April 12, 2024


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Researchers solve the mystery of a sea creature whose eyes evolved throughout its shell: ScienceAlert

Researchers solve the mystery of a sea creature whose eyes evolved throughout its shell: ScienceAlert

Small, scaly, unassuming chitons have eyes unlike any other creature in the animal kingdom.

Some of these marine slugs have Thousands of puffy little peepers Embedded in their segmented shellsAll with lenses made of a mineral called aragonite. Although small and primitive, these sensory organs called specimens are believed to exist Capable of true vision, Distinctive shapes And so is the light.

However, other chiton species have smaller “eye spots” that act like individual pixels, much like the components of an insect or mantis shrimp. Al Ain Complexand form an optical sensor distributed on the ketone shell.

A new study investigating how these different visual systems emerged has revealed the surprising evolutionary ingenuity of these rock-dwelling creatures: their ancestors hastily evolved eyes on four different occasions, giving rise to two very distinct types of visual systems today.

Although not quite as repetitive as crabs and their body plans, which have evolved at least five times, the study once again shows how evolution offers multiple solutions to basic problems, such as how to use light to avoid becoming lunch.

“We knew there were two types of eyes, so we didn't expect four independent origins.” He says Evolutionary biologist and lead author of the study Rebecca Varney of the University of California Santa Barbara. “The fact that chitons evolved their eyes four times, in two different ways, is very surprising to me.”

The visual systems of chitins consist of small, light-sensitive structures called caecilians (left; green) interspersed with either larger scalloped eyes (middle; blue) or smaller, more numerous eye spots (right; red). These structures are connected to the optic nerves via openings in the cortical plates. (Varney et al., Sciences2024)

To reconstruct this evolutionary history, the researchers compared fossils and analyzed DNA samples taken from specimens preserved at the Santa Barbara Museum of Natural History to piece together the chitons. Evolutionary tree.

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The analysis showed that the two visual systems evolved twice each in rapid succession. But curiously, the groups that arrived at similar visual structures were not the ones most closely related to each other; They were distant relatives, separated by millions of years.

Eye spots evolved into one group of chitons 260 to 200 million years ago during the Paleolithic. TriassicWhen dinosaurs first appeared, they were just outgrowing the first shell eyes developed by another group in the modern era Jurassic About 200 to 150 million years ago.

Then shell eyes evolved again between 150 and 100 million years ago, during the Paleolithic Cretaceous periodin the chitons Toniciinae and Acanthopleurinae, making it the newest lens eye ever to appear.

Finally, eye spots evolved again on a different branch of the chiton evolutionary tree late in the 20th century PaleogeneAbout 75 to 25 million years ago.

Close-up of bumps on ketone shell.  Some are darker in color, almost purple, others are light beige.
Chiton's eyes. Dark spots are eyes, small bumps are aesthetics. (Wyss Institute at Harvard University)

After piecing together a timeline, Varney and his colleagues remained curious about the possible conditions that drove this recurring evolution.

Chitins have openings in the cortical sheets through which the optic nerves pass. It has been shown that species with fewer slits tend to develop fewer and more complex scalloped eyes. On the other hand, chitons with larger clefts continued to form more numerous and simpler eye spots.

“Clarification of role [trait] “History in shaping evolutionary outcomes is crucial to our understanding of how and why personalities develop in predictable ways.” We conclude.

As for how these structures feed visual information into… Ketogenic brainThis is the focus of ongoing research.

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What we know so far, who Another recent studyis that in at least one type of chiton, the scalloped eyes are more complex Sending visual information for processing in a ring-shaped neural structure that runs around the entire body. The optic nerves connected to this ring sense the body's location based on which parts of the ring are activated.

The study was published in Sciences.