Circadian Rhythms in the Retina of the Atlantic Tarpon, Megalops Atlanticus
Kopperud, Kristin Leigh
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Light is a critically important environmental variable in the lives of marine organisms, affecting physiology and behavior acutely and dictating daily biological rhythms and seasonal reproduction in the long-term. In unpolluted natural environments, the daily light-dark cycle is a reliable environmental cue, but with today’s rapid coastal development, light pollution is an increasingly serious threat to living things, including marine fish. Alterations to the natural light-dark cycle may have profound implications for the health and longevity of organisms by interfering with the function of their internal timekeeping mechanisms. While extensive research exists on the harmful effects of inappropriate light on circadian function in humans, rodents and birds, comparable studies on marine fish are virtually nonexistent. The aims of this study were to determine the effects of light on biological clock function in the teleost retina, using the ecologically and economically important Atlantic tarpon (Megalops atlanticus), as a model. Teleost retinas undergo daily biochemical and anatomical modifications to increase visual efficiency, including changes in retinal sensitivity and acute repositioning of rod and cone photoreceptors for optimal light capture according to time of day (a phenomenon called retinomotor movement). While external influence such as lighting condition may certainly incite direct responses by the visual machinery, we sought to investigate the involvement of circadian clock(s) in driving these rhythms. To test the hypothesis that retinal sensitivity changes with time of day in M. atlanticus, sensitivity of the retina was measured periodically by electroretinography (ERG), following exposure to either a 12L:12D light-dark cycle (LD; N=18) or constant darkness (DD; N=18) for four days prior to testing. The intensity of light required to elicit a maximal ERG response was significantly greater during the day than the night in fish held in LD. To determine whether this cycle is driven by a circadian clock, ERG was performed at the same timepoints on juvenile fish held in 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 thus demonstrate that this cycle of retinal sensitivity is driven at least in part by an internal biological clock. Using immunofluorescence with opsin-specific primary antibodies and periodic sampling over 24 hours, we also observed rhythms of retinomotor movement (RM) in juvenile tarpon. Populations of fish were exposed to LD (N=18) and DD (N=18) as above, and two additional treatments simulating exposure to light at night: a four-hour advance of light onset (AdvL; N=18) and constant light (LL; N=18). RM was quantified by measuring photoreceptor (rod and cone) outer segment (OS) position with respect to the outer limiting membrane (OLM), and extent of RM was plotted over a 24-hr period for visualization of rhythms. Fish held in LD exhibited robust oscillations of RM over the course of the day; both rod and cone OS position changed significantly between light and dark hours. To elucidate circadian clock involvement in these rhythms, we investigated whether they persisted in the absence of external light stimulus. Constant darkness suppressed RM by rod OS. However, cone OS position continued to oscillate rhythmically, implicating clock involvement. Constant light, conversely, suppressed RM of both photoreceptor types. In addition, the rhythm remained synchronized to the advanced light cycle, with rod and cone OS position changing significantly between light and dark hours. These significant effects of light exposure at night on tarpon retinal clock function suggests that light at inappropriate times is likely detrimental to visual efficacy. These rhythms support survival behaviors such as prey capture and predator avoidance in these fish, thus these findings present a strong case for mitigation efforts of coastal light pollution.