Dive Dry with Dr. Bill

#294: Is There an Urchin Doctor in the House?

I developed a fascination for sea urchins when I was a senior in high school back in Chicago. I had already decided I wanted to be a marine biologist IF I ever grew up, but had no idea what area to specialize in. Then one night a few of my buddies and I drove to a theater in a neighboring city to watch the new James Bond film, "Thunderball." When the lovely Domino, played by Claudine Auger, stepped on a sea urchin while emerging from the water, and Sean Connery removed the spines from her feet; I knew then and there that I either wanted to become an echinoderm specialist... or a medical doctor specializing in urchin spine removal! When I began my studies at Harvard the following academic year, I was fortunate to find a mentor who was a world authority on these spiny skinned creatures. That saved me from medical school.

Now Ms. Auger and Mr. Connery were in tropical waters where sea urchin spines tend to be long and sharp, and often toxic. The urchins I encountered in the cold waters of Massachusetts had short spines and were not toxic, but were edible... well before I discovered sushi. When I came to Catalina, I discovered the convergence of the cold California Current and warm subtropical waters created a combination of different types of sea urchins. In fact, two of our local species (the red and purple urchins) are closely related to the green urchin of the waters near Boston, and one (the black urchin) is more closely allied with the ones seen in "Thunderball." However, I'm still waiting for a lovely "lady go diver" to step on their spines and call out to Dr. Bill for help.

Today's column focuses on the black urchin, also known as the Coronado, crowned or banded sea urchin. The Channel Islands off southern California are the northernmost part of its geographic range. It is known as far south as the Galapagos. Although they are common in shallower waters, their maximum depth has been reported as 360 feet. The juveniles may be found in the intertidal, especially under rocks and in small crevices. The adults remain partially hidden in protective cavities and crevices during daylight.

The dark black color of this urchin helps distinguish it from other common local species like the red, purple and white urchins. There may be hints of red in it, but nowhere near as intense as the color of the red urchin. The spherical exoskeleton or test of this urchin is fairly small, about two inches in diameter, and the spine length is 2-3 times the diameter of the test. The red urchin has a very large test and much shorter spines. The red urchin's spines feel smooth if touched carefully, while the black urchin's spines are serrated and rough if you run your finger down them. Although the spines are usually solid black, they may be banded with white, especially in the juveniles.

The black urchin is a member of the urchin family scientists call the Diadematidae. This family includes many of the shallow water tropical urchins. Although many of those are toxic, this species is not. However, the spines are brittle and may break off, embedding in the skin of an unwary bather, snorkeler or diver.

The black urchin has a varied diet. Although drift algae is a large part of it, especially during the day, they also munch on sponges, barnacles, bryozoa, tunicates and even scavenge on dead fish and other critters. Further south they even feed on corals. These additional menu items are often a consequence of their unique defense strategy against the vicious sheephead fish that feed on them when exposed. Urchins feed with a toothed structure known as the Aristotle's lantern. The calcium carbonate teeth are even used to drill shelter holes in the rock, and continue to grow as they are worn down by abrasion.

During the day, when sheephead are actively munching (and mating), the black urchin remains in its hiding place, extending its sharp spines outward to ward off the attacks of any fish that might attempt to feed on them. However, sheephead like many other wrasses, retire at night to sleep in shelters in the rocks or on the bottom. As night falls, the urchins come out of hiding and begin to graze in a small area surrounding their shelter hole safe from their major fish predator. Thus the area around an urchin shelter often has fewer species, and thus lower biodiversity, than areas where they have not ventured to feed.

Divers occasionally capture these and other urchins and break them open to feed sheephead, garibaldi and kelp bass. While there are areas in southern California where some urchin species have overpopulated, this is not the case on Catalina. I consider it a major "no no" to do this in our waters. It is ecologically unnecessary to control the urchin populations here since their natural predators, including sheephead and lobster, have not been over fished to the extent that they can't do the job themselves. It is my opinion that the practice of feeding marine life is not a good message for new divers. However, even I will feed the few remaining abalone in our waters with a nice, juicy blade of drift kelp!

© 2008 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 and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. 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," "Common Fish and Invertebrates of the Sea of Cortez" or the new "Sharks and Rays of Southern California" DVD's. Yes, take Dr. Bill home with you... we'll both be glad you did!

Black or Coronado urchin out in open at night and in protective hole with defensive spines during day; micro-
scopic view of spines showing the serrations and close-up of the teeth of the Aristotle's lantern in the mouth.

This document maintained by Dr. Bill Bushing.
Material and images © 2008 Star Thrower Educational Multimedia