ARTICLE 137    2015


by Jenny Edwards


When you keep watch for small things while walking on the beach it is amazing what you can find. The people who regularly pick up pieces of rubbish, such as tiny pieces of plastic, have very well trained eyes.


Maybe that is why Stacy and Donovan Wade noticed a little, roughly cone-shaped item that appeared to be made of bone or shell. They had found something that the ancient Greek philosopher and naturalist, Aristotle, described as being like a horn lantern. He was describing the mouth parts of a sea urchin and ever since these structures have been referred to as Aristotle’s Lantern.


Sea urchins, sea stars and their relatives have a five section, radially symmetrical design. This pattern has been around almost since complex life on earth began so it obviously works. Instead of two jaws with multiple teeth in each like us, urchins have five “jaws” or pyramids with one tooth in each. In many ways this arrangement is superior to ours.


The teeth are continually grown at the inner end of the lantern as the urchin absorbs calcium carbonate from the sea water and converts it to calcite – think of marble. The renewal process keeps up with wear and tear so that the teeth never become shorter. They are also self-sharpening at the biting edge. The microscopic structure of each tooth is layered so that the hard calcite crystals shear off with wear leaving the tooth as sharp as new.


Muscles in the five pyramids contract to push the teeth out to bite the algae on which they feed. Other muscles pull the teeth closed and inwards delivering the food to the oesophagus at the upper end of the lantern.


Some urchins even use their teeth to bore hollows into rock where they shelter from predators when not out feeding. Our local small urchins, Heliocidaris species, do this. In some locations such as Guerilla Bay there can be dozens per square metre.


Search “Aristotles Lantern” on the Internet and you will see more photos and drawings, including some of the ancient type of five sided lanterns that were used to protect candles from the wind.


Photos by Stacey and Donovan Wade  

Mystery object found at North Rosedale Beach

Separated to show the jaws and teeth.