[This diary was first posted to Daily Kos on August 11, 2006.]
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."
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.