👀 Why do eyes keep growing in adulthood?

How can the eyes of animals continue to grow after adulthood? This question, long without a clear answer, is newly illuminated by the study of a marine worm with astonishing visual capabilities. The discovery of a striking similarity with vertebrates opens perspectives on shared evolutionary mechanisms.

The marine worm *Platynereis dumerilii* is a model organism used to understand the development of eyes and the brain. Its camera-type eyes, comparable to those of vertebrates, provide it with sharp vision and persist in developing throughout its life. An international research team became interested in this phenomenon to uncover its secrets.


Analysis of these worms' retinas revealed a specific zone at its periphery where neural stem cells are concentrated. This region, named the ciliary marginal zone, is the site of active cell division during phases of eye growth. Nadja Milivojev, first author of the study, observed that these cells divide precisely at the edge of the retina, a characteristic also present in some vertebrates like fish.

Indeed, in fish and amphibians, the ciliary marginal zone produces new retinal neurons that allow for the expansion of the eye. The discovery of an analogous structure in the worm indicates that similar cellular solutions can emerge independently during evolution. Florian Raible, co-senior author, explains that this observation shows how the eyes of these invertebrates add photoreceptors and increase in size continuously.


Another unexpected aspect concerns the influence of environmental light on this growth. Molecular analyses highlighted that a light-sensitive molecule, c-opsin, modulates the activity of stem cells. Present in the worm's photoreceptor precursors, this molecule acts as a switch linking external light to cell proliferation. Thus, light is not only used for vision; it also participates in eye development (explanation at the end of the article).

These results help fill a gap in the understanding of ocular growth in invertebrates and vertebrates. They suggest that common principles might guide the evolution of sensory organs. New questions arise, particularly regarding the potential impact of artificial light on these biological processes. The study of stem cell systems in this worm could help understand how nervous tissues adapt and regenerate.


Kristin Tessmar-Raible, co-senior author, indicates that such discoveries are important for understanding the biology of living organisms. The article detailing this work was published in Nature Communications, offering a solid foundation for future investigations.

The influence of light on biology beyond vision

Light is perceived by living organisms through molecules called opsins, which convert light signals into biological responses. Traditionally associated with vision, opsins are actually involved in many processes, such as the regulation of circadian rhythms or the synchronization of sleep cycles. This functional diversity shows that light acts as a powerful environmental regulator.

Opsins are divided into several families, including c-opsins, found in vertebrates, and r-opsins, more common in invertebrates. The presence of a c-opsin in the worm *Platynereis* suggests that these molecules can have analogous roles in distinct evolutionary lineages. They often act as sensors that trigger cell signaling cascades, influencing, for example, stem cell division.

Beyond the eyes, light modulates aspects such as skin pigmentation or plant growth via photosynthesis. In humans, exposure to natural light is essential for maintaining a balanced circadian rhythm. Disturbances caused by artificial light, particularly blue light from screens, can interfere with these natural processes.

Research on the interaction of light and biology helps understand how organisms adapt to their environment. It also raises questions about anthropogenic impacts, such as light pollution, which could alter biological mechanisms in wild species. By studying models like the marine worm, scientists explore the deep links between the light environment and the development of organisms.