Featured in the newest Dialogue Magazine »

Special eyes

Horseshoe crabs don’t look mysterious and enigmatic, but they are. Normally one would not expect any very deep questions to be evoked concerning creatures which resemble miniature tanks, moving with ponderous dignity across the beach. But these marine creatures with shells, these “crabs”, are not actually miniature when compared to other animals of the seashore. These animals weigh as much as 4.5 kg (10 lbs) and they may grow to be 60 cm (2 ft) long. To find one such specimen would be memorable enough – but where there is one, there are generally thousands or hundreds of thousands. At the appropriate time in the spring, some beaches along the Atlantic seaboard from Maine to Yucatan Peninsula, are invaded by thousands or even millions of these apparitions.

They look sinister, these crabs with their lateral eyes projecting in a sort of perpetual frown from the smooth contours of the shell. They are not really threatening however. These creatures have merely come to the beach to lay their eggs. But not all of them manage to retreat safely back into the sea. Through the years, many of these animals have been captured for physiological study. It was in 1926 that H. Keffer Hartline began to study electrical impulses from the optic nerve of horseshoe crab eyes. From these studies some important principles about the functioning of eyes were discovered. As a result Drs. Ragnar Granit of Sweden and Americans H. Keffer Hartline and George Wald were awarded the 1967 Nobel Prize in Medicine.

During all those years of study, the lateral eyes had always been removed from the crab before the experiments were conducted on electrical impulses in the nerve leading away from the eye, or in the eye itself. Then in the 1970s a novel approach was tried. A team of scientists applied electrical probes to the eyes of intact animals. Imagine their surprise when they found that at night, sensitivity of the crab eye to light was increased by a factor of up to one million times that of the daytime response!! (Robert B. Barlow p. 90) Subsequent research showed that an internal twenty four hour clock (circadian rhythm) in each crab’s brain, controlled this amazing cycle. Even when crabs were kept in constant darkness for more than a whole year, their eyes still showed this circadian rhythm.

Not surprisingly, it has been discovered that this unique cycle of sensitivity to light has very complicated controls. For a start, the research team found that these changes involve a feature exactly opposite to other biological systems. Normally as sensitivity to a stimulus increases, so does the background noise (signals generated at random rather than in response to a genuine stimulus). The normal situation is like what happens when you turn up the volume on your radio. The volume goes up but so does the static (background noise). In the case of the crab’s eye however, the noise level goes down as the sensitivity increases. (Barlow p. 91).

A number of special features in the nervous system of the crab and in its eye are needed to bring about this special cycle. Firstly of course there must be a time keeping center in the brain. Two neurotransmitters, special chemicals which enable nerve cells to communicate with each other, apparently control the cycle of changes in the eye. As dusk sets in, the aperture widens in each component of the compound eye. In addition, in photoreceptor cells in each component of the eye, rhodopsin molecules (which react to light) are shifted closer to the aperture. This enhances the chances that each photon of light will set off a “quantum bump” with the receptor molecule. In addition, the bumps that do occur at night actually last longer than those during the day. This longer duration at night increases the likelihood that enough quantum bumps will occur at the same time to generate an electrical impulse which can be transmitted to the nerve cell. Changes in the ion channels in the photoreceptor cell membrane also enhance the opportunity for generating an electrical impulse. In addition it is absolutely essential that all these conditions are reversed during the day. Otherwise the animal would be permanently blinded at sunrise.

It is apparent that a very complex system is in place here. Firstly the time keeping mechanism in the brain must exert control of day or night phase in the eye by means of special neurotransmitter compounds in the optic nerve. The resulting changes in the eye include opening or closing of ion channels in the photoreceptor cell membrane, the moving of banks of rhodopsin molecules and changes in the sensitivity of these molecules. The system needs all these components to function, especially to protect the crab from being blinded by too much light during the daytime.


There seems little doubt that the horseshoe crab eye is unique in the animal kingdom. But what benefit does the horseshoe crab obtain from these amazing eyes? This animal finds its food by means of chemical stimuli (Daniel C. Fisher p. 203) and it has no predators (Barlow p. 93). The optical system of this animal however has been so intensively studied that there are equations available to describe its response both to static and to moving images. Recently generated computer models indicate that crab eyes are most sensitive to objects the size of fellow horseshoe crabs which move at speeds typical of these creatures. Tests with live animals in nature indeed confirm that these crabs see almost as well at night as during the day. Normally horseshoe crabs are not that interested in other members of their own species. In the springtime however, all this changes. The males develop a fascination for members of the opposite sex. During an ever so brief interlude in the spring, males use their wonderful eyes to identify potential mates. In the dark, as females move up the beach with the highest tides, males scramble to attach themselves (literally) to a suitable mate. Undignified free-for-all scrimmages develop as males jostle for position with the females. And thus reproduction is accomplished for another year.

So the wonderfully specialized eyesight of the horseshoe crab is useful to them only during the mating season. Such a fancy system is far too sophisticated for their actual needs. A chemical method of locating females would work just as well. They already use such a method to locate and pursue their food. From the viewpoint of evolutionary theory, the horseshoe crab is a most unlikely candidate for the development of fancy eyesight. This capability is much too peripheral to its lifestyle to expect that natural selection for this feature could have a significant effect on the population. All the possessors of beneficial eye mutations might well die off long before better eyesight would do them any good. In addition, any favorable mutations in the female gender would be wasted since only the males pursue a mate.

Varied ecology

Horseshoe crabs are astonishingly hardy. These animals can withstand days of drying, wildly fluctuating variations in salinity, and great swings in temperature (Ward pp. 138-139). They are also extremely tolerant of industrial pollution, so that on many shores in the eastern United States, horseshoe crabs are the last creatures left. During much of their lives however, adult horseshoe crabs live offshore in water up to 50 m (150 ft) deep, far from most fluctuations in the environment. In May, mature crabs( ages 5-7 for males, and 7-9 for females) instinctively migrate inshore at night during the highest tides of the year. They proceed inland as far as the water is able to carry them and then the female scoops out a nest. The male, having used his remarkable eyesight to locate a female, then fertilizes the eggs and the nest is covered over. Then while the eggs remain high and dry, development takes place. When the next really high tide comes several weeks later, the hatchlings move out of their nest with the tide waters. During their first year, the larvae grow slowly in the intertidal zone. Eventually, after many molts, they retreat as adults to the offshore regions – far from the effects of the tides which they will later need to track in order to reproduce successfully.

Long History

Some individuals have questioned whether perhaps in a distant evolutionary past, horseshoe crabs might have needed their eyesight to avoid predators. But this organism is the least likely candidate for a past which is different from the present. Experts differ on precisely how deep in the rocks, fossils identifiable as horseshoe crabs are found. Most agree however that horseshoe crabs are very old or “some of the most long-lived survivors on this planet” (Ward p. 137). Daniel C. Fisher claims alternatively that “none of them are known as fossils” ( p. 205). What Fisher, an authority on the horseshoe crab, is saying, is that the shape of the shell of living specimens is slightly different from those of fossil specimens. He places great emphasis on this and says therefore that there are no fossilized examples of the modern species. Ward comments in return: “To a less critical eye, the horseshoe crabs of that long-ago time look virtually identical to present day species. But Fisher found slight differences in the carapaces [shells] of the Jurassic and the modern species, and investigated how these differences would affect the animals’ swimming.” (Ward p. 148) It seems however that Fisher is placing undue emphasis on slight differences in shell contours. He himself in Eldredge and Stanley (p. 206) points out how difficult it is to ascertain the correct shape of the crab shell since compression by overlying sediments can modify this feature. What the fossil record tells us then is that horseshoe crabs of the past were like those that we see today. If the horseshoe crab, consisting largely of shell, is unattractive to predators today then it was also unappealing in the past.

Isolated Status

Not only are horseshoe crabs genuine “living fossils”, but in taxonomic terms, they are a very isolated group. Horseshoe crabs today are represented by a mere five species. Some authorities place these few representatives in their own class Merostomata. This group compares unfavorably with other classes in the phylum Arthropoda. The class Insecta for example contains about one million species, the Crustacea contain about 26,000 species and the Arachnida boast about 70,000 species. The actual numbers of species vary with the authority quoted, but the ratios of species numbers should remain about the same. In anybody’s book, horseshoe crabs are isolated taxonomically from other organisms in the huge phylum Arthropoda. These crabs would obviously not be considered an evolutionarily active or successful group, particularly as little variation has been found in the fossil record.

Horseshoe crabs constitute a prime example of stasis (no evolutionary change). However for evolutionists there is one major problem with this conclusion. The biology of horseshoe crabs is such that one would expect them to evolve rapidly, if evolution theory were correct. Populations which are generalists (able to tolerate a wide variety of conditions) are the ones which are expected to be able to adapt to changing conditions and to show change over time. The opposite is expected for populations which survive only under a narrow set of conditions. These are the specialists. The latter are expected to be most prone to extinction and least likely to exhibit change. Some generalist populations might unexpectedly lack the genetic variety necessary to enable them to change rapidly. However this is not the case with the horseshoe crab. Biochemical tests (electrophoresis) indicate that horseshoe crabs have very high levels of genetic variability such as one would expect to find in organisms with rapid rates of evolution (Fisher p. 205) and a study of the variation in appearance in various populations of these organisms (Riska 1981) also reveals an unusually high proportion of variation between separated populations rather than within any given local population. Again this phenomenon would be expected in populations capable of rapid change (divergence) – not in populations which show no change worth mentioning.

All the details concerning the horseshoe crab are opposite to the expectations of evolution theory. This is a hardy, tolerant organism which suffers neither from predators nor climatic extremes. They eat almost anything so they should not starve. Within their populations there is also much genetic variability. Such a generalist, genetically diverse taxon should be one of a rich profusion of similar species. Divergence should long have been the order of the day here. But horseshoe crabs remain taxonomically isolated although they are found in deep depths of rock. In that the fossil specimens are so similar to living species, it seems evident that their past ecology would be similar to today. Do they enjoy fancy eyesight today? Then they must have had the same capacity in the past. Evolution theory cannot account for the development of such a complex but peripherally useful system. An oscillating system which varies in sensitivity over the course of a day by a factor of up to one million times would require powerful selection. But this is an all or nothing system so selection would be ineffective anyway. There is no special reason other than intelligent planning why this creature is so gifted. What an interesting example of richness and variety in the creation. What an original method the males employ to find a mate. There is no reason to believe they have ever conducted their courtships in any other way.

Margaret Helder
January 1997

Subscribe to Dialogue