Spirals in Time: The Secret Life and Curious Afterlife of Seashells Page 19
As for the nautilid diet, their living descendants provide clues as to what they ate. During the day, chambered nautiluses stay hundreds of metres beneath the waves, then rise up at night into shallow coral reefs where they scavenge for the dead. And being seriously short-sighted, chambered nautiluses sniff rather than see their food. They have a pair of sensitive pits, called rhinophores, that help them pick up the whiff of a decomposing body from at least 10 metres (33 feet) away and track the odour plume in three dimensions through the water; scale that ability up to a human standing at the starting blocks of a 100-metre running track, and they could sniff a ripe Brie sandwich being eaten at the finish line. Ancient nautilids may have had a similar habit of smelling and groping their way towards dead food scraps, and it could have made them more resilient to changes in the water around them. Down in the deep, there would have still been plenty to nourish animals that weren’t too fussy about their food.
Recently, a new piece was added to the ammonite puzzle when Neil Landman, from the American Museum of Natural History in New York, pondered the importance of geography. He mapped out the global distribution of ammonites that lived towards the end of the Cretaceous, including a handful that survived the mass extinction – for a while, at least. The species that were swiftly snuffed out were ones that had relatively small ranges. By the same token, ammonites that hung on for a while longer generally occupied a wider sweep of the planet. It makes sense that species with smaller ranges are often more vulnerable to extinction. They have all their eggs in one basket, geographically speaking, and are more likely to get wiped out in one go, perhaps by a random event. Imagine a species of dung-eating insect living only in a single cowpat, and what happens if a cow happens to tread on that very turd.
Landman and his colleagues put their findings forward as good evidence that ammonites with a wider range were initially protected, although in the long run it was no guarantee of survival. Ultimately, all the ammonites went extinct (and no palaeontologist truly believes that ammonites could still be out there, somewhere, hiding in the deep). The dying ammonites left the nautilids alone to continue the ancestral line of shelled cephalopods, after almost 400 million years in the sea.
Having followed the rise and fall of the ammonites, let’s return to the question of ammonites and argonauts. Could argonauts have learned their shell-making skills from these long-lost ancestors? Nice idea, but there is a fundamental flaw – ammonites and argonauts probably didn’t exist at the same time.
We know of 10 extinct argonaut species from the fossilised remains of their delicate shells. The oldest is Obinautilus from the Oligocene around 29 million years ago, although some palaeontologists consider this to be a nautilid, which leaves the oldest fossil argonaut at a youthful 12 million years old. Meanwhile, the last known ammonites, as we’ve just seen, went extinct shortly after the mass extinction at the end of the Cretaceous, around 65 million years ago. More fossils could still be found to fill this gap but, as things stand, it looks highly likely that argonauts never actually encountered any living ammonites, let alone began copying their shells, and it’s now widely agreed that this almost certainly didn’t happen.
There really is only one plausible explanation for why argonauts have shells that resemble extinct ammonites: it is simply a striking case of convergent evolution. They look so alike because each evolved under the same selective pressure – to be streamlined underwater. Studies have shown that the ridges and ribs on argonaut shells reduce drag while they swim through water, stabilising them and limiting the amount they rock from side to side while they propel themselves along. The same thing would have applied to ammonites too, all those millions of years ago.
Even if argonauts didn’t model their shells on ammonites, the question of whether they parasitise some other creatures or make their own shells still needed to be answered. Back in the nineteenth century, argonauts commanded a huge amount of attention and discussion. Scores of scientific papers were written. A few rare preserved specimens of shells with their baffling occupants were passed around. Lord Byron even wrote about them in his poem The Island. A host of illustrious scientists held strong views on the argonaut debate, with Richard Owen, Jean-Baptiste Lamarck, Joseph Banks and Georges Cuvier among them. Not everybody was taken in by the stories of octopuses sailing around in stolen boats, and many argued that Argonauta and Ocythoe should be united as a single species, shell and shell-maker in one. Italian naturalist Giuseppe Saverio Poli examined young octopuses under a microscope and saw they were encased in a little shell, thus proving – he was convinced – that they were not parasites.
In the end, the issue was resolved by a now largely forgotten pioneer of marine biology, who devoted herself through the 1830s to uncovering the truth about these strange animals. Her story of the argonauts follows a meandering journey, taking in a princess’s wedding dress and a groundbreaking piece of technology along the way, and ending in the solution to the contentious puzzle of how the argonaut got its shell.
The lady and the argonauts
Jeanne Villepreux was born in 1794, a long way from the sea. She grew up in Juillac, a village in rural south-west France, the eldest child of Jeanne and Pierre. Not a lot is known about her upbringing, but her family seemed to be reasonably well off. Jeanne’s father was noted in local records as a shoemaker, shopkeeper, landlord and Juillac’s first policeman. When Jeanne was eleven her mother died, and her father remarried. It’s not known how well Jeanne got on with her step-mother, who was half her father’s age, but she stayed at home until she was 17 before setting off for a new life in Paris.
Chaperoned by her cousin and a herd of cows, Jeanne walked almost 300 miles to the capital. It should have taken around two weeks but the journey was interrupted, so it seems, when her cousin assaulted her and Jeanne sought refuge in a convent in Orléans. She finally made it to Paris and began a job as a seamstress, something she was clearly very good at because it wasn’t long before she took part in a royal wedding.
Jeanne was entrusted with embroidering the wedding dress of an Italian princess, Marie-Caroline, the Duchess of Berry, for her marriage to Charles Ferdinand D’Artois, a nephew of King Louis XVIII.
Among the congregation of French and Italian dignitaries was James Power, originally from the British Caribbean colony of Dominica, who had become a wealthy merchant based in Sicily. In 1818, two years after meeting at the royal wedding, Jeanne and James were married in Sicily. The couple settled in Messina, a port on the east coast, where Jeanne became a lady of leisure. She no longer sewed or embroidered dresses for a living, and she didn’t continue with such genteel pursuits to keep herself busy, as most other aristocratic ladies were expected to do. Instead she rolled up her sleeves and became a scientist.
On Jeanne’s doorstep was the Strait of Messina, the narrow gap between Sicily and the Italian mainland that connects the Ionian and Tyrrhenian seas. For mariners this is a dangerous place where ferocious currents race north and south, switching direction every six hours and sucking tides swiftly up and down. Much feared since classical times, the strait’s raging whirlpools and rocky reefs were personified as two sea monsters in Greek mythology, Scylla and Charybdis.
The six-headed shark-toothed beast, Scylla, guards one side of the strait. In Homer’s epic poem The Odyssey, the hero Odysseus narrowly escapes being devoured by Scylla, although several of his companions aren’t so lucky. In a later encounter, Odysseus drifts back through the strait on a raft and this time gets a bit too close to Charybdis, the whirlpool, who sucks up masses of water into her enormous mouth along with the unfortunate Odysseus; he clings on to his raft and waits until Charybdis belches him back out, spinning whirlpools across the sea. In another ancient story, Jason and his crew of Argonauts sail through the perilous waters between Scylla and Charybdis on their way back from stealing the Golden Fleece. They only survive their encounter because Jason convinces Thetis, a sea nymph, to guide the way.
When Jeanne arrived in Messina and b
egan pondering the legendary strait, she didn’t go hunting for menacing beasts such as Scylla or Charybdis. Instead she became entranced by some of the real creatures that inhabit these turbulent waters.
Jeanne’s interests in the natural world had already begun to lead her around the island, which she would explore for the next 20 years. She wrote a guidebook to the island’s wildlife, and studied caterpillars and butterflies, starfish, crabs and even Noble Pen Shells; she described watching an octopus wedging a stone between the pinna’s twinned shells before devouring the mollusc inside. Way ahead of her time, she came up with the idea of restocking overfished rivers with fish and crayfish. She also tamed a pair of pine martens that lived in her house while she observed their behaviour; she brought a tree inside for them to climb in, and live birds and squirrels for them to hunt. And it was a curious marine species that tempted her to embark on a revolutionary study. Jeanne realised that she was in the perfect place to answer a time-worn question: do argonauts borrow, steal or make their shells? She knew that to find an answer she had to do something no one else was doing. She would spend a lot of time with living argonauts.
Jeanne had a ready supply of these animals from the seas around Sicily; fishermen sometimes snagged argonauts in their nets and gave them to her, and she also ventured out and caught them herself. All she needed was a way of keeping them alive while she observed and experimented with them, so she devised a series of brand new observation platforms.
One was a simple box, later dubbed the ‘power cage’: four metres (thirteen feet) wide, two metres (six feet) tall and a metre (three feet) deep, with a door that flipped open on top and two glass observation windows so she could peer in. At each corner was a small anchor that fixed the contraption to the seabed at the shoreline. The cage walls were made of narrowly spaced bars that kept a fresh supply of seawater flowing through, but held the argonauts and their shells captive inside. Jeanne also built a glass tank in her house. It was the world’s first aquarium. Her inventions let Jeanne observe the marine world in a way that no one had ever done before, and she settled in for months and years of patient observation and learning.
Watching adult argonauts swim around her aquariums, she saw how easily they climbed all the way out of their shells, and how they aren’t permanently fixed inside like all the other molluscs with shells, including the chambered nautilus. She saw how the argonauts held on to the shells with their suckered arms, and noted that they never abandoned their shells altogether.
This was one of the pieces of accurate biology that Jules Verne included in Twenty Thousand Leagues Under the Sea. Professor Aronnax tells his servant, Conseil, about argonauts never choosing to leave their shells, even though they’re free to go at any time. In reply, Conseil remarks that Captain Nemo should have called his ship not the Nautilus but the Argonaut, because he too could leave, but chooses to stay confined inside.
Argonauts were clearly different from the other cephalopods Jeanne studied. Placing common octopuses inside her aquarium, they swiftly munched any food on offer before slipping their soft, unshelled bodies through the bars and slinking off into the open sea. This was something the argonauts never did, choosing to hang on to their shells and remain stuck behind bars. And when Jeanne took away their papery spirals, the argonauts died. She concluded that if they were borrowing shells from other animals, then surely they would have wandered outside the cage to try to find another one.
Fracturing their shells and leaving them in place, Jeanne saw that even though argonauts can’t make new shells, they do know how to a mend a broken one. Her injured argonauts rubbed the surface of their shells with silvery, web-like membranes at the end of two of their arms, which exuded a sticky substance that sealed up the cracks. Analysing the glue’s chemical make-up, Jeanne matched it to the calcium carbonate of the original shell.
Next, she tried breaking off small chunks of shell. After being inflicted with this new level of damage, an argonaut would spend hours sorting through bits and pieces on the aquarium floor, testing out shell fragments to find ones that were a perfect fit for the gaps; it would then glue the chosen pieces in place on its shell to complete the broken jigsaw puzzle.
The discovery that argonauts are equipped with these expert shell-fixing skills lent more support to Jeanne’s argument that they do indeed make their own shells and don’t simply steal them from other animals. But there was still one final part of the picture left to find: Jeanne needed to catch an argonaut in the act of actually making a shell.
Contrary to the reports made by Giuseppe Saverio Poli, Jeanne saw no sign of a shell when she examined unhatched argonaut eggs. However, she carefully watched as they hatched and grew up, and saw that when the young animals reached the size of a little fingernail, around nine millimetres (a third of an inch) across, they began to build their hard outer covering. As the argonauts got bigger, so did their shells.
Thanks to Jeanne’s extensive research, there was no longer any doubt that argonauts do indeed make their own shells, and that they do it in a completely different way from all the other molluscs. Instead of secreting a shell with their mantle, argonauts have shell-making glands at the end of those two arms that she observed repairing breakages; these are spread out into broad membranes (the very same ‘sails’ that Aristotle imagined argonauts unfurled to propel themselves over the seas).
All of these discoveries could have been lost and forgotten had Jeanne not kept up with her correspondence, and been good at publishing her findings. When Jeanne and James Power decided to leave Sicily and live in London, then Paris, they travelled overland. Meanwhile Jeanne arranged for the bulk of her papers and research equipment to be sent on afterwards by sea; everything was packed up and loaded onto a sailing ship bound for London. A short way into the voyage, off the French coast, disaster struck. The ship sailed into a storm and sank, sending Jeanne’s treasured collections into the ocean depths (the kind of romanticised disaster that rarely strikes scientists today, but perhaps a reminder to do regular data backups – the modern equivalent of avoiding a shipwreck).
Jeanne’s findings live on in pages of letters she wrote to Richard Owen at London’s Natural History Museum, and in the various papers and studies presented in scientific journals. However, her legacy as an early female scientist has faded and she is little remembered for her achievements. Her dedicated research saw her elected as a rare female member of many scientific institutions in Italy, France, Belgium and England, including a corresponding member of the Zoological Society of London. Few people nowadays have heard of the lady who spent years watching, probing and asking questions about this obscure but captivating group of animals.
In the years since Jeanne Power conducted her studies in Sicily, knowledge of argonauts and their way of life has continued to grow. We know they feed on various other animals that live up in the water column, including fish, jellyfish and sea butterflies; we know the females make their shells by laying down material on the inside and outside; we know that no two argonaut shells are exactly the same because of the way they patch them up (this makes it extremely difficult to identify species based on their shells alone); we know that when argonauts meet they sometimes cling to each other and form rafts. No one really knows why they do this, but it could explain why hundreds of them sometimes strand together on beaches.
We also know a lot more about the argonauts’ strange sex lives. Throughout her studies, Jeanne noted that she only ever found egg-producing female argonauts. Where were the sperm-making males? She was the first scientist to suggest that the worm-like objects found stuck to female argonauts could be something to do with the males. When Georges Cuvier originally spotted this peculiar appendage he identified it as a parasitic worm, and in 1829 named it Hectocotylus. Much later, following Jeanne’s suspicions, it transpired that these were not in fact worms at all but important mementos left behind by inconspicuous males.
Without doubt the less impressive of the sexes, male argonauts can be 12 times smal
ler and weigh 600 times less than females; they barely reach the size of a peanut. The males don’t make shells, but they do have an impressive trick up their sleeves. One of their eight arms is specially modified into a sperm-delivery organ. In other words, they have a penis on the end of an arm. What’s more, the male argonaut’s penis is detachable.
The word hectocotylus is now used for the arms of many male octopuses and squid that dole out packets of sperm to females. Amid a grabby clinch of arms and tentacles, the male will reach into the female’s body (in argonauts there is a cavity under the mantle; in other octopuses the male pokes into the female’s body just under her eyes). He detaches his wriggling, sperm-laden limb, which clamps on to her with suckers. Female argonauts will often collect and carry around the offerings from several males at once.
After dropping their penis some male cephalopods will grow a new one, but not male argonauts. They only get one shot. Their arm drops off, hopefully stuck to a receptive female, and shortly afterwards they die. Female argonauts, on the other hand, keep going and unlike their octopus cousins they can rear many clutches of young over the course of their lifetimes. Most mother octopuses deposit their eggs inside caves and crevices on the seabed. They will usually stick around to watch over their offspring, to fend off predators and keep their broods well oxygenated with wafts of clean water. A deep sea octopus has recently been seen in the Monterey Canyon off the Californian coast guarding her eggs for 53 months, by far the longest period of egg-brooding ever seen in any animal. After all that time, and possibly even longer, she will most probably die, as most female octopuses do, after their single, tremendous reproductive effort.