Th e Lungfi sh Urogenital System

Abstract

All three extant genera of lungfish, Australian, African and South American, appear to possess unremarkable, even 'degenerate eyes' when viewed externally. Th e eyes of the Australian lungfish, Neoceratodus forsteri, are slightly larger than those of the other species (seven African species in the genus Protopterus and the single South American species Lepidosiren paradoxa). N. forsteri seems to be the most visually-oriented of the extant lungfishes. All three genera of lungfish, however, possess remarkable and beautiful retinal adaptations, including, coloured oil droplets, multiple cone spectral sensitivities and large photoreceptor inner segments, making them more closely aligned in design to modern amphibians and other terrestrial animals, than to teleosts. Th e tetrapod-like retinal features of N. forsteri provide the capability for tetrachromatic colour vision and add to the debate on the phylogenetic origin(s) of lungfish. Th ey also suggest that the complex colour vision system of vertebrates on land, exemplified by birds, may have first evolved in the aquatic environment or at least close to the time when aquatic life emerged onto land. Other ocular adaptations in dipnoans include a non-spherical lens, the anatomical mechanism for accommodation, a mobile pupil and giant retinal cells. Th is eye design suggests a need to increase light flux, rather than for a reliance on high spatial acuity, a conclusion supported by the relatively low ganglion cell densities. Future work should certainly aim at a better understanding of the visual biology, behaviour and ecology of all lungfish, especially in light of their disappearing habitat worldwide. Both African and South American species also need a full description of their visual system before they are properly consigned to being 'less well developed', than N. forsteri.