FRANZ-ODENDAAL, T.A.; Dalhousie University, Nova Scotia, Canada: Growth, development, constraint and variation of the skeletal elements within the teleost eye

The sclera plays a supportive role in all vertebrate eyes and usually contains two skeletal elements. In bony fish (Teleostei) the scleral cartilage is restricted to an equatorial ring, unlike the situation in tetrapods. Some teleosts (not all) also undergo ossification in the anterior and posterior regions of this ring; these elements are the scleral ossicles. Basal Actinopterygii have a sclerotic ring composed of four elements, not two. A large scale analysis of whole-mount stained bony fish as well as radiographs were examined in order to determine variation and/or constraint of the scleral ossicles and scleral cartilage across living teleost groups, as well as whether there is any correlation between phylogenetic distribution and presence/absence of scleral ossicles. A closer examination of these elements in zebrafish (Danio rerio) and the alewife (Alosa pseudoharengus) was conducted, which included examination of growth series and histological sections. Results indicate that the shape of the scleral cartilage is uniform across the over 400 species examined. The shape and size of the scleral ossicles are however variable, although their number is highly constrained. Presence/absence of ossicles does not correlate with phylogenetic distribution. Results from the alewife, which has large scleral ossicles compared to zebrafish, show that eye diameter measured against body length shows allometric growth; the scleral ossicles increase in size (length) in proportion to eye growth. Histological analyses revealed some insights into the mode of ossification of the scleral ossicles in teleost eyes.