Nature’s Sunscreen: How Baltic Fish Protect Their Eyes From Ultraviolet Radiation

Fish living in shallow coastal waters face an invisible threat whenever sunlight penetrates beneath the surface: ultraviolet radiation can damage DNA, proteins and sensitive tissues, including the structures responsible for vision. A new study titled 'UV-protective sunscreen compounds and their differential accumulation across ocular tissues in fish eyes from Baltic Sea,' published in nature in Scientific Reports, reveals that several Baltic Sea fish species concentrate natural UV-absorbing chemicals in particular parts of their eyes, creating what appears to be a carefully positioned biological sunscreen system.

These compounds, called mycosporine-like amino acids (MAAs), are not thought to be produced by the fish themselves. They originate mainly in primary producers such as phytoplankton and seaweeds, enter food webs through feeding relationships, and eventually accumulate in fish. Their ability to absorb ultraviolet wavelengths, combined with their chemical stability and possible antioxidant properties, makes them particularly useful in environments where sunlight can become biologically damaging.

Six Baltic Fish Reveal Where Natural Sunscreen Is Stored

Researchers examined adult specimens from six common western Baltic Sea species: European flounder, European plaice, turbot, common dab, shorthorn sculpin and herring. Most were caught in shallow coastal waters at depths below 25 metres, environments where natural sunlight can be strong during spring and summer.

Instead of treating the eye as a single sample, the team separated it into four regions: the cornea, lens, vitreous body, and a combined inner layer containing the retina, choroidea and sclera, referred to as RCS. MRI and Optical Coherence Tomography helped researchers map the anatomy before dissection, while high-performance liquid chromatography was used to identify and measure the sunscreen compounds.

The imaging work also offered a detailed view of fish-eye anatomy. MRI provided undistorted whole-eye structural information, while OCT resolved fine corneal layers at micrometre scale, allowing researchers to connect biochemical measurements with specific anatomical regions more accurately.

Protective Chemicals Sit Between Sunlight and the Retina

Four MAAs dominated the eye samples: palythine, asterina-330, usujirene and palythene, together accounting for more than 97% of the measured compounds. Their proportions differed among species and tissues, showing that fish do not simply spread these chemicals evenly throughout their eyes. Asterina-330, for example, was absent from all tested eye tissues of common dab.

The clearest pattern was the concentration of MAAs toward the front and middle of the eye rather than the innermost tissues. In most species, the lens and vitreous body contained the highest levels, while the RCS region containing the UV-sensitive retina generally held much smaller amounts. Shorthorn sculpin also showed comparatively strong accumulation in its cornea.

This positioning could function like a series of UV filters placed in front of delicate retinal tissue. Light entering a fish's eye travels through the cornea, lens and vitreous body before reaching the retina, meaning MAAs concentrated along that pathway can absorb harmful wavelengths before they penetrate deeper into the eye. The compounds also absorb at different wavelengths, broadening the range of ultraviolet radiation that the eye can potentially screen.

Herring Stand Out With Exceptionally High MAA Levels

Herring showed the strongest sunscreen accumulation of all six species. Researchers measured total MAA concentrations of about 3.59 mg per gram of dry weight in the cornea, 3.81 mg/g in the lens, and 5.05 mg/g in the vitreous body, compared with only 1.68 mg/g in the RCS layer. Palythine dominated every herring eye region, accounting for roughly 65% to 73% of the total MAAs.

The five other species contained substantially lower total concentrations, ranging from approximately 0.03 to 1.13 mg/g dry weight across samples. Flatfish displayed relatively similar overall patterns despite their unusual development, during which one eye migrates to the opposite side of the head. The researchers raise the possibility that this distinctive metamorphosis and adaptation to different light environments could influence MAA distribution, though that connection remains unanswered.

The watery environment inside and around parts of the eye may make these compounds especially effective. The researchers describe a proposed 'button on a string' mechanism in which nearby water molecules help MAAs dissipate absorbed UV energy extremely rapidly as heat. The water-rich vitreous body and externally exposed cornea could therefore provide particularly suitable conditions for this protective chemistry.

The Sunscreen Food Chain Appears to Stop With Fish

A second part of the research asked whether these natural sunscreens continue travelling through the food web when fish are eaten. The scientists examined eyes from several fish-eating predators, including cormorant, harbor porpoise, harbor seal, grey seal, raccoon and otter. None showed detectable UV-absorbing compounds at the sensitivity of the analytical method.

The finding suggests that MAAs can move from primary producers through zooplankton into fish, yet their accumulation may effectively stop at the fish level rather than continuing into warm-blooded predators. Why remains unclear?

Differences in digestive chemistry, cold-blooded versus warm-blooded physiology or biological receptors responsible for transporting MAAs are among the possibilities raised by the researchers, but these explanations still need experimental testing.

Another major unanswered question concerns how fish move dietary MAAs from their digestive systems into such precisely selected eye tissues. Previous work has detected smaller amounts in organs including the gills, heart, intestine, kidneys, liver, skin and stomach, suggesting that some organs may act as transfer points before the compounds reach the eye. The molecular transport system itself remains largely unexplored.

Baltic coastal waters are being altered by warming, eutrophication and increased terrestrial runoff, with coastal darkening reducing how deeply sunlight and UV radiation penetrate. Less UV exposure could lower one source of eye damage, while reduced underwater visibility may make visually guided feeding and predation more difficult. For fish that depend heavily on sight, changes in the underwater light environment could reshape the balance between protection from harmful radiation and the need to see clearly enough to survive.