Wednesday, November 3, 2010

Fossorial Eyes

Many fossorial squamates in many different lineages have evolved reduced eyes. In the early 20th century Gordon Walls proposed that an ancestral snake went through a burrowing phase, lost or reduced their eyes, and then re-evolved larger, functional eyes when they adapted again to life on the surface. A variety of hypotheses have been proposed to explain eye-reduction in fossorial squamates and other animals, including: a lack of  a need to respond predators, sudden changes in environmental conditions, or visual accuracy. Exceptionally small eye balls have been said to have evolved to save metabolic energy, or atrophy in fossorial habitats, or reduced due to developmental anomalies. Pressure placed on the eyeballs as the animal burrows through soil has also been considered the cause of the reduction. Foureaux et al. (2010) examined  the eyes of four species of fossorial reptiles: three amphesbaenids (Amphisbaena alba, Amphisbaena mertensi, Leposternon infraorbitale) and the blind snake Typhlops brongersmianus (family Typhlopidae). Using a light microscope, as well as an SEM the authors visualized the minuscule eyes and did histological examinations. Although small, and generally considered to be degenerated, the 1–2 mm diameter eyes were covered with a thin, transparent scale that covers a conjunctival sac and corresponds to the spectacle of other snakes. The Blind Snake's eyes were oval, while the amphisbaenids' eyes were cup-shaped. The amphisbaenids' sclera was composed of cartilage, while the Typhlops' sclera was composed of connective tissue and striated muscle fibers. The retina of these fossorial species had all of the layers found in other vertebrates. Reduction in the size of the eyeball, the rudimentary cornea, an absence of the anterior chamber, the presence of a complex iris-ciliary body, and a lens with amorphous nucleate cells all correlate with a fossorial life style. The fact that the retina was intact and presumably functional suggests that the eyes of fossorial squamates may in fact function for light perception.

Foureaux, G., M. I. Egami, C. Jared, M. M. Antoniazzi, R. C. Gutierre, R. L. Smith. 2010. Rudimentary Eyes of Squamate Fossorial Reptiles (Amphisbaenia and Serpentes).The Anatomical Record 293(2)351–357.

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