A medium-sized chiton commonly known as black katy can sometimes reach a length of 12 centimeters. (Photo by Bob Armstrong)

A medium-sized chiton commonly known as black katy can sometimes reach a length of 12 centimeters. (Photo by Bob Armstrong)

On the Trails: A diversity of eyes

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)

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.

More in Sports

Snow is also a four letter word with more than one proper use. (Klas Stolpe / Juneau Empire)
Pure Sole: WTF with the F word

The F word. F… U… and no need for that third and… Continue reading

Jessica Larsen of the University of Alaska Fairbanks Geophysical Institute describes her research on Alaska’s Mount Churchill at the Fall Meeting of the American Geophysical Union on Dec. 9, 2024. (Photo by Ned Rozell)
Alaska Science Forum: The threat within an Alaska mountain

Mount Churchill stands in a white corner of the Alaska map, deceptive… Continue reading

Juneau-Douglas High School: Yadaa.at Kalé senior Dylan Sowa (35) beats Bartlett senior Wyatt Rust (13) in a faceoff earlier this season. Sowa scored four goals in Thursday’s 5-4 extra-time win over the Houston Hawks to open the 2025 ASAA Division II Hockey State Tournament at Soldotna. (Klas Stolpe / Juneau Empire file photo)
Crimson Bears open postseason by devouring Hawks on extra-time ice

Sowa wears big hat for sixth-seed JDHS in win against third-seed Houston at state hockey tournament.

Wasilla’s Layla Hays blocks a Colony shot in the Warriors win over the Knights in last season’s state title game. Hays, a senior, and her number-one ranked Warriors play JDHS inside the George Houston Gymnasium on Friday and Saturday. (Klas Stolpe / Juneau Empire file photo)
Sports on tap for this week feature current and future state champs

Sports fans living in Juneau, or visiting, will be treated to a… Continue reading

Glacier Swim Club athletes Valerie Peimann, Emma Fellman, Pacific Ricke, Lily Francis and Lucia Chapell dress for the cool pool weather at the 2025 Alaska Senior Championships in Soldotna last weekend. (Photo courtesy GSC)
Peimann, Fellman lead GSC at Alaska Senior Championships

Glacier Swim Club brings 11 swimmers to Soldotna meet, set club record.

Juneau-Douglas High School: Yadaa.at Kalé Nordic Ski Team and community cross-country skiers start the Shaky Shakeout Invitational six-kilometer freestyle mass start race Saturday at Eaglecrest Ski Area. (Klas Stolpe / Juneau Empire)
Crimson Bears cross-country skiers in sync

JDHS Nordic Ski Team tunes up for state with practice race

Thunder Mountain Middle School eighth grader Carter Day of the Blue Barracuda Bombers attempts to pin classmate John Croasman of War Hawks White during the inaugural Thunder Mountain Mayhem Team Duels wrestling tournament Saturday at TMMS. (Klas Stolpe / Juneau Empire)
Inaugural Thunder Mountain Mayhem Tournament makes most of weather misfortune

More than 50 Falcons wrestlers compete amongst themselves after trip to Sitka tourney nixed.

An adult double-crested cormorant flies low. (Photo by Bob Armstrong)
On the Trails: Some January observations

One day, late in January, a friend and I watched two Steller… Continue reading

In this file photo Juneau-Douglas High School: Yadaa.at Kalé seniors Cailynn, left, and Kerra Baxter, right, battle for a rebound against Dimond High School. The Baxters led JDHS in scoring this weekend at Mt. Edgecumbe with Cailynn hitting 23 on Friday and Kerra 28 on Saturday. (Klas Stolpe / Juneau Empire file photo)
JDHS girls sweep Mt. Edgecumbe on the road

Crimson Bears show road strength at Braves’ gym.

Most Read