Emerald Crab

Care at a glance

Overview

A hardy, easy-to-keep opportunist that is peaceful while food is plentiful but turns predatory when hungry. Keep only one per tank, since emerald crabs fight each other. Watch larger individuals around small fish, snails, hermits, and shrimp, and keep it fed to keep it honest.

In the wild

A shy, mostly nocturnal crab that clings to and hides in rockwork by day and comes out to forage over the reef at night. It is prized for eating bubble algae (Valonia) that few other animals will touch. Give it caves and overhangs to shelter in, and do not rely on it as a full cleanup crew because a well-fed crab often stops working algae.

Good to know

Mithraculus sculptus is a spider crab in the family Mithracidae, with a flattened glossy green carapace and spoon-tipped claws it uses to scoop algae. It reaches about 6 cm across the body with claws extending further, and lives roughly 2 to 4 years. Captive breeding is possible but very difficult, with larval survival typically well under 20 percent.

Did you know

It is one of the very few reef animals that reliably eats bubble algae, scooping it out with spoon-shaped claw tips, which is the main reason hobbyists tolerate its opportunistic streak.

Related species

• 

  Red Mithrax Crab

  Mithraculus forceps

  2 in (5 cm)11 gal (40 L)75–82°F (24–28°C)

• 

  Sally Lightfoot Crab

  Percnon gibbesi

  3.1 in (8 cm)30 gal (115 L)72–79°F (22–26°C)

• 

  Arrow Crab

  Stenorhynchus seticornis

  3.9 in (10 cm)30 gal (115 L)72–79°F (22–26°C)

• 

  Scarlet Zebra Hermit Crab

  Calcinus laevimanus

  1.2 in (3 cm)20 gal (75 L)72–79°F (22–26°C)

• 

  Electric Blue Hermit Crab

  Calcinus elegans

  2 in (5 cm)20 gal (75 L)72–79°F (22–26°C)

• 

  Halloween Hermit Crab

  Ciliopagurus strigatus

  2 in (5 cm)20 gal (75 L)72–79°F (22–26°C)