Black Pencil Urchin (Eucidaris tribuloides)

The Pencil Urchin is one of the most distinctive and prehistoric-looking invertebrates available in the marine hobby. Its brown to tan, sometimes mottled body is ringed by thick, blunt, stubby spines radiating outward in all directions, giving it a silhouette unlike any other urchin in the trade. Also known as the Mine Urchin and Slate Pencil Urchin, it is common for the spines of the Pencil Urchin to become encrusted with coralline algae, bryozoans, and sponges over time as this species remains stationary for most of the daylight hours, turning each individual into a living, growing piece of the aquascape. It is a hardy, characterful, and genuinely fascinating animal, but it comes with important caveats around reef compatibility that require full transparency before purchase. This is not the right urchin for every system, and choosing it for the right setup makes all the difference.

Why we love it

• One of the most visually distinctive urchin species available; the thick, crayon-like spine silhouette is immediately recognizable and unlike anything else in the cleanup crew
• Particularly effective against overgrown coralline algae on glass, overflows, and equipment; one of the only animals that will actively consume and control coralline growth in areas where it has become excessive
• Research suggests this species is technically a spongivore by preference, making it potentially useful in systems where encrusting sponge growth has become a nuisance on corals or rock
• Over time its spines become beautifully encrusted with coralline algae, sponges, and other growth, making each individual a unique and ever-changing part of the display
• Hardy, long-lived, easy to feed, and safe to handle; one of the more forgiving urchin species available for experienced hobbyists who plan the setup correctly

Suggested parameters

• Temperature: 24 to 26°C (75 to 79°F)
• Salinity: 1.025 to 1.026 SG; sensitive to sudden salinity changes and must be acclimated slowly
• pH: 8.1 to 8.4
• Alkalinity: 8 to 10 dKH
• Calcium: 400 to 450 ppm
• Magnesium: 1250 to 1400 ppm
• Nitrate: Low and stable; will not tolerate high nitrate levels; if water conditions are poor, it will shed its spines
• Phosphate: 0.03 to 0.12 ppm
• Copper-based medications are immediately fatal to all echinoderms and must never be used

Care and temperament

• Difficulty: Easy
• Temperament: Peaceful toward fish and most tankmates; will not actively attack anything
• Reef safe: With extreme caution; this is the most important section of this listing and must be read carefully before purchase
• Minimum tank size: 115 litres (30 gallons) with ample live rock
• This species spends most daylight hours sheltered in rocky crevices and moves actively at night; aquascaping must include stable, secure structures with accessible hiding spots and enough room for the urchin to navigate without constantly destabilizing rocks
• Sturdy, thoroughly secured rockwork is non-negotiable; this animal will dislodge rocks and unsecured items as it moves and this will not change with time
• Do not attempt to pry it out of a crevice or off of the rockwork; it will hold on so firmly that its spines will break before it releases its grip

Reef compatibility: be honest with yourself

This is where the Pencil Urchin requires the most careful consideration. The hobbyist community is genuinely divided on this species, and both experiences, reef-safe and not, are real and documented.

• By natural preference this species is a spongivore, but when preferred foods are unavailable it has been documented eating a very wide range of organic material; how reef-safe an individual proves to be depends heavily on how consistently well-fed it is
• Community reports suggest a preference for LPS corals when the urchin does eat coral tissue; SPS-dominant systems may be lower risk but are not guaranteed safe
• Even individuals that never eat coral tissue remain a physical risk to unsecured frags and corals due to bulldozing during nightly foraging
• Best suited to FOWLR systems, systems with soft corals only, or well-established reef systems where all corals are firmly glued, the urchin is consistently supplemental-fed, and the keeper is prepared to monitor its behaviour
• Not recommended for systems with prized LPS colonies or carefully maintained frag racks

Diet

The Pencil Urchin is primarily an omnivorous scavenger that will graze on algae covered areas and smaller invertebrates including sea squirts and sponges during its nightly forays.

• Grazes naturally on algae, coralline algae, biofilm, detritus, and sponge growth throughout the rockwork
• Nori sheets weighted with rubble or clipped near the rockwork as supplemental feeding
• Algae wafers and sinking herbivore pellets placed on or near the rockwork
• Clams and mussels on the half-shell and other meaty marine-origin foods are also accepted and should be offered periodically to a well-fed individual as part of a varied diet
• Consistent supplemental feeding is one of the most effective ways to reduce the likelihood of coral nipping behaviour; a well-fed Pencil Urchin has less motivation to investigate coral tissue

Compatibility and tankmates

• Compatible with most reef fish of all sizes; poses no threat through aggression
• Avoid triggerfish, pufferfish, and large predatory wrasses that can flip it over and attack its softer underside
• Use caution with small, slow-moving invertebrates that may be trampled or incidentally consumed during nightly foraging
• Keep one per system in most setups; multiple individuals increase physical disturbance and feeding pressure in the aquascape

Health and acclimation notes

Drip acclimation over at least 45 to 60 minutes is strongly recommended before introduction. The Pencil Urchin is very sensitive to high levels of copper-based medications and high nitrate levels; spine shedding is the primary visible indicator that water quality has deteriorated beyond acceptable levels. Once established in a stable, well-fed system with plenty of live rock and secure aquascaping, this is one of the hardier and more long-lived urchin species available in the trade.

What you are buying

• You will receive: 1 Pencil Urchin (Eucidaris tribuloides)
• Body colour ranges from light brown to reddish brown; spine colour varies and individual spines are frequently encrusted with coralline algae, sponge, and bryozoan growth in established systems
• Adult size reaches approximately 10 to 13 cm (4 to 5 inches) including spine span

Drip acclimation of at least 45 to 60 minutes is strongly recommended. All frags and rockwork must be firmly secured before introduction. This species is best suited to FOWLR or soft coral systems, or reef systems where all corals are glued and consistent supplemental feeding will be maintained. Copper-based medications must never be used in any system housing this animal.