True Crabs for the Reef Tank: Emerald, Pom Pom, Porcelain and Arrow Crabs Explained

In this guide

Unlike hermit crabs, true crabs are born with their own hard shell and never need to borrow one. This single difference changes how they behave in a reef tank. True crabs tend to be more specialised in what they eat and how they interact with their surroundings, and the four types covered here range from genuinely reef safe filter feeders to one species that Indian hobbyists should think twice about before adding to a coral heavy display. None occur naturally in Indian coastal waters and all reach the trade here as imports.

Emerald Crab

The Emerald Crab (Mithraculus sculptus), also called the Emerald Mithrax Crab, is kept almost entirely for one job: clearing bubble algae, a nuisance form of algae that most other cleanup crew members simply ignore. It has a smooth, rounded green shell and flattened, spoon shaped claws built for scraping algae off rock. It grows to around five centimetres and does best in a tank with plenty of live rock to hide in during the day, since it is more comfortable under lower, reef typical lighting.

Here is the frank part that many sellers gloss over. An Emerald Crab with bubble algae to eat behaves itself. An Emerald Crab that has cleared all the bubble algae and is left hungry will start nibbling coral polyps, soft corals and even LPS species, and once it develops a taste for coral it tends to keep going back even when other food is offered. Keep it well fed with dried seaweed or algae wafers once the bubble algae runs out, and treat any coral nibbling as a sign to rehome the crab into a sump or fish only tank rather than hoping it settles down. Avoid keeping it with triggerfish, puffers or hawkfish, since these will simply eat the crab. A minimum tank size of twenty gallons gives it enough space, since Emeralds are territorial with their own kind.

Pom Pom Crab

The Pom Pom Crab (Lybia spp.), also called the Boxer Crab, is one of the more unusual animals a hobbyist can add to a nano reef. It carries a small anemone in each claw, held up and waved like a pair of pom poms, hence the name. The anemones serve the crab in two ways: they are raised defensively when the crab feels threatened, and any food particles that drift into their tentacles get eaten. It is a tiny, slow moving crab, well suited to a small tank where it will not be bullied by faster tankmates, and it is considered genuinely reef safe, since the anemones it carries are too small to sting a coral seriously even on the rare occasion the crab brushes past one.

Feed it finely chopped meaty foods, frozen mysis or brine shrimp, delivered close enough for its short reach, since Pom Pom Crabs are not efficient foragers across an open tank. Keep tankmates peaceful and appropriately sized. Anything that would see a small, slow crab as a snack, wrasses, larger hermits, or boisterous damsels, is a poor match.

Porcelain Crab

Porcelain Crabs are not true crabs in the strictest biological sense, they belong to a separate family, but they are sold and kept alongside true crabs and deserve a mention here for exactly that reason. They are filter feeders, using delicate, fan shaped appendages to sweep passing plankton and food particles out of the water column rather than actively hunting or grazing. The Porcelain Anemone Crab (Neopetrolisthes ohshimai), with its pale red polka dot pattern on a translucent shell, is the type most often available, and it typically lives sheltered among the tentacles of a host anemone for protection, much as a clownfish does.

This is widely regarded by experienced reef keepers as one of the few genuinely, unambiguously reef safe crabs available in the hobby. It does not eat coral, does not bother snails, and does not grow aggressive with age. The trade off is that it needs a steady supply of suspended food in the water column, so it does best in an established tank with some level of planktonic life or regular target feeding with fine particulate foods. A skittish, shy temperament means it is usually seen at night or when the tank lights first come on, so do not expect a bold, constantly visible tankmate.

Arrow Crab

The Arrow Crab (Stenorhynchus seticornis) is instantly recognisable, with an arrow shaped body, a long pointed snout, and legs that can be three times the length of its body, giving it a spider like appearance that some hobbyists love and others find unsettling. It is popular for one specific job: hunting down bristleworms, which can overpopulate a reef tank and which few other cleanup crew members will touch. It is genuinely effective at this.

The honest caveat, and this needs to be said plainly rather than softened, is that Arrow Crabs are opportunistic predators, not simple scavengers. When bristleworms and other food run short, they will go after small, slow moving fish, shrimp, and even peck at coral polyps. Reports of Arrow Crabs attacking cleaner shrimp and small gobies are common enough in reef keeping circles that this should factor into your tankmate choices rather than be treated as a rare exception. Keep only one per tank unless you have a confirmed mated pair, since they fight territorially with their own kind. A tank of at least thirty gallons with plenty of live rock crevices to retreat into suits them well, and they are genuinely long lived and hardy once settled, often surviving five years or more. If your tank houses small ornamental shrimp or slow, docile fish you are attached to, an Arrow Crab is not the safest cleanup crew choice, however useful it is against bristleworms.

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