Spirals in Time: The Secret Life and Curious Afterlife of Seashells (36 page)

With so many endemic cone snails living in small areas of habitat, the spotlight falls on local conservation efforts; the future of each of these species will depend on what happens
at a local or a national scale. Meanwhile, another threat is looming on the horizon for all the cone snails, and the rest of the marine molluscs across the world, one that will need a global solution. The shifting composition of the atmosphere due to humanity’s carbon emissions means that some seashells could soon begin to simply melt away.

CHAPTER TEN

The Sea Butterfly Effect

A
sea butterfly flutters past. Its spiralling shell is translucent and colourless as though it were sculpted from glass. Inside, I see a cluster of cells twitching and contracting as its heart beats. Little wings stick out from the shell’s flared opening and flicker in energetic bursts, propelling it through the water in circles. It stops now and then as if to catch its breath, and I hold my own as I quietly watch, partly so as not to disturb it but also because this is the first sea butterfly I’ve seen and I can’t quite believe my eyes.

It quivers one more time and flits out of sight. I sit up and look at the shallow petri dish on the laboratory bench in front of me. I can just make out a tiny, whirling dot and suddenly feel like I’ve been Alice in Wonderland, peering through a tiny door into another world.

Earlier that morning, the sea butterfly had been swimming through clear, deep waters that surround the island of Gran Canaria. This parched volcanic outcrop lies 100 kilometres (60 miles) west of mainland Africa, at the same latitude as the desert border between Morocco and Western Sahara. I had come to meet Silke Lischka, a sea butterfly expert who had kindly agreed to help me find one of these beautiful, peculiar molluscs that could easily have sprung from the imagination of a storyteller. I desperately wanted to see one for myself, to check that they are real. And I wanted to see them now because their time might be running out. These fragile animals could one day soon begin to vanish from the seas, the early victims of climate change and a silent warning of troubles to come.

We had motored offshore on a black, inflatable research boat across the sea, flat like a swimming pool and only ruffled here and there by a gentle breeze. We found a good spot, stopped the engine, and Silke then lowered a plankton sampler into the blue water. Peeping over the side, I watched the rope paying out 15 metres (50 feet) or more, visible all the way as it dragged the white net down like a slender, upside-down parachute. On its return journey back to the surface the net sifted seawater, trapping anything bigger than a fine sand grain (70 microns, or 0.07 millimetres). Hauling the net back on board, Silke carefully unclipped the canister that had caught the siftings and tipped the contents, about half a litre of water, into a small screw-top barrel. I looked in and saw a blizzard of swirling particles, and immediately started imagining what we might have caught.

Six or seven times, Silke plunged the net down then dragged it back up, bringing in more minuscule treasures until she decided that we had enough to be getting on with. We kicked the engine into life and returned to land, passing flying fish that skittered through dry air on their improbable wings before plopping back down to where they usually belong.

Back in the laboratory at PLOCAN, the Plataforma Oceánica de Canarias, we sat diligently working through the plankton samples, pouring out small pools of seawater and examining their contents through microscopes with up to 40 times magnification. We had captured a fidgeting, living galaxy. There were masses of minute crustaceans called copepods, with bodies shaped like tear drops and some with a single, red cyclopsian eye; they paddled through the water on pairs of long whiskery appendages and turned endless pirouettes, chasing their tails round and round. Fuzzy tufts of cyanobacteria, or blue-green algae, drifted past like tumbleweed. I spied some
Noctiluca scintillans
. Under the microscope these dinoflagellates (a type of green algae) look like transparent peaches. At night, in their millions, they transform the seas into a glittering light show of bioluminescence. There was a tunicate larva with a small head and wriggling tail; how strange to think that, in time, it would settle onto the seabed, absorb its brain and become a plant-like sea squirt. I saw radiolarians like exquisite, many-pointed stars, pulsing cuboid jellyfish larvae, and foraminifera with coiled, chambered bodies that could be mistaken for miniature ammonites. But most splendid of all, I was quite convinced, were the gastropods with tiny wings.

For a while we saw no sea butterflies and I began to worry that I’d missed my chance, that it was too late in the season and the Atlantic had already become too cold and empty of food for them to still be hanging around. But we carried on, in hushed concentration, working our way through the barrel of seawater, until eventually Silke let out a little giggle and told me to come and take a look. She had found a small specimen of
Limacina inflata
(sea butterflies tend not to go by common names, only their scientific labels). Her find seemed to break the spell of the hiding sea butterflies, and suddenly plenty more showed themselves. Silke spotted a different species, not with a spiralling shell but with a delicate, conical tube instead. I began to get my eye in and found a sea
butterfly for myself and it felt all the more special. I was the first person ever to lay eyes on that particular tiny creature.

‘They look like little snitches,’ said Silke, chuckling. And they do. When J. K. Rowling created the game of quidditch, played on broomsticks by the pupils at Hogwarts School of Witchcraft and Wizardry, and the small golden ball with wings (which Harry Potter caught many times and swallowed at least once), I’d like to think she was inspired by sea butterflies. I watched them, transfixed, as they spun around, busily inspecting their shrunken sea as if they had somewhere important to get to. Soon, I became convinced that I was a natural-born sea butterfly-spotter. I spied sea butterfly larvae, which are so much smaller than the adults. Side by side they were pea and grapefruit. The young ones haven’t yet grown wings but have two lobes that are covered in tiny wriggling hairs and whir in circles, like an industrial floor-polisher. The movements of these energetic adolescents made the water around them glimmer and dance in a certain way that I learned to recognise and zero in on. And I found another minute mollusc with a spiralling shell that looked similar to the rest but with one important difference. I showed it to Silke and she raised her eyebrows at me, smiling; I knew I had earned brownie points. It was a heteropod, a distant relative of sea butterflies from another deep division of the gastropods. Unlike the sinistral sea butterflies, this one had a shell that twirled to the right.

Sea butterflies are also known as pteropods, the ‘wing feet’ creatures (just as pterosaurs were ‘winged lizards’). These most unlikely gastropods have wings instead of feet, which they use to swim through open seas worldwide, occupying the biggest living space on the planet. They are perhaps the most abundant animals that almost nobody has heard of.

Other pteropods, known as sea angels, also fly about underwater, but these have lost their shells. Instead, to protect themselves, their bodies are loaded with noxious chemicals that attackers soon learn to avoid. Their chemical defence is
so effective that small crustaceans called amphipods have learned to kidnap sea angels and carry them around, keeping them alive, like personal bodyguards. However, don’t be fooled by the angelic appearance of the sea butterflies’ shell-less relatives. Sea angels are compulsive predators that hunt exclusively for sea butterflies. They have keen eyesight to spot their prey, fast wings to pursue them, and suckered tentacles to grab them and wrench them out of their shells in a violent battle of angels and butterflies.

Sea butterflies themselves get their food in an altogether gentler fashion. They cast webs made of sticky mucus and – just like spiders – they trap their food. Among the things that often wind up in their nets are crustacean and gastropod larvae (including of their own kind), phytoplankton, and obscure, vase-shaped animals called tintinnids. When it’s ready, the sea butterfly hauls in the whole lot, eating its dinner, web and all.

Their gossamer webs are difficult to see but in the 1970s and '80s, two dedicated sea butterfly researchers found a way. Ronald Gilmer and Richard Harbison from Woods Hole Oceanographic Institution in Massachusetts spent a lot of time scuba-diving all over the world, tracking down these minute creatures and observing what they get up to in their natural habitat (many sea butterfly species grow large enough as adults to be seen with the naked eye). They would take a bottle of crimson dye with them and squirt drops into the water near sea butterflies to illuminate their webs. The animals would cast their nets and then hang motionless in the water – neither rising nor falling – giving Gilmer and Harbison the idea that sea butterflies might use their feeding apparatus to help them stay afloat, rather like the way that female argonauts use their shells.

Sneaking up and gently nudging them, the divers witnessed the sea butterflies’ escape response: they quickly jettison their web, then either flit angrily away or pull in their wings and drop into the depths. Sea butterflies are
good swimmers but they use up a lot of energy in the process. Many are negatively buoyant, and have to keep swimming or they sink. There are clearly benefits to be had from floaty nets, like tiny parachutes, that give them a break from all the incessant flitting.

There’s a lot we still don’t know about how sea butterflies move around their open ocean world. Silke shows me a video she shot of an Arctic species drifting through a large glass jar. She gently stirs the water and the sea butterfly stops beating its wings, holds them stiffly above its head and seems to ride the currents like a hawk on a thermal.

Sea butterfly procreation is especially curious. In some species there are separate males and females that will pair up, grab hold of each other’s shells, and swim together in spirals through the water for a minute or two while the male transfers sperm to the female. She will then lay strings of fertilised eggs, which she may carry around with her, stuck to her shell, before the young hatch and swim off. Meanwhile, some species are sequential hermaphrodites; they all start life as males then later switch sexes, becoming females. Early in the spawning season, when there are only male sea butterflies, they will mate with each other. Males undertake a mutual sperm exchange, then hold on to their partner’s donation until they turn into females. Then, all the new female need do is to fertilise her eggs using the donated sperm she’s saved up from her earlier, male-only encounter. It might initially seem like an odd way of doing things, but it makes sense in the big, wide open ocean where finding a partner of the right species
and
the opposite sex can be difficult: by swapping genders and having sex in this unusual way, the sea butterflies increase their odds of finding a suitable mate.

Pteropods are not the only gastropods that have abandoned the sea floor.
Janthina
is a genus of snails with vivid purple, spiralling shells that float on the sea surface, buoyed up by a raft of frothy bubbles.
Glaucus atlanticus
, known as the sea swallow, is a shell-less gastropod that also occupies this two-dimensional
world; it hangs upside down from the surface rather like a water boatman in a pond, with long fingerlike projections that store stinging cells scavenged from its favourite food, the Portuguese Man-of-war. Spanish Dancers, another no-shell gastropod, can usually be spotted crawling across coral reefs but occasionally they fling themselves into the water and swim along with flamboyant ripples of their mantle, like a flamenco dancer’s twirling skirts.

All of these gastropods are drifting and swimming through seas that are silently changing and many of them – especially the sea butterflies with their tiny, fragile shells – could soon find their world turning sour.

Silke Lischka
had come to Gran Canaria not to show me sea butterflies but to take part in a major, two-month research expedition, designed to help us understand more about what the future holds for these delicate molluscs and other minute sea creatures. Despite the gruelling work schedule, Silke had devoted her well-earned day off to helping me in my search, but she had to get back to studying what happens to sea butterflies when their watery world is threatened.

The problem of pH

For a little over two centuries people have been digging out and pumping up ancient black stuff from deep underground and using it to produce heat and light and food and to propel themselves about the place at ever-faster speeds. Burning all this coal and oil sends carbon dioxide in colossal quantities into the air where, together with other pollutants, it insulates the planet like a blanket, trapping the sun’s radiation and leading to the various complex effects of anthropogenic climate change. But not all the so-called greenhouse gases released from burning fossil fuels stay in the atmosphere. Around a third of all the carbon dioxide ever made by human activities has been absorbed into the oceans.