All light-sensing structures on the chiton shell, Varney explained, are attached to nerves, which pass through the shell slits to connect to the body\u2019s main nerves. The slits function as cable organizers, bundling sensory neurons together. More slits mean more openings through which nerves can run.<\/p>\n

It so happens that the number of slits is standard information that is recorded anytime somebody describes a new chiton species. \u201cThe information was out there, but without the context of a phylogeny to map it back to, it didn\u2019t have any meaning,\u201d Varney said. \u201cSo I went and looked at this and started seeing this pattern.\u201d<\/p>\n

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\u2019 visual systems.<\/p>\n

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.<\/p>\n

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 chiton\u2019s life as the edge grows. \u201cThe oldest eyes are those in the middle of the animal, and the most recently are added at the edges,\u201d Sumner-Rooney said. As a chiton, \u201cyou might start life with 10 eyes and finish your life with 200 eyes.\u201d<\/p>\n

As a consequence, the growing edge of a shell plate has to leave holes for new eyes \u2014 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.<\/p>\n

Much remains to be discovered about how chitons see the world, but in the meantime, their eyes are primed to become biologists\u2019 new favorite example of path-dependent evolution, Nilsson said. \u201cExamples of path dependence that can be really well demonstrated, as this case [is], are rare \u2014 even though the phenomenon is not only common, it\u2019s the standard way things happen.\u201d<\/span><\/p>\n<\/div>\n

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