Mollusk Eyes Reveal How Future Evolution Depends on the Past – Quanta Magazine

All light-sensing structures on the chiton shell, Varney explained, are attached to nerves, which pass through the shell slits to connect to the bodys main nerves. The slits function as cable organizers, bundling sensory neurons together. More slits mean more openings through which nerves can run.

It so happens that the number of slits is standard information that is recorded anytime somebody describes a new chiton species. The information was out there, but without the context of a phylogeny to map it back to, it didnt have any meaning, Varney said. So I went and looked at this and started seeing this pattern.

Varney saw that twice, independently, lineages with 14 or more slits in the head plate evolved eyespots. And twice, independently, lineages with 10 or fewer slits evolved shell eyes. She realized that the number of slits locked into place which kind of eye type could evolve: A chiton with thousands of eyespots needs more slits, whereas a chiton with hundreds of shell eyes needs fewer. In short, the number of shell slits determined the evolution of the creatures visual systems.

The findings lead to a new set of questions. One that the researchers are actively investigating is why the number of slits constrains the type of eye that can evolve. Answering that will require work to elucidate the circuitry of the optic nerves and how they process signals from hundreds or thousands of eyes.

Alternatively, the relationship between eye type and the number of slits might be driven not by the needs of vision but by the way the plates develop and grow in different lineages, Sumner-Rooney suggested. Shell plates grow from the center outward by accretion, and eyes are added throughout the chitons life as the edge grows. The oldest eyes are those in the middle of the animal, and the most recently are added at the edges, Sumner-Rooney said. As a chiton, you might start life with 10 eyes and finish your life with 200 eyes.

As a consequence, the growing edge of a shell plate has to leave holes for new eyes many small holes for the eyespots, or fewer larger holes for the shell eyes. Too many or too-big holes could weaken a shell to its breaking point, so structural factors might limit which eyes are possible.

Much remains to be discovered about how chitons see the world, but in the meantime, their eyes are primed to become biologists new favorite example of path-dependent evolution, Nilsson said. Examples of path dependence that can be really well demonstrated, as this case [is], are rare even though the phenomenon is not only common, its the standard way things happen.

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Mollusk Eyes Reveal How Future Evolution Depends on the Past - Quanta Magazine