Spirals in Time: The Secret Life and Curious Afterlife of Seashells Page 17
Most of the Noble Pen Shells I see are on the small side, about as wide as my outstretched hand. They are all young ones, not yet fully grown. It means this particular spot is an important nursery for the population, and a good indication that all is well for the pen shells of Sant’Antioco. I can’t tell for sure without spending days and weeks swimming around the entire island counting shells as I go and then ideally coming back some time later to see if things have changed. But the presence of juveniles is a sure sign that adults are nearby and they’ve been successfully breeding. These are probably not shells that Chiara will harvest, because they are too close to town, and to the gaze of prying eyes. Popping my head up above the surface, I see a coach-load of tourists drive past along the seafront, a few hundred metres away.
Not much is known about pen shells and their current status in the Mediterranean, following protection more than 20 years ago. A few scientific studies have mapped out their distribution and sizes, and there are signs of recovery and healthy populations. Their seagrass habitats are certainly under pressure still, in particular from rising sea temperatures, but pen shells do live elsewhere, too, in sandy, muddy environments that are far less threatened. To some extent the pen shells’ protection is a precautionary measure, a proactive step to make sure they don’t dwindle as they so easily could, rather than waiting for catastrophe to strike, by which time it might already be too late.
Back down below me, the Noble Pen Shells seem to shift and glide across the seabed but in fact it is the grasses and weeds that flutter in the breezy current around them, while the shells stay put. They are wedged firmly in place up to their middles in the soft sediment, anchored by their unseen byssal threads.
There is no doubt that sea-silk continues to enchant people, especially when they are regaled with worn-out fables told as if they were still true today. Surely, though, there are wonders enough to be had in the reality of these giant shells with golden beards. We can marvel at the tiny crabs that cohabit with the living shells, and the octopuses and fishes that move in when they die; we can ponder the strange mystery of who it was who first thought to tease out a pen shell’s fibres and spin them into silk; we can contemplate the spelling mistake made centuries ago that led to a deep-rooted case of mistaken identity; and we can admire the intricate embroideries made by the artisans of the more recent past and present.
Giuseppina and Assuntina Pes will keep working on their weavings but for the most part they will use alternative fibres, not sea-silk. Chiara Vigo will continue to run her museum, tell her stories and venture to the shore to gather more byssus when no one is watching.
The Noble Pen Shell is a rare thing indeed. It is a sea creature with something to offer but isn’t, for once, being plundered to meet human needs and desires. So it can only be a good thing that newly woven sea-silk remains an obscure, curious thread that gleams now and then on just one tiny island.
CHAPTER SEVEN
Flight of the Argonauts
I’ve never seen a living argonaut. Few people have. In a rare sighting in October 2012, fishermen accidentally caught a female argonaut while hunting for squid a few miles off the coast of Los Angeles. They brought the strange creature back to shore and gave it to a local aquarium. It was unusual for this tropical species to show up in temperate Californian waters. Staff at the Cabrillo Marine Aquarium assumed that she had been carried on a current sweeping up from the south and carefully placed her in a warm-water tank. For some time, the exhausted animal lay helplessly at the bottom and the aquarium keepers feared the worst. Then one of them thought to give her a helping hand towards the water surface. After that, the argonaut perked up, and started swimming around her captive home; she eventually began to eat, grabbing morsels of fish and shrimp offered to her.
A video posted online shows the captive Californian argonaut. Hovering in the water, her shell is iridescent with a bronzy-silver gleam and for the first few seconds it’s difficult to make out the animal inside. Then all of a sudden she pops out, revealing herself to be a delicate, shiny little octopus. She pulls out her eight arms, grabs hold of her shell and deftly spins it round before climbing back inside.
Argonauts are the only octopuses that live inside a shell. All the other members of the order Octopoda, around 300 in total, have embraced a soft, naked life. Now and then you might spot a common octopus peeping out from inside an empty clam shell. A video clip went viral a few years ago of an octopus in Indonesia picking up half a coconut shell and strutting off across the seabed, using its arms as legs. When it comes to full-time shell-living, though, it’s just the four members of the genus Argonauta: the Greater, Rough-keeled, Brown and Tuberculated Argonauts. They all look quite alike, with pale and thin shells, covered in ridges and rows of nodules. Depending on the species, their shells can be between five and thirty centimetres (two and twelve inches) across, while the animals inside are considerably smaller. Throughout their lives they cruise the upper highways of tropical and subtropical seas, way above the heads of their octopoid relatives, which mostly live close to the seabed, lolloping and swimming along but rarely venturing too far up into open water.
After a week of life in captivity at the Cabrillo aquarium, the argonaut gave everyone a big surprise. She was joined in her tank by thousands of tiny argonauts. It turns out she had been carrying fertilised eggs, and now they were starting to hatch.
It was all hands on deck as helpers were drafted in to count the new arrivals. Clerical staff were brought out from behind their desks, and visiting schoolkids were given a taste of scientific research. Over a course of a few days, the argonaut released a total of 22,272 minute hatchlings, each one only a millimetre across. Other videos, this time shot down a microscope, show some of the new argonauts. The twitching oval blobs are mostly transparent, with two big, dark eyes and a covering of spots that expand and contract; one minute they are patterned like a giraffe, the next they are peppered with tiny black dots. The flickering colours are made by chromatophores, cells embedded in the mantle that are filled with pigment granules and are concealed or revealed by minute muscles relaxing or contracting. The infant argonaut grapples with zooplankton and uses its little arms to shovel them into its mouth; it’s the first time such a tiny argonaut has been caught on camera tucking into its food.
Sadly, though, the Californian argonaut and her plentiful offspring didn’t survive more than a few weeks in captivity. The aquarium keepers couldn’t easily have returned her to the sea because the warm water current that delivered her to California had stopped and they were a long way from her normal tropical habitat. At around the same time, empty argonaut shells were found washed up on nearby beaches, suggesting there had been some sort of mass stranding. Even if the captive argonaut had been left at sea she might not have survived. At least this nomad had helped researchers gain new insights into these most enigmatic creatures.
People have known about and puzzled over argonauts for millennia. Two questions have confounded many great minds: what purpose does the argonaut’s shell serve, and where do their shells come from?
The name ‘argonaut’ stems from Greek mythology, and the band of heroes – the original Argonauts – who sailed on the ship Argo with Jason in search of the Golden Fleece. It was the Greek philosopher Aristotle who first wrote about their molluscan counterparts. He suggested they use their shells as boats to float on the surface of the sea, with their arms as oars to row themselves along, or two arms flattened and hoisted up as sails. The story was passed on and retold for centuries by naturalists, and writers who professed to have seen this strange scene for themselves. The sailing octopuses appear in Jules Verne’s 1870 novel Twenty Thousand Leagues Under the Sea. While held captive aboard Captain Nemo’s submarine, the Nautilus, marine biologist Professor Aronnax ponders the peculiar sight of hundreds of argonauts sailing across the waves, all holding their arms in the air like flapping ears.
An alternative common name for argonauts is the paper nautilus, because their light,
papery shells look a little like those of the chambered nautilus. As this name suggests, nautiluses have shells that are divided into chambers (argonaut shells, by contrast, have no inner chambers). As they grow, expanding their shells from the open end, nautiluses inch their body forwards, and periodically seal a chamber off behind them. A tube running between the chambers, called the siphuncle, then empties liquid from the new chamber by osmosis, and gases diffuse in. Nautiluses can adjust the fluid levels inside their shells, like a submarine’s ballast tanks, controlling their buoyancy and reducing the energy demands of active swimming. Like other cephalopods, nautiluses swim by jet propulsion in a two-stroke system: water is sucked inside the shell, then squeezed out through a funnel. Shifting the position of the funnel controls their direction to some extent; nautiluses swim hesitantly forwards but can scoot away backwards at much greater speed. When they feel threatened, they can withdraw inside their shells, and shut the opening with a leathery trapdoor called a hood.
On the inside nautilus shells are lined with mother-of-pearl, giving them their other common name, the pearly nautilus. On the outside, they’re decorated with ginger tiger stripes across the top, with some that fade to white underneath, as if on being dipped in the sea their markings had started washing off. There are four recognised species in the Nautilus genus, including the Belly-button Nautilus and the White-patch Nautilus. Two other species were shuffled across into a new genus, Allonautilus, because when living specimens finally showed up a few years ago they were thought to be rather too different from the rest. All of them have around 90 slim tentacles – the most of any living cephalopod – making them look like they’re eating a mouthful of spaghetti. They occupy tracts of deep, tropical waters, in the Indian and Pacific Oceans, and are rarely seen alive. When they die their empty shells bob to the surface and can drift to distant shores.
Empty shells were all people knew of nautiluses for a long time. Collectors adored their shininess and elegant whorls, and naturalists were desperate to get their hands on a complete specimen, soft parts and all. Paper nautilus shells, on the other hand, did occasionally show up with something living inside them, but this didn’t stop naturalists arguing over the identity of these little creatures.
On an ill-fated 1816 expedition to find the source of the River Congo, British naturalist John Cranch was fishing for specimens from the Gulf of Guinea off West Africa when he found several argonaut shells, complete with living occupants. They survived on board in a bucket of seawater for several days while Cranch observed them. He saw they could come all the way out of their shells, if they wanted to, and otherwise looked and behaved like octopuses: they had suckers that stuck to the side of the bucket, they swam around using a jet of water and their skin changed colour.
All of this was later reported by William Leach, curator of zoology at the British Museum, because Cranch himself died of fever, along with most of the crew, and never made it back from Africa. In honour of his lost friend, Leach named the argonaut species Ocythoe cranchi, but this was applied only to the soft animals, not the shells they were found in. Many eminent naturalists believed the octopuses didn’t belong with the shell but had killed and devoured the original occupant before seizing their vessel and sailing off. In their minds the octopuses were nothing but parasites.
Carl Linnaeus had named the shells Argonauta argo back in 1758, in the tenth edition of his book Systema Naturae, and in 1814 Constantine Samuel Rafinesque assigned the name Ocythoe antiquorum to the allegedly parasitic animals that were often found inside. John Cranch’s was a new species of parasitic octopus.
For a long time, a living specimen of the shell-making argonaut itself remained unknown. Presumably they were lurking down in the depths somewhere; perhaps they were some other kind of nautilus. The fact that none had been found wasn’t seen as a major problem, though; after all, chambered nautiluses were very rarely seen alive, but their empty shells were fairly common.
In 1828, English naturalist William Broderip reported in the Zoological Journal that a French collector in Marseille claimed to have found a real argonaut, not a hitch-hiking Ocythoe octopus. I can sense Broderip’s eyebrows twitching as he wrote this, and he stayed on the fence, pointing out that much remained to be known before coming down firmly on either side. But he still took a punt that in the long run the octopuses would probably be revealed as pirates, and not the industrious shipwrights of what he called ‘fairy boats’.
The idea of octopuses sailing around in stolen shells may sound like a fanciful Just So Story, but there are some even stranger ideas floating around that have made scientists stop and think. Rather than snatching shells from living species, maybe argonauts hijacked them from far more ancient creatures?
The small collection of living nautilus species is all that remains of an immense cephalopod dynasty. In modern seas, the most common cephalopods are the ones with no external shells, the octopuses, squid and cuttlefish. But in times gone by it was the shelled cephalopods that reigned supreme. Masses of animals that looked a lot like nautiluses romped through the oceans for hundreds of millions of years. Within that group, the most abundant and diverse of them all were the ammonites, and there were some that looked so eerily similar to argonauts, you might be persuaded they were cast from the same mould. By the late nineteenth century, a distinctly offbeat idea had come to light. What if naked octopuses originally borrowed or stole shells from ammonites? Did argonauts learn how to make shells by copying their ancient relatives?
This theory was first proposed in 1888 by German geologist Gustav Steinmann; it was revisited in 1923 by Swiss palaeontologist Adolf Naef, then again in the 1990s by Zeev Lewy from the Geological Society of Israel. They all imagined the ancestors of modern argonauts to have started out hiding inside empty ammonite shells. Then the argonauts somehow evolved the ability to fix up their borrowed shells, to mend holes and cracks. As the argonauts got better and better at repairing shells they eventually no longer needed a template at all, and could merrily continue shell-making without having to find an ammonite shell first.
Lewy went a step further, proposing that argonauts were in fact scavengers of recently dead ammonites, which he rather charmingly referred to as ‘post-necrotic floaters’. In other words the ammonite shell, complete with dead animal inside, floated to the sea surface and drifted around for a while. Lewy suggested that naked argonaut ancestors laid their eggs inside these post-necrotic floaters, leaving the new hatchlings to slowly eat their dead hosts and ultimately occupy the vacated shell.
To find out if there is any truth in these ideas and see if there is a link between argonauts and ammonites, we should jump back in time half a billion years to see where this all began. Down at the base of the cephalopod evolutionary tree sits a little creature that lived towards the end of the Cambrian. It was about the size of a pinky toe and wore a slender and slightly bent shell like a wizard’s hat. Charles Doolittle Walcott, of Burgess Shale fame, was the first to describe fossils of these animals (though they were from later deposits), and he named them Plectronoceras.
Plectronoceras is the oldest undisputed cephalopod (strange creatures called Nectocaris from the Burgess Shale itself could be cephalopods, although not everyone agrees on that). Their shells were divided into chambers, like nautiluses, and they may have spent much of their lives skipping across the seabed or wafting through shallow seas as part of the plankton. Following on from these modest drifters there were far more impressive, not to mention scarier, cephalopods to come.
Starting around 485 million years ago, the Ordovician was the next major stage in Earth’s history. The planet was a strange place compared to the way things are now. Temperatures were much higher, as were carbon dioxide levels, and most of the landmasses were clumped together into a massive super-continent, Gondwana, but nothing much lived there. Life was still largely confined to the oceans, where there was a mixture of living things that we could recognise today, plus a range of other, bizarre creatures.
Trilobites scuttled across the seabed; bivalves and brachiopods stayed put as they sifted the water for food; gastropods ambled past fronds of red and green seaweeds and colonies of coral. Above the seabed, early chordates called conodonts wriggled their eel-like bodies and gnawed at their food with the sharpest teeth that ever evolved; floating through the water were colonial creatures called graptolites that looked like delicate, saw-toothed tuning forks. For all of these creatures, one of the most dangerous things they were likely to encounter in Ordovician seas was an enormous shelled cephalopod.
The unassuming Cambrian cephalopod lineage went on to flourish in the Ordovician. They evolved into masses of new groups; some were tightly coiled, others had shells as straight as pencils. Incomplete remains have been found of gigantic straight shells from a creature named Cameroceras. Estimates of their full size range up to an astonishing 10 metres (more than 30 feet), as long as a double-decker London bus. These were formidable beasts, like a primeval apparition of a Colossal Squid, the main difference being that these ancient creatures lived inside the longest seashells ever to exist.
It’s generally thought that Cameroceras may have spent a good deal of time resting close to the seabed, pulling itself along with a cluster of arms and scooping prey into its mouth. Other straight-shelled cephalopods would have hung in the water with their heads down, grabbing prey from the bottom. Some evolved counterweights at the ends of their long shells and swam horizontally. Like giant spears, they could have shot through the oceans in pursuit of prey. Whichever way you look at it, the Ordovician saw the rise of the cephalopods.