Mespilia globulus

Acclimation: invertebrates cope poorly with changes in water parameters. Drip-acclimate it slowly, then watch it for 24 to 48 hours: urchins often take longer than other livestock to become active again. Urchins are sensitive to salinity and are among the first animals to show stress.

Setup: add it only to a well-established tank (at least six months old, or with established algae), with sand and plenty of algae-covered rock offering ledges, caves and crevices. As a rule of thumb AlgaeBarn suggests one urchin per 20 gallons (about 75 litres) and finds it better suited to small tanks than the larger urchins.

Tankmates: it is peaceful and considered reef safe; it does not eat corals. According to AlgaeBarn it is far less likely than other urchins to knock over frags or rockwork; like any urchin it can still disturb the substrate and other inhabitants, so glue your frags well. Urchins may also occasionally graze encrusting coralline algae. Avoid fish that prey on urchins.

Sensitivities and health: no copper treatment in a tank that houses an urchin. In the trade, an M. globulus that carries no debris is considered stressed or sick. Damaged spines grow back. SeaLifeBase rates it harmless to humans.

Water: aim for 22 to 25.5 °C, a specific gravity of 1.023 to 1.025, pH 8.1 to 8.4 and 8 to 12 dKH.

In the wild it grazes at night on the organic material and algae that settle or grow on rock, rubble and the bottom, swallowing calcareous sediment along the way.

In the aquarium it is a herbivore that grazes algae on the rockwork. When they run low, supplement with dried seaweed (nori) or macroalgae; you can fix the sheet to a rock with a clip or a rubber band. None of the sources consulted gives a feeding frequency.