Visual physiology underlying orientation and diel behavior in the sand beach amphipod Talorchestia longicornis

Talitrid amphipods employ vision for zonal recovery behaviors on sand beaches and for entraining circadian activity rhythms. Using a hierarchy of methods, we examined visual spectral and response-intensity functions in Talorchestia longicornis, a species in which orientation and rhythm entrainment are wavelength-specific behaviors. Microspectrophotometry, electroretinogram recording and behavioral assays were used to determine visual pigments, retinal spectral sensitivity and whole-animal spectral responsivity, respectively. Diel changes in absolute sensitivity were also investigated at retinal and whole-animal levels. Two receptor spectral classes were identified, with values for visual pigment λ(max) of 427 and 518 nm. Retinal spectral sensitivity varied with electrode position along the distal-proximal axis. Chromatic adaptation of distal and proximal photoreceptors resulted in sensitivity peaks at 430 and 522 nm, respectively. In accordance with identified visual pigments and spectral sensitivity, T. longicornis photobehavioral responsivity covered a broad range (420-580 nm). Collectively, a dual-pigment visual system underlies wavelength-specific behavior in T. longicornis, with the short-wavelength pigment likely to be localized in the distal R5 retinular cell. While response-intensity functions did not change over the diel cycle at the retinal level, behavioral photoresponsiveness varied between day and night. At a wavelength used by T. longicornis for celestial orientation (420 nm), photobehavior was heightened at night, potentially aiding in nocturnal orientation. By contrast, at a wavelength used to entrain its circadian rhythm (520 nm) and for routine visual tasks, photobehavior was heightened during the day, and spectral sensitivity matched to the twilight spectrum, facilitating crepuscular vision and entrainment by irradiance at sunrise and sunset.