Survivors: The Animals and Plants that Time has Left Behind. Richard Fortey

unaware of the gift they are providing to an animal many millions of years their evolutionary junior.

      Birds always attract devotees, and naturalists’ concerns for the welfare of the red knot probably accounts in turn for their anxiety about the state of the horseshoe crab population. If that were to fail, then so would the long migration of the attractive waders. A recent census estimated that there could be as many as seventeen million horseshoe crabs in the Delaware Bay area, and that concerns about their decline may have been exaggerated. Since the horseshoe crab has a range that extends along the shore north to Maine and south to the Yucatán Peninsula in Mexico the population is assuredly larger still, although the densest concentration of individuals is probably where I saw the heaving multitudes at Kitt’s Hummock and Pickering Beach. Delaware Bay is also where the mature crabs grow largest at maturity. Since there has, indeed, been a decline in red knot numbers the cause must lie somewhere else in its complex migration story. The weakest link in an ecological chain is always the critical one.

      It had always been a dream of mine to see throngs of jostling horseshoe crabs reach the climax of their life cycle. For more than three decades at the Natural History Museum in London I studied fossils of trilobites. This once important group of sea animals went extinct something like 260 million years ago, when the world was a very different place. Trilobites had once swarmed in all the ancient oceans, but now their remains have to be patiently collected by splitting open the rocks that have entombed their shelly remains. Like horseshoe crabs, trilobites are arthropods: animals with jointed legs and all the muscles and tendons tucked inside an exoskeleton. However, unlike horseshoe crabs, trilobites did not survive the mass extinctions that redesigned the biological face of our planet. It is astonishing to learn from unchallenged fossil evidence that relatives of Limulus were contemporaries of trilobites. That nocturnal scrimmage on the beach in Delaware might have happened many millions of years before; I might even have been listening to sounds that had been rehearsed in Palaeozoic times. There were relatives of the horseshoe crabs in the sea long before other arthropods, such as insects and spiders, had ventured onto land, or before crustaceans – shrimps, crabs and lobsters – had taken up the central roles in the ecology of the ocean they enjoy today. So it would not be incorrect to describe the animals thronging along Delaware Bay as primeval. Indeed, many scientists believe that Limulus is the closest living relative of trilobites themselves. Would the head-shield of the giant Cambrian trilobite Paradoxides, a fossil 510 million years old, have felt the same to the touch as the beached Limulus I restored to the sea in eastern America early in the twenty-first century? Like a horseshoe crab, a trilobite would surely have contemplated me through compound eyes set within its head-shield; its eyes are preserved in detail as fossils. Trilobite legs would have scraped against my mammal flesh with just the same spikiness as Limulus. It would have crept and it would have crawled, brother under its external skin to the hordes on Delaware Bay.

      1. A Silurian trilobite, Calymene blumenbachii, from the limestone quarries of Dudley, West Midlands, England.

      So a visit to Delaware is to me rather like a visit to the holy city of Rome to a Catholic. Naturally, I had to meet the Pope. The Pontiff of horseshoe crabs is Carl Shuster, in his tenth decade still a giant of a man: craggy faced, walking without the aid of a stick, with lively eyes beneath towering eyebrows, memory and curiosity undimmed, and only betraying his years by his deafness. Like all field biologists he wears a coarsely chequered thick shirt and blue jeans, hitched up with a stout belt. He was brought up during the Great Depression, when he had to run a farm, so he is himself a survivor, like the horseshoe crabs to which he has devoted his life. His father was a mathematician who gave succour to penniless intellectuals during the tough years, while young Carl raised asparagus, chickens and strawberries. He brought together all the current knowledge about his favourite animal in his book The American Horseshoe Crab. He is accompanied in Delaware by his former student, a man of boundless enthusiasm, Glenn Gauvry (himself an implausibly youthful sixty years), who coordinates much of the research on Limulus around the Bay area. Those volunteers who help with counting the crabs during their nocturnal orgies receive a handsome little pewter lapel pin as a record of their collaboration in the conservation project. Naturally, the pin features a horseshoe crab. It signifies membership of one of the more exclusive clubs in the world.

      Carl Shuster and his colleagues established the biological facts about horseshoe crabs that allow naturalists to understand how they fit into the ecology of the Atlantic coast. Limulus is a typical arthropod in that it must moult in order to grow, shedding its old coat as a kind of pale ghost of its former self, and growing a new and larger external covering. With a little vigilance it is possible to find one of the cast ‘shells’ on the beach: they are almost as light as tissue paper, for the animal recycles what it can. The newly emerged horseshoe crab is capable of moving immediately. In this it has the advantage over other marine arthropods in the area; freshly moulted blue crabs, for example, are virtually motionless until their new ‘shell’ hardens. They often hide away. Younger horseshoes resemble the older ones apart from being a little spinier. The surprisingly tough, but flexible exoskeleton is made of a chitinous material, similar to that forming the wings of beetles. A typical horseshoe crab takes ten years to reach sexual maturity, after which it does not moult again, but heads to the beach for reproduction. Only these mature animals partake in the littoral orgy, which is why one does not see any little limulids scuttling between the adults. They are not demanding animals: a fully mature animal might go for months without eating. When her time comes, a female may well lay 80–100,000 eggs, and enough of these survive the depredations of wading birds to secure the future generations. The greenish eggs are laid in the sand in batches of four to six thousand in spheres about the size of a golf ball; the female makes repeated visits on successive tides to complete her duties. Females can be recognised by the scars left behind by the mounting males: up to fifteen males may have a chance of fertilising the eggs of any female. Nonetheless, only about thirty-three eggs out of a million survive to adulthood. This means that at various stages of its life Limulus provides a lot of food for other animals. Loggerhead turtles are an important predator on the crabs, even when they are adults. Turtles, too, are animals with a long geological history, so they may have had eons to make something nutritious out of horseshoe crabs, which now seem all sinew and horn and little enough meat. The crabs themselves can survive on molluscs and carrion and almost any kind of scraps, but the strong inner parts of the legs can also crush thick shells if needs be. Although they seem to lurch in an ungainly way on land, under water the crabs are more streamlined and can move quite fast, even sculling on their backs. They can easily right themselves if they need to. In short, they are tough, jack-of-all-trades kind of creatures, built to last. They remind me in a way of a Volkswagen Beetle that I once owned (a beetle being another arthropod, of course) that carried on carrying on even though its coach-work was full of rusty holes, its suspension was down almost to the tarmac, and it often fired on only three cylinders.

      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.