Survivors: The Animals and Plants that Time has Left Behind

This analogy particularly came to mind when I saw a badly damaged horseshoe crab still trundling gamely onwards, even with a great hole punched right through its head. Looking over the beach more carefully there seemed to be a lot of these war veterans: lumps out of the thorax, broken tail spikes – clearly, it must take a lot to finish these creatures off. Glenn Gauvry pointed out to me what a great advantage for reproductive success this resilience would furnish. Such endurance is possible because the blood of Limulus polyphemus has exceptional clotting powers; the animal does not bleed to death because its blood coagulates and ‘walls off’ damaged areas. And the blood is blue. Does not the horseshoe crab begin to seem ‘curiouser and curiouser’, as Alice would have said? The blood of Limulus is blue because it is fundamentally different from that flowing in red-blooded creatures, like you and I and the kangaroo. Whereas we have haemoglobin as our oxygen-carrying pigment, which includes the element iron as an indispensable component, the horseshoe crab carries a copper-based molecule called haemocyanin to do a similar job. In nature, copper often comes with such a blue colour tag. The molecular structure of both these vital molecules is now known in detail, as is the way they move oxygen through the tissues, although this is not directly part of our story. But the Limulus narrative would not be complete without exploring the extraordinary coagulating properties of its blood a little further, because this affects the very survival of the species.

This discovery was made in 1956 by Fred Bang of the Marine Biological Laboratory at Woods Hole. He noticed how Limulus blood clotted dramatically when infected by a particular bacterium. Subsequent research showed that the crab’s blood had an extraordinary sensitivity to a vast range of micro-organisms that are found almost everywhere in nature – known as gram-negative bacteria. A few cubic centimetres of seawater may contain hundreds of thousands of these tiny organisms. Since some of these bacteria are also agents of disease in humans, this property was of immediate interest. It seems that a hypersensitivity to microbial enemies helps to protect the crabs in their natural habitat – as soon as the bacteria enter a wound their defences were up. Limulus has a very diffuse blood system compared with ours, with the interior of the animal bathed in blood, and lacking the defined circulation of veins, arteries, and capillaries we are used to in humans and other vertebrates. In horseshoe crabs the job of defending the works of the animal is given to one type of protective cell, the amoebocyte, which contains a mass of granules capable of promoting clotting. When a gram-negative bacterium is in its vicinity an amoebocyte will react by rupturing and then the granules are released. A clot follows and the infection is sealed off. Now we know why dented and holed crabs can totter on regardless. They have had hundreds of millions of years to come up with an effective response to some of their most dangerous and invisible enemies.

Twelve years later in 1968, Bang and his colleague Jack Levin had managed to prepare and extract the active principle ‘Limulus amoebocyte lysate’ (known as LAL) that clots human blood plasma when exposed to gram-negative bacteria. This is an extremely useful substance in medical diagnosis, readily used for the detection and measurement of the poisonous toxins (called endotoxins) belonging to the appropriate bacteria. Poisonous endotoxins are released into the host organism (you, me, or a horseshoe crab) when the bacterial cell wall ruptures. LAL is a highly sensitive chemical able to detect minute quantities of the offending substances. The LAL test is now widely applied, having been sold on to pharmaceutical companies for commercial manufacture. It has to be prepared close to the Limulus populations, but is then exported around the world. This means that there is a tremendous demand for the horseshoe crab’s blue blood. What had saved it from harm for millions of years now made it a desirable commodity.

The influence of this new industry has been the subject of some debate. Carl Shuster told me that recent estimates say that there may be as many as seventeen million adult crabs in the Bay. By 2003, some three million crabs were harvested for the pharmaceutical trade – an unsustainable quantity. The bird people were worried about the fate of the red knot and its fellow waders. Now the number has been reduced, and the technique for obtaining blood has been adapted so that it does not require the animals to be killed: they have become blood donors! Four companies have the right to what is called ‘bleed and release’, and this method has been applied to some 600,000 crabs per annum. Maybe it is possible to get the LAL and not threaten the horseshoe crabs, after all. Meanwhile, some ornithologists are sceptical, and feel that the companies, as beneficiaries of the local species, should put something back into safeguarding the regional marine habitat for the benefit of all parties, crabs included. Given the long time taken for the crabs to reach sexual maturity there is always a lag before the effects of a population slide can be observed when the summer high tide hits the beach. But there are certainly some very vigilant people on the case.

There was another threat to the welfare of the horseshoe crabs. They were harvested in great numbers to use as bait to catch the giant marine snail or whelk, known up and down the Atlantic coast as conch (Busycon). Big, pinkish conch shells are a familiar item in every seaside knick-knack shop; put to the ear they allow the listener to ‘hear the sea’. Roadside stalls sell the shells for a modest price in the small villages around Delaware Bay. The molluscs inside the shells have a loyal following among connoisseurs of seafood. The only time I tried them I found the flesh very tough. A crab split in two is a pungent treat for the big snails, however, and fishermen along the coast from Delaware to New Jersey are well aware of the power of this lure. The US Fisheries Commission limited the number of crabs to be used to 100,000 in New Jersey, and later introduced a moratorium, but some fishermen moved northwards to Massachusetts or even Maine (the northern limit of the crabs’ distribution) to continue their trade. A method of sustainable fishing must be worked out, and there are hopeful signs. On Delaware Bay, for example, the use of a different kind of trap employing mesh bags has cut bait use by 50 per cent. One cannot help feeling that the horseshoe crabs deserve to prosper unhindered. There may seem to be endless crowds of them jostling for a space on the sand, but the example of the American passenger pigeon comes to mind: countless millions of the birds were slaughtered in the nineteenth century until the species finally became officially extinct in 1914. How dreadful to contemplate the thought that an animal that has survived in readily recognisable form from the early days of the dinosaurs might become extinct in the cause of being a special garnish on a plate of fruits de mer. It is fortunate that its blue blood is so valuable.

Limulus polyphemus is not alone. There are three additional, Asian species of living horseshoe crabs, but none of them can be found in anything like the profusion of the North American form. Tachypleus tridentatus has been given protected status in Japan, where it lives on the Seto Inland Sea. Its shallow-water habitat there is under threat, and increasing industrialisation on the Seto Sea seems unlikely to spare it. Various attempts have been made to conserve the crab, but none of them has solved the problem of making this ancient animal at home in the modern world. Nonetheless, it has an important place in Japanese history, as its form is believed to have inspired classical samurai masks, and brave warriors were supposed to be reborn in the guise of horseshoe crabs. The contemporary artist Takeshi Yamada has made a modern interpretation of masks based upon the ancient icon. A related species, Tachypleus gigas, is fairly widespread in eastern Asia. I saw the fourth species, Carcinoscorpius rotundicaudatus, while I was on fieldwork in Thailand more than a decade ago. As its second, species name would indicate to those with a smattering of Latin, this particular type has a rounded tail spike compared with other horseshoe crabs. When I first set eyes upon Tachypleus, the poor crab was in one of those tanks in a restaurant where delicacies are displayed before consumption, along with several sad-eyed fishes and a dying lobster. I was puzzled by what there could possibly be to eat on the horseshoe crab, because I knew from dissecting its American relatives that they are not exactly meaty. If the species in the tank was indeed a relative of trilobites this might be my first and last chance to taste something resembling my own speciality. When the cooked item finally arrived I felt a twinge of conscience, for it transpired that large, yolky eggs hidden under the head-shield provided the delicacy, which was only available while the gravid females came inshore. I felt that I was consuming the next generation just to satisfy my curiosity. The eggs came served in a thin sauce on a mountain of noodles. They had a rather overwhelming rancid-fishy taste; I am not anxious to repeat the experience.

2. A growth series of juvenile Tachypleus tridentatus, the Asian species of horseshoe crab, collected from tide strand lines in Deep Bay estuary, Hong Kong. (The scale is a 15 cm/6 in ruler.)

As for the place of the horseshoe crab in the tree of evolution, the Latin name of the horseshoe crab I saw in southern Thailand might offer a clue. Carcinoscorpius means ‘crab scorpion’, and that is what these curious creatures are: superficially crab-like relatives of the scorpions. They are the most primitive living examples of a great group of arthropods that includes all living spiders and mites as well as scorpions, and several less familiar kinds of animals. This group is referred to as chelicerates, after those special appendages – chelicerae – located on the head-shield, which they all share in one form or another. All the relatives of the horseshoe crab I have mentioned live on land. But the cradle of life was the sea, and Limulus and its relatives take us back to the far, far distant days when the land surface was barren of larger organisms.

In the darkness along Delaware Bay the scratching percussion of the crabs provides an unmusical accompaniment on an imaginary journey backwards in time: to an era well before mammals and flowering plants; a time before the acme of giant reptiles, long before Tyrannosaurus; backwards again through an extinction event 250,000,000 years ago that wiped nine-tenths of life from the earth; and then back still further, before a time of lush coal forests to a stage in the earth’s history when the land was stark and life was cradled in the sea; a time when a myriad trilobites scuttled in the mud alongside the forebears of the horseshoe crabs. The trundling, heaving, inelegant not-so-crabs along Delaware Bay are messengers from deep geological time.

A palaeontologist would naturally want to track the history of the horseshoe crabs back into the distant past. A few years ago I visited the famous quarries in Bavaria, southern Germany, where the Solnhofen Limestone of Jurassic age, 150 million years old, had been excavated. Great opencast pits scour the gently rolling countryside revealing thin slabs of cream-coloured limestone, where each bed represents the former sediment surface. The limestone provides the perfect fine-grained stone for the manufacture of lithographic printing plates; this is still a popular medium with artists today, but had an even greater use in the past for graphic illustration. The Germans called this kind of rock ‘plattenkalk’, which is an appropriate name because if a fossil turns up it will be laid out on the surface of the slab like a fish on a very flat plate; and some of the fossils are, indeed, those of fishes. The most famous fossils from the Solnhofen Limestone are skeletons belonging to the early bird Archaeopteryx lithographica, complete with feathers, but they are very rare – only one turns up in an average decade. Some other fossils are quite abundant, like those of little sea lilies (Saccocoma). The Solnhofen Limestone is thought to have accumulated in a warm lagoon, or a series of lagoons, not far from a biologically diverse land habitat, but with periodic influxes of waters from the sea. From time to time the lagoon became salty enough from evaporation to poison living organisms, and its deeper parts were depleted in oxygen sufficiently to deter scavengers. The result is the outstanding preservation of delicate animals. When sticky mud was exposed, animals could get trapped upon it, such as delicate flying reptiles or dragonflies. Operating together, these special conditions preserved a huge cross section of Jurassic life. One of these animals is a horseshoe crab called Mesolimulus walchi. It really is remarkably similar to our living Limulus polyphemus. At first glance it looks as if it had just wandered in from Delaware. One has to look hard to notice that its marginal spines are longer than in our living blue-bloods, and there are a few other minor differences. Nobody could doubt that this species, too, trundled through the shallows, nor that it carried its eggs under its head-shield. To that showy new upstart – a feathered bird – it may already have seemed archaic.

Up to this point I have avoided describing the horseshoe crab as a ‘living fossil’. This is not only because I am chary about using a phrase that is a paradox and an oxymoron rolled into one, but also because it is a misleading description. Charles Darwin himself was cautious when he introduced the term in the phrase quoted at the start of this book. Despite what I have just said about Mesolimulus it is not exactly the same as Limulus. Consider everything we have learned about our living horseshoe crab. It is woven deeply into an ecology that is utterly different from that in the Jurassic. Millions of birds of many species depend on the horseshoes’ eggs every year, whereas its old relative was probably irrelevant to the life cycle of what is often called ‘the first bird’. Limulus has adapted to many changes of circumstances: new predators, new climates, and now humankind. It is a winner in the lottery of life, and not just because of its long family tree. ‘Living fossil’ seems to imply a negative judgement somehow, as if the poor old organism was just about tottering along on its last legs, having hardly changed in tune with a changing world, awaiting an inevitable end. A similar misplaced judgemental tone is often applied to dinosaurs. ‘We mustn’t be dinosaurs! We must change with the times!’ is a mantra of commerce. The dinosaurs were actually superbly efficient animals, and their extinction was most likely a combination of external factors (a drastic meteorite impact is favoured by many) that had nothing to do either with their virtues or lack of them. They were animals of the wrong size living in the wrong places at the wrong time. Bad luck! Meanwhile, the living fossils trundled on through the crisis because … well, we will come to that.

Modifications are happening at the genomic level all the time. There really is no such thing as ‘no change’; the very flexibility of the DNA molecule is what has kept natural selection on its toes for thousands of millions of years. Nor is change in DNA necessarily related directly to any change in the appearance of an animal. Many mutations accumulate in the large fraction of the genome that apparently does not do much work in the specification of proteins, or initiating developmental changes, or any of the other vital, active stuff. These mutations might well be irrelevant to the kind of changes in shape or colour that indicate the appearance of new species. A living fossil may indeed have accumulated many changes at the molecular level that have not even been expressed in its surface appearance, which is the phenotype that has to face the world. Fluctuations in gene frequency are the stuff of life, but they don’t map one-to-one on skeletons and limbs, which are the usual stuff of fossils. So a little caution in terminology is wise.

There is also a temptation to think of the living fossil as if it were a true, surviving ancestor. When the coelacanth fish was discovered it was presented in the popular press as ‘old fourlegs’ as if it were just about to march onto land on its stumpy fins as a thoroughgoing tetrapod. Not only does this scenario happen to be wrong, but the likelihood of any such ancestor surviving unchanged to the present day through many millions of years is also exceedingly remote. Time, chance, and competition will see to it that change is inevitable. What can be said without demur is that the ancient survivor and its other living – and more evolutionarily advanced – relatives will have shared a common ancestor, and that the features of the living fossil will be closer to those of that ancestor. The discovery of ancient fossils more or less similar to the survivor will date the appearance of the whole animal group to which they belong, and point up the changes that must have happened through geological time along the subsequent branches of the evolutionary tree. The survivors from the early days carry with them a package of information revealing primitive morphology, development, and biochemistry that can illuminate histories that would otherwise be hidden from us. Fossils never preserve blue blood. The ‘living fossils’ may not be the ancestor, but they are survivors carrying a precious legacy of information from distant days and vanished worlds.

Hence Limulus allows us to understand something about deep branches in evolution. It is far from unique. If every descendant species had simply replaced its predecessor, the history of life would be like one of those patients described by Oliver Sacks who live perpetually in the present day, constantly erasing the memories of yesterday. Fortunately, life is not like that. Deep history is all around us. In the life of the planet, the latest model does not always invalidate the tried-and-tested old creature. Groups of organisms that originated long, long ago, in very different worlds, have been able to evolve and adapt alongside their more recent cousins and second cousins. The story of life is almost as much about accommodation as it is about replacement. To look at a living horseshoe crab is to see a portrait of a distant ancestor repainted by time, but with many of its features still unchanged. This book reflects my interest in living survivors from the geological past and what they can tell us about the course of evolution. I have spent the last few years seeking out animals and plants that have helped to illuminate our understanding of the history of life. Wherever possible, I have visited these organisms in their natural habitats; none has proved less than fascinating. Observing how they survive today has allowed me a glimpse of their biology and provided clues about the reasons for their longevity. I have carefully selected the old timers I visited because I wished to understand their biology in depth; I have had passing encounters with several more. A few organisms proved too rare or inaccessible for me to discover personally – the coelacanth comes to mind – and then I have relied on the accounts of others. I shall relate many of these case histories to those of their fossil relatives, which is only to be expected of a palaeontologist. This will illuminate the vital fourth dimension – time. I soon discovered that there were too many potential candidates for inclusion, and I am obliged to mention some of them only briefly. I believe it is better to deal with a smaller number of organisms in detail than swish around vaguely with a broad brush. My specialist friends will probably complain that I have left out their particular favourite beast or weed, and my answer is that these survivors have lasted so long that they will almost certainly still be around for someone else to champion in the future.

Consider scorpions, for example. In some ways they are as impressive as horseshoe crabs as survivors. I have met them several times in my fossil collecting career, usually hiding beneath a log or a rock, for many of them stalk their prey at night and stay out of sight by day. In the Oman desert I once disturbed a huge, black knobbly scorpion that came running at me with its tail-sting erect, while I backed off in the opposite direction, gibbering foolishly. My Omani companions laughed and told me that its sting (in the tail of course) was relatively mild. A few hours later I lifted up a rock slab and nestled beneath it was a small yellowish scorpion with a flattened, side-wound sting. I was about to poke at it with my geological hammer when my companions tugged at my arm. ‘Don’t touch, it’s a real killer!’ The smaller, insignificant-looking creatures can often be the most deadly. The spike on Limulus’ tail and the sting on the scorpion’s are closely related structures, and indeed both animals belong to the same great arachnid group. The scorpion learned the trick of arching over the very end of its body to dart poison into enemy or prey. Encased within an external skeleton (cuticle) that prevents evaporation with outstanding efficiency, the scorpion has been able to live far away from water; some species specialise in surviving in the driest places on earth. Scorpions started out as sub-aqueous creatures like Limulus, and only later did they acquire the skill of living on land. Back in Devonian times, 400 million years ago, their relatives, the sea scorpions (eurypterids) were the largest invertebrate predators ever to have lived, some as long as a man. There are fossils from the Carboniferous age that really do look like living scorpions, at least to the non-specialist. They, too, have faced out extinction events that have blasted greater and more glamorous animals. The scorpion is built into mythology as a sign of the zodiac; it features on Roman mosaics, and in the Bible (1 Kings 12:11): ‘my father hath chastised you with whips, but I will chastise you with scorpions’. So one could argue that the scorpion’s connection to human culture is more pervasive than that of the horseshoe crab. My choice of organisms has been guided by the place they occupy in the tree of life, rather than by their innate charisma or significance in folklore and culture. The horseshoe crabs earn a special place in this natural history because their relatives root down to the beginning of the diversification of animals. The early Permian Palaeolimulus is clearly a horseshoe crab, for all that it predated the great extinction that put paid to most species at the end of the Palaeozoic Era, 250 million years ago. Limulitella is present in 242-million-year-old (Triassic) strata dating after the great trauma, evidence of their survival. Those distinctive tracks left by the tips of the legs, and the trail of the tail, that I observed in Delaware have been found as fossils even in the absence of the body itself. There were horseshoe crabs crawling among the coal swamps of the Carboniferous (Pennsylvanian), a little better segmented perhaps than the beasts on Delaware Bay, but carrying the distinct signature of their ancestry. In 2009 my Canadian colleague, David Rudkin, announced the discovery of the oldest typical horseshoe crab in rocks of Ordovician (approximately 450 million years) age, thus taking these simple arthropods back before all the major extinction events that have rocked the Phanerozoic biosphere. Whatever the magic ingredient for survival is, the horseshoes clearly have it in spades. ‘The meek shall inherit the earth’ may be an appropriate motto for their longevity.

3. Gustav Vigeland’s sculpture of a survivor, the scorpion, Frogner Park, Oslo, Norway.

Let me describe these early days in more detail. I have already remarked that the horseshoe crabs had set out upon their distinctive path before the first land plants had advanced upon harsh and barren shores; although recent discoveries suggest that a few simple plants may have already ventured onto mud flats. These had probably not yet been followed by insects or spiders. There were fishes already, of a primitive cast, but they had no ambitions then to invade the land, although a few species may have nudged into waters that were not fully marine. The seas abounded with trilobites, which occupied every ecological zone from shallow shores to ocean deeps. These prolific arthropods must have been many times more abundant than the early relatives of the horseshoe crabs. They evidently had an advantage at the time. This may have been the evolution of their robust dorsal ‘shell’ of calcite, which allowed them to develop spines, armour and a tough anchor for muscles, as well as an ability to roll up into tight, impregnable balls when threatened. Trilobites soon learned an array of different feeding habits; some were predators, some ate soft mud, others swam in the open seas. They died out some 255 million years ago. By contrast, the relatives of Limulus may have stayed conservatively on the sea floor as scavengers and predators. The horseshoe crabs on Delaware Bay have an exoskeleton of chitin, which is a natural polymer that is quite tough and flexible, although no substitute for stony calcite. But the horseshoe crab has turned what might have been a weakness to advantage by developing an exceptional immune system. Survival in the long term may depend on more subtle features than armour alone.