Circadian rhythms are well known to biologists, with hundreds of studies analyzing fundamental links between sunlight, cellular clocks, hormones and metabolism function.

But for the first few billion years of Earthly life, it wasn’t just solar cycles that mattered. Lunar and tidal cycles were just as important, and for modern marine creatures they still are. Yet these cycles have received only a smidgen of scientific attention.

“When you look at the literature of circadian and lunar rhythms, they were equally prominent in the literature” until the early 1980s, said evolutionary neurobiologist Kristin Tessmar-Raible of Austria’s University of Vienna.

That’s when the first circadian clock gene was cloned in a fruit fly, allowing scientists to manipulate its function in a common model organism, and focus shifted. “Everything switched in modern molecular biology to what you could look at in fruit flies and mice. Those only have circadian rhythms,” she said.

In a research review published in the March Bioessay, Tessmar-Raible and Florian Raible, a molecular biologist at the University of Vienna, describe the ubiquity of lunar and tidal cycles in ocean creatures, and the still-embryonic understanding of how those cycles work.

Their own interest was sparked several years ago in work on Platyneereis dumerilii, a marine worm known to evolutionary biologists as a living fossil, last sharing a common ancestor with vertebrates 600 million years ago. They found a previously unknown, light-sensitive cell deep in the worms’ brains, far from any light.

How many different clocks can you have? It’s an open question.

Mystified at its location, they researched the worm’s natural history, and learned that their wild spawning cycles occurred in time with lunar cycles. At conferences with marine biologists, Raible and Tessmar-Raible learned of a vast literature on animal behavior and lunar cycles.

From algae to jellyfish to worms to crustaceans to mollusks to fish, examples abound of behaviors that change according to moon and tide. Molecular research is just beginning now, and questions abound. Raible and Tessmar-Raible’s most basic question is how the lunar clock mechanisms work — and, indeed, how many different clock mechanisms there are.

“How many different clocks can you have? It’s an open question,” said Raible. “You can imagine that the inputs could differ between species. It doesn’t have to be light. It could be the pressure of the water. This is all up for investigation. It’s going to be very interesting to see and compare between species. It may be the same system, or there may be several independent systems that have evolved.”

Another question is how lunar clocks don’t interfere with circadian clocks, and vice versa. Yet another is whether land-dwelling creatures still have lunar clocks. It’s not uncommon for complex terrestrial vertebrates to share features with ancient marine ancestors; in humans, female reproductive cycles may correlate with lunar cycles, though evidence is mixed.

However, Raible and Tessmar-Raible note that many other animals’ reproductive patterns show no connection to the moon, and warn against speculation.

To them, understanding lunar cycles is less about investigating potential terrestrial analogues than coming to a deeper understanding of ocean creatures, which — despite humanity’s landed perspective — dominate Earthly life.

“First we want to understand how these things work in organisms that really have lunar clocks, and see which molecules are involved,” said Tessmar-Raible. “And then, are they really involved in vertebrates? Do we have them, and what are they doing? Let’s see.”