84.3 Friday, Jan. 6 Visual ecology of bi-lobed eyes in an Antarctic euphausiid COHEN, J.H.; Univ. of Delaware email@example.com
Big-eye krill, Thysanoessa macrura, are common luminescent Antarctic euphausiids with a circumpolar distribution extending from coastal waters into the subantarctic. They are omnivores often occurring in dense aggregations in the upper 400 m, and are prey for fish, whales and seabirds. Each T. macrura compound eye consists of discrete dorsal and ventral lobes. Based on eye morphology, Land (2000) proposed that dorsal eye lobes of midwater animals detect dark objects silhouetted against a bright background of downwelling light, while ventral eye lobes detect bright luminescent flashes against a dark background. In the present study, extracellular electroretinogram recording was used to measure aspects of the visual physiology of dorsal and ventral eye lobes in T. macrura collected near Palmer Station on the West Antarctic Peninsula to test whether sensitivity and temporal resolution of each eye lobe support these different ecological roles. Distinct spectral sensitivity maxima were observed for dorsal and ventral eye lobes, 463 nm (dorsal lobe, n=3) and 492 nm (ventral lobe, n=7), which is consistent with the dorsal lobe as a silhouette detector and the ventral lobe as a luminescence detector. V-logI functions were obtained from dark-adapted dorsal and ventral eye lobes; resulting log K values suggest the dorsal silhouette detector is more sensitive to light than the ventral luminescence detector. Temporal resolution as measured by critical flicker fusion frequency for dark-adapted eyes at 0.7°C was similar for dorsal silhouette-detecting lobes (17 Hz) and ventral luminescence-detecting lobes (20 Hz). Collectively, spectral sensitivities most strongly support different ecological roles for dorsal and ventral eye lobes in T. macrura, which provides physiological evidence for Land’s hypothesis, but contrasts with another bi-lobed euphausiid.