Electroretinographic Measures of Vision in Horseshoe Crabs with Uniform versus Variegated Carapaces

Abstract

The carapace characteristics of horseshoe crabs (Limulus polyphemus) have been correlated by others with male mating performance and it has also been suggested that such mating differences might depend on visual differences. The effect of carapace character on horseshoe crab vision was therefore noninvasively investigated using the electroretinogram (ERG) of the lateral compound eye as a measure. Male Limulus were dichotomized on three carapace dimensions: clear versus dark eyes, light versus dark carapaces, and presence versus absence of barnacles. All dichotomizations gave similar results: No crab was ERG-blind. Analyses based on ERG magnitude yielded trivial sensitivity differences between these groups. However, analyses based on ERG latency indicated that response speeds exhibited reliably different trends between groups: Although responses to dim flashes were similar for all animals, increasing flash intensity produced significantly greater changes in the latencies of the ERGs in uniform males than in variegated males. This interactive dependence of ERG speed differences on flash energy cannot be the result of greater light absorption by the variegated specimens'' darkened corneas. The visual capacities of variegated specimens are therefore somewhat altered, but certainly not absent. Equalizing animal size between groups did not measurably affect these results, implying that age was not a factor. The previously reported correlation of an individual male''s appearance with its performance in mating competitions is extended by these data to include its sensory functioning as well.

Adaptation-Dependent Differences in Electroretinographic Latency Patterns in Uniform and Variegated Horseshoe Crabs

“…This question was addressed physiologically by using a reliable, noninvasive biological signal, namely the electroretinogram (ERG). This visual signal demonstrated [1] that all individuals could see, at least in the ERG sense, and measurements of response size exhibited no significant difference in light sensitivity. However, response speed measures revealed a latency interaction: responses to dim flashes had similar latencies in all animals while variegated animals exhibited significantly slower latencies to bright lights.…”

“…As a result, by the time an adequate number of repetitions have been delivered and an average ERG has been collected, the eye has necessarily been exposed to a long train of visual stimuli whose energy and interstimulus interval (ISI) have determined its state of adaptation. And the 10-second ISI used by Wasserman and Cheng [1] was certainly short enough to produce LA, particularly at higher stimulus energies.…”

“…1 by Wasserman and Cheng [1] for photographs). Some individuals have quite uniform carapaces while others are variegated.…”

“…Two possible mechanisms were suggested [1] as the basis for this interactive result: (1) it could be caused by a corresponding change in the neural mechanisms that mediate photoreceptor transduction of light into bioelectric signals, or (2) it could be due to changes in the accessory screening pigments that surround the photoreceptors. Although these screening pigments only act as light-absorbing filters, their attenuation of the visual stimulus will produce latency changes because neural latencies are affected by stimulus energy.…”

“…An individual Limulus with a mottled carapace that is representative of the variegated animals used in the present study. Uniform animals, which had been illustrated in figure 1A of Wasserman and Cheng [1], had lighter unmottled carapaces and generally also had a clear lateral eye exhibiting a frank pseudopupil. As in Wasserman and Cheng [1], this illustration was derived from an Ektachrome photograph which was processed using Adobe Photoshop.…”