Adaptation-dependent differences in electroretinographic latency patterns in uniform and variegated horseshoe crabs

The carapaces of horseshoe crabs (Limulus polyphemus) differ. Some individuals have uniform carapaces and clear eyes while others have variegated carapaces and dark eyes. These differences have been reported to be correlated with latency differences in the electroretinogram (ERG) of the lateral eye. Such a result might have had a neural basis in the mechanisms underlying visual transduction but it could also have reflected a visual screening pigment difference. A direct experiment was therefore designed to choose between these two hypotheses by varying the relative state of adaptation. The results were as follows. In uniform animals, dark adaptation had the kind of effect seen in single photoreceptor cells - latencies were longer in dark-adapted eyes and latencies were also longer for dim flashes. However, variegated animals showed a significant adaptation interaction: in light adaptation, dimmer flashes produced the usual effect, namely a longer ERG latency, while in dark adaptation, latencies were close to equilatent, being within experimental error of each other for both flash energies. These data make it unlikely that the photoreceptor transduction mechanism is the locus of the visual differences between the two types of animals. Instead, they are consistent with an interaction of screening pigment effects with photoreceptor transduction effects.