Moon jelly and golden jellyfish, Raja Ampat, West Papua, Indonesia

© Alex Mustard/Minden Picture

Sunshine on a moon jelly. World Jellyfish Day

Today we're celebrating World Jellyfish Day—even though these creatures are best described as otherworldly. Despite their alien qualities (not to mention their inability to vocalize), jellyfish have a lot to say to us about the history of life on Earth.

For example, species like the moon jelly and smaller golden jellies in our picture possess primitive eyes that only detect the presence of light, like the nourishing sunbeams they're chasing here. But some of their cousins, like the box jellyfish, boast several complex eyes with corneas and retinas. These differences help biologists understand how the human eye evolved to process such dazzling spectacles as the jellyfish's delicate dance.