An omnivore, in addition to bubble algae, it will eat other foods introduced into an aquarium, and has been known to attack small and slow fish, but it is mostly a scavenger and a consumer of bubble algae. It grows to about 2 to 2.5 inches in size and has a green, flat and compact body with legs that are hairy and often pick up sand. Ideally, you should introduce the emerald crab into a mature aquarium with ample amounts of bubble algae. A single specimen in a 29 gallon nano tank can take care of a bubble algae bloom in a few months. Supplement the emerald crab with dried seaweed, a high quality pellet food and meaty items such as mysid shrimp or other meaty food items. Your tank setup should include plenty of rock work and places for the emerald crab to hide. Don’t be surprised if your emerald crab disappears for a week at a time. It is in there busily working on the algae. It is often just difficult to spot them, especially if they are not moving. If you have a bubble algae problem, the emerald crab will more than likely take care of it rather easily, but be aware that once the bubble algae is completely gone, you will have to supplement its diet.