Plankton Has Eye Made of Domesticated Microbes

The oceans are full of eyes. Giant squid scan the depths with the world’s largest ones, which are oddly similar to those of the sperm whales that hunt them. Mantis shrimps watch for prey using eyes that work like satellites. Starfish stare through the tips of their arms, chitons look up through lenses made of rock, and scallops peer at the water through dozens of eyes with mirrors inside them. But to see the strangest eyes of all—eyes so weird that we can’t even be sure that they are eyes—you have to squint.

These maybe-eyes belong to a group of rare, free-swimming algae called warnowiids. Each consists of a just one round cell, so small that a few hundred of which could fit in this full stop. Under the microscope, each warnowiid contains a conspicuous dark dot. This is the ocelloid. It consists of a clear sphere sitting in front of a dark red strip, and has components that resemble a lens, an iris, a cornea, and a retina.

Eyes are meant to be animal inventions. They’re supposed to comprise many cells. They are icons of biological complexity. And yet, here’s a non-animal that packs similar components into its single cell...