Ocular lens morphology is influenced by ecology and metamorphosis in frogs and toads

The shape and relative size of an ocular lens affects the focal length of
the eye, with consequences for visual acuity and sensitivity. Lenses are
typically spherical in aquatic animals with camera-type eyes and axially
flattened in terrestrial species to facilitate vision in optical media
with different refractive indices. Frogs and toads (Amphibia: Anura) are
ecologically diverse, with many species shifting from aquatic to
terrestrial ecologies during metamorphosis. We quantified lens shape and
relative size using 179 microCT scans of 126 biphasic anuran species and
tested for correlations with life stage, environmental transitions, adult
habits and adult activity patterns. Across broad phylogenetic diversity,
tadpole lenses are more spherical than those of adults. Biphasic species
with aquatic larvae and terrestrial adults typically undergo ontogenetic
changes in lens shape, whereas species that remain aquatic as adults tend
to retain more spherical lenses after metamorphosis. Further, adult lens
shape is influenced by adult habit; notably, fossorial adults tend to
retain spherical lenses following metamorphosis. Finally, lens size
relative to eye size is smaller in aquatic and semiaquatic species than
other adult ecologies. Our study demonstrates how ecology shapes visual
systems, and the power of non-invasive imaging of museum specimens for
studying sensory evolution.