Meeting Abstract
Caecilians are an ancient radiation of mainly fossorial amphibians with reduced visual systems (their name being a reference to their being blind). Adult caecilian amphibians eyes are small, covered by skin and sometimes by bone, with retinal cells that may only contain rod opsins, associated with scotopic (dim light) vision. Here we give an overview of what is known of the structure, function and evolution of the caecilian visual system. Morphological comparisons suggest different degrees of visual system reduction probably associated with different degrees of dedicated subterranean burrowing lifestyles, with independent losses of many components of the visual system or their modification to serve other functions. Spectral sensitivities of some caecilian photoreceptors have been determined with microspectrophotometry and visual pigment genes sequenced. Rod opsins regenerated in vitro with 11-cis retinal give pigments with spectral sensitivity peaks at 493nm or below, short-wavelength shifted in terms of the maximum absorption of light when compared with other amphibian lineages. The significance of this shift and its molecular mechanism remain to be determined. Because of the obvious potential for regressive evolution to lead to convergent losses that might confound morphological phylogenetics, comparative morphological data on caecilian eyes has been a focus for studies of methods of character coding and of detecting character independence. Additionally, these data have been used to address the utility of discrete character data in analyses of disparity. However, there are many inaccuracies in the underlying observations due in part to histological artefacts. We illustrate some of these problems and our recent attempts to use non-invasive X-ray microscopy (ZEISS 520 Versa) to provide more reliable observational data for use in such synthetic investigations.
