Spirals in Time: The Secret Life and Curious Afterlife of Seashells (19 page)

Dan Harries, from Heriot-Watt University’s Scientific Dive Team in Edinburgh, knows Flame Shells well. He tells me they can be easy to miss. ‘Occasionally they’ll come to the entrance to their nests and you’ll see them,’ he said. ‘But usually they’re hidden away.’ Instead, to get your eye in, you need to start looking out for suspicious bumps and lumps that shouldn’t really be there on a flat plane of tide-swept gravel and sand. If you give the seabed a gentle prod, you’ll notice it is soft and spongy.

Then there are all the animals that live among the Flame Shells. Thickets of bristle stars (leggy relatives of starfish) will congregate on a Flame Shell reef, thousands of them waving their arms in the water, while an occasional worm called a sea mouse will snuffle past with its luxuriant coat of iridescent spines. Sea sponges, soft corals and sea firs (relatives of jellyfish that look like miniature evergreen trees) are all devotees of Flame Shell reefs. In the midst of an ever-shifting substrate, they help themselves to the stable surfaces created by the reef where it would otherwise be impossible to get a grip. Clustered together, the clams and their nests transform the seabed from a featureless expanse into a bustling community.

A team of scuba-divers, including Dan, recently discovered the world’s biggest Flame Shell reef. Dodging the vessels in
a busy shipping lane, the divers sank down beneath the waves of the sea inlet, Loch Alsh, that runs between the Isle of Skye and the heaving backdrop of the Scottish Highlands. As the tides rise and fall each day they suck water in and out of the narrow channel that links the loch to the deep open water of the Outer Sounds of Raasay, making this an ideal spot for current-loving Flame Shells.

Dropping down and mapping out the Loch Alsh seabed, Dan and the dive team found Flame Shells
everywhere
. The reef they discovered covers roughly 75 hectares (almost 200 acres), an area equal to almost 3,000 tennis courts. It’s home to an estimated 100 million Flame Shells.

‘We were a bit curious as to why no one’s noticed them before,’ Dan admitted to me. A theory currently being tested is that these shelly reefs might naturally come and go. As the clams aren’t cemented in place on the seabed, there’s nothing stopping them upping sticks, moving on and building more nests elsewhere.

To create homes for other creatures doesn’t necessarily require millions of molluscs, gathered together in great reefs and beds. Solitary seashells can also form important habitats. There are fishes and octopuses that lay their eggs inside empty seashells; on land, mason bees use snail shells as nests. The Belligerent Rockshell doesn’t wait around for the other snail to die before turning its shell into a nest. Its victims are vermetid snails (known also as worm snails) that fix their tubular shells onto coral reefs like a random squeeze of toothpaste, with no mathematical elegance and with the open end peering up like an alien eye on a stalk. First the rockshell will suck the vermetid snail out of its shell, leaving behind a smear of eerie blue goo; it then turns around and lays its eggs inside the newly vacated tube. Charming.

Another group of animals that make use of second-hand shells are especially well known, perhaps because scientists and non-specialists alike find them endearing and fascinating
in equal measure. These are crustaceans that seem to think they are molluscs, and have become experts at bringing empty seashells back to life.

Quietly watch over a tide pool and you might spy a seashell behaving strangely. Instead of sitting very still or perhaps gliding slowly and smoothly along, it scuttles in bursts, dashing forwards for a short way, then hunching down when it thinks danger is near. And if you pick up one of these curious shells there’s a possibility that instead of an inquisitive soft tentacle peeping out you may be greeted by a sharp pinch.

Most crabs make their own shells. They construct a suit of body armour, which they shed and replace throughout their lives, each time finding somewhere safe to hide while their new outfit dries and hardens around them. Nevertheless, close to 1,000 living species of crab don’t bother with that. They have permanently lost their shells, and have instead evolved ways to take advantage of empty seashells. These are the hermit crabs, and they’ve been borrowing shells for a very long time.

In 2002 an unusual fossil shell was found in Speeton, a village in Yorkshire, England not far from tall cliffs that overlook the North Sea. The shell was an ammonite, an extinct cephalopod that swam through far more ancient seas, in the Lower Cretaceous around 130 million years ago. After it died it sank down to the seabed where a crab scuttled past, picked it up and climbed inside. It was Dutch palaeontologist René Fraaije who spotted the perfectly preserved body of this hitch-hiker inside its ammonite home with its claws peeping out. This is the oldest known hermit crab, and the only one found inside an ammonite so far.

A naked hermit crab is a bizarre sight. It has a soft, extended abdomen that twists to a point, making it look like some sort of grotesque shrimp. A crab that lives inside coiled
gastropod shells – as many species do – will push its bendy rear end into the empty shell and hold on tight, gripping the central pillar; it then retreats inside, plugging the shell opening with its claws, which have evolved to be just the right shape. These are trap doors that bite.

Other hermit crabs will grab on to a single, disarticulated bivalve shell – a clam or a cockle perhaps – and hold it over their head like an umbrella. Some specialise in long, narrow tusk shells. Their pincers are rounded to form a perfect plug for the entrance of their tubular homes. The one thing that hermit crabs never do is kill the occupant of a shell before moving in. They only adopt vacated shells and never consume their hosts: they wait for other animals to do that first.

The majority of hermit crab species live in the sea, and they have evolved a finely tuned sense of smell that draws them to the places where molluscs are being eaten. Particular peptides are produced when enzymes in predator-spit begin to digest mollusc meat; these waft through the water, and when a hermit crab picks up the scent, it marches off in search of a shell that will be abandoned any minute, just as soon as the predator has finished its dinner. Finding new shells is a critical part of being a hermit crab, and they devote a lot of time to this single pursuit. By not making their own shells hermits avoid the costs of construction, but it means that as they grow bigger they will keep on outgrowing their homes. Like Goldilocks, hermits are constantly on the lookout for the perfect shell: not too small, otherwise they won’t fit inside, and not too big, otherwise the shell is too heavy and unwieldy to carry around.

Curiosity about how an animal evolves to rely entirely on the leftovers of another has led many scientists to watch hermit crabs very closely. These scientists are behavioural ecologists, a bunch who devote themselves to understanding why other animals do what they do. Those who specialise in hermit crabs tend to spend their time tinkering secretly with shells, numbering them, swapping them over, offering
new ones and all the while watching how the crabs respond. From detailed behavioural studies one thing is becoming clear: hermit crabs do often live up to their name, and they can be quite antisocial.

For one thing, they have no qualms about stealing each other’s shells. Suitable shells can be in such short supply that hermit crabs are permanently at risk of being evicted, and when two of them meet, a number of things can happen. The etiquette of a crab-to-crab encounter usually begins with a ritualised show of claws as the duo try to settle things without a fight. A larger crab will hold out its claws so its opponent knows exactly what it’s dealing with (claw size is a good indication of overall body size and hence fighting ability and strength). This can sometimes end in surrender. The loser drops its shell and runs off naked; the victor can then take its time, inspecting the empty shell and perhaps trying it on for size before deciding if it wants to move house. If the situation is more evenly balanced, the slightly smaller crab might put up its dukes, thrusting its claws forwards repeatedly, probably in the hope that this will intimidate its aggressor and make it back off. But sometimes a scuffle is inevitable, and a hermit battle kicks off.

Crabs will wrestle each other, checking out whether their opponent’s shell really is worth the effort. If it is, one crab will climb on to the other and repeatedly hammer the shell with its claws. Eventually, either the attacker runs out of energy and gives up or the defender has enough and relinquishes its shell.

Things are different for the dozen or so species of hermit crabs that live on land. Being high and dry, they accept that seashells are in especially short supply, and these hermits will sometimes have to make do with whatever they can find, perhaps a piece of wood with holes in or a discarded plastic bottle. In Madagascar, land hermit crabs have been seen waiting at the base of a crumbling cliff and picking up
hollow fossil shells that occasionally drop out. On beaches, land hermits bustle to the tideline in the hope of finding a new home among the flotsam and jetsam, but there can be so many crabs around that most of the suitable shells will already be occupied. The severe housing shortage forces these crabs to socialise.

Whenever a land hermit crab is lucky enough to come across an empty shell (sometimes because a behavioural ecologist put it there) and if no one else is around, it will stop, take a closer look and probably try on the new shell for size. If it likes what it finds it will keep the new home and continue on its way. However, if the shell is too big the crab won’t pass on by, but will sit quietly next to it, sometimes for as long as 24 hours. In that time other crabs will probably amble past and wonder what’s going on. Then a spontaneous hermit party breaks out. Don’t get too excited, though, because the main thing that happens when hermit crabs get together is they start forming queues.

A gaggle of hermit crabs clustered around a big empty shell will sort themselves out into a size-ordered line with the biggest at one end, leading to the smallest at the other. This orderly formation is called a vacancy chain, and people form them too, of jobs and houses. The crabs work out who goes where by clambering around and feeling up each other’s shells. Sometimes, if there are lots of hermits in the area, several queues will form around a single, large vacant shell and then things get a bit more interesting: a tug-of-war ensues.

The biggest crabs will wrestle over the coveted empty shell while the little ones further down the line will shift queues like supermarket shoppers speculating on which checkout will move fastest. Eventually, one queue will win control of the empty shell and, in a flurry of claws, everybody in the successful line moves house. Each crab slips out of its old shell and into the newly abandoned shell of the crab one place ahead of it in the queue. They all get a new shell, one
size bigger, and quickly scuttle off, once again going their separate ways. Behavioural ecologists have worked out that forming vacancy chains provides benefits for all the crabs involved; adding just one new shell can efficiently provide new homes, of just the right sizes, for a whole gang of hermits.

Behavioural ecologist
Mark Laidre
took on the enviable task of studying hermit crabs on the beaches of the Osa Peninsula on Costa Rica’s verdant Pacific coast. In one experiment he coaxed the hermits out of their homes and gave them either new seashells or old ones previously worn by other crabs. From the outside, these two shell types seem to be similar in size, but second-hand shells have a larger entrance, and they’re bigger on the inside because previous occupants have excavated them (they secrete chemicals that soften the calcium carbonate, then scrape away layers inside). When Laidre gave crabs new shells, they were usually too big to fit in and part of their bodies stuck out, leaving them vulnerable to attack by predators. In contrast, the hermits given previously occupied houses were mostly doing just fine tucked up inside their shells. As well as being more spacious, the remodelled shells are lighter and easier to carry around. Laidre put hermit crabs on little treadmills and measured how out much energy they use up carrying new and old shells. He found that crabs have a much easier time strutting around wearing second-hand shells.

The nub of the problem is that only the smallest, youngest crabs are able to move into new shells and begin the long task of digging out the interior, and they will only do this as a last resort, when they can’t find a pre-used shell. On the beaches, there is a booming second-hand market in remodelled shells that become ecological heirlooms, passed on between many successive hermits.

In the sea, hermit crabs don’t bother remodelling their shells. For one thing, seawater buoys up their shells, effectively
making them lighter. Marine hermits also need their adopted homes to be as strong as possible to protect them from all the ocean predators, including plenty of other crabs, that have become specialists in cracking their way into tough molluscs. Excavating a shell and making it bigger, but weaker, just isn’t worth the effort.