The secret life of shells

Something wasn’t right about the sounds of the morning.

There was something out of place: a sound I hadn’t noticed in the post-sunrise bliss of my hammock but, once my ears began to tune in, I realised that it had been there all along. It was a soft organic clattering; hard things knocking and scraping in the sand beneath the veranda of our beachside bungalow.

Here on Medjumbe, a tiny island in northern Mozambique’s Quirimbas archipelago, it doesn’t seem appropriate to rush things, not even to satisfy curiosity, and so I swayed the hammock gently, and closed my eyes. I’ve always been fascinated by the way removing one sense heightens the others, and I’d closed my eyes deliberately to tune into the noise below the veranda. The sound was a fine scraping, a dragging, a chinking – similar to the way shells clink in my pockets during a walk on the beach.

When I peered over the edge of the hammock, I realised that it actually was the sound of shells – about 30 of them, all with hermit crabs inside. The little critters were dragging their mobile homes over broken pieces of shells, labouring over twigs of driftwood and the exposed, tangled roots of trees. The crabs didn’t seem to be going anywhere in particular while stitching their delicate trails into the sand.

Hermit crab on a beach in Mozambique

I didn’t know it then, but these little hermit crabs will change shells between five and 10 times in their lifetime, finding a suitable home each time they outgrow their old one. The criteria: a shell that’s large enough for them to withdraw into, but light enough to carry around, explains Prof Charles Griffiths, director of the Marine Biology Research Centre at the University of Cape Town. The hermit crabs need the protection of shells in order to survive because, unlike the crabs you see scuttling along the edge of the tide, their abdomens are soft.

A seashell becoming home to a hermit crab is a fine marine example of “recycle, reduce, reuse”. The shell’s first incarnation is as the exoskeleton of a mollusc, which is an invertebrate – an animal with no backbone – that has a soft body protected by a hard case (like snails, clams and oysters). But the mollusc didn’t begin its life with a shell on its back. Depending on the species, most of these little organisms hatch as larvae from eggs, then feed on floating food particles until they’re ready to sink to the sea floor and begin the process of metamorphosis.

How the mollusc grows its shell in this phase and through the rest of its life is a complex chemical process that takes place via the mantle, where proteins are secreted. The proteins form a structure onto which calcium ions (collected from the mollusc’s environment through its gills, gut and epithelium) bind, forming a complex matrix of calcium carbonate crystals. The shell grows in layers from the outer edge, and folds in the mantle affect the sculptural appearance of the shell, forming ridges, spines or scales.

The chemistry of shell growth is an intricate process, the exact details of which are not completely clear to scientists, nor are they very relevant to the average beach-combing holiday-maker. But the results of the process have had humans fascinated for centuries. We’ve collected shells to use them as money, as tools, as jewellery, ornaments, weapons, fertiliser and musical instruments. We’ve even used them as a form of entertainment, and a reason to go on holiday.

Cowrie shell on a beach in Africa

Roy Aiken, president of the Conchological Society of South Africa, “discovered” shells when he was five years old. Roy’s family was holidaying at Umtentweni on the South Coast when his father picked up a cowrie, and curious Roy asked him what kind of creature lived inside it*. “My dad had to go and find an answer to this and from that time on, we never looked back,” says Aiken. “We spent many happy times at the coast, walking miles together on South Coast beaches in search of new shell species.”

*(What does live inside a cowrie? A type of mollusc called a gastropod, a single-footed animal. These sea snails are often nocturnal; they are usually found under rocks and on sheltered reefs from low-tide pools up to a depth of more than 400 metres.)

 Over the past 50-odd years, Aiken has collected species of shells, families of shells, micro shells (those that are smaller than 5mm when they are at adult size) and, lately, has been searching our coastline for shells that may be new species to science. “Southern Africa is a wonderful area for collectors,” he says. “In the warmer places in northern KwaZulu-Natal, you can collect many beautiful Indo-Pacific species. Further south, you’ll come across an increasing number of endemic species, and it is this unique ecological niche at the base of Africa that attracts a great deal of interest from collectors.”

The amount and types of shells that you find on a beach are usually influenced by the proximity of offshore reefs, which is where many mollusc species live, as well as the prevailing water currents and whether these carry shells towards a particular beach, says Griffiths. Along Southern Africa’s east and west coastlines, the types of shells found are mostly determined by the ocean currents: the warm Agulhas current which flows south from tropical East Africa along the east coast, and the cold Benguela system on the west coast. “We tend to find tropical and subtropical species on the east coast, warm temperate species on the south coast and cold temperate species on the west coast,” explains Prof Dai Herbert, chief curator of molluscs at the KwaZulu-Natal Museum in Pietermaritzburg. To occasional amateur shell-collectors like me, that translates as a wide variety of shells on the east coast, many shells endemic to South Africa on the south coast and less variety, but often a great abundance, of shells on the west coast.

I have so many happy memories of childhood holidays spent combing South Coast rock pools and beaches for colourful shells, and making necklaces with the ones I gathered that had holes in them. (Those little holes are often made by marine predators which “drill” into the shells to feed on the inhabitants.) As an adult, I still find it hard to step over an interesting shell without picking it up and as a result, a small collection of cowries is now growing on the veranda of my beachside bungalow. I can never resist picking up the occasional cowrie, with the thought of taking it home for my mum, who has always treasured the shells.

But I am not the only one who’s collected cowries in these parts.

Ibo island in Mozambique

North of Medjumbe is Ibo island, once an important trading post on the slave, gold and ivory route and that’s now a crumbling collection of forts, merchant houses and mansions. The buildings, which have their roots in Arabic, Portuguese and Indian architecture, are beautifully neglected in a photogenic sort of way: bougainvillea clamber frantically up crumbling walls, flaking paint as they progress. Fig trees drop their roots where they find most appropriate, without any regard for roofs, doorways and coral walls. It feels almost like a ghost-town… except that people still live there.

On Ibo, there is a building known as “the cowrie house”; its entire façade is covered by neat rows of similar-sized cowrie shells. The story goes that the house belonged to a Portuguese trader whose wife collected a shell for the façade every day that he was away. It’s not surprising that the lonely woman found so many enormous cowries on her walks around the island: Ibo is one of 32 islands in the islands in the Quirimbas National Park, said to be one of the richest marine environments on earth.

Cowrie house on Ibo Island in Mozambique

Cowries, the same shell that sparked Aiken’s interest in conchology, have been a favourite among collectors for centuries and the shells have used for divination, as currency (Ghana’s currency, the cedi, is actually named after the cowrie), jewellery, as fertility symbols, and as decorations. What has collectors particularly enamoured with the shells is that there are so many different species, which usually have glossy shells and beautiful colour patterns. The gloss is something pretty unusual in shells, and they’re like this because when alive, the mantle extends over and completely encompasses the shell. “This means that the surface of the shell is kept free of encrusting organisms and remains glossy,” says Herbert.

Cowrie shell on a beach in Africa

The colour of a shell is laid down as new material is added to it during growth, which takes place at the edge of the mantle, around the shell aperture. “Cells are genetically programmed to switch the production of pigment on and off in a very specific manner so as to produce the colouration and pattern characteristic of the particular species,” explains Herbert. Depending on the species, the colour may be for camouflage, for temperature regulation, for protection or to strengthen the shell. Some of the colours you see in a shell are determined by the mollusc’s food source, and colour could also be derived from the mollusc’s waste products. If the pigment is secreted continuously, there will be a coloured spiral tracing the growth pattern of the shell, but if the pigment is secreted sporadically, the colour shows as spots or radial bands.

My shelly musings continue as the early morning sun grows in strength. I wonder what treasures have been left by the night’s high tide. Where have they come from? What colours will they be? How many shells are out there today? The clattering of the hermit crab, I realise, has dissipated and the fluid lap of low-tide waves is luring me from the gentle curve of my hammock. There is a trail of pale shells along the high tide line to be followed, and on the other side of the island, there are rock pools to explore.

 

High Live the inflight magazine for British Airways This story was first published in British Airways’ inflight magazine, High Life. It was the cover story for
the February 2014 issue.

The secret life of shells

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