Electroretinographic measures of vision in horseshoe crabs with uniform versus variegated carapaces.

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.