Ancient eye pigments shaped how fish see, fossils reveal

Palaeontologists at University College Cork have found that the evolution of fish is linked to the shape, location and abundance of tiny melanin-rich granules called melanosomes. Remarkably, these microscopic structures survive in 300-million-year-old fossil fish, suggesting that melanosomes have played a key role in vertebrate evolution since the major fish groups emerged around 445 million years ago. The study is published in Nature Communications.

Melanin is best known as the pigment that gives skin and hair their colour, but it also plays an important role in protecting internal organs from harmful substances. In vertebrates, melanin is stored in melanosomes, which come in different shapes and sizes. Surprisingly, little was previously known about the biological function of these differences, or how melanosomes vary among fish species.

To address this gap, the team painstakingly measured roughly 23,000 melanosomes from the eyes, skin and internal organs of 12 modern fish species belonging to the major fish groups: jawless fishes, cartilaginous fishes and bony fishes. They then compared their findings with 3,000 fossilised melanosomes preserved in 300-million-year-old fish fossils from the Mazon Creek biota in Illinois, USA.

The comparison revealed striking differences in eye melanosomes between the major living fish groups. Bony fish possess both meatball-shaped and sausage-shaped melanosomes, whereas all other fish groups examined have only the meatball-shaped variety. Because the same distinctive patterns of melanosome shape appear in the fossil fish, the researchers conclude that these differences in eye structure and vision have been important ever since these groups split, roughly 445 million years ago.

According to Dan Cirtina, PhD researcher and lead author of the study, the different melanosome shapes are linked to vision. In bony fish, sausage-shaped melanosomes in the retina appear to sharpen vision and help control how much light reaches the light-sensitive cells in the eye, although this process is relatively slow. Cartilaginous fish such as sharks and rays, by contrast, have lost these retinal melanosomes entirely, allowing more light to reach specialised reflective crystals at the back of their eyes so they can see better in low-light conditions.

Instead of relying on slow melanosome movements, cartilaginous fish depend more on rapid adjustments in pupil size to control incoming light, helping them quickly adapt to changing light levels as they swim between shallow and deep waters. This contrast between the two groups illustrates how different lineages arrived at distinct optical solutions to the same problem of regulating light reaching the retina.

The team also found that African lungfish have unusually large and abundant melanosomes in their internal organs. These findings support other evidence that melanin protects tissues against physiological stress. The researchers suggest this is an important adaptation that may have enabled early vertebrates to survive outside water and transition to life on land, linking pigment biology to one of the major events in vertebrate history.

Prof. Maria McNamara, Professor of Palaeontology at University College Cork and senior author on the study, said the findings show that melanin is much more than a pigment used to colour skin and hair. In her view, the shape and distribution of melanosomes are closely linked to major changes in fish evolution, including how fish see, how their bodies function and how they adapt to the environment. The fossil fish, she noted, show that these relationships are extremely ancient and are part of our evolutionary history.

Because the research combined measurements from living species with exceptionally preserved fossils from a single 300-million-year-old biota, the study offers a rare deep-time perspective on the sensory biology of early aquatic vertebrates. The work was published in Nature Communications on 27 October 2026 under the title “Melanosome evolution shaped sensory biology and terrestrial adaptations in early aquatic vertebrates”.

Subject of Research: The role of melanosome shape and distribution in the sensory biology and evolution of vertebrate fish

Article Title: Palaeontologists reveal the hidden role of eye pigments in vertebrate evolution

Article References: Palaeontologists reveal the hidden role of eye pigments in vertebrate evolution. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

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Keywords: melanosomes, melanin, fish evolution, vertebrate evolution, vision, fossils, Mazon Creek, retina, lungfish, palaeontology, Nature Communications, Palaeontologists