Emerald Crab (Mithraculus sculptus): Care, Diet and Safety

A few green bubbles show up on a rock in the back corner. A month later there are thirty of them, wedged where tweezers will not reach. That is the moment most people buy an emerald crab, and it is a reasonable move as long as you know what the crab will and will not do.

The emerald crab, Mithraculus sculptus (still sold under the old name Mithrax sculptus), is a small spider crab from the western Atlantic. Its range runs from the Bahamas and southern Florida to northern Brazil. It is one of the species in our Florida Keys clean up crew field guide, and it is the only animal in that guide we would point at bubble algae.

Quick facts

What an emerald crab eats

An emerald crab is an omnivore that gets sold as an herbivore. In the wild it lives on algae-covered rock and rubble, in shallow backwaters and on seagrass beds. It uses spoon-shaped claw tips to scrape algae off rock and to break open bubble algae (Valonia and Ventricaria). It also scavenges leftover food and dead animals, and it will take live prey if it is hungry and the prey is slow.

That second half of the diet is why reputations differ so much. A crab with plenty of algae and scraps on the rock behaves well. A crab in a clean tank goes looking for food, and it is not picky about where it finds it.

Does it really eat bubble algae?

Sometimes, and less well than its reputation suggests. A 2008 study by Joana Figueiredo and colleagues, published in the Journal of the Marine Biological Association of the UK, tested emerald crabs against bubble algae in aquaria. Medium and large crabs ate more bubble algae than small ones. But when the crabs were also offered pellet food, they chose the pellets 77% of the time, and algae eating dropped. The authors also filmed crabs tearing bubbles open and releasing the material inside into the water, which may help the algae spread.

So the practical advice is simple. Add the crab for bubble algae, then stop feeding pellets directly at the rock where it lives. Remove the large bubbles by hand outside the tank, and fix the nutrients that grew the algae in the first place. A crab is help, not a cure.

Reef safety: the honest caveats

Most emerald crabs live their whole lives in a reef tank without touching a coral. Some do not. The pattern hobby references describe is consistent: the risk goes up as the crab gets bigger and the tank gets cleaner. TFH Magazine notes that large crabs, around 3 to 4 inches across the legs, may start eating small fish. Reef Builders reported a crab nipping a clownfish’s fins and described accounts of crabs tearing up zoanthids and soft corals at night.

• Buy small to medium crabs.
• Keep one in a tank with real algae on the rock, not a spotless one.
• If damage starts showing up overnight, watch the tank with a red light after dark before you blame a fish.
• A crab that turns on its tankmates can be moved to a sump or refugium, where it is useful and harmless.

How many emerald crabs per gallon

Our clean up crew calculator only adds emerald crabs when you tell it you have bubble algae, and it holds them back from tanks under two months old. The count works out to about one crab per 40 gallons, with a minimum of one and a cap of three. Crabs that run out of algae will find something else to eat, so err low.

Acclimation

Float the sealed bag for about 15 minutes to match temperature. Then drip tank water in slowly to match salinity, because invertebrates are sensitive to salinity swings. Never add the shipping water to your tank. Release it near rock.

Copper is toxic to crabs. Never add one to a tank that has had copper in it.

Lifespan, size and molting

The shell grows to about 1.5 inches long. There is no reliable published lifespan for this species. Like all crabs it molts as it grows, and it may hide for a few days around a molt. An empty shell that looks like a whole dead crab is often just the molt. Leave it in place for a day; the crab may eat it to recover minerals.

How we find them in the Florida Keys

Emerald crabs live in the shallow, algae-covered rock and rubble that line the Keys, and down to about 177 feet according to the published range. They hide in crevices by day and come out at night. We collect them by hand under our Florida certificate and condition them in our Key West systems before they ship.

FAQ

The Underwater Gardener is a reef store in Key West, Florida. Our Florida Keys clean up crew is collected by our team under Florida certificate #AQ0254072 and conditioned in our Key West systems before it ships. Livestock ships UPS Next Day Air on Tuesdays and Wednesdays.