Long-Spined Sea Urchin in Cozumel
Long-Spined Sea Urchin (*Diadema antillarum*) in Cozumel: how to identify it, why it matters to the reef, and why not to touch it.
A black ball of very long, very thin, needle-sharp spines that move when a shadow passes. Nothing else on a Cozumel reef looks like it.
Quick ID
| Scientific name | Diadema antillarum |
|---|---|
| Group | Echinoderms — Urchins & Sea Stars |
| Typical size | Spines to 30 cm |
| Depth range | 1–25 m |
| When to see it | Year-round |
| Active | Night |
| Conservation status | Least Concern |
| Notes | Venomous — do not touch · Best seen on night dives |
Where You Will See It In Cozumel
Crevices and rubble at Chankanaab, Paradise Reef and Molas Shallow Reef, mostly hidden by day and out grazing at night.
Reliable sites: Chankanaab Reef, Paradise Reef, Molas Shallow Reef.
Behaviour
Grazes algae, and in doing so keeps algae from smothering young coral. A Caribbean-wide die-off in 1983 removed more than ninety per cent of the population and is one of the main reasons Caribbean reefs shifted towards algae dominance. Recovery has been slow and uneven, and a second die-off event hit the region in 2022.
Commonly Confused With
- Rock-boring urchin — Rock-boring urchins have short thick blunt spines and sit in eroded hollows.
- West Indian sea egg — Sea eggs are white-spined with a dark body and often carry debris on top.
Diving Etiquette And Safety
Spines are brittle, barbed and mildly venomous, and break off in skin. This is the strongest reason not to steady yourself on the reef in shallow water, and to watch your knees on a shore entry.
Part of the Cozumel marine life guide · see Cozumel dive sites · plan your Cozumel dive trip
Photo: Greg Grimes, CC BY-SA 2.0, via Wikimedia Commons.
