The first eye appears in the fossil record in a trilobite (Olenellus fowleri) over 550 million years ago (Schwab, 2017), and just a short 45 million years later, post Cambrian explosion, we find the earliest example of a camera-type (lens and retina) lateral eye belonging to Metaspriggina walcotti (Lei et al., 2026), a jawless, thumb-sized fish and one of the world's oldest vertebrates (Perkins, 2014). Since these early beginnings, like fish themselves, the fish eye has radiated and adapted to many different environments. For example, opsin (protein that allows light sensing) genes. Unlike mammals, amphibians, birds and reptiles, fish are host to a vast array of opsin genes, facilitated by gene duplications (Cortesia et al., 2015), allowing them to even perceive ultraviolet light (Losey et al., 1999). Diversity is not only found on the molecular level, of course. Take the gross morphological placement of flatfish eye, and the staggering migration it takes across its head (Brewster, 1987). When you want to find the extremes of fish biology, the deep sea is never a bad place to look. Take, for example, the study we highlighted recently on backward swimming in deep-sea fish (Perry, 2025; Priede & Jamieson, 2025). For fish eyes, it is no different. For example, you may have heard of rods and cone cells, two neat binary cells in the retina that convert light into signals for the brain. Well, deep-sea fish have been found with a hybrid mishmash of these cells, shattering a century-old paradigm found in textbooks (Fogg et al., 2026). Other anomalies include the silver spinyfin (Diretmus argenteus) which has the highest number of visual opsins of any vertebrate (Musilova et al., 2019). Moreover, eye adaptations found in the deep sea, such as tubular eyes, have evolved independently multiple times (Musilova & Cortesi, 2025). Rod cells, opsins and tubular eyes aside, adaptations to the darkness also include changing the size of your light-gathering apparatus. That is exactly what Pinti et al. (2025) explored in this issue, who tracked how eye size, relative to body size, changed with increasing depth. Using an incredible repository of images, they were able to measure eye diameter, eye area and fish length from 5639 individuals, representing 551 species and 37 orders. Although there was a broad array of investment in eye size across species, the trend showed that this investment peaked at around 500 m. Such a result has never been calculated before, with previous (inflated) estimates based on anecdotal evidence or a small number of species. After the 500 m depth threshold, eyes began to reduce in size, as investment in expensive visual structures no longer gave the same benefits in terms of resource acquisition or predator avoidance. These results reveal more about the mysteries of the deep, but more than that, they pinpoint eco-evolutionary drivers of deep-sea diversity, pushing our understanding beyond spectator amazement and into data-driven eco-evo processes.
William Bernard Perry (Thu,) studied this question.