Blue-eyed coral crab

Geographical Origin and Habitat: This species is widely distributed throughout the tropical coral reefs of the Indo-Pacific, ranging from the East African coast and the Red Sea to French Polynesia and Hawaii. It lives in an obligate symbiotic relationship with small-polyp stony (SPS) corals, particularly within the genera Pocillopora, Acropora, and Stylophora, at depths from the surface down to 30 meters.

Taxonomy: Belongs to the family Trapeziidae and the genus Trapezia. Commonly known as coral crabs or guardian crabs. This species is morphologically distinguished by its smooth, trapezoidal carapace and vibrant blue eyes. Their evolutionary biology is tied to their coral hosts, feeding on coral mucus and organic detritus trapped by the coral colony.

Social Behavior: Typically lives in monogamous pairs that occupy a single coral colony. They exhibit aggressive territorial behavior towards conspecifics or competing crabs, defending their specific coral home. In return for shelter, they protect the host coral from destructive predators, such as the crown-of-thorns starfish (Acanthaster planci), by pinching its tube feet.

Coloration and Sexual Dimorphism: The body displays a uniform bright orange, pinkish, or reddish-orange color. The most striking feature is their eyes, which are a brilliant, luminous light blue or turquoise. Males and females look alike, but mature females can be identified by their wider, rounded abdominal flap used to carry and protect egg masses.

Tank Setup: Because this crab is an obligate coral symbiont, the aquarium must contain healthy, live colonies of branching SPS corals, preferably Pocillopora or Acropora species. The minimum tank size depends on the requirements of the host corals, starting from 80-100 liters (approx. 20-25 gallons). The aquascape must prioritize coral health and stability.

Diet and Feeding: It feeds primarily on the mucus secreted by the host coral and organic particles trapped within the colony's branches. In captivity, it benefits from target-feeding fine coral foods (planktonic foods, particulate powders) and frozen cyclops or rotifers near the host coral. It does not harm or consume the actual tissue of a healthy coral.

Water Quality: Water parameters must meet strict reef standards required by SPS corals: temperature 24-26°C (75-79°F), pH 8.1-8.4, specific gravity 1.023-1.025, KH 8-10, calcium 420-450 ppm, and magnesium 1300-1350 ppm. Nitrates and phosphates should be kept close to zero. Requires high water flow and intense lighting for coral health.

Compatibility and Tankmates: Completely reef-safe and peaceful. Never house with predatory fish that hunt small crustaceans, such as hawkfish, large wrasses, triggerfish, or puffers, as they will easily pick the crab out of the coral branches. Excellent tankmates include peaceful reef-dwellers like chromis, gobies, and cardinalfish.

Aquarium Breeding: Mature females frequently carry orange egg masses under their abdomen in home aquaria. However, raising the larvae in captivity is highly challenging due to their planktonic larval stages. The larvae are easily removed by standard filtration systems and protein skimmers before they can settle.

Risks and Diseases: The primary threat to this crab is the health of its host coral. If the coral suffers from tissue necrosis (RTN/STN) or bleaching, the crab will lose its shelter and primary food source, leading to starvation. Like all crustaceans, they are highly sensitive to copper-based treatments and sudden shifts in water parameters.