Supplementary Material for: Sexual Dimorphism of the Electrosensory System: A Quantitative Analysis of Nerve Axons in the Dorsal Anterior Lateral Line Nerve of the Blue-Spotted Fantail Stingray <b><i>(Taeniura lymma)</i></b>

Quantitative studies of sensory axons provide invaluable insights into the functional significance and relative importance of a particular sensory modality. Despite the important role electroreception plays in the behaviour of elasmobranchs, to date, there have been no studies that have assessed the number of electrosensory axons that project from the peripheral ampullae to the central nervous system (CNS). The complex arrangement and morphology of the peripheral electrosensory system has a significant influence on its function. However, it is not sufficient to base conclusions about function on the peripheral system alone. To fully appreciate the function of the electrosensory system, it is essential to also assess the neural network that connects the peripheral system to the CNS. Using stereological techniques, unbiased estimates of the total number of axons were obtained for both the electrosensory bundles exiting individual ampullary organs and those entering the CNS (via the dorsal root of the anterior lateral line nerve, ALLN) in males and females of different sizes. The dorsal root of the ALLN consists solely of myelinated electrosensory axons and shows both ontogenetic and sexual dimorphism. In particular, females exhibit a greater abundance of electrosensory axons, which may result in improved sensitivity of the electrosensory system and may facilitate mate identification for reproduction. Also presented are detailed morphological data on the peripheral electrosensory system to allow a complete interpretation of the functional significance of the sexual dimorphism found in the ALLN.