Lamarck's Pebble Crab

Geographical Origin and Habitat: Widely distributed throughout the tropical Indo-Pacific, extending from East Africa and the Red Sea to Australia, southern Japan, Hawaii, and French Polynesia. It primarily inhabits shallow rocky reefs, coral rubble zones, and intertidal flats where it can find immediate cover under stones.

Taxonomy: Belongs to the class Malacostraca, order Decapoda, family Xanthidae, and genus Xanthias. First described as Xantho lamarckii by H. Milne Edwards in 1834, it serves as the type species for the genus Xanthias. It is classified as a true crab within the diverse group of xanthid crabs.

Social Behavior: A peaceful, cryptic, and mostly nocturnal invertebrate. It spends the day hidden deep inside crevices of live rocks or among dead coral branches. It is non-aggressive towards tankmates and does not pose a threat to small fishes or other peaceful reef inhabitants.

Coloration and Sexual Dimorphism: The carapace color is variable, ranging from light brown, beige, or cream to shades of reddish or orange with irregular dark brown spots. The claw tips are dark brown or black. Sexual dimorphism is characterized by the shape of the abdomen, which is much wider in females to carry eggs.

Tank Setup: Can be easily housed in nano reefs starting from 40 liters (10 gallons). Requires a rockwork arrangement with plenty of small caves, crevices, and natural hiding spots. Unlike hermit crabs, they do not require empty snail shells since they grow their own rigid exoskeleton.

Diet and Feeding: An omnivorous scavenger and detritivore. It feeds on microalgae, organic detritus, small worms, and uneaten fish food. In the aquarium, it will readily accept sinking pellets, algae wafers, and frozen foods such as mysis and brine shrimp.

Water Quality: Requires stable marine parameters: specific gravity between 1.020 and 1.025, pH 8.1-8.4, and temperature 22-28°C (72-82°F). Regular monitoring of calcium, alkalinity, and magnesium is vital, along with trace iodine supplementation to ensure healthy molting.

Compatibility and Tankmates: 100% reef-safe and an excellent addition to the aquarium clean-up crew. Compatible with peaceful fish, corals, and other small invertebrates. Do not keep with large predatory species like triggerfish, puffers, or hawkfish.

Aquarium Breeding: While females frequently carry orange egg masses under their abdomens, breeding is not feasible in standard aquariums. The larvae hatch as free-swimming zoea, which are filtered out by mechanical equipment or quickly consumed by fish.

Risks and Diseases: The most common health issue is molting failure due to a lack of calcium or iodine. They are highly sensitive to sudden changes in salinity and temperature, and even trace amounts of copper-based medications will be lethal.