Vision in Limulus mating behavior. ROBERT B. BARLOW JR. (Syracuse University), LEONARD C. IRELAND, AND LEONARD KASS
Although the Limulus visual system has been studied extensively for more than 50 years, the role of vision in the animal's behavior is not known. We investigated the problem by studying Limulus mating
behavior, which is the only well known behavior the animals exhibit in their natural habitat. Along the
Eastern coast of the North Atlantic Limuli move in from deep water, pair off, and build nests near the
water's edge at high tide. We observed the activity of males in the vicinity of cement castings placed 4 m below the high water line on the South side of Mashnee Dike, Cape Cod, MA. Cement castings were made of an adult female carapace (27 cm width), a hemisphere (29.5 cm diameter), and a cube (16.5 cm/edge), with the exposed surface area of the hemisphere and cube equated to that of the female carapace (1365 sq cm). Each casting was painted either black, grey, or white.
Male Limuli are attracted to the cement castings. The degree of attraction depends on both the form and the shade of the castings. We determined the degree of attraction by counting the number of males in contact with the castings during eight high tides, four at night and four during the day. Total number of contacts counted was 6988. The number of males was highest around the female models and lowest around the cubes. Thus Limulus can discriminate form. When the shade of the castings is taken into account, black castings yielded the highest percentage (40%) of male contacts both during the day and at night. The attraction to grey and white depended on time of day. Combining the data on both form and shade showed that black Limulus was the most attractive casting and white cubes the least.
We conclude that Limulus can discriminate both form and shade during mating behavior. The discrimination of form may result from information transmitted by several sensory systems. The discrimination of shade requires vision.
Supported by NSF grants BNS 8025519 and 7719436 and NIH grant EY00667.
Ultraviolet responses of the Limulus median ocellus. LARRY A. WESTERMAN (Syracuse University) AND ROBERT B. BARLOW, JR.
The median optic nerve of the horseshoe crab, Limulus polyphemus, discharges nerve impulses in response to ultraviolet illumination of the median ocellus, in situ. The impulses are conducted by only about 10% of the 300 nerve fibers from the twin ocelli. The remaining nerve fibers are silent in the cut nerve preparation. The spectral sensitivity of single ocellar nerve fibers was determined from threshold responses to 5-see flashes of narrow-band chromatic stimuli. The spectral sensitivity function reaches a maximum at 380 nm. The incident energy density at the corneal surface for a threshold response of I spike/see is 2 X 10-' µW/cm2/nm at 380 nm (stimulus bandwidth 10 nary). The ocellar response is more than 5 log units less sensitive at 520 nm. The electroretinographic response of the ocellus also reaches maximum sensitivity in the UV gamma Max 360-380 nary), but in addition shows a secondary maximum, 1.5-2.5 log units less sensitive, at 520 nm. No similar visible sensitivity is recorded from single ocellar nerve fibers.
Intensity coding in the optic nerve discharge is graded over a range of at least 8 log units of light intensity. The steadystate firing rate in response to long (10-see) stimulu delivered to the dark-adapted ocellus is approximately linear when intensities are near threshold. The rate reaches a sloping plateau at 3-4 log units above threshold, and then positively accelerates for intensities greater than S-6 log units above threshold. In one ocellar nerve, the firing rate did not saturate for incident flux up to 3.7 x 10 -14th photons/(sec.cm-2) at the corneal surface (380 nm) which produced a steady-state discharge of 51 spikes/sec.
At night the sensitivity of the lateral eye of Limulus is increased by the action of a centrally-located
circadian clock. Previous work shows that illumination of the ocellus increases the efferent optic-nerve activity generated by the clock. The spectral sensitivity of the enhancement effect matches that of ocellar nerve response over the wavelength range of 300-640 nm. We conclude that the median ocellus functions primarily as a UV receptor that can enhance the sensitivity of the lateral eye at night. Research supported by NIH grant EY00667 and NSF grant BNS7719436.
Circadian changes in visual sensitivity of Limulus: Behavioral evidence. MAUREEN K. POWERS (Vanderbilt University, Nashville, TN) AND ROBERT B. BARLOW, JR.
Physiological and anatomical measures show that the sensitivity of Limulus lateral eyes fluctuates daily: at night, single photoreceptors are more sensitive, amplitude of the electroretinogram increases, and the pupillary aperture enlarges. These changes are controlled by a circadian clock which is located in the brain and transmits efferent nerve activity to the lateral eyes. We report here that the animal's response to light also fluctuates daily and that the magnitude of change in behavioral threshold corresponds to the magnitude of change in physiological threshold.
Freshly collected male Limuli were securely restrained in an aerated seawater aquarium located in a lightproof shielded cage. Pilot work had shown that current pulses delivered to the fleshy hinge area between anterior and posterior carapace produced an unconditioned change in the rate or amplitude of tail and gill movements. Tail movements were monitored by attaching a bead thermistor to the middle of the tail, and gill movements were monitored by placing a thermistor underneath the animal. After 12-24 h habituation to the apparatus, a classical conditioning procedure was used to associate a 10 sec light stimulus to the lateral eye with shock once every S-10 min until the animal responded to the light. Nine out of fourteen Limuli responded satisfactorily within 50 trials. Animals were generally less responsive if collected more than 7 days before the experiment.
Visual sensitivity was measured in 5 trained Limuli during the day and the night, following at least 24 h in darkness, by presenting light of different intensities and computing the percent response to each intensity. In every case, behavioral responses were elicited during the night with stimuli that had been below threshold during the day. The three animals with the most complete data were 1.0-1.5 Iog units more sensitive at night than during the day, which is about the difference in sensitivity observed in physiological experiments.
Our results are the first to demonstrate a behavioral effect on the circadian rhythms within the Limulus visual system. The data show that there is a strong correspondence between behavioral and physiological changes in sensitivity, and they imply that the efferent input from the brain to the lateral eyes plays a significant role in regulating the visual sensitivity of the behaving animal.
Supported by a URC grant from Vanderbilt University, and by NIH grants EY03352, EY0667 and NSF grant BNS 7719436.