The blue coral crab, an oceanic dwarf beauty, is an unpopular crab species that can be a great addition to your marine aquarium. Although this species of crab is fairly old, it is relatively new to the aquarium hobby, which is why not many people talk about it or are aware of the proper conditions for keeping them.
They are relatively simple to take care of, with a few essential exceptions. 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, and Guard Coral Crab |
| Scientific name | Trapezia Cymodoce |
| Care level | Easy |
| Reef compatible | Yes |
| Sleeping nature | Nocturnal |
| Diet | carnivorous |
| Size | 2-3 cm |
| Color | Blue, grey, red, and 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
The blue coral crab lives associated with hard corals of the shallow sea water.
The crab can be found in a wide region, including 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, Palaw, 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 and 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 cleaners 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 a carapace size reaching up to 1.5 cm, and they have large orange claws that are covered with hairs and have 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; they also protect the corals, let alone harm them. They live in pairs 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, again from October to February, and disappear in July to September. The males travel in the night to find a better coral head.
- Filter feeding: Blue coral crabs are filter feeders, meaning they obtain their food by filtering small particles, such as plankton, algae, and detritus, from the water; hairy claws help with this.
Care Guide for blue coral crab
Tank setup
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 some gravel; there is no need for a thick substrate as the crabs do not have contact with it, but it should be thick enough to hold the corals.
Decoration
You already know about the basic things: Pocillopora and Acropora corals. If you do not have these corals, prefer any hard species available to you. If you don’t have hard corals, go for soft corals, anemones or rocks. But don’t make them depend on them permanently, as it can badly affect 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 same temperature, and don’t forget to use a thermometer.
Set the light for 12 hours at night and 12 hours at day to mimic the natural light; fixing the light to mimic their natural day and night cycle is preferable.
The dkh of the water should be kept between 8 and 12, with a pH and sg of 8.1–8.4 and 1.023–1.025.
Blue coral crab tank mates
They are quite peaceful and sociable crabs that won’t harm your corals, being reef-safe. So choose carefully small and peaceful invertabrates or fish that won’t harm or tease the crab.
The best would be a group of the same or mixed species or another opposing 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 – demands sea urchins
Each of them has a different host preference, is the same size, and is peaceful. So nobody will harm anybody. None of them are tank cleaners, so better keep some tank cleaner hermit crabs.
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, blood worms, small insects and frozen food, as well as flakes and pellets. You can also try specialized foods for crustaceans and fish.
