# The Free-Running Circasemilunar Period Is Determined by Counting Circadian Clock Cycles in the Marine Midge Clunio Marinus

## TL;DR

The study shows that the marine midge Clunio marinus uses its circadian clock to track semilunar rhythms, challenging previous assumptions about independent clock mechanisms.

## Contribution

The study reveals that the circasemilunar clock in Clunio marinus depends on the circadian clock, challenging the idea of independent operation.

## Key findings

- The free-running circasemilunar period in Clunio marinus changes with diel T-cycle length, supporting a day-counting mechanism.
- Circasemilunar emergence periods under LD 6:6 and LD 12:12 are comparable, indicating reliance on endogenous circadian oscillations.
- Disrupting the circadian clock with constant light eliminates the free-running circasemilunar rhythm.

## Abstract

Semilunar rhythms are found in numerous marine organisms, but the molecular mechanism and functional principles of endogenous circasemilunar clocks remain elusive. Here, we explore the connection between the free-running circasemilunar clock and the circadian clock in the marine midge Clunio marinus with three different chronobiological assays. First, we found that the free-running circasemilunar period of the adult emergence rhythm in C. marinus changes linearly with diel T-cycle length, supporting a day-counting mechanism. Second, under LD 6:6, periods of circasemilunar and circadian emergence were comparable to those under LD 12:12, indicating that the circasemilunar counter in C. marinus relies on endogenous circadian oscillations rather than external T-cycles. Finally, when desynchronizing the circadian clock with constant light, the free-running circasemilunar emergence rhythm disappeared as well, suggesting that it requires a synchronized circadian clock. These results oppose the long-held view that C. marinus’ free-running circasemilunar clock operates independently of the circadian clock. In a broader evolutionary context, our results strengthen the idea that the circasemilunar clocks of dipterous insects are based on different functional principles compared to the circasemilunar or circalunar clocks of marine annelids and algae. These divergent clock principles may indicate multiple evolutionary origins of circasemilunar and circalunar clocks.