Acoustic communication, auditory processing, and neuropeptide modulation of sensory systems in coral reef fishes

Assessment of the underwater sensory environment is critical to the reproductive success of fishes, and detection of sound stimuli is important for acoustic communication during social interactions. The neural centers for auditory processing are subject to modulation by neuropeptides such as gonadotropin-releasing hormone (GnRH) and arginine vasotocin (AVT). The extensive non-hypophysiotropic projections of GnRH and AVT suggest they function as neuromodulators of sensory and sensorimotor processing to mediate behaviors. This dissertation examines aspects of the acoustic communication and auditory processing abilities of the sound-producing Hawaiian sergeant damselfish Abudefduf abdominalis , and tests several predictions of the sensory neuromodulation hypothesis, which states that GnRH and AVT influence the processing of sensory information prior to integration with motor control circuits. Hawaiian sergeant fish produce low frequency, low intensity sounds during close range agonistic and reproductive behaviors. Spectral characteristics of sounds match the frequency range of hearing measured by auditory evoked potentials and single neuron recordings in the hindbrain and midbrain, as predicted by the acoustic communication hypothesis. Central auditory neurons also modulate to playbacks of complex natural conspecific sounds with lower thresholds compared to tones. GnRH somata in the terminal nerve and midbrain tegmentum showed sex and seasonal plasticity in number and size that were correlated with changes in GnRH-immunoreactive (-ir) fiber densities in the sensory torus semicircularis, tectum and vagal lobe. Reef fishes also use visual cues during social interactions, and GnRH and GnRH receptors were localized to the retina and visual brain of four different perciform fishes. Sex and seasonal plasticity in AVT somata number and size was also observed within all three AVT-ir cell groups, and fiber densities in the torus semicircularis and vagal motor nucleus were correlated with temporal changes in the number of gigantocellular-POA somata. GnRH and AVT axons were found in many sensory and motor regions throughout the brain. Neurophysiology experiments also showed that GnRH inhibits midbrain auditory neuron activity. These data provide support for the acoustic communication hypothesis in the Hawaiian sergeant fish, and for the sensory neuromodulation hypothesis that GnRH and AVT have the potential to influence visual and octavolateralis processing in the fish brain.