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Sea urchins are Echinoderms, spiny-skinned animals related to starfish and sea cucumbers.  Echinodermata is a rather small phyla of animals and is unique in that virtually all members of this group are strictly marine.

They also, as a group, employ nearly every type of feeding method exhibited by animals.  Sea stars are predators, crinoids are filter feeders, as are some sea cucumbers.  Many types of sea cucumbers are deposit feeders (swallowing sand, digesting the organic material and passing the undigestible bits).  Sea urchins alone are mainly herbivores.  The animal above is my local Purple Sea Urchin (Arbacia punctulata).

Being marine animals the main plant life urchins consume would naturally be algae.  There are two basic forms; macro-algae (seaweeds) and micro-algae.  Micro-algae is best known as the slippery scum that coats coastal rocks that make walking in the intertidal zone so treacherous. In order to eat algae an animal must have a way of either cutting the macro-algae into swallowable bites or scraping the micro-algae off its substrate.  Sea urchins have a tool that can accomplish both tasks.

Like all echinoderms, urchins have a five-part body plan.  Sea stars have five arms (or multiples of fives), sea cucumbers have five sets of branched tentacles.  Although it may not appear so from looking at a live urchin (see photo above), they also have a body that is divided into fives.  This can best be seen when looking at the dead skeleton, called a "test".  Count the sections in the tests pictured below.  Five.

The mouth of the urchin is a structure unlike anything else found in the animal kingdom.  It is basically a set of five calcium plates located in the center of the underside of the body.  The term used for this structure is "Aristotle's Lantern", named for the description the 4th century B.C.E philosopher and naturalist gave it based on its resemblance to horn lanterns used during his time.  In his book "Historia Animalium" Aristotle wrote:

"In reality the mouth-apparatus of the urchin is continuous from one end to the other, but to outward appearance it is not so, but looks like a horn lantern with the panes of horn left out."

   
Horn lantern, left.  Urchin mouth, right.

This mouth isn't simply five teeth, but consists of a surprisingly complex combination of calcium plates, muscles and connective tissue which combine to form a structure that is adaptable enough to chew seaweed into bits (picture a caterpillar eating a leaf), scraping and swallowing a cell-thick layer of micro-algae off a rock, or even biting off chunks of flesh from a dead fish (although their main diet is plants, urchins are opportunistic omnivores and will scavenge dead animals if they come across them.  They are also cannibalistic, but only on the dead of their own kind.)

The muscular system attached to the five teeth enable the animal to move the whole structure up and down as well as side to side.  So when grazing algae the mouth first retracts into the body a bit, muscles work to separate the five teeth (opening the mouth wide), push the structure forward and close the teeth together (gripping the plant) and even moving it side to side to tear a piece off of the plant.  Muscular contractions within the mouth itself then move the food into the stomach.  As simple as all this sounds, it really is an extraordinary organ used by such a simple invertebrate.

The muscular system associated with the lantern is unbelievably powerful, and I'll give three examples of this. First, in many areas the grinding action of sea urchins scraping algae off of rock or coral is a major contributor to erosion (specifically, this process is known as "bioerosion").  A swarm of urchins (and they do travel in swarms) can reduce rocks to sand over the course of just a few years.  Second, they now have to be taken into account when building concrete pilings used for bridges since places with large populations of urchins can actually undermine the sturctural integrity of the cement.

Third is a personal experience.  When keeping these animals in captivity care must be taken that no electrical wires are in the water.  I use submersible filters in many of my marine displays which have a thick coating of plastic covering the wiring.  Urchins, scraping algae growing on the wires, will chew right through to the copper. I learned this the hard way when, reaching in to an urchin tank, I got a shock that knocked me on my ass.  Below is one of my urchins scraping micro-algae growing on the glass of its aquarium.  Sucton cups on the ends of the hundreds of tube feet secure the animal to the glass as the mouth parts go to work.

When keeping urchins alive in captivity you obviously need to provide them with proper nutrition.  After getting really tired of collecting seaweed in the dead of winter I decided to try store-bought greens a few years back.  I placed a big leaf of romaine lettuce in their tank and watched as they swarmed all over the leaf and devoured it in a matter of hours.  When I came back the next morning I found a tankful of dead and dying urchins.

I had a suspicion of what happened, so I collected dozens more specimens but this time fed them romaine purchased from an organic food store.  Again they devoured the leaf and the next day they were all fine.  Even after being washed, the pesticide residue left over from the grocery lettuce was enough to kill them overnight. My urchins now eat $5 heads of lettuce.

In some areas urchins are in a coevolutionary battle with a species of algae known as Desmarestia. Coevolution is a response by one species to the adaptive behaviors of another. In this case seaweeds like Desmarestia are heavily grazed upon by urchins.  This algae has evolved a defense whereby it actually produces small amounts of sulfuric acid in its cells.  When eaten by an urchin the acid dissolves parts of the calcium mouthparts, making them unable to grab and tear off parts of the plant's blades.  Urchins in areas where this algae grows feed on other species instead.

Fun Fact: In addition to romaine lettuce, I've discovered my urchins like dried sushi wrappers more than anything else.

Other diaries in this series can be found here.

Originally posted to Mark H on Fri Aug 11, 2006 at 06:23 PM PDT.

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Comment Preferences

  •  Tip Lantern (15+ / 0-)

    I'll hang around as long as possible to answer any questions as best I can.  And yes, I know algae aren't true plants, but they do photosythesize and that's good enough for me.

  •  The story about the lettuce (5+ / 0-)

    makes the point about eating organic in the starkest possible way. Thanks for a fascinating diary!

  •  Beautiful Pics! (3+ / 0-)
    Recommended by:
    station wagon, kraant, Cato come back

    I think the sea urchins you have are very lucky to have you taking care of them! 5 bucks a head for organic lettuce, yeah, they're worth it aren't they?

    I did a bit of scuba diving many years ago, sea urchins were fascinating and beautiful.

    Thanks for the wonderful, interesting diary, I look forward to the next one!

  •  Thank you (0+ / 0-)

    I enjoy your diaries so much. They are terrific!

    Oh and Mark, and your blog isn't too shabby.  I have it in my bookmarks.

    ;)

  •  Mmmmm... Uni. n/t (0+ / 0-)

    "Good idea Chuck, but Syrup won't stop 'em." Firesign Theater, Everything You Know is Wrong.

    by 3card on Fri Aug 11, 2006 at 07:25:32 PM PDT

  •  SusanG (10+ / 0-)

    Thanks for the rescue! And me with my lost TU status today. And "Enough about us" you wrote earlier was top notch.

    •  I read this too late (4+ / 0-)
      Recommended by:
      WI Deadhead, greeseyparrot, Fabian, eastmt

      to be able to tip your original Tip Lantern, so here you go.

      I haven't commented before, but I do read every one of your Marine Life entries, and have found them all fascinating.  I hope you don't think that just because there aren't a lot of comments doesn't mean that only a few people are reading.  I find that you do such a good job of giving complete explanations that there aren't really any questions left to ask, and I generally feel silly just posting a "Good job!"  I imagine other dKos users feel the same way.

      Tonight, though, I can second another poster's comment about the organic lettuce anecdote - I'm going to forward that to people I know with an interest in healthy eating, give them some more ammo.

      I'm also wondering if the San Francisco Bay has a large population of sea urchins, seeing as how we have a few rather important bridges in the area...

      •  re: San Fransico Bay (7+ / 0-)

        I'm on the East coast so I'm not very up to date on pacific urchins. However, my understanding is that when sea otters were heavily hunted (they are a major predator on urchins) the urchin population exploded causing problems with the kelp beds.

        As far as bridges go, this would mostly apply to small bridges with narrow supports. A bridge near me that was replaced had extensive damage due in part to bioerosion from sea urchins.

        •  Nice series Mark. I only got to (2+ / 0-)
          Recommended by:
          greeseyparrot, Fabian

          this after the rescue. One thing I have learned about urchins is, that as sashimi, they are the single worst tasting thing on the planet. :)

          Happiness depends on being free, and freedom depends on being courageous. - Thucydides

          by Clive all hat no horse Rodeo on Sat Aug 12, 2006 at 09:24:29 PM PDT

          [ Parent ]

        •  Here's another for your lost TU (2+ / 0-)
          Recommended by:
          gogol, WI Deadhead

          And thanks for these wonderful diaries.

          Glad you added that disclaimer about the algae aren't plants thing. It was bugging me - bio prof here.

          If I ever get a chance to teach organismal bio again, I can't wait to share some of your diaries with my students - hope that's OK with you.

          Sea urchins . . . my background is developmental biology, and I spent a summer taking a course at the MBL in Woods Hole (one of the cooler experiences of my life). One particularly good memory is mixing sea urchin sperm and eggs in a petrie dish and just watching . . . for several hours . . . the first cell divisions in sea urchin embryos.

          The only thing Republicans do well is take our tax dollars and transfer them to the rich, instead of providing the services we thought we were paying for.

          by Janet Strange on Sat Aug 12, 2006 at 09:27:10 PM PDT

          [ Parent ]

  •  Sea Urchin Harvesting... (2+ / 0-)
    Recommended by:
    WI Deadhead, Fabian

    One of the more interesting adventures I shared was buying a share in an urchin harvesting boat, an 83' former ocean tugboat with provisions for four people to walk the bottom in ten to fifteen feet of water, breathing through air hoses and scooping up urchins with a grabber tool into mesh bags.

    On board was a crew of six who did nothing but scoop out the eggs and flash freeze them for shipment to  Japan. In 1970 we got sixteen bucks a pound at our end. God knows what they cost over there.

    More sex enhancement, I think, was the reason for the price. Where the urchins were they were in vast numbers. I think they were actually some kind of invasive species. This was the west side of the San Juan Islands, between the Puget Sound (Seattle) and the Straits of Juan de Fuca. We worked out of Port Townsend, but it didn't last long. Too many regulations, between the big boat, the diving, seafood, and international trade.

    Fun though. I don't like the taste either.

    Faith or evidence. Decide.

    by ormondotvos on Sat Aug 12, 2006 at 11:11:41 PM PDT

  •  MarkH, a wonderful diary and still more kudos (1+ / 0-)
    Recommended by:
    greeseyparrot

    for diary rescue.

    I voted for  Mermaid's Purse but Fins would have been my second choice.

    So please do  Mermaids, but please also give Fins a second chance.

    And since I never have a problem saying the obvious, Keep Up the Good Work!

  •  Darn, I really need to learn... (1+ / 0-)
    Recommended by:
    Fabian

    to remember to check my Hotlist, I would have missed this if it weren't for Diary Rescue.
    Great piece as always, I really appreciate your work, thank you so much.

    Behold the Lambs of Kos

    by greeseyparrot on Sun Aug 13, 2006 at 12:04:04 AM PDT

  •  This is such a wonderful article (0+ / 0-)

    It makes me want to know more.  

  •  red tide victims (0+ / 0-)

    Nice article ... but I was expecting the obligatory 'red tide wiping out urchins' near the end.

    I don't have to tell you we're strip mining the seas and I'm sure you're aware of the consequences ... but others may not be.

    "Religious bondage shackles and debilitates the mind and unfits it for every noble enterprise" - U.S. Constitution author and fourth President James Madison

    by Iowa Boy on Sun Aug 13, 2006 at 06:16:45 AM PDT

  •  Mouth Structure (0+ / 0-)

    So when grazing algae the mouth first retracts into the body a bit, muscles work to separate the five teeth (opening the mouth wide), push the structure forward and close the teeth together (gripping the plant) and even moving it side to side to tear a piece off of the plant.

    Like a post hole digger?

    post hole digger

    -7.50 -6.56 | Why is it that those who can remember that those who forget history are bound to repeat it are bound to repeat it?

    by cmanaster on Sun Aug 13, 2006 at 08:17:43 AM PDT

  •  probably too late to mention this but (0+ / 0-)

    a friend of mine at the dawn of molecular biology wanted to isolate a non-vertebrate actin gene. Standard strategy for isolating a gene then was to find a cell that expressed a lot of the protein. He knew that the tube feet of sea urchins were rich in actin, so he went to Woods Hole and arranged to get urchins from MBL (Marine Biological Labs). With their agreement he would let the urchins settle onto a sheet of glass and then pull them off - the tips of their tube feet stayed behind but the urchins were ok. He scraped the tube feet residue off with a razor blade and isolated the RNA, and returned the urchins to MBL for most of what he paid for them. He called this operation "Rent an Urchin."

    "In theory there is no difference between theory and practice. In practice there is."

    by Wee Mama on Mon Sep 04, 2006 at 12:34:47 PM PDT

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