Abstract:
The G. Typhlonarke is endemic to New Zealand and is represented by two species, T. aysoni and T. tarakea. Both are blind electric rays with concealed eyes, one dorsal fin, and pelvics modified for walking or burrowing. T. avsoni has a circular, thin disc, while that of T. tarakea is ovoidal and thick. Both species are marine and benthic in shallow water. Their skeletal features and musculature are typically rajiform but illustrate quantitative modification correlated with their benthic habit. Their eyes are hidden beneath the skin and display near-normal development of all the ocular layers, but the lens is not represented. It is suggested that this is due to a failure of the lens evocator mechanism rather than to degeneration of a normally-formed eye during functional development. The latter is the condition in all other known blind fishes. The brain in T. tarakea and T. aysoni shows reduction of structures concerned with vision, and increased development of other sensory centres. The latter is regarded as compensatory for the lack of vision. The divergence of the genus is believed to have occurred late in the evolution of the family, and to have been initiated by the failure of the lens evocator mechanism, resulting in the imposition of blindness with compensating sequelae such as enfeeblement of the natatory mechanisms, modification of the pelvics etc.
