The arrow crab also commonly known as spider crabs are reef safe and an interesting addition to your reef aquarium, they can grow to 6 inches in size and are great for cleaning up leftover food in your tank. By cleaning up leftover foods and other wastes they help process this material and keep your tank clean. They are an interesting addition to your tank and a marvel to watch! They can be aggressive to coral banded shrimp when food is not available.