Slate Pencil Urchin

The slate pencil urchin is easily told apart from other sea urchins by the few thick, blunt-tipped spines growing from its small, round body. The body (test) is reddish brown and rarely exceeds 5 cm; the spines are a little longer than the body. These spines are not venomous and do not pierce like needles. Over time the spines become covered with coralline algae, sponges and small organisms; this is natural.

It is a hardy and inexpensive species; in fish-only aquariums and reefs with robust stony corals it helps clean up algae and detritus. However, it is not considered fully reef-safe: being omnivorous, when hungry it grazes on sponges, tunicates, the coralline algae on the rocks and sometimes coral tissue, and while roaming it can knock over unsecured coral frags. Heterocentrotus mamillatus from the Indo-Pacific, sold as the "red pencil urchin," is a different and much larger species. Specimens in the trade are wild-collected.

It occurs in the Western Atlantic from Florida and the Caribbean to Brazil, and along the West African coasts in the Eastern Atlantic. It lives on shallow reefs, in rock crevices and among coral rubble. It hides in holes during the day and comes out at night to feed.

• Volume: at least 120 liters; caves to hide in and algae-covered live rock to graze on.
• Water: salinity 1.023–1.026 specific gravity, pH 8.1–8.4, temperature 23–27°C; nitrate should be low.
• Reef compatibility: Reef-safe with caution. Rocks and coral frags should be firmly secured; it is not recommended for aquariums with a valuable collection of coralline algae and sponges.
• Acclimation and medication: Acclimate by the drip method for at least 2 hours; it is sensitive to salinity differences. Copper-based medications are lethal.

Omnivorous. It grazes algae and biofilm on the rocks and also eats detritus and carrion. When algae run short, offer nori clipped to a rock, algae-based granules and, a few times a week, small mysis or chopped seafood. Regular feeding makes it less likely to turn to corals and sponges.

Nocturnal and peaceful; it leaves fish and mobile invertebrates alone. It climbs the glass and rocks with the fine tube feet between its spines; during the day it is usually wedged into a crevice with only its spines visible.

Good companions: clownfish, tangs, damselfish, gobies; cleaner shrimp, snails, hermit crabs.

Avoid: triggerfish and pufferfish, large wrasses; unsecured coral frags; areas with valuable sponges and coralline algae.

Kept singly. In large aquariums a second individual can be added only if there is enough algae and food; otherwise competition for food increases the risk of damage to corals.

The sexes are separate; it releases eggs and sperm into the water and the larvae develop in the plankton. It is not bred in home aquariums.

• Spine loss: losing a few spines is normal; shedding many spines and pale patches on the body are signs of poor water quality.
• Starvation: may show as inactivity and spine loss; if algae are insufficient, supplementary food is essential.
• Sudden deaths: copper and rapid salinity changes.

Males and females cannot be told apart externally.