Mollusks and the Homes They Build

Rob Hardy

September 29, 2015 8:43:28 AM

Did you ever hold up a seashell to your ear to hear the winds and waves within?  You get a hint that a shell isn't just a shell, but a portal to distant worlds.  People have valued shells for their beauty, but they have also made myths about them with themes of birth, sex, and death.  They have used shells as decorations for sending off the dead, and used them for money, and more.  The lore of shells, and plenty of the science behind marine mollusks, are the subjects of Spirals in Time: The Secret Life and Curious Afterlife of Seashells by marine biologist Helen Scales.  She says of her book, "It is my attempt to set the record straight, to throw out the novelty knick-knacks and reinstate seashells to their rightful place as glorious objects that can tell us so many things."  The beauty of mollusk shells is undeniable, and their variety comes from how evolution has formed them not only as protection for the creatures inside, but also to help the creatures eat, move, hide, fight, and more.  Scales's accessible account has lots of facts and explanations, and her enthusiasm for her subject is a delight. 

Shells were the first known jewelry.  Dog whelk shells were discovered in a cave in Morocco, but they were not fossil deposits.  About 100,000 years ago they were brought from the sea 25 miles away.  Studies on them show that the cream-colored shells had been decorated with red pigment, and had been pierced with holes, with patterns of microscopic wear that showed they had been strung together.  Shells may have been used as tools before being used as jewelry, but using them for decoration, and not for an obvious practical purpose, signifies something important about human advancement.  The jewelry was probably regarded as valuable, and indeed shells were used for a long time as money.  They are good for this purpose; it is hard to counterfeit them, they are tough, they are easy to handle, and specific shells, if appointed as currency, can be counted upon to come in consistent sizes and weights.  It was not just the coinage of tribes; in the eighteenth century British fleets imported forty million cowries to Africa a year for the slave trade.  The cowry currency declined when slavery was made illegal, but was still used for purchases of such things as palm oil through the nineteenth century. 

The diversity of particular shells is astonishing.  Heart Cockles have tiny transparent windows that let light in.  The light is admitted for the purpose of powering photosynthetic microbes which have a symbiotic relationship with the mollusk.  The microbes are protected within the shell, and the mollusk gets a ready supply of food from its little interior farm.  Some shells get used in the way of oyster knives and jemmy their way into an oyster dinner.  Some shells get used as drills, into wood; shipworms are not worms but clams, and have sunk fleets.  Some shells get used as battering rams to smash into other mollusks.  The Clusterwink shell serves as a little lampshade for lights within.  If you give the snail a prod, it produces a blue-green light that makes the whole shell glow.  This might startle a predator away, or it also may alert other predators in the area about the presence of an intruder, a sort of burglar alarm. 

Here is an aspect of mollusks I had never before known: textiles.  The enormous bivalve known as Pinna nobilis, or the Noble Pen Shell, looks like a huge mussel standing on the bed of the Mediterranean Sea.  To stand that way, such a shell requires guy wires, a thousand slender threads to anchor it, each about as wide as a human hair.  These hairs have been woven into thread and made into cloth.  This sounds fanciful enough, and people have exaggerated how "sea silk" has been used for powerful sails for ancient vessels or for royal robes for Roman emperors.  But sea silk is a real, if rare, fabric; the British Museum has a glove made of the material which belonged to its founder, Sir Hans Sloane.  Sea silk weaving is still done.  Scales goes to a little island off southern Italy where a local woman demonstrates how she cleans and combs and spins the threads.  Sea silk was always rare, and now is even more so; the pen bivalves are endangered, and the woman can only get new threads when a fisherman pulls up a dead specimen. 

We aren't the only species to appreciate sea shells.  Hermit crabs use them as homes, and swap for bigger ones as the crabs grow larger.  Hermit crabs never kill a mollusk for its shell; they are only on the lookout for empties.  They do steal the shells from other hermit crabs, battering away until the current owner has had enough and abandons its shell.  A hermit crab coming upon a suitable empty will exit its current shell and enter the new one, but if the shell is too big, it will sit by it.  This will draw the attention of other hermit crabs, who will sort themselves out into a "vacancy chain," a queue which will eventually enable each crab to move up one size of shell.  Then having accomplished the group swap-around, they all disperse to their separate ways. 

Many mollusks are good for humans to eat.  In fact, we eat something like sixteen million tons of them a year.  There are clams, mussels, and oysters, of course, but in some places people eat whelks or conchs.  We have run into a problem with the algal blooms triggered by our farming nutrients washed into the sea.  Not only are these generally bad; specifically, mollusks can eat the algae, and maybe they will thrive on them, but they will make themselves poisonous to humans.  That sort of shellfish poisoning is much more common than it was only a few decades ago. ( I learned that one danger posed by mollusks is imaginary, that of giant clams: "Their reputation as dangerous mantraps is utter nonsense with no record of anyone ever getting a part of themselves stuck inside one of these enormous bivalves.")  Mollusks do more for us than taking a place on our tables.  Mussels glue themselves to rocks using an enormously powerful adhesive which is now the basis for development of synthetic bio-glues that would be invaluable in gluing tissues together during surgeries.  The Geographic Cone Snail can have beautiful patterns, but collectors of the living specimens must beware; the sting is often fatal to humans.  The toxins, however, can be used and modified to become medicines; a neurotoxin has potential for being a powerful pain medicine. 

So we should value our mollusk fellow-creatures, and ensure their well-being.  Naturally, there are pages here about the difficulties posed by global warming and ocean acidification.  But also, in a final few pages, Scales tells us about the danger of buying ornamental shells.  A shiny, pristine shell you can buy was taken from an animal that was killed to get the shell.  Shells from animals which die naturally quickly become beaten or bored into.  We know more about the trade in mollusks for food than we do that for decorations, and there is little regulation of the ornamental trade, which is a huge business.  Anecdotal reports indicate that there are shortages of ornamental specimens caused by over-harvesting.  You are less likely to have an ecological effect if you pick up your own shells, but Scales warns not to take live specimens (especially the deadly cone shells), to return any rocks you move to look for shells, and to refrain from taking every specimen you find.  It's good advice, advice I had never seen anywhere else.  It's a unique slant on ecological matters, from someone obviously knowledgeable and who obviously cares.