The Elements of Evolution (44-17-3) Chiton


Nature provides a multitude of examples of multifunctional structural materials in which trade-offs are imposed by conflicting functional requirements. ~ Chinese materials scientist Ling Li et al

Chitons are marine mollusks that arose 500 MYA. They have a segmented shell connected by muscular tissue that allows them to roll up into a protective ball. Chitons can move with surprisingly speed. If they choose, chitons can cling tightly to irregular surfaces.

Chitons do not have a brain and not much of a nervous system. But they are aware of their surroundings and act accordingly: getting themselves picked up by passing waves when they want, using ambush tactics on prey, and reacting to threats of predation.

What Nature has perfected is to use comparatively simple, cheap starting materials and turn them into an exquisite, multifunctional material. ~ German materials scientist Peter Fratzl

While most chiton are nocturnal, some have busy days. Of these, 100 of the 940 extant species have up to 1,000 aragonite eyes embedded in shell crevices. This plethora of peepers let chitons watch the action around them: their minds putting together the multitude of inputs into a single image.

It actually forms a shockingly clear image. ~ Sönke Johnson

Aragonite is a variety of calcium carbonate (aka calcite or calspar) and is one of the most common minerals on Earth. Calcite is a major constituent of limestone, chalk, and marble. It is also the building material for animal shells, exoskeletons, and eggshells. And, in the case of chiton, eyes.

There is a drawback to chiton calcite eyes. The lenses create weak spots in the shell’s armor.

In order to see, they had to back off on mechanical protection. ~ American evolutionary biomechanist Sheila Patek

Chiton eyes nestle in protective grooves, with shell protrusions that partly compensate for the eye spots.

Chitons might see better with slightly larger eyes. But that apparently would compromise shell integrity too much.

Sometimes we assume Nature is perfect. But more often than not it is a perfect compromise. ~ English biologist Andrew Parker