Emerald crab

Lore & Background

The emerald crab has a flat, shiny green carapace that is longer than it is wide, finely sculpted with whitish material on its projections. Its large chelae (claws) are also green, spoon-shaped, and tipped with white, while the walking legs are paler, hairy, and often encrusted. It grows to about 4 cm in length. This species ranges from the Bahamas and southern Florida to northern Brazil, at depths down to about 54 meters. It is found in varied habitats but may be most abundant in back reef environments. It is often seen among branches of corals such as Porites furcata and on turtle grass (Thalassia testudinum). Mithraculus sculptus is largely nocturnal, hiding in caves, crevices, and under rocks during the day. It is a scavenger that also feeds on algae. It tolerates both high and low temperatures and can withstand strong currents by clinging to the substrate with its legs. When threatened, it hides beneath the extended tentacles of the sun anemone (Stichodactyla helianthus).

Reader's Guide

In the reef ecosystem, Mithraculus sculptus occupies a role as a nocturnal scavenger and herbivore, feeding on algae and organic debris. It is often found among branching corals and seagrass blades, where it consumes encrusting organisms. Its ability to tolerate strong currents and temperature extremes allows it to thrive in dynamic back reef habitats. In the aquarium trade, this crab is kept for its compatibility with other reef species and its appetite for bubble algae (Valonia ventricosa). Research indicates it can control excessive bubble algae growth, with best results when its diet is supplemented with mysids rather than pelleted food. However, if underfed, it may consume coral polyps or small fish. Its strong legs make it difficult to remove from an aquarium once introduced.

Did You Know?

• The emerald crab's carapace is longer than it is wide and finely sculpted with whitish material on the projections.
• It can withstand strong currents by using its legs to cling to the substrate.
• When threatened, it often hides beneath the tentacles of the sun anemone (Stichodactyla helianthus).
• Research shows it can control bubble algae growth in aquaria, but best results occur when its diet is supplemented with mysids, not pelleted food.