Horseshoe Crabs and Vision

At the turn on the century, little at all was known about how vision worked. Scientists of course knew that the eyes were responsible but exactly what happened when light entered the eye and how the brain actually received the information was a mystery.

The compound eye of Limulus has served as a model for studying vision since the early part of the century and before we describe some of that work it deserves an introduction.

Arthropods, like Limulus, possess eyes, but the eyes can vary in complexity. Some are simple, consisting of a few photoreceptors, while others are large and can resolve images. In all of the arthropods, the exoskeloton (the "shell") contributes the lens portion of the eye. As a result, the focus of the eye is fixed as it is part of the outer skeleton of the animal.

Some arthropods possess a type of eye known as a compound eye. The compound eye is made of smaller, simple eye units, called ommatidia. Each ommatidia is composed of a cornea, which as was previously mentioned is formed from the outer exoskeleton. This cornea acts as a lens to focus light into the eye. Following this is an element called the "crystalline cone" which serves as a second lens. It is produced by adjacent cells, usually four in number.

The cone tapers to a receptor unit called a retinula which focuses the light into a translucent cylinder called the rhabdome. The rhabdome is surrounded by light sensitive, or retinular cells. There are generally seven similar retinular cells and one eccentric cell. It is the inward-facing portions of these cells in fact, which form the rhabdome. The rhabdome is the common area where light is transmitted to the reticular cells. Each of these cells is connected to an axon and since each ommatidia consists of seven or eight reticular cells, there are this number of axons which form a bundle from each ommatidia. These axons then form connections with other nerves to create an optic ganglion which passes the visual signal to the brain. Each ommatidia passes information about a single point source of light. The total image formed therefore is a sum of the ommatidia fired. This resultant image can be thought of as a series of dots, just like a computer image is composed of a series of discreet dots (pixels). The more pixels, the better the picture.

Why Limulus

One of the basic tenets of research at the MBL is the use of marine organisms as simple models of more complex biological systems. This is based on an assumption that we can use what we learn from these simple systems in our understanding of the more complex systems we might find in human beings. This has proven to be true and is no different in our understanding of the principles of vision. As H. Keffer Hartline, whom we will learn of more in a moment, used to say to his students when studying vision, "avoid vertebrates because they are too complicated, avoid color vision because it is much too complicated, and avoid the combination because it is impossible."

Limulus provided MBL researchers with a near-perfect model for studying vision. First, it is large, easy to find and easy to handle. As invertebrates go, it is a giant. It possesses both simple and compound eyes. For a marine animal it is also quite hardy and can be safely kept out of water for relatively long periods of time. The compound eyes are relatively large and the optic nerve, which connects the eyes to the brain is not only enormous, up to four inches long, but also lies just below the carapace. For early researchers who wanted to eavesdrop on the signals travelling between eyes and brain one could scarcely design a better animal!

[NEXT - Hartline discovers Lateral Inhibition and wins the Nobel Prize]

References used in the Vision sections of this site.

Hartline., H.K., Wagner, H.G., Ratcliff, Floyd, Inhibition in the Eye of Limulus, Journal of General Physiology, 1956, 39:5 pp.651-673

Westerman, L.A., Barlow, R.B., Ultraviolet responses of the Limulus mediann ocellus, Biological Bulletin, 1981 161 352-353

Barlow, R.B., Ireland, C.I., Kass, L., Vision in Limulus mating behavior, Biological Bulletin, 1981 161 339-340

Powers, M.K., Barlow, R.B., Circadian changes in visual sensitivity of Limulus: behavioral evidence, Biological Bulletin, 1981 161 350-351

Hubbard, Ruth. Retinene Isomerase, Journal of General Physiology, Vol 39, No. 6 pp.935-962

Wald, G., Human Vision and the Spectrum, Science, 1945, 101, 653

Wald, G., Life and Light, Scientific American, Oct. 1959, pp 92-108

Invertebrate Photoreceptors, A Comparative Analysis, Jerome J. Wolken, Academic Press, NY, 1971

Kimbel, R.L., Poincelot, R.P., Abrahamson, E.W., Chromophore Transfer from Lipid to Protein in Bovine Rhodopsin, Biochemistry 1970 9:8 1817

Westerman, L.A, Barlow, R.B, Ultraviolet responses of the Limulus median ocellus, Biological Bulletin General Scientific Meetings. 161:3 352-353

Barlow, R.B, Ireland, L.C., Cass, L., Viision in Limulus mating behavior, Biological Bulletin General Scientific Meetings. 161:3 339-340

Sargent, William., The Year of the Crab., W.W. Norton & Company 1987