# Multiple Pathways of Visual Adaptations for Water Column Usage in an Antarctic Adaptive Radiation

## TL;DR

This study explores how Antarctic notothenioid fish evolved different visual adaptations for living at various depths in the ocean, revealing unexpected patterns in eye size and light detection.

## Contribution

The study identifies multiple evolutionary pathways for visual adaptations in notothenioids, challenging existing models of visual diversification in aquatic species.

## Key findings

- Eye size diversification accelerated nearly 20 million years after the initial notothenioid radiation.
- Closely related notothenioid species often show the highest eye size divergence.
- Independent opsin tuning site changes occurred repeatedly across the phylogeny, not linked to habitat depth or eye size.

## Abstract

Evolutionary transitions in water column usage have played a major role in shaping ray‐finned fish diversity. However, the extent to which vision‐associated trait complexity and water column usage is coupled remains unclear. Here we investigated the relationship between depth niche, eye size, and the molecular basis of light detection across the Antarctic notothenioid adaptive radiation. Integrating a phylogenetic comparative framework with data on eye size and depth occupancy, we provide support for an acceleration in the rate of eye size diversification nearly 20 million years after the initial radiation. Our results further reveal that levels of eye size divergence are often highest between closely related taxa. We further analyzed opsin tuning site sequences and found changes representing repeated instances of independent tuning site changes across the notothenioid phylogeny that are generally not associated with habitat depth or species eye size. Collectively, our results strongly support that multiple evolutionary pathways underlie the diversification of visual adaptations in this iconic adaptive radiation.

By investigating the diversification of the visual system in notothenioids, we reveal lineages at equivalent depths often depict extremes in eye size with striking convergences in tuning sites between distantly related species that are likely driven by the unusual environmental conditions of the Southern Ocean. This finding represents a striking departure from the expectations of visual diversification in other aquatic lineages and raises new research questions that simultaneously challenge prevailing models of neural diversification while offering a new perspective on visual diversification during adaptive radiation.