Bicolor Sea Urchin

Geographical Origin and Habitat: Salmacis bicolor is native to the tropical waters of the Indo-West Pacific Ocean. Its distribution ranges from the coasts of East Africa and Madagascar, through India and Sri Lanka, to Indonesia, the Philippines, and northern Australia. It inhabits sheltered sandy lagoons, coral rubble zones, muddy sea beds, and coastal reefs at depths of 2 to 40 meters.

Taxonomy: Belonging to the family Temnopleuridae. It features a spherical calcareous test that is slightly flattened at the poles. The spines are of medium length and are arranged in neat rows along the ambulacral plates, creating a regular geometric structure.

Social Behavior: It is a peaceful, slow-moving benthic scavenger. Like other shallow-water sea urchins, it exhibits covering behavior, picking up pieces of shells, macroalgae leaves, and small stones using its tube feet to cover its test for camouflage and UV protection.

Coloration and Sexual Dimorphism: The spines show a highly distinctive banded coloration, which is the source of its scientific name: they typically feature a purple, red, or orange base with contrasting green or yellowish tips. There is no external sexual dimorphism; males and females are morphologically identical.

Tank Setup: Requires a mature marine aquarium of at least 100 liters (30 gallons). The enclosure should have ample sandy areas and stable live rock for grazing. Provide loose shells and rubble on the substrate for its covering behavior. Ensure rockwork is secure, as the urchin can dislodge loose coral fragments.

Diet and Feeding: A voracious benthic herbivore. It feeds by scraping microalgae, filamentous green algae, and organic biofilm from rocks and aquarium glass. If the aquarium is very clean, supplement its diet with dried seaweed (nori) sheets and vegetable-based sinking pellets to prevent starvation.

Water Quality: Maintain stable water parameters: temperature 23-28 °C, pH 8.1-8.4, and salinity at SG 1.022-1.025. Monitor calcium levels (minimum 400 ppm) to ensure proper test maintenance. It is highly sensitive to copper-based medications, high nitrates, and low oxygen levels.

Compatibility and Tankmates: 100% reef safe and peaceful toward all fish, corals, and other invertebrates. Do not house with specialized urchin predators, such as triggerfish, puffers, and large wrasses, which can chew through its spines. Safe to keep with other peaceful tankmates.

Aquarium Breeding: Breeding in home aquariums is not documented. It is a broadcast spawner, releasing gametes into the water column. Rearing the delicate planktonic larvae requires specialized sterile tanks and microalgae cultures, making it impractical for hobbyists.

Risks and Diseases: Rapid spine loss is a critical sign of stress, starvation, or chemical shock (such as copper exposure or sudden salinity drops). Never expose the urchin to air during transfer, as air bubbles can get trapped in its water vascular system, leading to death.