Emerald Crab

Emerald Crab (Mithraculus sculptus) — Clean‑Up Crew Spotlight Takeaway: The emerald crab is a hardy, algae‑eating scavenger best known for controlling bubble algae, but it needs the right environment and feeding to stay reef‑safe. 🦀 Identity & Natural Behavior A small Caribbean mithrax crab, typically 1.5–2 inches across when mature. Recognized by its deep green coloration, flat carapace, and cupped claws adapted for scraping algae. Primarily nocturnal; spends the day tucked into rockwork and becomes active after lights out. 🌿 Diet & Utility in Reef Tanks Famous for eating bubble algae (Valonia spp.), one of the few invertebrates that reliably do so. Also consumes: Film algae Hair algae Detritus Leftover food In tanks with low natural food, they must be supplementally fed (nori, pellets, small meaty bits) to prevent opportunistic behavior. 🧪 Reef Compatibility Generally reef‑safe, with important caveats: Well‑fed emeralds remain excellent citizens. Underfed or oversized individuals may: Nip at fleshy LPS (acans, euphyllia, scolys) Steal food from corals Pick at small fish while they sleep (rare but documented) Keeping them small and well‑fed is the key to long‑term safety. 🪨 Tank Requirements Minimum tank size: 20 gallons Rockwork: Essential—provides grazing surface and hiding spots Water parameters: Temp: 76–78°F Salinity: 1.025–1.026 pH: 8.1–8.4 Stable alkalinity and low nutrients preferred Social behavior: Usually peaceful Multiple emeralds can coexist in larger tanks with abundant rock In smaller tanks, males may spar 🔍 Sexing Males have larger, more pronounced claws and a narrower abdominal flap. Females have a wider abdominal flap for carrying eggs. 🧭 Best Use Cases in a Reef Shop or Customer System Bubble algae outbreaks Tanks needing a hardy, low‑maintenance algae grazer Systems with plenty of rock and microalgae Customers wanting a “character” in the CUC—emeralds are charismatic and visible ⚠️ Notes for Customer Education “Reef‑safe with caution” is the honest framing. Feed them if the tank is very clean. Remove individuals that grow unusually large or become bold. They are not a solution for major bubble algae infestations alone—manual removal plus nutrient control is still required.