When I first heard of optogenetics, I figured they had the idea completely backwards. optogenetics uses light from outside the organism to trigger actions inside cells in the organism. I thought the idea would be to put light-emitting genes inside organisms, and then "read out" the underlying biology. I learned only recently that somebody is working on adding fluorescence to cuttlefish to see "when neurons fire" (with spatial info), among other thiings: https://www.science.org/content/article/scientists-engineere...

Of course my long game is to add cuttlefish-style "pulsing, glowing" tattoos to humans.