Thick red spines like blunt crayons, up to 10 cm (4 in) long on a test of 8 cm (3 in), wedged into a crack in the surge zone. Nothing else here looks remotely like it, and the spines are so heavy that a live animal is startlingly solid in the hand.
Those spines are a wedging tool, not a weapon. The urchin jams itself into a hole with them and locks in place, which is how a slow animal survives in the top two meters of a reef where the surge would roll anything loose. It comes out at night to graze algae and returns before dawn.
They are also used by people, and have been for a long time: heavy, uniform and easy to shape, the spines were used as writing implements, files and jewelry, and the name slate pencil urchin is literal.
And they are a record. The spines lay down growth bands and their chemistry tracks the water the animal grew in, so fossil and archaeological spines are read as a temperature archive of shallow reefs going back thousands of years.
- thick blunt spines like red crayons, far heavier than any other urchin's here
- spines triangular in section and often algae-covered at the tips
- wedged into a crack by day, in the top few meters of reef
- test to 8 cm, spines to 10 cm
