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.