TankBud

Emerald crab

Mithraculus sculptus

Also known as Green Clinging Crab, Green Emerald Crab, Mithrax Crab

SaltwaterCrab

Emerald crab (Mithraculus sculptus)
Mark Loch · CC BY-SA 2.0

Care at a glance

Temperature
75–82°F
24–28°C
pH
8.0–8.4
Adult size
2.4 in
6 cm
Minimum tank
11 gal
40 L
Temperament
Mostly peaceful
Social behavior
Solitary
Tankmates
With caution
Swim level
Bottom
Diet
Omnivore
Difficulty
Easy
Breeding
Hard
Rarity
Very common
Reef safe
Reef safe with caution
Family
Mithracidae
Origin
Tropical western Atlantic, from Florida and the Caribbean south to northern Brazil, on rock and reef rubble

Overview

The emerald crab is a hardy, easy-to-keep opportunist for the bottom of a saltwater tank. It is mostly peaceful.

Did you know

The emerald crab is one of the very few reef animals that reliably eats bubble algae (Valonia). It scoops the algae out with spoon-shaped claw tips. That habit is the main reason hobbyists tolerate its opportunistic streak.

Caring for the emerald crab

Keep an emerald crab in 11 gal (40 L) or larger, with caves and overhangs to shelter in. Hold the water at 75 to 82°F (24 to 28°C) and a pH of 8.0 to 8.4.

The emerald crab eats algae, and it stays peaceful while food is plentiful. It turns predatory when hungry, so keep it fed. Do not rely on it as a full cleanup crew, because a well-fed crab often stops working algae.

Keep only one per tank, since emerald crabs fight each other. Watch larger crabs around small fish, snails, hermits and shrimp. Treat it as reef safe with caution.

Captive breeding is possible but very difficult. Larval survival is typically well under 20 percent.

Good to know

The emerald crab, Mithraculus sculptus, is a spider crab in the family Mithracidae. It has a flattened, glossy green carapace, and its claws stretch wider than the body. It lives roughly 2 to 4 years.

In the wild

The emerald crab lives in the tropical western Atlantic, from Florida and the Caribbean south to northern Brazil, on rock and reef rubble. It is shy and mostly nocturnal. It clings to and hides in rockwork by day and comes out to forage over the reef at night.

Common questions

What size tank does an emerald crab need?

An emerald crab needs at least 11 gal (40 L). It spends most of its time in the bottom.

What temperature and pH does an emerald crab need?

An emerald crab is comfortable at 75–82°F (24–28°C) with a pH of 8.0–8.4.

What does an emerald crab eat?

An emerald crab eats algae and stays peaceful while food is plentiful. It turns predatory when hungry, so keep it well fed.

How big does an emerald crab get?

An adult emerald crab reaches about 2.4 in (6 cm).

How does an emerald crab breed?

An emerald crab can be bred in captivity, but it is very difficult. Larval survival is typically well under 20 percent.

Related species