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The eyes of troglobitic urodeles are reduced in size compared to their epigean relatives (reviewed by Brandon, 197Ia), but in no case is a thorough analysis of growth and variation in size available. Measurements of the eyes of Eurycea tridentifera (Mitchell and Reddell, 1965) and Haideotriton wallacei (Brandon, 1968) indicate that one parameter of eye reduction of these animals is considerable individual variation and bilateral asymmetry. The significance of these observations is not clear, however, since in no case has it been determined whether or not such variation reflects genotypic differences. The inherent difficulties in genetic analysis among urodele amphibians preclude a quick answer to this question. Typhlotriton spelaeus is the only troglobitic urodele that attains sexual maturity after metamorphosis; the others are paedogenetic (Brandon, 1966; 1971 a). In addition, T. spelaeus larvae have functional eyes that usually degenerate after metamorphosis (Eigenmann and Denny, 1900; Stone, 1964; Besharse, 1972). These observations show that the minimal genotype necessary for the development of functional eyes is present in T. spelaeus and that such genetic factors affecting degeneration as may be discovered must have their effect on a terminal developmental stage. We have suggested (Besharse, 1972; Besharse and Brandon, in preparation) that structural degeneration of the eyes of T. spelaeus results from generalized cessation of eye growth and maintenance. Our observations on eye size and its variation are compatible with that hypothesis. In the present report we analyze growth and variation in size of T. spelaeus eyes compared to those of the more visually oriented species Eurycea ludfuga. Eyes of larval T. spelaeus are smaller than those of E. ludfuga at the beginning of the larval growth period but, in most cases, increase in size at a rate (relative to body size) comparable to those of E. ludfuga during larval life. Postmetamorphic cessation of eye growth in T. spelaeus, during the time when the eyes are degenerating structurally, is accompanied by increased individual variation and bilateral asymmetry of eye size.

Size and growth of the eyes of the troglobitic salamander Typhiotriton spelaeus