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Marine worms can distinguish sunlight from moonlight

Marine bristle worms use a unique protein to differentiate between sunlight and moonlight, according to a new study led by Johannes Gutenberg University Mainz (JGU).

This discovery centers around an atypical cryptochrome protein (Cry) that plays a crucial role in synchronizing the marine organisms’ internal lunar calendar with the moon’s phases, particularly vital for their reproductive cycles.

Focus of the study 

Cryptochromes are light-sensitive proteins found in various organisms, crucial for several biological processes. The marine bristle worm Platynereis dumerilii utilizes a specialized Cry protein, known as L-Cry, to discern sunlight from moonlight and various moon phases. 

This capability is essential for aligning their reproductive activities with the full moon, operating on an internal monthly calendar or circalunar clock.

The researchers employed the cryo-electron microscopy platform at the University of Cologne to visualize the three-dimensional structure of the L-Cry protein under different lighting conditions. 

What the researchers learned 

These structural analyses, combined with biochemical investigations primarily at Mainz University, revealed that L-Cry forms a dimer (two subunits linked together) in the dark, but under intense sunlight, it disassembles into monomers (single subunits).

This behavior is notable for two reasons. First, the spatial arrangement of L-Cry’s two subunits in the dark is unique, differing from arrangements seen in other Cry proteins. 

Second, the direction of light-induced changes in L-Cry is opposite to that observed in other Cry proteins, which typically transition from monomer to dimer in the light.

Professor Eva Wolf of the JGU Institute of Molecular Physiology, who led the study, explained the significance of these findings.

Critical insights 

“Our findings could explain how L-Cry manages to distinguish between sunlight and moonlight: Intense sunlight always activates both subunits of the dimer simultaneously, which initiates its breakdown into individual subunits. The significantly weaker moonlight, however, statistically only activates one of two subunits,” explained Professor Wolf.

These insights have expanded our understanding of L-Cry’s function as a photoreceptor, distinguishing it among the diverse Cry proteins known for various functions, including sensing Earth’s magnetic field in birds.

Challenging research 

Hong Ha Vu, a doctoral candidate and significant contributor to the study, highlighted the challenges of the research. 

“Working with light-sensitive proteins is always a challenge,” said Hong Ha Vu. “When preparing the L-Cry proteins for analysis, we need to carry out all experimental processes in the dark or under specifically defined red light conditions to prevent unintentional pre-activation of these very light-sensitive proteins.”

“For the functional characterization of L-Cry, it is also necessary to use lighting conditions similar to underwater natural sunlight and moonlight illumination of the kind that the bristle worms encounter in their natural habitat.” 

“Only then we can compare the specific properties of L-Cry in its role as a sunlight and moonlight receptor with those of other cryptochromes.”

Study implications 

“Our investigations have provided important new insights into how this most unusual sunlight and moonlight receptor works,” said Professor Wolf. 

“Furthermore, our structural and molecular mechanistic insights into L-Cry’s function have opened up future avenues of research that should help us better understand the still largely unknown molecular processes involved in synchronization of the circalunar clock with the moon phases.”

Marine bristle worms 

Marine bristle worms, belonging to the class Polychaeta, are a diverse group of generally marine annelid worms. They are known for their distinctive parapodia (fleshy protrusions on each body segment) that bear many bristles made of chitin. 


These worms are found in various environments, from the coldest ocean temperatures of the abyssal plain to the extreme heat near hydrothermal vents. They inhabit all depths of the oceans, with more than 10,000 species described.

Body forms

Polychaetes exhibit a wide range of body forms and lifestyles. They can be brightly colored, iridescent, or even luminescent. Typically less than 10 cm in length, they can range from 1 mm to 3 m. 

Their bodies are segmented, and each segment has parapodia, which are used for movement and often act as the primary respiratory surfaces. Their diets vary, including predators, herbivores, filter feeders, scavengers, and parasites. 

The mouth of polychaetes varies depending on their diet and can include a pair of jaws and a pharynx that can be everted rapidly for feeding.

The outer body wall of a polychaete consists of a simple columnar epithelium covered by a thin cuticle. Underneath this lies a layer of connective tissue, a layer of circular muscle, a layer of longitudinal muscle, and a peritoneum surrounding the body cavity. 

Key characteristics 

Most polychaetes have a simple but well-developed circulatory system, with contractile blood vessels, and some species have rudimentary hearts.

Polychaetes exhibit a variety of interesting reproductive strategies. Many species show bioluminescence, and some have complex eyes capable of sophisticated vision. 

Their nervous system is relatively advanced compared to other annelids, with a large brain and various sensory organs, including eyes and statocysts​​​​.

The study is published in the journal Nature Communications.

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