| Summary: | Living elasmobranchs occupy every major aquatic ecosystem throughout the world (Compagno 2003; Compagno et al. 2005). Sensory ecology can be a good determinant in comprehending the processes occurring between an organism and its natural environment (Weissburg and Browman 2005). By utilizing ecophysiological tools, insight into the adaptive responses of the sensory systems to their ever-changing ecological niche can help explain behavioral and life history characteristics (Hueter 1991; Litherland 2009). Aquatic animals show structural and physiological adaptations in their visual sense specific to the ecological requirements of their habitat (Hart et al. 2004), implying that vision is an important modality.
The visual system of the smooth dogfish (Mustelus canis, family Triakidae) was examined using corneal electrophysiological methods to determine the visual spectral range, irradiance sensitivity, and speed of vision (flicker fusion frequency, FFF). The smooth dogfish, a shallow water bottom feeder inhabiting inshore waters along the eastern United States, was found to be extremely sensitive to dim light (-3.1- 0.1 log light intensity), and have a slow FFF (13 Hz), thus being well adapted to the scotopic conditions of the turbid coastal inshore waters. This prompted a second set of experiments focusing on the chromatic adaptations of the photoreceptor cells and retina function following light adaptation. Light adaptation increased the photopic threshold by 2.0 log light units of intensity (LLI). However, the temporal resolution was not dramatically increased (to 17 Hz), indicating that the retinal integration time is very slow for this species under all circumstances. The spectral sensitivity peak for M. canis (470 nm) was found to be significantly blue-shifted in comparison to other members of the Triakidae family (Crescitelli et al. 1995; Sillman et al. 1996).
Smooth dogfish appear to forgo high spatial and temporal resolution for the enhancement of photon capture. The sandbar shark inhabits the same inshore estuaries during the summer months but has a visual system with a higher temporal resolution (FFF, 54 Hz) and a brighter photopic threshold (1.2 LLI-50% max) (Litherland 2009). Furthermore, other elasmobranch or telelost species inhabiting similar photic environments also exhibit faster temporal resolution; little skate (FFF, 30 Hz), weakfish (FFF, 40 Hz), red drum (FFF, 50 Hz), spotted sea trout (FFF, 60 Hz), and Atlantic croaker (FFF, 58 Hz) (Horodysky et al. 2008; McComb et al. 2010).
Coastal seas tend to contain more dissolved organics and particulates than the clear oceanic waters of the epipelagic and pelagic zones (McFarland 1986), therefore the retina of smooth dogfish has adapted to be extremely sensitive to dim light, has a long integration time, a low flicker fusion frequency and temporal resolution, and retinal cells that are able to adjust to changing light conditions. All of these factors contribute to the visual system to provide optimal visual ability to enable smooth dogfish to accurately exploit its surroundings.
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