Meeting Abstract

P2.53  Monday, Jan. 5  Investigating the mechanisms responsible for the evolutionary origin of a greatly hypertrophied cypriniform ceratobranchial 5 SOU, E.*; HERNANDEZ, L. P.; George Washington University, Washington DC 20052 phernand@gwu.edu

Significant hypertrophy of the pharyngeal jaws and concomitant loss of oral jaw teeth characterize most cypriniform fishes. While cichlid pharyngeal jaws have received much attention from evolutionary morphologists, understanding the evolution of the hypertrophied ceratobranchials that comprise the cyprinid pharyngeal jaws has received little attention. Here we use both a phylogenetic approach as well as an ontogenetic analysis of growth to examine the origin and evolution of the hypertrophied ceratobranchials. A phylogenetic survey of basal teleosts and Ostariophysans to determine the relative size of ceratobranchials early in teleostean evolution has never been undertaken. Our findings suggest that it is only at the base of Cypriniformes that we see a greatly hypertrophied ceratobranchial 5. Indeed in several more basal clades, including Gonorhynchiforms (sister to Cypriniformes) and Clupeomorpha, the fifth ceratobranchial is markedly smaller than more anterior ceratobranchials. Interestingly other otophysan groups are also characterized by a smaller ceratobranchial 5. Using the zebrafish as a model cypriniform we also examined growth of all ceratobranchials from their first origin as cartilaginous elements at early larval stages until adulthood when all elements have undergone ossification, in order to better understand the developmental mechanisms via which such changes in ultimate size may have occurred. Growth rate of all ceratobranchials changed significantly during larval development, with a marked increase in growth rate corresponding to the onset of mineralization. Both the timing of initial differentiation as cartilaginous elements, as well as increased growth rate, appear to be involved in the generation of the hypertrophied jaws.