PHYLOGENETIC AND EVOLUTIONARY PERSPECTIVES ON AN ENIGMATIC ORGAN - THE BALANCER OF LARVAL CAUDATE AMPHIBIANS

The balancer is a larval appendage whose taxonomic distribution is lim ited to approximately half of the species found in three of the ten fa milies of salamanders. These organs are small projections from the reg ion of the jaw joint that are present before limbs develop; they appar ently function in mechanical support and chemical adhesion. Balancers are typically associated with species that are pond breeding, as oppos ed to stream breeding, and are therefore assumed to be adaptive in sti ll water and non-functional in flowing water. However, many exceptions to this generalization exist. We explain this unusual distribution by combining morphological and natural history data with a phylogenetic analysis. Because the balancer has been little studied, we summarize t he literature on morphology, development, variation, function, ontogen etic fate, ecological distribution, and homology. Using immunohistoche mical methods we find that the balancer is a complex organ. The balanc er contains a layer of type II collagen that may function as a non-bon y skeleton and is innervated by the ramus mandibularis of the fifth cr anial nerve. We hypothesize that balancers evolved as a synapomorphy o f advanced salamander families, after the divergence of the Sirenidae, and that they have been lost independently many times. Homoplasy is u nidirectional involving loss of the balancer alone, according to this hypothesis.