Circadian rhythms of retinal sensitivity in the Atlantic tarpon, Megalops atlanticus

The Atlantic tarpon (Megalops atlanticus Valenciennes, 1847) is a visually-guided marine predator that may be active at any time in the 24-hr light-dark cycle despite dramatic changes in light intensity over the course of the day. To test the hypothesis that retinal sensitivity changes with time of day in M. atlanticus, possibly under the influence of a circadian clock, populations of fish (three populations of 6 fish per lighting treatment for two treatments = 36 fish total) were exposed to different lighting treatments and sensitivity of the retina was measured periodically by electroretinography (ERG). The intensity of light required to elicit a half-maximal ERG response was significantly greater during the day than the night in fish held in 12L:12D light-dark cycles (LD). To determine whether this cycle of retinal sensitivity is driven by an internal timekeeping mechanism, ERG was performed at 4-hr intervals over the course of 24 hrs in constant darkness (DD). Sensitivity was significantly higher during subjective night than during subjective day, though the rhythm in DD was damped relative to the cycle in LD. These results show that retinal sensitivity is much higher at night than during the day in a light-dark cycle, and that this cycle of retinal sensitivity is driven at least in part by an internal biological clock. Such endogenous timekeeping mechanisms enhance survivability by allowing organisms to anticipate change in their external environments. Rhythms of retinal sensitivity are likely important for survival by supporting prey capture, predator avoidance, and reproduction, but they may be disrupted by abnormally-timed exposure to light, including artificial light at night.