The classic black long-spined urchin — a strikingly mobile pincushion of fine, banded, mildly venomous spines, marked by five white dots and a bright orange anal ring. It is the most effective algae grazer in the hobby, but its long, brittle, stinging spines make it hazardous to handle and a risk in tightly aquascaped reefs.
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Educational only. KinStation content is reviewed by licensed veterinarians but cannot replace an in-person exam. Always consult a licensed veterinarian or board-certified specialist for diagnosis, treatment, or any decision affecting your pet's health.
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Test about 5-7 cm (2-3 in); needle-thin spines commonly 10-15 cm and up to ~30 cm long.
Lifespan
4–10 years
Social needs
solo
Native region
Indo-Pacific (Red Sea to Japan and Australia)
Origin
Old World
Climate
🌴 Tropical
Water type
🌊 Marine
Family
Diadematidae
Genus
Diadema
Part of the Sea Urchins
Spiny echinoderm grazers prized as reef clean-up crew for mowing down film, hair and nuisance algae. Most are reef-safe but may dislodge loose corals, and all are highly intolerant of copper and sudden salinity changes.
How this animal changes through its life — each stage often has its own care, diet and space needs.
Photo coming soon
Larva
Most marine invertebrates hatch into microscopic planktonic larvae (such as the zoea of crustaceans or the bipinnaria/veliger of echinoderms and mollusks) that drift and feed in the water column. The larva looks nothing like the adult and undergoes major reorganization.
Photo coming soon
Juvenile
After settling out of the plankton, the juvenile takes on a recognizable miniature of the adult body plan — a tiny shell, a small star, or a translucent shrimp. Crustaceans grow by molting, shedding the exoskeleton to enlarge.
Photo coming soon
Adult
Adults reach full size and reproductive maturity with the species' mature shell, shape, or coloration. Many continue to molt or grow throughout life, and some show sex differences in size or claw/appendage shape.
Color & pattern variants
Natural variants occur in the wild; selectively bred (man-made) variants were developed in captivity.
Natural
Black (adult)
The familiar all-black adult form with five white test spots and a vivid orange anal ring.
Banded juvenile
Juveniles show pale-and-dark banded spines that darken with age — a natural life-stage difference, not a separate morph.
Habitat & enclosure
House a single specimen in an open, established reef or FOWLR tank of at least 55 gallons (210 L) with room for its long spines to move without skewering corals or being snapped against the glass. Keep stable tropical reef parameters: temperature 72-80F (22-27C), pH 8.1-8.4, salinity SG 1.024-1.026, alkalinity 8-11 dKH, calcium 400-450 ppm and low nitrate.
It ranges across the Indo-Pacific from the Red Sea to Japan and Australia, on reef flats and lagoons. Provide caves and overhangs it can wedge into by day; it grazes more at night. Moderate flow and any reef lighting are fine.
Substrate
Live rock with caves and ledges is the priority — it grazes the rock and shelters in crevices by day. Any sand or bare-bottom base works beneath; give it structure it can tuck its spines into.
Equipment & setup
Standard reef gear is sufficient: skimmer, live-rock biofiltration, heater and moderate flow. Avoid tight, fragile aquascapes and delicate corals near the open swimming area, since the spines will reach them.
Diet
A prolific herbivore and arguably the best algae mower available to the hobby, grazing film, hair, turf and even tougher macroalgae off rock. Supplement with nori on a clip or algae wafers if the tank's algae can't keep up, since a hungry Diadema will scour rock bare quickly.
Behavior & temperament
Peaceful toward fish and corals, but a mobile hazard: the long spines can dislodge or puncture corals and clams, and the urchin constantly repositions. It is most active at night, sheltering in crevices by day, often with its orange ring and iridescent blue test visible. Keep one per tank unless very large; small fish (cardinals, juveniles) sometimes shelter among its spines in the wild.
Health
CAUTION — the hollow, brittle spines are mildly venomous and deliver a painful sting with swelling; they snap off easily and embed in skin, so always use thick gloves and tongs and never handle it bare-handed. Health-wise it is very sensitive to copper, salinity swings and air exposure like all echinoderms; spine loss signals starvation or water-quality problems. Stings are usually self-limiting but can be serious for sensitive individuals — seek medical advice if spines are deeply embedded or reactions are severe. (Educational only, not medical or veterinary advice.)
Tips, DIY & hacks
Drip-acclimate slowly and move it only with a rigid container and gloves — never net it or grab it. Add it to control a serious algae outbreak in a roomy tank, then watch that it doesn't strip the rock or skewer corals. Keep a sting first-aid plan (warm-water immersion, careful spine removal) in mind before you buy.
Reviewed and signed off by: KinStation Editorial — pre-launch draft (pending DVM review) on 2026-06-09