Auditory Evoked Potentials of the Loggerhead Sea Turtle (Caretta caretta)

Repulsion from hopper dredges using auditory stimuli is one frequently proposed solution for reducing incidental mortalities of sea turtles.However, before this tactic can be assessed, research must first be performed on the auditory mechanism of sea turtles, an area underdeveloped in the literature.In this study, threshold for response to stimuli and the effects of stimuli and white noise on the threshold were determined for the loggerhead sea turtle, Caretta caretta.3 5 juvenile loggerhead turtles caught in the Chesapeake Bay were used in this study.A computer capable of delivering stimuli and receiving bioelectric activity via electrodes implanted in the loggerhead sea turtle was used.Either a low frequency broadband click or tone bursts (250, 500, 7 50 or 1000 Hz) were deliver by a bone vibrator to the turtle's tympanum.Intensity and frequency of stimulus was manipulated for the threshold experiment.Rate of stimulus presentation and intensity of white noise were manipulated for the rate and masking experiments, respectively.The maximum sensitivity was in the low frequency region of at least 250 to 1000 Hz with a maximum sensitivity at 250 Hz of -24.4 dB re: 1 gravity unit.The broadband click produced clear auditory response with a mean threshold of -10.8 dB re: 1 gravity unit and 8.5 dB re: 1 dynes/cm2.In the rate experiment, interpeak latencies for peak I and peak V were significantly dependent on rate.In the masking experiment, signal to noise ratios ranged from -3.5 to -8.5 dB (x=-5.2± 2.4).The broadband click stimuli elicited synchronous neural activity of the hair cells and was determined to be the most efficient stimulus to use when recording threshold from the loggerhead sea turtle.An increase in the stimulus rate resulted in the disruption of neural synchrony and thus interpeak latencies increased with rate of stimulus.Finally, loggerheads appear to be able to resolve the stimulus through a high level of white noise.These techniques of auditory evoked potentials may be utilized in two fields of applied research: the development of an acoustic repelling device and the identification of diseases of the brain of sea turtles.