| Summary: | Over evolutionary time, cetaceans (whales, dolphins, and porpoises) have accumulated many visual adaptations in response to life in an aquatic environment. However, many gaps remain to be filled in our knowledge of the form and function of cetacean eyes. Micro-computed tomography (micro-CT) is a high-resolution X-ray imaging method that is emerging as a powerful tool for studying morphology. Eyes are well-suited to this type of analysis because the components of the eye differ in density enough to be easily visualized by micro-CT. In the present study, eleven cetacean eyes representing three families and at least nine species were scanned, with morphological measurements taken from the rendered images. These data were combined with data from two previous studies (Lisney and Collin, 2019; Miller et al., 2013) in order to investigate how cetacean ocular morphology varies between clades, how the eye scales with body mass, and whether ocular morphology is affected by ecological variables such as dive depth. Cetaceans in general had proportionally smaller eyes than one would expect given their large body mass. Mysticetes (baleen whales) were found to have significantly thicker scleras (i.e., eye walls) and may have smaller lenses than odontocetes (toothed whales) relative to eye diameter. While the function of the thickened sclera remains unknown, odontocetes may have larger lenses to increase sensitivity while foraging at depth. Overall eye shape was found to correlate to maximum dive depth, with deeper-diving cetaceans having eyes that were flattened along the axial diameter. The functional purpose of this adaptation is unknown. These results point to interesting morphological differences between clades of cetaceans and begin to shed light on how ocular features have been shaped by ecological factors such as diving.
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