Closeup of three moon jellyfish

The eyes of the moon jelly (Aurelia aurita) include pigmented cells that sense light. Credit: Getty

Evolution

Eye evolution came easy for simple sea creatures

Family tree shows that jellies and their relatives evolved eyes independently at least eight times.

Jellyfish and their kin have no brains and make do with rudimentary nervous systems. But an analysis now shows that these simple sea creatures evolved eyes multiple times, transforming basic precursor cells into a wide range of useful visual systems.

Using DNA sequences, Natasha Picciani and Todd Oakley at the University of California, Santa Barbara, created an evolutionary tree of Cnidaria, the large grouping — or phylum — that includes jellyfish, sea anemones and corals. They then incorporated information about the species’ light-sensing abilities. The team found that the common ancestor of today’s cnidarians could probably detect light and dark, but lacked specialized eyes.

Among the descendants of that eyeless ancestor, at least eight separate evolutionary events gave rise to eyes, including some with lenses and others with simple structures called eye cups. At least two major types of cnidarian eye use different molecular systems for light detection, suggesting that different lineages co-opted ancestral genes independently to enable vision.