Meeting Abstract
Marine mammals and reptiles are characterized by an array of morphological features that are considered to be adaptations towards secondary aquatic lifestyles. One of such features is found in the eye: it has been noted since the 19th century that the sclera, the fibrous protective outer layer of the eye, is unusually thick in marine mammals and reptiles. However, while the role of the thick sclera has been hypothesized about, differences in the thickness of sclera across different mammals have never been rigorously quantified. Here we show that the sclera of aquatic and semiaquatic mammals is thicker than the sclera of terrestrial mammals even when eye size is accounted for, using a sample of 70 species and 128 individual specimens. This is true for both the sclera at the posterior pole of the eye and sclera near the cornea. We also saw that phylogeny has a strong effect on this relationship, and therefore must be accounted for when determining variance in scleral thickness between habitat groups. We argue that the most plausible hypothesis is that increasing intraocular pressure as a method of visual accommodation causes increased scleral thickness, though other aspects of aquatic life like temperature and pressure may play small roles.
