Spirals in Time: The Secret Life and Curious Afterlife of Seashells (24 page)

People have done strange things with pinna pea crabs. There are ancient recipes listing pea crabs as an ingredient for soup. They have also been a source of moral guidance, with the belief that we could all be a bit more selfless and cooperative like them. An ancient Greek book from the second century called
The Interpretation of Dreams
informs couples that they will have a long and happy marriage if they dream about the pinna shells and pea crabs that live so harmoniously together. Strange dreams indeed.

I scoot from pinna to pinna, but none of them seem to be occupied by little crabs or shrimp. Inside a dead shell, gaping and still, there is a dark shadow of a fish lurking. The basilisk blenny slowly retreats like a shadowy face pulling back from
the window of an abandoned house, not wanting you to know that you are being watched.

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.