title: Limulus vision in the ocean day and night: effects of image size and contrast.
abstract: Male horseshoe crabs, Limulus polyphemus, use their eyes to locate mates day and night. We investigated their ability to detect targets of different size and contrast in a mating area of Buzzards Bay, Cape Cod, MA. We found that males can see large, high-contrast targets better than small, low-contrast ones. For targets of the same size, animals must be about 0.1 m closer to a low-contrast target to see it as well as a high-contrast one. For targets of the same contrast, animals must be approximately 0.2 m closer to a small target to see it as well as one twice as large. A decrease of 0.05 steradians in the size of the retinal image of a target can be compensated by a four-fold increase in contrast. About 60% of the animals detect black targets subtending 0.110 steradians (equivalent to an adult female viewed from about 0.56 m), while only 20% detect targets subtending 0.039 steradians. This study shows that horseshoe crabs maintain about constant contrast sensitivity under diurnal changes in light intensity in their natural environment. As a consequence of circadian and adaptive mechanisms in the retina, male horseshoe crabs can detect female-size objects about equally well day and night.
authors.affiliation: Department of Bioengineering and Neuroscience, Syracuse University, USA.
authors.forename: E D
authors.initials: ED
authors.lastname: Herzog
authors.forename: M K
authors.initials: MK
authors.lastname: Powers
authors.forename: R B
authors.initials: RB
authors.lastname: Barlow
mesh_terms: D000221:Adaptation, Ocular / Q000502:physiology; D000818:Animals; D002940:Circadian Rhythm / Q000502:physiology; D015350:Contrast Sensitivity / Q000502:physiology; D005260:Female; D006737:Horseshoe Crabs / Q000502:physiology*; D008297:Male; D009792:Oceans and Seas; D011597:Psychomotor Performance; D012858:Size Perception / Q000502:physiology; D014785:Vision, Ocular / Q000502:physiology