Robert Barlow
Bob Barlow is a student of Hartline who continues to explore vision in Limulus at the MBL and who studies how vision influences behavior. He found that male horseshoe crabs are able to distinguish shapes and shades of color. He placed concrete casts of adult female crabs, a hemispherical cast and a cube both of which approximated the size of a female crab. He also painted these casts either black, gray, or white. He found that the crabs were attracted to the castings and that they showed preference to the black crab-shape and the white cube received the least attention. This all was going on during the nightly mating in the shallow waters of the full moon. What seemed fairly simple in concept; crabs locating females, turned out to be a highly complex interaction of the compound, the simple eyes, and a biological clock located in the brain of the crab.*
Barlow reported that the simple eyes, the ocelli, are sensitive to ultraviolet light. He also found that the sensitivity of the ocellus increases at night through the action of a "centrally located circadian clock." He concludes that the "median ocellus function primarily as a UV receptor that can enhance the sensitivity of the lateral eye at night."
What actually happens is that the ocellus sends signals to the lateral (compound) eye of Limulus and effectively suppresses lateral inhibition at night. Lateral inhibition increases the ability to see borders but it makes vision less efficient in dim light. Suppressing lateral inhibition allows the animal to see better in dim light. Ultraviolet light is what sets off this suppression.
The key to understanding what all this is that the nights when there is the most ultraviolet light available to detect is on the full moon! This is exactly the time the horseshoe crabs are spawning and exactly when the males need their dim light vision to effectively discriminate females from things like rocks, lobster bouys, and even the wayward foot of a scientist! *
Abstracts for the following articles are available by clicking the title.
Westerman, L.A., Barlow, R.B., Ultraviolet responses of the Limulus mediann ocellus, Biological Bulletin, 1981 161 352-353
Barlow, R.B., Ireland, C.I., Kass, L., Vision in Limulus mating behavior, Biological Bulletin, 1981 161 339-340
Powers, M.K., Barlow, R.B., Circadian changes in visual sensitivity of Limulus: behavioral evidence, Biological Bulletin, 1981 161 350-351