Long-spined sea urchin
Diadema setosum
— Where the name comes from
Diadema is the band Persian kings wore around the tiara, from the Greek diádēma, from diadéō, “to bind around”: here it names the ring of iridescent blue spots and the orange point the animal carries at the centre of its back, like a crown. Setosum means “bristly”, which is a euphemism for spines thirty centimetres long and as thin as needles.
— Facts
- Group
- Echinoderms · Echinodermata
- Order
- Sea urchins
- Habitat
- Coral reefs and shallow rocky bottoms
- Range
- Indo-Pacific; entered the Mediterranean through the Suez Canal
- Size
- Test 6–9 cm; spines up to 30 cm
- Diet
- Algae
- Lifespan
- 4–6 years
— Characteristics
- 01
The spines are hollow, very brittle, and snap off inside the skin.
- 02
It points its spines towards an approaching shadow.
- 03
The orange dot on its back is an anus, not an eye.
— What sets it apart
It sees without having eyes. It has no visual organs, but its whole surface is covered with light-sensitive cells, and the spines themselves act as a screen: the animal compares the light arriving from different directions — shaded by its own spines — and derives from it a kind of coarse vision, enough to aim its spines at an advancing shadow and to head for dark crevices. It is a compound eye the size of the whole animal. It entered the Mediterranean via Suez, and in 2022 a sudden die-off caused by a protozoan wiped out its Levantine populations within months.
