The blue coral crab, an oceanic dwarf beauty, is an unpopular crab species that can be a wonderful addition to your marine aquarium. Despite being a pretty old species, this crab is new to the aquarium hobby, and that’s why there are few people talking about it and who know the right requirements for keeping them.
Aside from a few essential requirements, blue coral crabs are very easy to care for. Here you’ll learn everything you need to know about blue coral crabs to keep these tiny pets thriving.
Quick overview of blue coral crab
| Names | Blue Coral Crab, Red Coral Crab, Lilac Coral Crab, Guard Coral Crab, サンゴガニ |
| Scientific name | Trapezia Cymodoce |
| Care level | Easy |
| Reef compatible | Yes |
| Sleeping nature | Nocturnal |
| Diet | carnivorous |
| Size | 2-3 cm |
| Color | Blue, grey, red, orange |
| Habitat type | Marine |
| Depth | 0-59 m |
| Optimal temperature | 23°C–29°C (73°F–85°F) |
| Temperament | Peaceful |
| Mating | Indirect sperm transfer and courtship ritual |
| Lifespan | Around 3 years |
Habitat and distribution (coral guard crabs)
The blue coral crab lives in association with hard corals in shallow seawater. The crab can be found in a wide region: the Red Sea and throughout the Indo-West Pacific. The species is spotted in many places, such as Egypt, Papua New Guinea, the South Pacific Ocean, Taiwan, Palau, Fiji, Indonesia, the Maldives, India, and many other countries.
Blue coral crabs’ association with corals indicates their habitat is under threat as the corals are being destroyed by pollution and other external reasons.
Some facts about blue coral crabs
- Mutualistic Symbiosis—Trapezia cymodoce has a symbiotic relationship with Pocillopora and Acropora corals/SPS corals, where they both (the crab and the coral host) benefit from each other. The crab gets shelter, protection (from the branching calcareous skeleton), and mucus and coral tissue (food sources) produced by its coral host. Also fallen detritus that is trapped by the coral polyps.
In return, the coral gets cleaner of excess sediment and protection from Acanthaster and other predators, given by the blue coral crab, which attacks the predator with sharp and large pincers.2
- Appearance—Trapezia cymodoce are pretty small crabs with the carapace size reaching up to 1.5 cm. Blue coral crabs have large orange claws that are covered with hairs and a blue-grey carapace.
The males have a narrow, elongated abdomen with two unequal pairs of pleopods (primarily used for swimming), and the females have a flat and wide abdomen with four pairs of pleopods. Both males and females of the blue coral crabs reach sexual maturity at a carapace length of 0.4–0.5 cm and become fully mature above this size.1
- Behavior—The blue coral crabs are completely reef safe; also, they protect the corals, let alone harm them. They live in a pair of male and female, with a single pair in a single Pocillopora coral
The females carry eggs throughout the year. Juveniles appear twice a year, from March to June and again from October to February, and disappear from July to September. The males travel in the night to find a better coral head.
- Filter feeding—Blue coral crabs are filter-feeding crabs, meaning they obtain their food by filtering small particles, such as plankton, algae, and detritus, from the water; hairy claws help them do this.
Care Guide for blue coral crab
Tank setup (red coral crab)
Size
Take a 20-gallon tank where some corals and a small group can fit easily. Increase the size considering the density of the population.
Substrate
Take any typical sand substrate and mix in some gravel; there is no need for the thick substrate, as the crabs do not have contact with it, but it should be thick enough to hold the corals.
Decoration (red coral crab)
You are already familiar with the basic types of corals, specifically Pocillopora and Acropora. If you do not have these corals, prefer any hard species available. If you don’t have the hard corals, go for soft corals, anemones, or rock. However, avoid making them rely on these permanently, as it can negatively impact their health. Bring the required corals as quickly as possible.
Now make the tank more natural with some aquarium stones, caves, and plants.
Temperature and water requirements
The temperature for blue coral crabs should be kept between 23°C and 29°C. Ensure the tank mates share the same temperature, and don’t forget to use a thermometer.
Set the light for 12h night and 12h day to mimic the natural light; fixing the light that mimics their natural day and night cycle is preferable.
The dkh of the water should be kept between 8 and 12, with the pH and sg of 8.1-8.4 and 1.023-1.025
Blue coral crab tank mates
These crabs are quite peaceful and sociable, and they won’t harm your corals, making them reef safe. So choose carefully small and peaceful invertebrates or fish that won’t harm or tease the crab.
The best would be a group of the same or mixed species or another opposite gender of blue coral crab.
You can consider keeping them with other crabs like:
- Gaudy clown crab – demands palythoas and zoanthids
- Harlequin crab – demands sea cucumbers
- Zebra crab – demans sea urchins
Each of them has different host preferences but is the same size and peaceful. So nobody will harm anybody. None of them are tank cleaners, so you better keep some tank-cleaner hermit crabs, such as the zebra hermit crab, the thin-striped hermit crab or the yellow-tip hermit crab.
What should you feed your blue coral crab
They can live on scraps of food, zooplankton, and mucus stuff, but you have to ensure they get a healthy diet.
Blue coral crabs are considered carnivorous crabs, so giving them vegetables or fruits is not recommended, though all crabs are omnivores.
Give them brine shrimp, prawns, bloodworms, small insects, and frozen food as well as flakes and pellets. You can also try specialized foods for crustaceans and fish.
Frequently Asked Questions
What is a blue coral crab?
A blue coral crab is a small, peaceful marine crab known for its striking blue-grey body and orange hairy claws. Scientifically named Trapezia cymodoce, it lives in a mutual relationship with hard corals like Pocillopora and Acropora, helping clean and protect them. It’s reef-safe, easy to care for, and makes a unique addition to marine aquariums.
Is the blue coral crab edible?
No, blue coral crabs are not considered edible. They’re tiny, reaching only 2–3 cm, and are more valuable in coral reef ecosystems or aquariums than on a plate. Their role in protecting corals makes them ecologically important rather than a food source.
