The emerald crab, Mithrax sculptus, is one of the more popular herbiverous crabs on the market. Their dark, emerald green color makes them a fairly attractive crab, but it's their algae cleaning habits that really make them popular. They are famous for their ability to remove bubble algae (valonia) which can plague a reef tank, but they also remove most other unwanted fleshy algaes that may grow in your tank. They are compatible with each other and with most other fish and invertebrates, including corals, sponges, and anemones. The average size we sell has a carapace length of about a half an inch, but bigger or smaller ones are available upon request for no extra charge.