Living Fossils: The Horseshoe Crab Story

Living Fossils Continue to
Amaze and Surprise Scientists

by Margaret Helder, PhD

There are quite a number of living fossils around the world, including the Ginkgo and dawn redwood trees in China, and animals like the horseshoe crab and the coelacanth fish. In this two-part series, we will look at a couple of famous ones.

The classic definition of a living fossil is an extant organism that closely resembles fossil specimens considered to be many millions of years old.¹ In a world where presumably most other organisms were evolving, why did these organisms not change over time as well? Does this mean that their genetic controls stayed relatively constant, or did they exhibit plenty of genetic diversity that never led to obvious changes? Alternatively, is the time since the original creatures were buried and fossilized actually of short duration, only thousands of years rather than millions?

On the beach, in the lab, in the rocks

The horseshoe crab (Limulus polyphemus) is considered to represent a particularly spectacular case of a living fossil. Live specimens can be found in the thousands or hundreds of thousands along the Atlantic coast of the United States at certain times of the year. They may weigh as much as 4.5 kg (10 lbs) and grow to 60 cm (2 ft) long. These creatures became particularly famous when in 1967 Drs. Ragnar Granit of Sweden and Americans H. Keffer Hartline and George Wald were awarded the Nobel Prize in Medicine for their studies on electrical impulses moving along the large optic nerve in these creatures. From these studies the scientists elucidated the primary physiological and chemical processes in all eyes.

Some time after the initial studies, other scientists applied electrical probes to the eyes of intact horseshoe crabs. Imagine their surprise when they found that at night, the sensitivity of the horseshoe crab’s compound eye to light increases by a factor of up to one million times that of the daytime response!² This is a most remarkable circadian rhythm not found in other creatures. It requires some very specialized design features to make it work.³

Taxonomic isolation

It so happens however that the horseshoe crab has other claims to fame as well. In taxonomic terms, these creatures are a very isolated group, which means there is nothing else like them. Horseshoe crabs are represented today by a mere four species. Some authorities place them in a class Merostomata along with the quite different Euripterids (sea scorpions) which are known only from fossils. In terms of numbers of living representatives, the Merostomata compare unfavorably with other classes within the phylum Arthropoda [i.e., animals with a chitinous exoskeleton and jointed appendages]. The class Insecta, for example, contains possibly one million species. The Crustacea contain about 42,000 species and the Arachnida, some 100,000 species.  In anybody’s book, horseshoe crabs are isolated taxonomically from other organisms within the huge phylum Arthropoda. These animals (not true crabs) would obviously not be considered an evolutionarily active or successful group, particularly as little variation has been found in the fossil record. Why, therefore, do they survive to this day?

The four living horseshoe crab species include: the Atlantic  (Limulus polyphemus) from the Atlantic coast of North America, the mangrove (Careinoscorpius rotundicauda), the Indo-Pacific (Tachypleus gigas), and the tri-spine  (Tachypleus tridentatus) from south and east Asia. Despite the different genus names, the creatures all look very much alike.

Living fossil status of the horseshoe crab

As far as their status as living fossils is concerned, horseshoe crabs certainly have a very solid claim to it. As Nong et al. pointed out in early 2021:

The oldest actual fossils of horseshoe crabs date to the Ordovician period ~450 million years ago (Mya), and remarkably, extant species remain relatively unchanged morphologically since this extremely ancient date.⁴

Not only are these species very similar, but a recent discovery of a horseshoe crab from Poland (Limulus darwini) means that the genus Limulus itself is considered to have survived in almost unchanged form for about 148 million years. Thus Kin and Blazejowski declare: “Accordingly, L. darwini is here regarded as the oldest known representative of the genus.”⁵

The explanation often given for such long term stasis is that these creatures have evolved extremely slowly. Given that most evolutionists consider evolutionary change to be inevitable, especially if long periods of time are involved, they typically insist that the process cannot be avoided.⁶ The slow-evolution explanation, therefore, looks like special pleading. Some evolutionists try to hide the problem with terminology. Niles Eldredge in 1984 summarized the situation as one of “rates of change.” He clarified this to mean that apparent slow rates of change may apply mainly to appearance, but not necessarily to the underlying genetics.⁸ George Gaylord Simpson in 1944 gave the name bradytely to the slow process of change characteristic of living fossils.⁷  Others have suggested that living fossils be renamed stabilomorphs (creatures with stable appearance), Perhaps the reason is that such jargon does not have the same attention-getting cachet of the term ‘living fossils.’⁹

Highly variable genetics

What has been revealed about the genetics of Limulus? In 1981, Riska studied the amount of morphological variation evident in local populations along the Atlantic coast. Between populations in the area, he found that coefficients of variation within populations were perhaps even higher than in most organisms (such as mammals). This was very surprising. In fact, between localities “Limulus seems to be more variable than other animals. In the present study this percentage ranges from about 50% to 73%. These are unusually large values.”¹⁰ Such a condition of higher diversity between populations is thought to be characteristic of taxa which are evolving rapidly – but Limulus, according to its phenotype (outward appearance), is not obviously changing.

The same situation of unexpectedly high diversity was found with physiological characteristics which more closely reflect the status of the genes. Kin and Blazejowski point out that “Levels of polymorphism and heterozygosity for allozyme loci in Limulus are comparable to those found in much more rapidly evolving organisms.”¹¹ In terms of obvious differences, the horseshoe crabs should display considerable diversity between living and fossil taxa. But they do not.

Revelations from the genome

In the modern era of genetic sequence, we now have the luxury of comparing actual genes in extant organisms. It turns out that there is amazing diversity between horseshoe crab species. Homeobox genes, for example, are particularly relevant for comparing organisms since these genes specify cell identity and positioning during embryonic development. A subset of these genes, the Hox genes, specify the actual body plan. In general, biologists consider that organisms will display very few differences between the homeobox genes (and the Hox genes) since they are so critical to development. If something goes wrong early in embryonic development, the result is invariably lethal. It seems to make sense, therefore, that organisms would not invest effort in changing those genes.¹²

When we come to the horseshoe crabs however, a shock awaits us. There is amazing diversity between these genera in the numbers and placing of Hox genes. The research by Nong et al. says,

For C. rotundicauda and T. tridentatus, the number of Hox genes was found to be 43 and 36, respectively. In C. rotundicauda we found that there are five Hox clusters, with Hox genes on additional scaffolds; while in T. tridentatus, there are three Hox clusters, again with other Hox genes scattered across different scaffolds…. [For] L. polyphemus … there are four Hox clusters with additional Hox genes located on different scaffolds.”¹³

For comparison, consider that in the scarlet millipede (Trigoniulus corallinus) there are only 12 Hox genes and in the fruit fly (Drosophila melanogaster) there are 8 Hox genes.¹⁴ In regard to Hox gene diversity, the horseshoe crab is more like mammals which exhibit 39 Hox genes in 4 clusters on four different chromosomes. When one considers that all the horseshoe crab species look very much alike, and that fellow arthropods display such low numbers of these genes, the differences in Hox gene numbers and clusters between horseshoe crab species were a massive surprise to everyone.

Thinking outside the Hox

Why are the genes of the different horseshoe crab species so different? Kin and Blazejowski speculate that the horseshoe crabs did not need to change, so they didn’t.

[T]heir level of adaptation, the quality of their adaptive strategy is so high (so effective), that small changes which had to continuously occur over several millions years (in the case of L. Darwini, at least 148 Ma) did not result in any significant morphology variations…. it implies… that they (as a group) could afford to “reject” any further changes.¹⁵

This does not make sense even within evolutionary theory. How could these creatures, as a group, decide anything? How could they reject the inexorable force of evolution that Darwinians believe in? It was unexpected to find that the horseshoe crabs exhibit far more genetic diversity than many others creatures which they presume have continuously evolved. It looks as if the horseshoe crabs have continued to churn out genetic variations simply to stay the same!

This is not the only possible conclusion. If the time interval is short since all the fossils were catastrophically entombed, then the amount of contained diversity would not necessarily result in outward changes. The important issue is not rate of change but the time interval available for change to occur.

In a future article, we will examine another famous living fossil: the coelacanth.

References

1. Chris T. Amemiya et al. The African coelacanth genome provides insights into tetrapod evolution. Nature 496 #7445 pp. 311-316. See p. 315.
2. Robert B. Barlow, Jr. 1990. What the brain tells the eye. Scientific American April pp. 90-95. See p. 90.
3. Barlow pp. 90-95.
4. Wenyan Nong et al. 2021. Horseshoe crab genomes reveal the evolution of genes and microRNAs after three rounds of whole genome duplication. Communications Biology 4:83 pp. 1-11      https://doi.org/10.1038/s42003-020-01637-2       See p. 2.
5. Adrian Kin and Blaze Blazejowski. 2014. The Horseshoe Crab of the Genus Limulus: Living Fossil or Stabilomorph? PLoS ONE 9 #10: e108036. 1-15.  See p. 3. https://doi.org/10.1371/journal.pone.0108036
6. Niles Eldredge. 1984. Simpson’s Inverse: Bradytely and the Phenomenon of Living Fossils. In Niles Eldredge and S. M. Stanley eds. Living Fossils. Springer Verlag. New York. pp. 272-277. See p.
7. G. Simpson. 1944. Tempo and Mode in Evolution. New York. Columbia University Press.
8. Eldredge p. 272.
9. Kin and Blazejowski p. 7.
10. Bruce Riska. 1981. Morphological variation in the horseshoe crab Limulus polyphemus. Evolution 35 #4 pp. 647-658. See p. 655-656.  (italics his).
11. Kin and Blazejowski p. 7.
12. For example, consider the case of the homeobox genes in the coelacanth (a living fossil fish) when compared with ray-finned fish and four-footed creatures. There are great differences in morphology, but only slight differences in homeobox genes among all these creatures. Amemiya et al. 313.
13. Nong et al. 4.
14. Nong et al. 4.
15. Kin and Blazejowski p. 8. Some phrases omitted for sake of brevity.

Comment: If the evolutionary timeline were true, there would be a 148 million year gap between the youngest fossil horseshoe crab and those alive today. This means that horseshoe crabs were living by the millions without leaving a trace in the fossil record through the time of the dinosaurs, through the Cretaceous extinction and all the way to the present. They look virtually unchanged over all that time and three times further back, to 450 million years ago. In the same amount of time, fish supposedly evolved into reptiles and mammals evolved into scientists. Something seems drastically wrong with this belief. We appreciate Dr Helder’s insights into this topic. —Ed.

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Margaret Helder completed her education with a Ph.D. in Botany from Western University in London, Ontario (Canada). She was hired as Assistant Professor in Biosciences at Brock University in St. Catharines, Ontario. Coming to Alberta in 1977, Dr Helder was an expert witness for the State of Arkansas, December 1981, during the creation/evolution ‘balanced treatment’ trial. She served as member of the editorial board of Occasional Papers of the Baraminology Study Group in 2001. She also lectured once or twice a year (upon invitation) in scheduled classes at University of Alberta (St. Joseph’s College) from 1998-2012. Her technical publications include articles in the Canadian Journal of Botany, chapter 19 in Recent Advances in Aquatic Mycology (E. B. Gareth Jones. Editor. 1976), and most recently she authored No Christian Silence on Science (2016) which promotes critical evaluation of scientific claims. She is married to John Helder and they have six adult children.