Atlant’s Column: Skeleton Eyes

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Welcome to the new section on which explores, analyzes and explains microscopical structures:

Calcitic Microlens Plate

Brittle stars look almost the same as sea stars, but they are only as big as an outstretched human hand. Their evolutionary history goes back 500 million years into the past, which means they belong to the dinosaurs of the seas. Since then, they have changed little. Their current design is almost the same as during primeval times.

Text by science writer Atlant Bieri
Photography by Martin Oeggerli

In one of these vintages researchers have discovered a piece of high technology. The brittle star species Ophiocoma wendtii has the ability to detect light and shade. This in itself is nothing special, but for years the researchers found no organs that looked like eyes. As they gave the bone plates on the surface of the arms a closer look using an electron microscope (SEM), the structure revealed arrays of spherical blobs on the surface of the bony structure. At first they seemed to have no function, but in cross-section, they looked like perfectly shaped lenses.

Could this be the long-sought eyes? The current knowledge about optics spoke against it. Lime or calcium for that matter is one of the optically problematic substances. Although calcium is transparent in crystalline form, any light beam passing through the crystal is divided into two different beams. With such a lens of brittle star would see everything twice.

Moreover, the light is not focused in a single focal point, but at different locations behind the lens. The result is a blurred image. This as “spherical aberration” known phenomenon is plaguing the manufacturers of high precision glass lenses, such as those used for cameras. They solve the problem by grinding the curvature of the lenses not uniformly round but instead grind tiny dents or bumps at precisely calculated locations. Thus all the optical errors can be compensated.


Fly proboscis

Triumph of the Brittle Star

This of course is a very expensive and complex business. Is it possible that a brittle star brings thousands of lenses of only 40 micrometre in diameter into the right shape to eliminate all visual errors? The researchers put them to the test. They cut off the lens layer from the central disc and put them on a light-sensitive photographic plate. After exposure, a sharply defined black spot appeared precisely under each lens. The lenses worked perfectly; the brittle star triumphed. After more accurate measurements the researchers found out that the light is focused at a point approximately five micrometres behind the lens. There, they later also found the nerves that respond to light stimuli and make brittle star flee into a dark corner as soon as a predatory fish approaches.

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Continue reading Atlant’s Column: Stealth Eyes, December 2012, or go back to the archive