Long Spined Urchin

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The Long Spined Urchin (Diadema setosum) is a distinctive marine invertebrate known for its exceptionally long, thin, dark spines and rounded body. It often displays iridescent blue spots and a characteristic orange ring around the anal cone, making it easy to recognise.

Native to tropical Indo-Pacific waters, this urchin spends much of its time moving across live rock and reef surfaces in search of algae. Its long spines provide protection from predators and allow it to navigate around the aquarium, so sufficient open space and stable rockwork are important.

Long Spined Urchins can be useful algae grazers in established marine aquariums. However, they may also consume desirable coralline algae and can dislodge unsecured coral frags as they move. They are generally considered reef compatible with caution, rather than completely harmless to every reef setup.

Handle with extreme care. The spines are brittle and sharp, can penetrate skin, and may break off if touched or crushed. Never grasp the urchin by its spines; use an appropriate container and avoid direct contact.

Quick Care Guide

• Scientific Name: Diadema setosum

• Common Name: Long Spined Urchin / Long-Spined Sea Urchin

• Family: Diadematidae

• Type: Marine Invertebrate — Sea Urchin

• Adult Size: Body approximately 8–10 cm; total span with spines can exceed 30 cm

• Temperament: Peaceful

• Care Level: Moderate

• Minimum Aquarium: 250–300 L recommended

• Temperature: 24–27°C

• pH: 8.1–8.4

• Salinity: 1.023–1.026 SG

• Diet: Herbivore

• Food: Hair algae, film algae, seaweed and nori

• Tank Level: Live rock and aquarium surfaces

• Reef Safe: Yes, with caution

• Suitable For: Established marine and reef aquariums

• Aquascape: Secure rockwork with open grazing areas

• Important: Extremely sharp, brittle spines; sensitive to copper and sudden water changes

Frequently Asked Questions

Is the Long Spined Urchin reef safe?

Generally, yes. It is an effective algae grazer, but it can knock over unsecured coral frags and may consume desirable coralline algae. 

What does a Long Spined Urchin eat?

It feeds primarily on algae growing on rockwork and aquarium surfaces. Supplement its diet with nori or other suitable marine algae if natural growth is insufficient. 

How big does it grow?

The body can reach around 8–10 cm, while the long spines can extend the overall diameter considerably, sometimes beyond 30 cm. 

Are its spines dangerous?

Yes. The spines are extremely sharp and brittle, and some species have venomous spines that can cause painful injuries. Avoid direct contact and never handle the urchin by its spines. 

Can it live with corals and marine fish?

It can coexist with many reef inhabitants, provided the aquarium has enough algae and stable conditions. Secure loose corals and avoid predators such as triggerfish and pufferfish that may attack urchins. 

Why is my Long Spined Urchin losing its spines?

Spine loss can indicate stress, starvation or poor water quality. Check salinity, temperature, pH and other water parameters promptly, and ensure it has sufficient food. 

Common Mistakes

• Handling the urchin directly or brushing against its spines.

• Adding it to a new aquarium with insufficient algae.

• Failing to secure coral frags and loose rockwork.

• Allowing it to starve after algae levels decline.

• Exposing it to copper-based medications.

• Making sudden changes to salinity or temperature.