The Enemy Bunker

A group of purple sea urchins I found on my dive. I call them "the enemy" sometimes because they have a nasty habit of eating the holdfast of kelp. Without a holdfast in place, the kelp just floats away, to eventually die and rot. Since until recently mankind had an equally nasty habit of killing the sea urchin's natural predators, they went out of control. They have destroyed entire kelp forests, creating what is called an "urchin barren".
Oh, and a little random fact: urchins, like many echinoderms (sea stars, sea cucumbers, sand dollars), have radial symmetry. This means you can chop one in half which ever way you want, and those halves will look pretty much identical. I think they actually have something called "pentametrous symmetry", which means that there are only five ways you can chop them in half with that result.
Photo taken while diving at Honeymoon Cove, California.