Dive Dry with Dr. Bill

#233: Slate Pencil Urchin

Over the past few weeks, I've been editing all my footage of the spiny skinned marine invertebrates that marine biologists like myself call echinoderms. That scientific name translates simply to... what else, spiny skinned. I'm getting ready to create a new commercial television series I hope to sell to mainland cable networks on kelp forest ecology. So, I've had starfish (sea stars if you prefer), brittle stars, sea cucumbers and sea urchins on my mind. Last week it was the eviscerating sea cucumber Holothuria. This week I'm going to pick on another member of that group.

Occasionally while diving Catalina's waters I encounter a critter that I don't associate with our temperate waters, but with warmer subtropical and tropical ones. The finescale triggerfish and scythe butterflyfish are two excellent examples. Whenever I see them, I think of my dives in Mexico's Sea of Cortez. I frequently see the triggerfish there, fatter and sassier than our local individuals. Although I haven't seen the scythe butterflyfish there (they are generally much deeper that far south), I see plenty of its relatives. The subject of today's column, the slate pencil urchin, is a species of sea urchin I frequently saw down south, but never expected to see in our waters. Yet last year while diving a site we now call Blue Car Wreck between Pebbly Beach and the East End Quarry, I encountered one!

When I returned to my office after the dive to research this discovery, I was surprised to find that Santa Catalina Island is actually listed as the very northernmost location in this species' range. From here, it extends south to Ecuador and the Galapagos Islands. Perhaps I'll see a few on a trip I hope to make down there next year... assuming I can rob our local bank enough times to finance that dive adventure! I don't remember encountering the slate pencil urchin in our waters before, so this was one of those unexpected discoveries that keep me diving here day-after-day, year-after-year... at least until I can afford to move to the tropics! No 7mm wetsuit required there, and certainly no dry suit for this manly man (read: human male lacking a functioning nervous system to sense water temperatures).

The slate pencil urchin is very different from our local sea urchins, even the black or Coronado urchin which is also found in Mexican waters. Instead of sharp, menacing spines like the sea urchins in our area, this species has very thick, blunt, pencil-like spines. They are quite distinctive, making this species easy to identify. There are ten rows of 5-8 spines each. The color of the test or exoskeleton ranges from purple to reddish-brown. A local field guide states it gets up to six inches in diameter, but others report it up to 10" in warmer waters. These urchins wedge between or under rocks and into crevices at depths from the low intertidal to about 450 feet. The one I discovered was tightly hidden among several rocks when I saw it. I carefully moved the rocks away to film the individual, and then returned it and the rocks to their previous position.

The common name, slate pencil urchin, is actually used for a number of species residing all around the globe. They are similar in appearance to the one in our waters. The scientific name of ours is Eucidaris thouarsii with the species name supposedly in honor of an 18th-19th century French botanist Louis-Marie Aubert du Petit-Thouars who specialized in orchids. I'm not sure what those beautiful plants have to do with this rather pedestrian echinoderm, but beauty is in the eyes of the beholder (and if I be a holdin' her, she be a beauty). The strange thing is that the sea urchin was named in 1846, and the French botanist was not born until 1853. Go figure. I think someone who researched this species before me had his or her historical facts mixed up.

If you look at the underside of the urchin in the accompanying image, the mouth is easily seen in the center. There are five hardened calcium carbonate "teeth" forming what looks like a beak. It is used to scrape algae off the rocks in our area, but may also be used to feed on certain corals in warmer waters. Hmmm... I guess I really do need to go back to the tropics to further research these fellers. Like many urchins, this species is believed to remain hidden during the day, out of sight of predators, then come out at night to feed. Of course I use the same tactic on occasion, but it is by the light of the full moon!

Evolutionarily speaking, the slate pencil urchins are the oldest living group of sea urchins. It is said that all other groups evolved from them. Perhaps this is the reason its spines seem adapted for defense only by wedging the animal tightly into rocks so predatory fish and other species cannot dislodge them. More evolutionarily advanced sea urchins usually have sharply pointed spines which serve to better protect the animal while it is out in the open. Of course whenever I think of sea urchins with sharp spines, I think of James Bond... that is the real Bond, Sean Connery... pulling the spines out of lovely Domino's foot in 1965's "Thunderball." I don't think the slate pencil urchin would be asked to return for a second casting call in a remake of that movie. Of course I'm always willing to play the part of the dashing, debonaire Bond. Yes, I realize that's something of a stretch, but...

© 2007 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays. Please help me climb out of self-imposed poverty... buy my "Munching and Mating in the Macrocystis," "Great White Sharks of Guadalupe," "Calimari Concupiscence: Mating Squid, " "Playful Pinnipeds: California Sea Lions," "Belize It or Not: Western Caribbean Invertebrates, Fish and Turtles," "Gentle Giants: Giant Sea Bass" or "Common Fish and Invertebrates of the Sea of Cortez" DVD's. Yes, take Dr. Bill home with you... we'll both be glad you did!

Slate pencil urchin wedged in rocks and on top of the reef;
views of the underside, mouth and teeth

This document maintained by Dr. Bill Bushing.
Material and images © 2006 Star Thrower Educational Multimedia