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Marine Biological Laboratory, Woods Hole, Massachusetts 02543
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 animal’s 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 level of illumination at the water’s surface was 1.0 cd/m2. After giving the animals time (
30 min) to acclimate to the new trough without sand, we videotaped their behavior in the vicinity of a high contrast (black) cylindrical object (diameter: 6.5 cm) placed in the center of the trough. After 5 min, the object was removed for 5 min or replaced with a transparent object of the same size. This sequence of 5-min test intervals was repeated for about 1 h, and then the animals were returned to their sand-filled troughs. We digitized the video recordings at 2 frames/sec and traced the paths of animals on a transparent sheet attached to the monitor. Using NIH Image software, we also measured their "distance of closest approach" to the objects (4). The distance at which animals began to turn from the object ("turning distance," see Ref. 4) was difficult to judge with precision and was therefore not measured. All animals appeared about equally active. We did not track specific individuals.
Figure 1 shows that juvenile horseshoe crabs avoided the black object (a), but not the transparent one (b). Only twice did an animal contact the black object, whereas animals contacted and circled the transparent object many times. In the absence of an object, they moved about through all areas of the trough. For purposes of demonstration, we show only 30 traced paths for each of the two conditions containing targets. Each traced path begins and ends near the sides of the trough because we did not include the animals’ movements along the sides of the trough.
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This study presents the first evidence that juvenile horseshoe crabs can see. Their avoidance of high-contrast objects is similar to that observed for adult females (4). The previous study suggested that adult females migrating to shallow waters to build nests search for unoccupied areas and thus avoid dark, female-size objects. Juveniles may turn away from dark objects because they represent potential predators. Adult males avoided dark objects only if they were held overhead (R. Barlow, pers. obs.). Adult males may view overhead objects as predators in the same way that juveniles view dark objects on the bottom. We cannot relate the specific behaviors we observe to the sex of the 1-year-old juveniles because their sex is not known. Horseshoe crabs acquire external sexual features when they reach maturity, at about six years of age. Perhaps all the juveniles we tested were females and avoided high-contrast objects as they do in adult life. Or the juveniles we tested could have been a mix of males and females, and not yet at the time in life when males change their response to visual objects in front of them from avoidance to attraction.
Supported by grants from the National Science Foundation, National Eye Institute and the National Institutes of Mental Health. C. McGuiness and S. Meadors received REU Fellowships from the National Science Foundation.
Footnotes
1 Boston University Marine Program. 
3 University of South Carolina.
4 Humboldt-Universitädat, Berlin
5 SUNY Upstate Medical University.
Literature Cited
This article has been cited by other articles:
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  A. Garm, M. O'Connor, L. Parkefelt, and D.-E. Nilsson Visually guided obstacle avoidance in the box jellyfish Tripedalia cystophora and Chiropsella bronzie J. Exp. Biol., October 15, 2007; 210(20): 3616 - 3623. [Abstract] [Full Text] [PDF]
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 C. Ridings, D. Borst, K. Smith, F. Dodge, and R. Barlow Visual Behavior of Juvenile Limulus in Their Natural Habitat and in Captivity Biol. Bull., October 1, 2002; 203(2): 224 - 225. [Full Text] [PDF]
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