The effects of photoperiod on growth rate and circulating thyroid hormone levels in the red drum, Sciaenops ocellatus: evidence for a free-running circadian rhythm of T-4 secretion

Previous studies with red drum and other species have indicated that diurna l rhythms of circulating thyroid hormones (thyroxine, T-4, and 3-5-3 ' -tri iodo-L-thyronine, T-3) are synchronized to the light cycle, and not to time of feeding. In this study we set out to address the effects of various lig hting regimes on thyroid hormone levels in the red drum. The first experime nt was undertaken to determine the effects of long and short photoperiods o n diurnal thyroid hormone rhythms, growth rate, feed efficiency and food co nsumption. Red drum raised under a long photoperiod (16L:8D) grew significa ntly larger and exhibited greater feed efficiency than their short photoper iod (SL:16D) counterparts. There were no changes in food consumption or the diurnal profile of plasma thyroid hormones, e.g. increased peak amplitude or duration, that would explain this increase in growth rate and feed effic iency. The second experiment was undertaken to determine if diurnal thyroid hormone rhythms in the red drum originate from an endogenous circadian clo ck. To address this question, red drum were housed under a 12L:12D photoper iod and fed once daily at variable times before the lighting was switched t o constant dim illumination for up to 3 days. The rhythm of circulating T-4 levels persisted for two complete cycles with constant amplitude in fish t hat continued to be fed during constant dim illumination, and did not appea r to entrain to feeding. The T-4 rhythm also persisted for three complete c ycles under constant conditions in feed-restricted fish, although with a di minished amplitude over time. To our knowledge, these data provide the firs t evidence for a free-running circadian rhythm of plasma T-4 levels in a fi sh. These findings implicate the involvement of an endogenous circadian clo ck that determines when the hypothalamo-pituitary-thyroid axis is activated . (C) 2001 Elsevier Science Inc. All rights reserved.