Urchins are really into rock

I'm thinking that maybe there is a career in sea urchin dentistry.

While working at Sepok Point today, I came to realize that most of the biomass in sea urchins in the Verde Island Passage is arguably in the form of Echinometra mathaei, also known as the rock boring urchin.  The concept of a boring urchin is not new to my colleagues here, who are very gracious in tolerating my windy stories about what I think are remarkable animals.  However, Echinometra really does bore -- right into rock.  It does this in part by abrasion from its formidable spines, but mostly by using its teeth.  Urchins actually have five teeth, mounted in a 5-part, radial jaw that can open and close like the chuck of a drill.  This jaw apparatus is known as Aristotle's lantern, for reasons that are obscure and part of a strange history of urchin nomenclature.  But we'll let that go for now or this blog entry will end up being a treatise, not a nature nugget.  Suffice it to say that the mouth of the urchin is situated on the bottom surface of the globose body, and large enough to allow protrusion of the Aristotle's lantern so that it can chew on, um... the rock.

The 5 teeth of an urchin are sharp and chisel-shaped at the tip. Although they are largely made of limestone, the teeth are hardened by "doping" this limestone with magnesium, a process known as dolomitization.  The term, incidentally, derives from an Italian mountain chain known as the Dolomites.  The mountains are very hard limestone with... high magnesium content.  Who would have thought there was a connection between sea urchin dentition and mountains in Tyrol?

Anyway... these hard little teeth can do a lot of damage to rock, especially over the lifespan of a sea urchin.  Here is the culprit, removed from his (or her -- it's hard to tell from the outside) boring life in the rock:

In spite of the hardened teeth and fierce-looking spines, these little fellows are gardeners.  Sort of.  They don't chew straight down into the rock, but make a channel, or a groove in the stone.  This channel is enlarged as the animal grows, and as it harvests its food.  This consists of algae growing in the channel.  Bare surfaces don't stay that way for very long in the sea.  Algae is a very quick colonizer of newly exposed surfaces, including those chewed to nakedness by busy little urchin teeth.  As the urchin chews, it removes a bit of the rock along with the algal food, thereby doing two jobs -- making a protective channel in which to live, and getting nutrition from the algae growing inside this home.  Soon, the rock can look a bit like Swiss cheese:

When the algae at one end of the channel are all eaten, the urchin moves along to the other, allowing regrowth of the plant cover.  By the time the urchin gets to the end of the channel, there is enough regrowth to make it worthwhile to move slowly back to the other end again, munching the newer algae as urchin inches along.  The rock is hard, but hey, the urchin has all day.  There is some evidence to suggest that individual urchins can keep this up for decades.  In light of that supposition, it's no wonder that a perfectly good coastline can start to look like this for much of its length:

Which brings us to the question of urchin dentistry.  Now that I think of it, maybe the idea of being an echinoid dental practitioner needs some rethinking.  Besides being very hard, sea urchin teeth are advanced along the inside of the jaw as they wear out, providing fresh tooth tip as the old tip erodes away.  Urchin teeth are very cleverly designed as long shafts of tightly packed, minute plates such that as these flake off the worn end, they leave a fresh, sharp edge.  No need for night guards or other hideous plastic dental aides to prevent wear from gnashing or grinding.

One could say that the teeth are boring, but never dull.

Along some parts of the Mabini coast, I have noticed that young Echinometra start out in less boring accommodations.  They crawl into a dead barnacle, which has become a shell of its former self:

Presumably as they outgrow this living situation, the urchins crawl out of the barnacle and start a new channel, or get into an old groove left behind by an urchin who has gone on to that great cheeserock in the sky.

I should mention that there is an urchin that does burrow straight down into the rock, making a cylindrical shaft in which it can live like Timmy stuck in a shallow well.  This urchin, Echinostrephus aciculatus, has no need for rescue from Lassie, though, as it is perfectly shaped to fit into its tiny well, with the spines sticking out ever so much:

How Echinostrephus manages to make this amazing vertical shaft, which is several times as deep as the urchin is high, is not fully understood.  Presumably they use their Aristotle's lantern as well.  The shaft of the boring is long enough that when disturbed, for example by an urchin-hunting echinoderm biologist, the urchin quickly skootches (to use the highly scientific term) down into the hole.  The animals are nicely designed for this.  The body is slightly conical, with a narrow end pointing downwards.  This end is furnished with lots of strong tube feet that pull the urchin downward.  The spines on the bottom of the body are very short, as opposed to the ones on the more exposed top, as can be seen in this laboriously extracted captive, flipped upside down (i.e. mouth upwards):

How this stationary urchin makes it's living is not understood either.  Some claim that it snags bits of drifting food with the long, exposed spines.  One has to question whether that is enough.  They can't farm like the Echinometra does, because it's too dark down there in the recesses of the bore hole.

As usual, nature leaves me with no recourse but a fascinated shrug.