I’ve done a post on sea urchins before, but it was only a broad overview. Today I’d like to focus on one particular genus of sea urchins, the slate pencil urchins. I saw a picture of these guys and just knew I had to write about them.

A nice picture of a pencil urchin out of the water. Image by Mountainamoeba, CC BY 2.0, via Wikimedia Commons

There are five species of slate pencil urchin, all in the genus Eucidaris. They are found in the Indian, Pacific, and Atlantic Oceans, as well as in the Caribbean Sea. They like warm tropical waters, and are usually found in shallow areas, though some can be spotted as deep at 500m.

The most obvious part of pencil urchins is their spines — though I’m not sure we should call them spines. To me, spines should be nice and sharp, and pencil urchins’ definitely are not. They’re more like the eraser end of a pencil than the sharp end. Their bodies can grow to be 6 cm in diameter, with the spines growing to be about the same length.

The (relatively) giant, blunt spines of pencil urchins are used for a number of different purposes. They help deter predators, and also provide the urchins with camouflage, as algae and seaweed get attached to the spines. The urchins use the spines to move around, as well as to anchor themselves in rock crevices for protection.

Here’s what they look like in the water. Image by François Michonneau, CC BY 3.0, via Wikimedia Commons

These sea urchins don’t have a very exciting diet, feeding mostly on algae and detritus. Sometimes they get bored of eating the same old algae, and will prey on sponges and small invertebrates. Delicious! They have relatively strong jaws that are capable of handling their varied diet.

Like a lot of marine invertebrates, pencil urchins are pretty lazy when it comes to reproduction. They simply release eggs and sperm into the water and cross their spines that the gametes will meet and fertilize one another. Larval pencil urchins are equally slothful, drifting on ocean currents until they are ready to settle on the ocean floor and begin adulthood.

I can’t decide whether pencil urchins look hilarious or super cool. Maybe it’s a bit of both. They look a bit like mines, which is pretty neat. Still, those blunt spines are quite silly. Ah well, at least they’re interesting!

Cover image by Nhobgood (talk) Nick Hobgood, CC BY-SA 3.0, via Wikimedia Commons