Long-spined sea urchin (Diadema setosum) care guide

Group
Other invertebrates
Other names
Long-spined urchin, Black longspine urchin, Diadema urchin
Family
Diadematidae
Origin
Indo-Pacific from the Red Sea and East Africa to Australia and Japan; reefs, sand flats and seagrass. Also now in the eastern Mediterranean
Water
saltwater
Maximum size
25 cm
Temperature
22–27°C
pH
8.1–8.4
Hardness
SG 1.023–1.025, dKH 8–12
Minimum tank
300 litres
Care level
intermediate
Temperament
peaceful
Reef safe
with_caution

Care

The long-spined urchin has a black body (test) up to about 7 cm across and very long, thin, hollow, black spines, sometimes banded brown, giving an overall size of about 25 cm. Key features are five white spots on the body, blue spots and lines of iridescent blue, and a bright orange ring around the anal cone on top that can look like an eye. Safety: the spines are brittle, break off easily in skin and are hard to remove, and the venom causes pain and swelling (often compared with a bee sting). Wear thick gloves and use tools when working in the tank, and handle the urchin with a container, not hands. Set-up: a large tank (300 litres or more) with open rock and room for the spines. It is mostly nocturnal, hiding in caves by day and roaming at night, and it can move surprisingly quickly on its spines. It may knock over loose rock and frags, and it scrapes coralline algae as well as nuisance algae, which is why it is marked with caution for reefs. Water: stable, high-quality water. Losing spines is a sign of poor water quality or stress. It is useful in aggressive fish tanks where most invertebrates would be eaten, as fish leave it alone. Copper: copper-based medicines and some plant fertilisers are toxic to shrimp, snails and most invertebrates. Never dose copper in a tank holding this animal, and check fertiliser and medicine labels before use. Sources: LiveAquaria (liveaquaria.com/products/longspine-urchin-black); Wikipedia (en.wikipedia.org/wiki/Diademasetosum). Checked October 2026.

Diet

Herbivore. Grazes algae and seaweed from rock, mostly at night, including film and hair algae and coralline algae. Supplement with dried seaweed (nori) clipped to the rock if algae is scarce.

Breeding

Sexes are separate and look alike. It spawns by releasing eggs and sperm into the water, sometimes linked to full moon or warm water above 25 °C. The planktonic larvae are not raised in home aquariums.

Compatibility and tank mates

Peaceful with fish and corals. Marked with caution because it bulldozes frags and loose rock, scrapes coralline algae and can trap or spike small fish. Triggers and some puffers may attack it. Small fish and shrimp often shelter among its spines in the wild.

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

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