MACESIC, L. J.; Florida Atlantic University: Morphometrics and behavioral function of the electric organs of Bancroft�s numbfish, Narcine bancroftii

All elasmobranchs have the ability to detect electricity, however only skates (Rajiformes) and electric rays (Torpediniformes) are capable of bioelectrogenesis. Within these orders, skates emit weak electric organ discharges (EODs) involved in communication, whereas electric rays emit strong EODs during prey-capture. Of the Narcinids, Bancroft�s numbfish, N. bancroftii, produces both strong and weak EODs from large, main electric organs and small accessory organs respectively; however, the function of these discharges remains unknown. Prior to investigating the behavioral function of the EODs, morphometrics were studied to examine possible sexual dimorphisms or ontogenetic changes in the electric organs. Morphometrics included: disc width, total length, total mass, main electric organ mass and surface area, accessory electric organ mass, and number of main electric organ electroplaques. The proportion of EOD generating mass relative to the total body mass, or, the electro-somatic index (ESI, expressed as percent), was calculated for both main and accessory electric organs (MESI and AESI respectively). Mean MESI of N. bancroftii was 13.69� 0.63% SE (n=15) and mean AESI was 0.10 � 0.01 SE (n=15). Main and accessory electric organ masses and the number of electroplaques in the main electric organ correlate positively with size morphometrics. In addition, the AESI demonstrates positive allometry, whereas the MESI is negatively allometric. None of the morphometric characters were sexually dimorphic. Electrophysiological experiments will characterize both the main and accessory EODs throughout ontogeny. Preliminary results of behavioral experiments suggest that the main EOD functions solely in defense, and not in predation. Future studies will test the use of the accessory EOD in intraspecific communication.