The stress physiology of extended duration tonic immobility in the juvenile lemon shark, Negaprion brevirostris (Poey 1868)

Tonic immobility (TI) is a reversible coma-like stasis inherent to a variety of terrestrial and aquatic taxa, including elasmobranchs, yet virtually nothing is known about its underlying neurological and physiological processes in any taxa. The purpose of this research was to investigate the physiological effects of TI on the juvenile lemon shark (Negaprion brevirostris). Eight juvenile lemon sharks were subjected to four, three-hour treatments during which blood was sampled at 0, 30, 90 and 180. min, over a 6. week period. Treatments were differentiated by the method of maintaining the shark, either in TI, or allowed to swim freely between blood samples and the presence or absence of a pre-treatment exercise period designed to simulate the capture induced exhaustion that usually precedes the use TI in the field. The results suggest that TI is an inherently stressful experience, which magnifies the degree of perturbation observed in a number of blood chemistry parameters. It is thought that TI induced a short term reduction in ventilatory efficiency, which appeared to be countered by a series of compensatory mechanisms that include increased ventilation rates, and maintenance of the primary stress response. TI remains one of the most enigmatic areas of biology for all taxa and further research into its underlying psychological, physiological and neurological processes is recommended.