Frederick A. Dodge

Limulus Vision in the Marine Environment

Horseshoe crabs use vision to find mates. They can reliably detect objects resembling potential mates under a variety of lighting conditions. To understand how they achieve this remarkable performance, we constructed a cell-based realistic model of the lateral eye to compute the ensembles of optic nerve activity (“neural images”) it transmits to the brain. The neural images reveal a robust encoding of mate-like objects that move underwater during the day. The neural images are much less clear at night, even though the eyes undergo large circadian increases of sensitivity that nearly compensate for the millionfold decrease in underwater lighting after sundown. At night the neural images are noisy, dominated by bursts of nerve impulses from random photon events that occur at low nighttime levels of illumination. Deciphering the eye’s input to the brain begins at the first synaptic level with lowpass temporal and spatial filtering. Both neural filtering mechanisms improve the signal-to-noise properties of the eye’s input, yielding clearer neural images of potential mates, especially at night. Insights about visual processing by the relatively simple visual system of Limulus may aid in the design of robotic sensors for the marine environment.

Visually guided behavior of juvenile horseshoe crabs.

The horseshoe crab, Limulus polyphemus, has long been an admirable model for vision research. More than 70 years of research on the physiological properties of the Limulus lateral eye have uncovered fundamental mechanisms of visual function common to many animals, including humans (1, 2). Less attention has been given to the role of the lateral eyes in the animals behavior. Initial field studies showed that adult males use their eyes to find mates, whereas adult females avoid mate-like objects (3). Our attempts to study these behaviors in the laboratory were not successful because adults do not exhibit them in captivity (R. Barlow, pers. obs.). We therefore turned our attention to juvenile Limulus and report here an investigation of their visually guided behavior both in the field and in the laboratory. We first studied visually guided behavior of juvenile crabs on tidal flats (0.3-1 m depth) of the North Monomoy Island Wildlife Refuge, Chatham, Cape Cod, Massachusetts. Because juvenile as well as adult animals are most active on the submerged flats during high tides, we restricted our observations to these periods. We selected 1-year-old juveniles, born in the spring of 2000 (stages VI to X; prosoma widths: 16-39 mm). Their compound lateral eyes contain from 500 to 600 ommatidia, or about half the number of the adult eye. When a moving juvenile crab was located, we placed a high-contrast cylindrical object (7.6 cm diameter; 15 cm high) on the bottom 15 to 45 cm in front of the animal. Twenty-three of the 26 animals tested changed direction and avoided the object; the other 3 stopped and buried themselves. A low-contrast, gray object of the same size and placement evoked avoidance behavior in 14 of 20 animals. Five animals continued straight and hit the object, and one stopped and buried itself. Most animals appeared to respond to objects placed in front of them because they could see them, with the black object being more visible than the gray one. However, we could not eliminate the possibility that they detected a disturbance in the water when the objects were placed in front of them. To examine the visually guided behavior of juveniles under more controlled conditions, we placed them in shallow seawater troughs (40 cm X 50 cm; 3 cm water depth; 2 cm sand depth) under ambient diurnal lighting in the Marine Biological Laboratory, Woods Hole, Massachusetts. To test the animals, we transferred 10 of them to a trough of the same size, containing seawater (3 cm depth) but no sand. The lack of sand prevents them from burying themselves and enhances their locomotor activity. We simulated the illumination of an overcast cloudy day by reflecting light from a white diffusing surface located above the trough. The

Growth, Visual Field, and Resolution in the Juvenile Limulus Lateral Eye

1. Ratliff, F. 1974. Studies on Excitation and Inhibition in the Retina. The Rockefeller University Press, New York. 2. Barlow, R. B., J. M. Hitt, and F. A. Dodge. 2001. Biol. Bull. 200: 169-176. 3. Barlow, R. B., L. C. Ireland, and L. Kass. 1982. Nature 296: 65-66. 4. Powers, M. K., R. B. Barlow, and L. Kass. 1991. Visual Neurosci. 7: 179-189. i of males and females, and not yet a the time in life when l s change their response to visual objects in front of them from

EVALUATION OF CIRCADIAN RHYTHMS IN THE LIMULUS EYE

The visual system of the horseshoe crab, Limulus polyphemus, shows a remarkable circadian rhythm in sensitivity. About the time of sunset, a circadian oscillator in the brain of the animal generates efferent optic nerve signals to the lateral eye modulating both its structure and function. The overall increase in sensitivity nearly compensates for the reduction in ambient light at night (1, 2). High nighttime visual sensitivity appears to be important for mating, a visually guided behavior that the animals can accomplish as well at night as in the day (3). The mechanisms underlying the elegant adaptation of the Limulus eye to dim nighttime illumination include anatomical changes in the retina that increase photon catch at the expense of spatial resolution, and physiological changes that enhance the summation of photon events at the expense of temporal resolution. Here we evaluate further the effect of the circadian clock on two

Circadian Rhythms in Limulus Visual Sensitivity Compensate for Day-Night Changes in Light Intensity.

At night a circadian clock transmits efferent optic nerve activity to the lateral eyes of Limulus polyphemus, increasing their sensitivity to light. The animal’s use of vision to find mates at night suggests that the clock’s effect on retinal sensitivity compensates for the nighttime decrease in ambient illumination. To test this possibility we recorded activity from optic nerve fibers in situ while the animal was exposed to day-night changes in illumination in its natural environment. Using a waterproof recording chamber we monitored the maintained activity of single or small clusters of optic nerve fibers. Placing an animal at a meter depth near the shoreline of Woods Hole, Massachusetts, enabled us to monitor optic nerve activity with a remote amplifier and recorder connected to the animal via a tether. A single optic nerve fiber typically responds to ambient daylight illumination at rates ranging from 4 to 14 impulses/s. In one experiment the steady-state firing rate was 4.7 ips in the early evening when the ambient intensity was 85 cd/m 2 . At 8 p.m. the rate increased to 9.2 ips as the light intensity decreased to 0.054 cd/m 2 . The rate was 11.8 ips under bright illumination the following day (22,000 cd/m 2 ) and dropped to 5.3 ips at 0.040 cd/m 2 that night. Similar results were recorded from animals that were placed in an aquarium next to a south-facing window in Woods Hole. The nearly 1,000,000fold decrease in ambient illumination at night appears to be associated with only about a two-fold decrease in optic nerve response. We conclude that the circadian clock’s input to the lateral eye nearly compensates for the diurnal changes in light intensity. Supported by NSF, NEI, NIMH, RPB and the Lions of Central New York.