What it means to be an eye is so much broader than we originally thought. ~ American zoologist Sönke Johnson
Light is a rich information-carrying medium, so it is unsurprising that vision evolved early and in most forms of life. Vision independently evolved over 50 times. Though sight itself is functionally convergent, there are an astonishing variety of vision systems, including eyes that are not at all apparent.
Many invertebrates and vertebrates have different vision subsystems, each tuned to one specific task. ~ English biologist Innes Cuthill et al
Erythropsidinium is a single-celled alga. It has an organelle eye that it uses to catch prey and avoid predators. The eye detects polarized light. Erythropsidinium‘s eye is so extraordinary that no one believed the biologist who first described it over a century ago. But then, it is hard to explain how an organism can see when it has no nerves nor a brain.