How do animals distinguish between sunlight and moonlight?

Moonlight ocean

A special protein helps bristleworms keep track of the phases of the moon. © steve-goacher/ iStock

The phases of the moon are not only relevant for stargazers, but also for the behavior of many different animal species. Bristleworms living in the sea, for example, use the moon to coordinate their reproduction so that they all swim to the surface at the same time and mate there. Biologists have now discovered how they achieve this synchrony: a special light-sensitive protein in the worms changes its structure depending on whether the sun or moon is in the sky, thus helping the animals arrive on time for the mass orgy.

The lunar cycle is an important clock for numerous marine organisms. Among other things, it helps brown algae, fish, corals and turtles to synchronize their behavior and come together in time for mating. Bristleworms of the species Platynereis dumerilii also “date” each other with the help of the moon. If this decreases and the nights become darker, the worms, which are only a few centimeters long, swim en masse to the surface of the water and, under the cover of darkness, release sperm and egg cells, which then find each other and mature into worm larvae.

A protein as a moon sensor?

But how does this inner lunar calendar of the bristleworms work? Previous experiments have shown that a protein called L-Cry could be central to this. It belongs to the group of light-sensitive cryptochromes, which also help plants, for example, to recognize the time of day and to make optimal use of the available sunlight. The L-Cry of the bristleworms is also apparently able to distinguish between different light valences and thus distinguish between sunlight and moonlight. But how exactly the L-Cry protein manages to do this is still only partially understood.

Researchers led by Hong Ha Vu from the University of Mainz have now, for the first time, taken a closer look at the molecular mechanisms of this inner “moon clock”. To do this, they exposed isolated L-Cry proteins to the simulated light conditions of sunshine or moonlight that penetrates the sea surface and used so-called cryo-electron microscopy to examine how the three-dimensional structure of the cryptochrome protein changed depending on the simulation.

Unusual structural change as key

The result: The two subunits that make up L-Cry proteins were linked together in moonlight, but separated from each other in sunlight, as the researchers report. Such behavior has not yet been observed with any other cryptochrome. When the structure of these light-sensitive proteins changed in past experiments, it was always the other way around: the subunits were separated at night and connected during the day. It is not yet entirely clear what added value this reversed process brings to the bristleworms. But Vu and his team already have a guess as to how exactly the restructuring will work.

The researchers assume that the increased photon flow from sunlight first stimulates the coenzyme flavin adenine dinucleotide (FAD), which is associated with the L-Cry, after which it accepts an electron from the amino acid tryptophan. In this altered state, the FAD could ultimately cause the subunits of L-Cry to separate and thus change its structure. Or to put it another way: “The intense sunlight always activates both subunits in the protein at the same time, which initiates the disintegration of the dimer into the individual subunits. In significantly weaker moonlight, however, only one of two subunits is statistically activated,” explains co-author Eva Wolf, also from the University of Mainz. However, exactly how the change in protein structure tells the bristleworm when it is mating time still needs to be examined in more detail.

Source: Johannes Gutenberg University Mainz; Specialist article: Nature Communications, doi: 10.1038/s41467-023-42708-2

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