Sea urchins: Green and Black, Round and Flat, Delicious and Deadly

Yu. M. Yakovlev

The first feeling one gets when seeing a sea urchin is bewilderment: is it an animal at all?! Indeed, its appearance evokes the images of unusual toys made by Nature just for fun. The spherical shape, the rigid shell constructed out of oddly-shaped plates, straight rows of hinged spines… The impression is intensified by the sea urchin's immobility. Even the movement of its spines seems to be actuated by a hidden engine, with a weakened spring or nearly dead battery. By the way, when getting better acquainted with the object under a binocular magnifier, one can notice some moving pipes, suckers, tweezers… Hydraulics at work! In short, it is just a prickly, absolutely noiseless, mechanical toy.

The feeling that you are watching a curiously made thing and not a living being does not disappear after dissecting the latter: the inner space, which you can observe after the transparent cavity liquid - neither blood nor lymph - has flown out, is filled with radial yellow-orange segments, resembling tightly packed plastic containers. No plasticity whatsoever, the triumph of radiality!

In the center of the bottom surface of the shell one can notice quite an intricate device. It is the mouth organ of the sea urchin, which is called Aristotle's lantern. Inside the inner mechanics one can discern some green-grey interlacements of soft wide pipes, which is nothing less than its digestive system.

And at this moment the real nature of the sea urchin is beginning to emerge: the pipes contain not fuel but remnants of algae, its basic food. Some species of sea urchins demonstrate a more sophisticated menu: sponges, bryozoans, and colonial sea squirts. Sea urchins also eat detritus, which contains microscopic algae and microorganisms.

This is what I remember of my first encounter with these unique animals, though with time sea urchins became for me just common sea inhabitants.

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