Coils of time – evolution of the chambered nautilus
Peter D. Ward
It’s not easy studying the nautilus, a creature that lurks in the depths of the ocean and emerges only at night to prowl the coral reefs. But the rewards are great: discovering just how old living fossil can be.
Every evening across the immense expanse of the tropical western Pacific, millions of white-shelled, dinner-plate-size mollusks begin an epic voyage. They rise from their day-time resting place–the dark, muddy ocean bottom a thousand feet or more deep–and slowly swim upward to shallow coral reefs where they feed for the night. These animals, the chambered nautiluses, look like snails with tentacles–their closest living relatives are the octopus and squid. More than that, though, they bear the look of a creature from a bygone era, over the past 500 millions years–before, during, and after the age of dinosaurs–more than 10,000 related species have roamed the seas. But in the 65 million years since the dinosaurs died out, the family to which the chambered nautilus belongs has gradually diminished. Today only a few species still exist, and they remain poorly known. Only recently have we learned some of the key facts about nautiluses, facts as simple as their extraordinary nocturnal voyages.
This new knowledge has caused paleontologists such as myself to ask new questions: Is this nightly behavior a holdover from the age of dinosaurs, when great scaly marine lizards preyed on the shelled denizens of ancient seas? And, more important, is the chambered nautilus itself only a recent descendant of an ancient lineage–or a true living fossil that actually lived during the age of dinosaurs and endured through the long roll of time unchanged?
The nautilus has commanded scientific attention at least since the time of the ancient Greeks, who were intrigued by the unique, beautiful partitions of its shell. When cut in half, the nautilus shell describes an unwinding spiral intersected with beguiling regularity by pearly chambers. In the 1600s the great English natural historian Robert Hooke received a chambered nautilus shell (even then a great rarity) and wrote the first learned treatise about its shape. Without ever seeing a live specimen, he correctly deduced that the chambers of the shell held gas rather than animal flesh and thus gave the creature buoyancy.
Centuries would pass before other scientists could observe the chambered nautilus alive in the wild and begin to learn its secrets. The nautilus looks like nothing familiar: it swims just above the seafloor, moving like a peculiar fish; it has tentacles and a jet propulsion system somewhat like those of other cephalopods–the class of mollusks that also includes octopuses, cuttlefish, and squid. But there are great differences. The chambered nautilus has about 90 tentacles rather than the Y or 10 of other cephalopods, and its peculiarly primitive eye lacks a true lens. And no other cephalopod has anything remotely resembling the nautilus shell. The animal itself is housed in the last chamber in the shell’s spiral, with only its head and many tentacles visible. As the nautilus grows, it adds a new chamber and moves into it, leaving the old one behind.
In 1975, I made the first of what turned out to be annual trips to the Western Pacific to study these odd creatures. My first goal was to observe the chambered nautilus in its natural environment along the seaward edges of coral reefs. In New Caledonia, where my studies began, local fishermen reported seeing them in the shallowest reef regions at night, but never during the day. Like the neighboring Great Barrier Reef of Australia, the immense New Caledonian reefs form great vertical walls extending down many hundreds of feet from the warm surface of the coral sea to the cold, dark, muddy bottom. During daylight hours, the nautiluses lurk in these depths, out of sight and danger from shell-breaking predators such as turtles and triggerfish. But with the coming of night, as the faint daytime light reaching the thousand-foot depths fades to complete blackness, they begin to stir. They follow the slope of the bottom toward shallower water, swimming just above the mud, sand, and coral rubble, eventually reaching the sheer vertical rocky walls that mark the base of the reef itself. Undeterred by these great coral skyscrapers, whose tops may still be 500 feet overhead, the nautiluses continue their voyage, ascending like silent hot-air balloons. Finally, after journeying for several hours, they arrive in the shallows and spend the night roving the reef, using their sense of smell to find carrion, lobster molts, and hermit crabs. With the first approach of daylight, they swim back over the reef walls and fall down into darkness.
Starting from Noumea, the capital city of New Caledonia, my colleagues and I would travel the 12 miles across the wide lagoon, through the heavy afternoon waves whipped up by the trade winds. We would arrive at the seaward side of the barrier reef just before sunset, about the time the nautiluses, far beneath us, would begin their journeys. Anchoring our boat, we shone large underwater lights to mark its position; in the utter blackness of the tropical night, we’d need to find it after our long, deep dives. Finally, at about 8 P.M., we would splash into the darkness and follow our beams of light down into the sea.
Then we waited, a small band of humans hanging weightless at hundred-foot depths, sweeping the black water with dive lights to catch a glimpse of an ascending nautilus. Sooner or later a solitary animal would rise into view, to be caught in the beam of our dive lights (unlike us human divers, the nautiluses always traveled alone). As we followed them up the reef walls and into shallower water, they seemed oblivious to our presence and our lights, changing neither direction nor speed. I would later discover that they make these enormous vertical migrations by water-jet-powered swimming alone, without any help from an altered buoyancy of their chambers. Like squid, they draw water into a body cavity, passing it over the gills that extract oxygen, and then push it out through a funnel beneath their mouth to jet forward.
By the mid-1980s many different research projects had amassed a lot of new information about this enigmatic animal, but had not led to any definitive answer about its age. The genus Nautilus belongs to a family of similar creatures, which collectively are called nautiloids. Most textbooks of the time suggested that while the chambered nautilus shell resembled the shells of extinct nautiloids from the age of dinosaurs, the living species were very recently evolved, a few children orphaned from an ancient family. According to this view, the chambered nautilus might date back only a million years or less. If this were the case, the natural history we were observing might be recently evolved as well, and thus of little use in interpreting the lives of the ancient nautiloids. To learn more about the true ancestry of the nautilus, we needed to study its genes and fossils.
Around this time, the technology of DNA sequencing was beginning to offer new ways to discern the shape of an organism’s family tree; it was even allowing researchers to estimate by using a “molecular clock” when different lineages branched apart. But DNA sequencing was not easy for the chambered nautilus. To conduct such studies, you need tissue taken from living animals, not shells. Thus each living species of nautilus had to be sampled. But researchers had been debating for decades just how many species there actually are. They tried to identify living nautilus species in the same way that paleontologists identify fossil species, by finding distinguishing structures on their anatomy. And they tended to seize on the slightest difference–a minor variation in the sutures of the shell, for example–to declare a new species. The result was that by the late 1980s the genus Nautilus contained 11 more or less agreed-upon living species.
To see how these so-called species were all related, it was necessary to get their tissue and read their genes, but who knew how many different chambered nautiluses we had to find to see their full diversity? To track down the many possible species was a huge undertaking–far more work than could be accomplished by any individual. So in 1983 I joined forces with paleontologist Bruce Saunders of Bryn Mawr College to discover just how many species did exist, and to discover their age as well.
As we began to study more and more specimens, it became clear to us that far too many of the alleged species were much too narrowly defined. But there were a few of them that–from their shells at least–did seem as if they might be truly distinct. One of these standouts, Nautilus scrobiculatus, commonly known as the king nautilus, had never been caught alive and had been known only from its shell. The shell of the king is not as tightly coiled as the shells of other living nautiluses; it has a very large and open central depression. In cross section, the king nautilus shell is square rather than rounded, and it is ornamented on the outside by unique cross hatchings. Of its soft anatomy, however, only a single tantalizing clue existed: at the turn of the century an English zoologist named Arthur Willey found a rotting carcass within the shell of a stranded king nautilus on a New Guinea beach. The surface of the animal’s soft parts seemed to be covered with bizarre large, warty protuberances, but the body was too rotten for Willey’s observations to be trusted. He continued to search for the elusive king nautilus but never succeeded in catching one alive.
Its eventual captor turned out to be Bruce Saunders in May 1984. And when he sent me a telegram announcing his success, I immediately booked a flight for New Guinea.
Nautilus fishing is an agonizing endeavor. The stakes are so high, and the vagaries of boats, weather, and the oceans so stressful, that trapping these creatures is never relaxing. I was particularly anxious as Saunders and I peered over the side of our boat, watching the faint outline of a nautilus trap appear as it was winched up from its thousand-foot resting place. Our traps were made of steel rods covered with chicken wire and baited with fish. About three feet on a side, they had a small opening that allowed a nautilus to enter, much as a crab trap does. We had baited this trap the night before and then lowered it with a long line. Now, as we raised it to the surface, I was agitated and impatient. Soon the trap was only 20 feet below us, and I could see that it held nautiluses. But what kind? Finally it broke the water, and as we muscled it over the side of the boat, I saw the familiar brown-and-white striped shells of large Nautilus pompilius, a common species. But tucked into a corner were two creatures alien to me and nearly everyone else, which at that time had been seen alive by only two humans, Bruce Saunders and his field assistant. They were king nautiluses.
I was taken completely by surprise. The upper part of the body was covered with thick, fleshy tubercles that you never see on a chambered nautilus, and its color and pattern were quite different as well. But the most striking feature was the shell: it was covered with a thick, shaggy orange fur. No one had ever seen this fur coat on the shells of king nautiluses that had washed ashore. All the animals Saunders and I had caught up to this point–from Fiji in the east to Palau in the west–were minor variations on a chambered nautilus theme, differing only in their size and shell. But the king nautilus was altogether different.
Over time, as we trapped more king nautiluses, studied them, and released them back into the sea, we learned that their behavior was just as unusual as their appearance. While a typical chambered nautilus can survive out of water for an hour, for example, the king nautilus can’t be exposed to the air for even a few minutes. Later, back in the United States, Saunders and I performed a series of dissections on the soft parts of the king and chambered nautiluses. We discovered that the gills of a king are about half the size of those of a chambered nautilus and thus can’t build up large oxygen reserves, which explains their sensitivity to being out of water. These and other distinctive features of their soft anatomy confirmed that they were a special animal, but we were still left wondering how special they were. Did they still belong in the chambered nautilus genus? How had they branched away from the other living nautiluses? To answer these questions, we needed the genes of these creatures.
By 1986, Saunders and I had collected tissue from nautiluses around Fiji, Samoa, Australia, New Guinea, the Philippines, Palau, and New Caledonia. Two independent sequencings of their genes yielded the same remarkable result there are only two distinct groups of nautiluses. One is composed of the king nautilus, which appears to have descended from the chambered nautilus about 15 million years ago; the other group is composed of all the other so-called nautilus species.
If the genetic evidence is accepted, it means that the long-agreed classification of the living species has crumbled. The king nautilus represents a different genus altogether, while the differences in shell morphology of the other “species” seem to be useless in telling them apart. We had gone from 11 living species belonging to one genus–Nautilus–to two genera, Nautilus and our newly recognized genus, with only two or three species between them. We gave the king nautilus a new scientific name, Allonautilus, which is Latin for “other nautilus.”
These surprising results made us wonder if nautiloid fossils might also have some secrets to reveal. Unable to study extinct nautiloid DNA, we had to figure out a new way to classify these animals based on their shells alone. Previous studies of nautiloids had classified them on relatively few features, and we hoped that we might find more to examine. Luckily for us, just such a trove of distinctive new characters had been uncovered by Neil Landman of the American Museum of Natural History in New York.
The chambered nautilus hatches at a very large size: it emerges from its egg with seven fully formed chambers and a shell diameter of more than an inch, making it the largest invertebrate at hatching in the world. (Indeed, it may have been this trait that allowed it to survive the great Cretaceous mass extinction, for the nautilus appears to lay its eggs in very deep water, where they take a year to hatch. Juveniles or unhatched eggs might have survived in a deep refuge when the great comet ending the age of dinosaurs, 65 million years ago, turned all the shallower oceanic regions into a toxic, heated cauldron of extinction.) When a living chambered nautilus emerges from its egg, it stops growing temporarily, and this pause leaves a distinct groove in its shell. Since the shell wraps around itself as it grows, these earliest stages are always preserved in the middle. Landman began dissecting fossils to see if similar marks were found in extinct species as well. He discovered that not only did these marks occur, but many other features also.
Saunders and I combined Landman’s new characters with the classical ones and then began to study their occurrence in living and extinct nautiloids. Both of us had been taught that present-day nautiluses are the most recently evolved of the 10,000 nautiloids that have swum through the oceans over the past 500 million years. Thus we expected them to have a lot of features that had evolved relatively recently. To our surprise, we found that today’s chambered nautilus appears to be extremely primitive–rather than being a descendant of some fairly recently evolved nautiloid, the chambered nautilus evolved much earlier. It may even be the ancestor of most of the nautiloids present on our planet for the last 75 to 100 million years.
Our fossil analysis also supported the DNA results: the king nautilus is much more recently evolved than the chambered nautilus and is probably an evolutionary offshoot. There was only one problem: If the chambered nautilus is so old, why are there no fossils of it? Until the late 1980s, only a single chambered nautilus fossil had ever been found, in Russia. It had been claimed to be 40 million years old, but because no one had found another specimen from the same fossil beds, many paleontologists began to wonder if it had been mislabeled.
It turns out that there are many fossils we can now confidently place in the chambered nautilus genus. The first to point this out was Richard Squires of California State University at Northridge. Squires had a collection of nautiloid fossils from 50-million-year-old rocks in Washington State, and in 1988 he published a paper describing these as the oldest chambered nautilus fossils ever found. Their shell shape certainly looked like that of the chambered nautilus, but Saunders and I now knew that shell shape alone is not enough. What about their other characteristics?
We soon found out. Cutting open the fossils, Saunders and I discovered that not only Squire’s 50-million-year-old specimens but also some of my own 100-million-year-old ones from California do in fact have hatching stages identical to those of the familiar chambered nautilus of today. They too hatched at more than one inch in diameter, with seven fully formed chambers. In every way they are virtually identical to the living chambered nautilus. The creature that swims in our oceans today is the same one that was swimming around 100 million years ago. It takes its rightful place with the coelacanth, the horseshoe crab, and a few other species as the Methuselahs of our planet.
Tonight, as the sun falls once again into the warm tropical sea, members of the ancient nautilus lineage will again begin their long climb into the shallows. With the knowledge we have now gained, we can’t help wondering if we are seeing the ancient influences of extinct predators such as giant marine lizards, other cephalopods called ammonites, and still other strange, vanished creatures. Did the nautilus originally take up its hiding place in the dark to escape inhabitants of the shallow oceanic regions of sun and light? Is that the price it paid for its near-immortally? And does the descendent of the chambered nautilus, our newly defined genus Allonautilus, make these same nightly migrations? There are still mysteries left to probe.
The living fossil we call nautilus survived the great cosmic collision that killed the dinosaurs, and many other changes in the 65 million years since that catastrophe. Its presence on Earth through all this time gives us one more small peek into the workings of evolution, which we are learning can tick slowly as well as at more staccato rates. The long voyages of the nautilus, from the depths into the shallows each night, are thus a perfect metaphor for its evolutionary history, which comes up to our world unchanged from the great depths of time.
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