Temporal Resolution and Spectral Sensitivity of the Visual System of Three Coastal Shark Species From Different Light Environments

Elasmobranchs have radiated into nearly every aquatic habitat on earth including the deep benthos, brightly lit coral reefs and even the murky freshwaters of the Amazon. As such, they experience tremendous variability in ambient lighting conditions which have resulted in remarkable adaptations within the visual system. Therefore, elasmobranchs represent an excellent group with which to compare and contrast visual function. Elasmobranch eyes rival higher vertebrates in function and complexity and aspects of the visual system are correlated with environmental factors. In this study, the visual temporal resolution (eye speed) and scotopic spectral sensitivity of three coastal shark species (bonnethead Sphyrna tiburo , scalloped Sphyrna lewini , and blacknose shark Carcharhinus acronotus ) were investigated by electroretinogram. Temporal resolution was quantified under photopic and scotopic conditions using response waveform dynamics and maximum critical flicker-fusion frequency (CFF). Photopic CFFmax was significantly higher than scotopic CFFmax in all species. The bonnethead had the shortest photoreceptor response latency time (23.5 ms) and the highest CFFmax (31 Hz), suggesting that its eyes are adapted for a bright photic environment. In contrast, the blacknose had the longest response latency time (34.8 ms) and lowest CFFmax (16 Hz), indicating its eyes are adapted for a dimmer environment or nocturnal lifestyle. Scotopic spectral sensitivity revealed maximum peaks (480 nm) in the bonnethead and blacknose sharks that correlated with environmental spectra measured during twilight, which is a biologically relevant period of heightened predation.