• Welcome to ScubaBoard

  1. Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

    Benefits of registering include

    • Ability to post and comment on topics and discussions.
    • A Free photo gallery to share your dive photos with the world.
    • You can make this box go away

    Joining is quick and easy. Login or Register now by clicking on the button

DIVE DRY WITH DR. BILL #820: THE PENCIL THICK URCHIN

Discussion in 'California' started by drbill, Jul 13, 2019.

  1. drbill

    drbill The Lorax for the Kelp Forest Scuba Legend

    # of Dives: 2,500 - 4,999
    Location: Santa Catalina Island, CA
    22,341
    5,127
    113
    DIVE DRY WITH DR. BILL #820: THE PENCIL THICK URCHIN

    Jimmy Buffet, a musician I've grown to like more as I pass 40 (plus a few decades), sang about a pencil thin mustache. You youngens do know what a pencil is, don't you? It's an ancient writing instrument that preceded the iPhone. Now I've never considered a pencil to be thin, perhaps because I still occasionally use ones from my grandfather's coal company which date back to 1942... five years before I was born.

    No, I'm not going to write a column about pencils. However, there is a species of spiny skinned critter (echinoderms such as starfish, sea cucumbers and the like) known as the slate pencil urchin (Eucidaris thouarsii). I was pleased to see it was first described by Louis Agassiz of Harvard way back in 1846. He founded Harvard's Museum of Comparative Zoology where I did my undergraduate research. Actually there are a number of species throughout warmer waters of the globe (oh how I wish) and the one I've seen here in Catalina waters is rarely sighted north of the Gulf of California (Sea of Cortez), Mexico. Its distribution extends down to Panama. As far as I can tell from my research, Catalina is the only one of the eight Channel Islands where it has been seen.

    My sighting of this species was at the warmest end of Catalina, the SE end. We were diving Blue Car Wreck north of the East End Quarry back in mid-June of 2006 when I spotted one in a rocky reef on the inside of the kelp line. I knew what it was immediately as I had filmed this species down in Mexico previously. Initially I was surprised to find it in our waters, but discovered that it was occasionally sighted here.

    As you can see from the images, the spines on this species are much thicker than on most sea urchins. Had it been the species present in 1965's movie Thunderball, James Bond would never have had the pleasure of sucking the spines out of the lovely Domino's foot. Fortunately for him (but not for her), the species in the film was probably a relative of our local black or Coronado urchin (Centrostephanus coronatus) which does have sharp spines like most sea urchins.

    The spines 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 500 feet. The one I discovered here as well as the ones I filmed in Mexico were tightly hidden among rocks. I carefully moved the rocks away to film the individual here, and then returned it and the rocks to their previous position. It is said that this species will move out of the rocks at night when many potential predators are asleep.

    From the perspective of evolution, the slate pencil urchins are the oldest living group of sea urchins. Yes, even older than Dr. Bill! 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 the sharply pointed spines which serve to better protect the animal while it is out in the open.

    Like most urchins, this species uses its five-part jaw structure (referred to as Aristotle's lantern) to scrape algae off rocks. This structure is of hardened calcium carbonate like the test or exoskeleton of urchins. Apparently in warmer waters, this and other related species may also feed on coral. I wonder if it is for the one-celled photosynthetic zooxanthellae in the coral tissues or the polyp itself.

    Sadly, I've only filmed this one pencil urchin in our waters. Its rarity may be related to our usually colder waters here at the very northern tip of its distribution. Perhaps it only arrives on Catalina during warm water episodes like El Niños. The SE end of the island where I spotted this individual is among the warmest regions in the Channel Islands.

    Oh, and for my readers who are dying to hear about their sex life, here goes. These urchins are either male or female (gonochoric). Fertilization is external even though the dull spines wouldn't be as much of a problem as those on sharper-spined species! Eggs may be brooded. Upon hatching the larvae enter the plankton where they may drift for several months before settling down. A thrilling life story, isn't it?

    © 2019 Dr. Bill Bushing. For the entire archived set of over 800 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page

    Image caption: Images of slate pencil urchin from Catalina waters and the much more common southern species, the Coronado or black urchin with thin, sharp spines.

    DDDB 820 pencil thick urchin sm.jpg
     
    Bob DBF likes this.

Share This Page