Reef Tank Janitors - Part 2

When it comes to picking out true crabs for your tank, be sure of what you are getting. Even though great scavengers, many species will attack and eat other crustaceans, as well as invertebrates. Species that grow large may attack fish, and they can be rather destructive in an aquarium, much like the 7-11 Crab for example. You want to choose crabs that remain small, and have very little if any negative impact on other marine life.

• The Emerald Green Crab (Mithrax sculptus) lives among rocks and feeds on algae, especially filamentous and Valonia (Bubble) types. This crab only grows to about one inch in size, and is tauted to be one of the best reef safe crabs to have for chowing down on algae.

• Another good herbivorous species is the Sally Lightfoot Crab. The "true" Sally Lightfoot species is the Grapsus grapsus found in the Galapagos Islands, which most likely won't be the one you find in fish stores. The Percnon gibbesi is found in Caribbean and other tropical waters around the world, is also labeled a Sally Lightfoot, and often referred to as a Nibble Spray Crab. This crab is said to be very shy and may hide for quite some time in your tank, only being seen on occasion until it becomes familiar with its environment.

• A strange looking species is the Arrow Crab (Stenorhynchus Seticornis). This crab is a good scavenger, but is said to be notorious for pulling Feather Duster Worms out of their tubes, and may pick at other invertebrates. Not a species considered to be completely reef safe.

• Not really crabs at all, but distantly related to scorpions, ticks and land spiders, Horseshoe Crabs have their own classification, Class Merostomata, in the Phylum Arthropoda. The most common species is Limulus polyphemus, found along the Atlantic and Gulf coasts of the U.S., and three species of the genera Tachypleus and Carcinoscorpio found in southern and eastern Asian waters, from Japan to Indonesia and India. Feeding on worms, mollusks and other small invertebrates buried in sand or mud, these animals mostly come out during low light or night time hours to scavenge. Unfortunately even though excellent sand sifters and scavengers, they can grow to three feet in size, which makes them rather destructive little bulldozers, and they are carnivorous. For these reasons these animals are not considered to be reef safe. However, IF provided an appropriate environment and you have a tank big enough to keep one, because it spends most of its time burrowing into the substrate, it makes an excellent sand stirrer.

To learn more about these and other true crabs, refer to our Crab Care & Photo Resources.

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