The onset of cortisol synthesis and the stress response is independent of changes in CYP11B or CYP21 mRNA levels in larval red drum (Sciaenops ocellatus).

Although cortisol plays an important role in teleost development, the onset of cortisol production and the cortisol stress response in teleosts remain poorly understood. Here we have reported basal cortisol levels and the development of the cortisol stress response in larval red drum (Sciaenops ocellatus). We isolated partial nucleic acid sequences encoding two key corticosteroidogenic enzymes, CYP11B and CYP21 and assessed ontogenetic patterns of their mRNA levels relative to basal and stress-induced cortisol production. Basal cortisol was first detected 3 days post-hatch (DPH) and reached a maximum at 9 DPH. Cortisol did not increase in response to an acute stressor prior to 6 DPH. From 6 DPH forward, stress caused significant increases in larval cortisol content. Stress-induced cortisol levels in 6-9 DPH larvae were highest 1h post-stress. In larvae 11 DPH and older, the highest cortisol measurements occurred 0.5h post-stress. Elevated cortisol was still evident after 3h in 6 DPH larvae. From 11 DPH onward, basal cortisol levels were reestablished in larvae by 1h post-stress. CYP11B and CYP21 transcripts were detected in red drum 12h prior to hatching and in all post-hatch larvae examined. Changes in CYP11B and CYP21 mRNA levels did not occur in association with the ontogenetic appearance of cortisol, or the onset of the stress response. As larvae developed, the dynamics of the cortisol stress response matured from a low magnitude, slow recovery response, to a response similar to that observed in juvenile and adult fish.