Mar 3, 2024 - Science

Mollusk eyes point to fork in evolution's path

Chiton tuberculatus, a chiton that uses many eyespots to achieve spatial vision on a beach in the US Virgin Islands.

Chiton tuberculatus, a chiton that uses many eyespots to achieve spatial vision, on a beach in the U.S. Virgin Islands. Photo: Douglas J. Eernisse

An ancient group of marine mollusks evolved two different ways to see — and the visual trajectory a species took may depend on a seemingly unrelated body feature, a new study suggests.

Why it matters: The findings weigh in on a fundamental question and fierce debate in biology — why does evolution follow the path it does, and can it be predicted?

  • The sweet spot for studying this question is when the same features — in the case of these mollusks, systems for seeing — evolve from different starting points, what's known as convergent evolution.

Context: Chitons are marine mollusks that live on rocks, typically in shallow water or intertidal zones. They have eight plates that overlap to form their shells and don't appear to have changed much over their 450 million years on Earth.

  • Scientists knew chitons have tiny sensory organs called aesthetes embedded in their shell plates. Aesthetes have light receptors that make them sensitive to light but they don't give the animals vision.
  • These light receptors likely evolved into eyes in some chiton species.
  • Except it happened in "two totally different ways," says Rebecca Varney, an evolutionary biologist at the University of California, Santa Barbara, and co-author of the new study.

How it works: Some chiton species have shell eyes that form images using lenses made of limestone.

  • Others have clusters of photoreceptors known as eyespots that are attached to the aesthetes and allow the animals to respond to shadows.
  • Still other species haven't evolved either system and make do with aesthetes.
  • Scientists have proposed the visual systems evolved stepwise from aesthetes to eyespots to shell eyes, but the new research suggests a different path.

What they did: The team created a phylogenetic tree of chitons and mapped when eyespots and shell eyes emerged on it.

  • They found the "visual systems evolved separately in chitons at least four times: two lineages through eyespots and two other lineages through shell eyes," they write in the journal Science.
Many shell eyes (black spots) are visible on the shell of the chiton Tonicia lebruni.
Many shell eyes (black spots) are visible on the shell of the chiton Tonicia lebruni. Photo: Anthony Draeger

Zoom in: When the researchers looked at the morphology of the different chitons, they found a correlation between the visual system and the number of slits a species has in its shell plates, where nerves from the shell connect to the rest of the body.

  • Chitons with eyespots have more slits than chitons with shell eyes.
  • These slits appear to be "critical junctions" that set the species' visual course. Varney says there are likely others.

The intrigue: Chitons' shell eyes "represent the most recent origins of camera-type eyes known," the authors write. These eye systems are similar to those in humans and other vertebrates.

  • They also found eyespots in chiton may have originated even more recently and quickly — within 7 million years, far faster than another estimate that the evolution of the eye in vertebrates took place over 30 million years, they write.
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