The Emerald Mithrax Crab is a commonly kept crustacean and is widely known for its ability to help control outbreaks of bubble algae in reef aquariums. These crabs are, however, omnivorous and will not hesitate to help clean up any excess or leftover meaty foods or waste that can settle on or in various surfaces of the tank. Emerald Crabs tend to stay hidden away during the day, emerging from small crevices or ledges to hunt and scavenge for algae and other debris at night. Although generally peaceful, in the absence of sufficient food, Emerald Crabs may demonstrate opportunistic behavior and attempt to predate on smaller invertebrates, fishes, and sometimes corals, so it is imperative to keep them well fed in established systems.

Introduction to Emerald Crab (Efficient Algae Controller & Cleanup Crew)

Emerald crabs (Mithraculus sculptus) are hardy, algae-eating invertebrates that play a crucial role in controlling nuisance algae in reef aquariums. Known for their bright green exoskeleton and powerful claws, these scavengers specialize in consuming bubble algae (Valonia spp.), making them a valuable addition to any cleanup crew. These crabs are low-maintenance and adaptable, often seen climbing rock structures or hiding in crevices during the day before emerging to scavenge at night.

Care Requirements

Care Level: Easy – Adaptable and low-maintenance Minimum Tank Size: 10+ gallons Aquarium Setup: Requires ample rockwork for hiding and foraging Emerald crabs are nocturnal scavengers, spending most of the day hiding in rock crevices before emerging at night to graze on algae and organic debris.

Diet & Nutrition

Emerald crabs are omnivorous and require a varied diet to thrive. Diet: Omnivorous – Prefers algae, detritus, and leftover food Recommended Foods: Bubble algae (primary natural food source) Hair algae, film algae, and uneaten fish food Supplemental seaweed and algae-based pellets if algae is scarce If food becomes limited, emerald crabs may opportunistically scavenge, so regular supplemental feeding ensures they remain reef-safe.

Lifespan & Growth Rate

Lifespan: 2-4 years Growth Rate: Moderate – Typically reaches 2 inches across As they grow, emerald crabs shed their exoskeleton through molting, which is a natural process that may make them appear vulnerable for a short time.

Temperament & Compatibility

Temperament: Peaceful to Semi-Aggressive – Generally peaceful but may show territorial behavior if underfed Reef Safe? With Caution – Completely safe for corals but may prey on small invertebrates if hungry Avoid housing them with aggressive crabs, triggerfish, puffers, or large wrasses, as these species may harass or eat them.

Common Challenges & Considerations

Opportunistic Feeding: If food is scarce, they may target small shrimp or slow-moving invertebrates. Territorial in Small Tanks: Best kept in larger systems with plenty of food and hiding spots. Molting Vulnerability: Newly molted crabs hide until their shell hardens, reducing visibility for a short period. To ensure safe and effective cleanup, provide a well-established tank with algae and detritus.

Water Conditions

Temperature: 74-80°F dKH (Alkalinity): 8-12 pH: 8.1-8.4 Specific Gravity: 1.024-1.026

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