A fish has been found - the only invertebrate known to science - that uses mirrors, not lenses to focus light for its eyes.|
"At these depths it is flashes of bioluminescent light from other animals that the spookfish are largely looking for. Interestingly, this might not be a workable solution for other critters; part of the issue is the depth (and darkness) in which the class of fish lives.
"The diverticular eyes image these flashes, warning the spookfish of other animals that are active, and otherwise unseen, below its vulnerable belly."
The mirror uses tiny plates, probably of guanine crystals, arranged into a multi-layer stack.