Emerald crab care

Quick reference

What is an emerald crab?

Mithraculus sculptus is a small crab sold mainly to eat bubble algae (Valonia, also called Ventricaria).^{[1, 2]} LiveAquaria says it will eat bubble algae and many other nuisance algae, and Kurtz calls it an opportunistic omnivore that also scavenges dead animals.^{[2, 3]}

Feeding

Here the study and the sales pitch part ways. In Figueiredo and colleagues' trials, emerald crabs offered a choice picked pellets over bubble algae 77 % of the time and frozen mysids 69 % of the time, and only pellets significantly cut how much algae they ate; bigger crabs ate more algae than small ones.^[1] The authors concluded the crab may be a less effective control than previously thought, especially when pellets are fed, and might do better fed mysids.^[1] Kurtz's own group in a 75 gallon tank never made much of a dent in his bubble algae.^[3] LiveAquaria says many keepers supplement with dried seaweed, chopped meaty foods and pellets;^[2] if algae control is the point, skip the pellets.^[1]

Aggression and spacing

LiveAquaria rates it semi-aggressive.^[2] Kurtz reports his crab tearing chunks from a clownfish's fins, and hobbyist reports of crabs tearing up zoanthids and other soft corals; attacks are most likely at night.^[3]

Tankmates

LiveAquaria lists it as reef compatible but warns that an underfed crab may turn to corals, invertebrates or small fish.^[2] Kurtz says it may not be entirely trustworthy around soft corals, and suggests moving a problem crab to the sump or refugium.^[3]

Disease and quarantine

The same study filmed crabs tearing bubble algae cells and releasing cell fluid containing juvenile cytoplasmic spheres, which may spread the alga.^[1] Pull large bubbles out by hand instead of relying on the crab to burst them.

Troubleshooting

Common mistakes

• •Expecting it to clear a bubble algae outbreak.^{[1, 3]}
• •Feeding pellets while relying on it for algae control.^[1]
• •Ignoring reports of attacks on soft corals.^[3]

Equipment & food for this species

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Inverts need stable salinity — use a refractometer, not a hydrometer.

Emerald crab FAQ

Do emerald crabs eat bubble algae?

Often, but they prefer other food: in a controlled study they chose pellets 77 % of the time and mysids 69 % of the time over bubble algae.^[1]

Are emerald crabs reef safe?

Usually when well fed. LiveAquaria lists them as reef compatible but says hungry crabs may eat corals, invertebrates or small fish;^[2] Kurtz has seen one attack a clownfish.^[3]

How big do emerald crabs get?

About 2.5 inches, according to LiveAquaria.^[2]

What do emerald crabs eat?

Algae, scavenged food and almost anything you feed your fish.^{[2, 3]}

Does popping bubble algae spread it?

Possibly: the study filmed crabs tearing bubbles and releasing fluid with juvenile cytoplasmic spheres that may spread it.^[1]