erimo (erimo) wrote,

Sunday sea urchin

A regular feature of PZ Myers' blog Pharyngula is his Friday cephalopod. Allow me to introduce- Sunday sea urchin. I prefer a shorter name for sea urchins, uni, the Japanese word. Ronald Shimek [1, 2] writes that uni "have one of the most complex and architecturally interesting feeding structures known in the animal kingdom. Just inside the mouth is a complex arrangement of both fused and articulating ossicles, supporting five continuously growing teeth [which can exert tremendous force due to simple mechanical advantages]... They meet at an acute angle and the action of eating serves to sharpen the teeth... Such a strong feeding structure developed really for only one reason, to eat calcareous algae." (There are examples of uni eating through lead, copper, iron, and concrete.) This whole assemblage is given the somewhat misleading term "Aristotle's lantern". I like the perspective of the teeth given in the picture below, but to understand the actual dimensions of this structure a view of it from within an uni is indispensable.

Interestingly, uni jaw structures were used to determine the longevity of the red uni, with surprising results. "The largest reported red sea urchins, with body diameters over 19cm, are from British Columbia, Canada, (Bureau, 1996) and with estimated jaw lengths of about 2.8 cm would be expected to be around 200 years old (Fig. 2A)." [1]

I'd have never suspected any of this about these small round animals with a water-vascular system. And yet these are just a few unique traits among many that uni possess. Below is a photo of an uni's jaws and teeth, you may be able to identify some of the features.

Edit: cross posted to invertebrates (view entry).
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