This study resulted from the identification of gaps in the literature pertaining to the morphological
descriptions of the tadpoles occurring at high altitudes in the Drakensberg Mountains in South Africa.
These tadpoles are exposed to low temperatures, high desiccation risk, elevated ultraviolet radiation,
competition, and predation and inhabit the clear, flowing streams and marsh areas of the mountain.
Highly varying environmental conditions caused tadpoles to have considerable intraspecific variation.
The high degree of plasticity necessitated extensive descriptive studies of tadpole morphology in order to
document intraspecific variation and set up reliable keys for species identification. Specified adaptations
to the extreme montane conditions are present in tadpoles of certain species. An especially interesting
adaptation is the elygium, a hemispherical pigmented area above the eye, which apparently protects the
retina from harmful ultraviolet radiation. There are no known studies of elygium plasticity in tadpole eyes
in relation to variation in ultraviolet radiation. Particular attention was given to the functionality and
cytology of this structure. Detailed measurements of tadpoles of six frog species of the high altitude
Drakensberg Mountains were made. Morphological adaptations were described on the basis of these
measurements. The cytological origin of the elygium of Amietia vertebralis was revealed through
histological and cellular ultrastructure studies. The change in elygium morphology over time was studied
as a function of ultraviolet intensity by exposing tadpoles to different levels of ultraviolet radiation. From
the detailed morphological descriptions a more reliable binomial key was constructed, which made it
possible to distinguish between Amietia umbraculata and A. vertebralis. A new amended definition of the
epidermal elygium can now be given as an area of melanophores originating from the pigmented
epithelium of the retina, forming a hemispherical shape from the dorsal margin of the iris. It is positioned
in such a way as to protect the retina when light enters directly from above. This empirical study of the
functional significance of the elygium showed that elygium morphology was considerably plastic, and
that there were differences in elygium area and base length in the presence or absence of UVB radiation.
In the presence of high UV radiation tadpoles produced an elygium with a broader base rather than longer
elygia with a larger area. A wider elygium base shaded the pupil more effectively, thus protecting the
retina from harmful UV radiation. The presence of a ventral elygium was also discovered. / Thesis (M.Sc. (Environmental Science))--North-West University, Potchefstroom Campus, 2011.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nwu/oai:dspace.nwu.ac.za:10394/4464 |
| Date | January 2010 |
| Creators | Kruger, David Johannes Donnavan |
| Publisher | North-West University |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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