Pencil urchin (Eucidaris tribuloides) care guide

Group
Other invertebrates
Other names
Slate pencil urchin, Mine urchin, Club urchin, Cidarites tribuloides
Family
Cidaridae
Origin
Western Atlantic from North Carolina to Brazil, Caribbean and Gulf of Mexico; eastern Atlantic islands; introduced in Malta
Water
saltwater
Maximum size
13 cm
Temperature
22–26°C
pH
8.1–8.4
Hardness
dKH 8–12, SG 1.023–1.025
Minimum tank
114 litres
Care level
intermediate
Temperament
peaceful
Reef safe
no

Care

The pencil urchin, or slate pencil urchin, is a Caribbean and Atlantic urchin with a brown, sometimes mottled body and thick, blunt spines that stick out in all directions like pencils or the spikes of a sea mine. As it ages the spines often become covered with coralline algae and other growths. It reaches about 13 cm including the spines. It belongs to the cidaroids, an ancient group of urchins. It is a slow-moving, nocturnal animal that wedges itself into crevices or under rock by day, anchored by its big spines, and rarely wanders far. At night it forages for algae lawns and small invertebrates such as sponges and sea squirts, and it also eats corals in the wild. Keep one in a tank of at least 114 litres (30 US gallons) with plenty of live rock to graze and many hiding places. Build rockwork solidly, as it can push rocks over. It does not tolerate high nitrate, copper or poor water quality; stressed urchins shed their spines. Never try to pull it out of a hole, as you will break its spines before it lets go. Copper-based medicines and some plant fertilisers are toxic to shrimp, snails and most invertebrates. Sources: LiveAquaria (liveaquaria.com/products/pencil-urchin); Wikipedia (en.wikipedia.org/wiki/Eucidaristribuloides). Checked October 2026.

Diet

An omnivore. Grazes algae and film on rock and eats small invertebrates such as sponges, sea squirts and coral polyps. Feed dried seaweed (nori) on a clip or weighted down, and offer some meaty foods such as chopped prawn or mussel and sinking pellets.

Breeding

Sexes are separate but cannot be told apart from the outside. It spawns into the water, often in time with the seasons and the full moon (peak spawning in late summer and early autumn in the Florida Keys). It is extremely difficult to breed in an aquarium.

Compatibility and tank mates

Not reef safe: it can eat and damage corals, especially soft corals and small-polyp stony corals, and knocks over frags. Peaceful with fish and most mobile invertebrates. Better in a fish-only tank with live rock. Triggers and some puffers may eat it.

Last updated 8 October 2026. Community-edited care guide on Aquatic-Auctions.

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