The red emerald crab, aka ruby crab, is an algae eater and brings color and personality to your reef tank. Known for their striking red hue and unique behavior, these crabs are not only eye-catching but also incredibly helpful in maintaining a healthy aquarium ecosystem.
Learn everything you need to know about the red emerald crab, including care, diet, and habitat to keep your crab thrive.
A quick overview of red emerald crab
- Common names: red emerald crab, ruby crab, ruby red emerald crab, ruby red mithrax crab, red mithrax crab
- Scientific name: Mithraculus sculptus
- Care level: Easy
- Temperament: Semi-aggressive
- Diet: Omnivore
- Max Size: Up to 2.1 inch (carapace 1.5 inch)
- Temperature: 77-78, ph 7.7-8.2
- Reef Safe: yes
- Lifespan: 4-6 years in captivity
- Color: red
Some unique facts about red emerald crab
If you’re considering adding a red emerald crab (Mithraculus sculptus) to your tank, there are a few things to know. While similar to the green emerald crab, red emerald crabs have a distinct red body. These crabs are nocturnal, staying hidden in caves or crevices during the day and only venturing out at night when they feel comfortable. Though not as common as the green variety, red emerald crabs are still widely available.
The color of emerald crabs can change depending on their diet and environment. Crabs that eat mostly green algae tend to remain green, while those fed other types of algae, like nori, can develop a reddish hue. There’s also a naturally red species, Mithrax ruber, which is often confused with red emerald crabs.
Red emerald crabs are not just about their color; they grow larger and can be more aggressive than their green counterparts. They’re also strong and can cling to rocks, making them resilient against strong currents in their natural habitat. So, while they are fantastic for controlling algae in your tank, be prepared for a bit more feistiness compared to green emerald crabs.
Remember, though they may look relaxed during the day, it’s just their nature to rest and conserve energy before their nighttime activities.
Habitat and distribution
Mithraculus sculptus, also known as the red emerald crab, thrives in tropical reef environments, mostly found in the Caribbean Sea and Gulf of Mexico. They prefer offshore reefs, where they make their homes in crevices, caves, and under rocks. These crabs range from Florida to Brazil, usually dwelling at depths of up to 54 meters (177 feet). Their habitat often includes back reefs, where they can hide and scavenge in safety.
Appearance
Mithraculus sculptus, or the red emerald crab, has a striking appearance that makes it a standout in any aquarium. Its body is a shiny red, with hairy legs that complement its vibrant color. The claws are tipped with white, giving them a distinct look. The crab’s eyes are also red, adding to its bold and eye-catching appearance. The overall design of this crab, with its shiny red shell and white-tipped claws.
Red emerald crab reef safe
Red mithrax crabs (Mithraculus sculptus) are generally considered reef-safe and can live peacefully with many reef species, including seahorses. They help control algae, particularly bubble algae, and clean up leftover food. However, as they grow, they may become more aggressive or destructive, especially if underfed, and could start nibbling on corals or small fish.
Red emerald crab care
Tank setup
Red mithrax crab tank size
For a 30-gallon tank, you can comfortably keep 1 to 6+ red emerald crabs, depending on the space and other tank mates. A larger group might be fine as long as you have enough room for them to explore and hide. They do best at 10-liter cylindrical-conical design.1
Substrate
The substrate doesn’t need to be anything fancy, but it’s important to provide a good base for them to roam. I recommend a sandy or rocky substrate where they can burrow and hide when they feel threatened or need rest.
Decoration
Red emerald crabs love tanks with plenty of rocks and hiding spots. Include structures like live rock, caves, or coral to give them places to explore and feel safe. These crabs are nocturnal, so they need spots to retreat to during the day when they prefer to rest.
Water Parameters
Red emerald crabs do best in stable water conditions. Keep the salinity between 1.024-1.026, with a temperature range of 77-78°F. Maintain a pH level between 7.7 and 8.2 to ensure they thrive. These conditions mimic their natural environment, making them feel at home in your tank.
What do red emerald crabs eat?
Red emerald crabs are omnivores, feeding on algae, leftover food, and even fleshy macroalgae. Their real strength lies in consuming bubble algae, which many aquarists struggle to manage. If you provide a steady food supply, such as seaweed, chopped shrimp, or quality pellets, they’ll be content. However, underfed crabs might start nibbling on corals or small tankmates.
Larger red emerald crabs tend to eat more algae than smaller ones, but their algae consumption drops significantly when pellets are added to their diet due to the higher energy content. While these crabs can help control algae, they may be less effective if pellets are included regularly. Switching to mysids might improve their algae-eating efficiency, making them more effective at keeping bubble algae under control. Also, be mindful that they may unintentionally spread algae while feeding, potentially worsening infestations.
Red emerald crab tank mates
They’re social creatures and enjoy the company of their own kind, but it’s important to make sure your tank is large enough to accommodate them. A group of 3-4 can be a great addition, giving them room to interact and explore without overcrowding.
When it comes to tankmates, emerald crabs generally get along well with other peaceful creatures like conches and snails. They’re unlikely to cause trouble with these animals, so you can safely add them to your tank without worrying about aggression.
Note: As red emerald crabs grow, they shed their outer skeleton and hide while the new one hardens. They remain vulnerable during this time.
