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Marine Biological Laboratory, Woods Hole, Massachusetts 02543
1 Cornell University.
2 Occidental College.
3 SUNY Upstate Medical University.
A horseshoe crab, Limulus polyphemus, begins life with approximately 6 ommatidia in each of its lateral eyes. When the animal matures, 15 or 16 molts later, each eye will have close to 1000 ommatidia. New ommatidia are added at each molt (1), and optic nerve fibers must also develop between new ommatidia and the brain if they are to be functional visual units. Therefore, determining the retinal location of all added ommatidia may help us to understand how subunits of the optic nerve (2) develop and how retinotopic maps form in the central visual pathways (lamina and medulla) of the brain (3). We already knew that new ommatidia are added to the anterior edge of the eye (4, 5) but it was not known if they were added elsewhere. In this study we closely examined all regions of the lateral eye before and after the Stage IX molt.
Retinas of two Stage IX specimens of Limulus, 3 years of age, were scarred with a sharp metal needle. The scars, placed along the anterior and ventral edges and at five central locations in August of 2001, would serve as landmarks for assessing the growth of the retina when the animal molted to Stage X. Ommatidia in every scar appeared damaged, and the pigmented tissue often clung to the scarring needle when it was withdrawn. After the scarring, the juveniles were maintained under natural diurnal lighting in a shallow seawater trough (40 cm x 50 cm; 10 cm water depth; 2 cm sand depth) at the Marine Biological Laboratory, Woods Hole, Massachusetts. In May of 2002 both animals molted.
We studied the development of the eyes by viewing the post-molt animals and molted carapaces with a Nikon SMZ-2T dissecting microscope. In spite of the trauma incurred by the deep scarring, the cornea and ommatidial array of the post-molt animal appear normal, as do most of the individual ommatidia. A few ommatidia where we expected scars appear stunted, but we have not investigated these areas anatomically because we do not want to sacrifice the animals.
Although scars were not readily apparent in the post-molt animal, we observed that naturally occurring patterns of ommatidia are retained from the pre-molt animal. Thus we could identify all ommatidia in the post-molt animal that were also present in the molt. We used the patterns, together with fragments of retinal pigments that remained inside the cornea of the molt (a result of scarring), as landmarks to completely reconstruct the right eye of a Stage IX molt (prosomal width, 30 mm). Using this reconstruction of the molt as a base, we added to it every ommatidium in the Stage X post-molt animal (prosomal width, 37 mm) that was not present or distinguishable in the molt (Fig. 1).
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The eye grows asymmetrically. As Waterman (1) noted, the eye grows more in width than in height. We measured a 33% increase in width and a 15% increase in height. The greater increase in width is accounted for by new ommatidia added to the anterior edge together with an increase in ommatidial diameter. The growth of ommatidia is also asymmetric. Ommatidia in medial and posterior regions of the eye of the molt increased 15% in diameter, while those in the anterior portion increased 25% or more. Because no ommatidia are added to the dorsal or ventral parts of the eye, the increase in eye height results only from an increase in ommatidial diameter. Indeed the number of ommatidia in the vertical direction (26 ommatidia) is the same as that of adults (2, 3), indicating that at least after Stage IX changes in view in the vertical direction are complete.
Our reconstructions confirm previous findings that relatively small ommatidia are added at the anterior edge in vertical columns (4, 5). They also show conclusively that ommatidia are added only at the anterior edge. Waterman’s (1) original conclusion, that the eye grows by adding ommatidia at the posterior edge, is in error.
Because the lateral eyes of the horseshoe crab grow by adding ommatidia to the anterior edge, the animal’s view of the world changes dramatically as it matures from a Stage I trilobite (
6 ommatidia) to an adult (1000 ommatidia). At the Stage IX molt, over 100 small ommatidia are added to the anterior edge, and the existing ommatidia increase in size. After molting, the optic axes of the ommatidia change, and an increasing percentage of ommatidia "look" posteriorly (5). How the brain accommodates optic nerve fibers from new ommatidia, and how soon after addition new ommatidia begin sending visual information to the brain, remain to be determined.
Supported by grants from the National Science Foundation, National Eye Institute, and the National Institute of Mental Health. K. Smith and C. Ridings received REU Fellowships from the National Science Foundation.
Literature Cited
This article has been cited by other articles:
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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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