This essay was launched by reading an almost unintelligible (to me) scientific paper about chiton eyes. Nevertheless, that paper led to others, and here I have summarized some of the related natural history of chiton eyes. And that led me to consider briefly about how other organisms see…
Chitons are familiar to all of us who wander into the rocky intertidal zones. They typically have eight hard, articulated shells or plates in a row, giving them a kind of armor over their backs. There’s a muscular foot underneath, used for creeping about and for clamping down firmly when a chiton is threatened. Threats often originate from potential predators, such as otters, sea stars, gulls and oystercatchers, crabs, humans, and even some fish.
How does a chiton know when a predator is close? Chitons do not have eyes on their heads (except very briefly as larvae), unlike most animals that have eyes. They have light-sensitive structures on their hard, dorsal shells that are connected via slits in the shell edges to a ring-shaped nervous system under the edge of the armor. That allows a chiton to react to changes in the light above it. Very useful if a predator approaches.
There are three kinds of light-sensitive structures on a chiton’s shells. All chitons have miniscule “aesthetes” distributed all over their shells (even gumboots, with that thick leathery mantle over the back?). They may have several functions and appear to be the most basic of three kinds of light-sensitive structures. Other structures, called eyespots, are less numerous than aesthetes, but are also distributed over the dorsal shells. These may create an array of pixels somewhat like that of the compound eyes of insects.
Lined chitons are small and often graze on coralline algae. (Photo by Mary Anne Slemmons)
Still less numerous are “shell eyes,” which are true image-forming eyes, with a lens that transmits and focuses light on the retina. The lens is made of aragonite (a very hard form of calcium carbonate), quite unlike the protein-based lenses of other molluscs such as octopuses and of vertebrates. Shell eyes are tucked in the little valleys on the surface of the shell, which helps protect them from abrasion. They are reported to function in both air and water.
Even more marvellous is the finding that image-forming shell-eyes evolved in two different chiton lineages, at different times in history. And so did eyespots — in two other different chiton lineages at two different times in history. Altogether, that’s four independent evolutionary origins of eyes among chitons.
Image-forming eyes are known in several other kinds of animals. Vertebrates typically have them, using the transparent cornea and a crystalline lens to transmit and focus light on a retina; focusing involves changing the shape of the lens. Other animals (fishes, octopuses) can focus by moving the lens to various distances from the retina. Changing the shape of the lens or moving it back and forth requires certain little eye muscles. Lacking those eye muscles, predatory snails reportedly cannot focus well, even though they have a sort of light-refracting lens at the ends of their tentacles (perhaps they work best at certain distances from objects?).
Box jellyfish have 24 eyes, some of which can form images using lenses and retinas; they are reported to be used to orient the critter in its mangrove habitats. Deep-sea fishes called barreleyes have two upward-oriented image-forming eyes, equipped with lens and retina. Some of these species also have a secondary kind of eye, with a retina, that gathers light using not a lens but mirrors; these mirror-eyes are oriented downward and sideways; they may be used for observing bioluminescent organisms. Bay scallops have dozens of eyes around their edges, each with a retina, a gelatinous, soft lens, and a set of mirrors. Focusing of light rays on the retina may be accomplished by changing the curvature of the mirrors or of the eye itself. Some small, deep-sea crustaceans called ostracods are reported to use flexible mirrors to reflect light to a retina; vision may be assisted by a thin lens. I wonder how well these animals see their surroundings.
I have here ignored the compound eyes of insects and many invertebrates, which are composed of numerous separate light-processing structures. They apparently do not form really clear images, although they are very good at detecting movement. That might make another essay someday…
• Mary F. Willson is a retired professor of ecology. “On The Trails” appears every Wednesday in the Juneau Empire.
