Arctic Green Sea Urchin
Strongylocentrotus droebachiensis
Atlantic Rock-boring Urchin
Echinometra lucunter
About
The rock-boring urchin never wanders far. It grinds its own hiding place directly into solid rock and spends its life defending that one small pit.
Also known as: red rock urchin
- Depth
- up to 45 m
How to tell it apart
Its elliptical, rather than round, shape sets it apart from most other reef urchins, but the clearest sign is behavioural: finding an urchin sitting inside its own hand-sized pit ground into solid rock is a strong indicator of this species.
Appearance
This urchin has an elliptical, rather than round, test (the hard shell that forms its body). It carries moderately short spines with wide bases and sharp tips, usually black, while the test itself ranges from black to deep brownish-red, often more reddish on the upper surface than underneath.
Habitat
This species lives on shallow rocky ground and coral reef, usually in very shallow water close to the surface but occasionally down to 45 m. It favours spots with strong water movement, wedging itself under rock slabs, broken coral, or seagrass, especially along high-energy shorelines.
Behaviour
Using the teeth around its mouth, this urchin grinds a bowl-shaped pit into the rock beneath it and shelters there through the day, emerging at night to graze on algae growing within easy reach of its burrow. It defends this pit against other urchins of its own kind. It is preyed on by the king helmet sea snail, and small gobies often shelter underneath its test.
Did you know?
Small gobies often hide underneath this urchin's shell, using its spines for shelter from predators.
Log it in Into the Blue
Describe what you saw and we name it, then keep every species you find in your Oceandex.
