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Agressive Pearl-Bearing Clam

© Форпост Северо-Запад / Горный музей

A hundred years ago or so, people risked their lives to get their hands on pearls. Not only it was dangerous to dive without gear, but some giant molluscs could grow that big they could actually kill.

As is known, pearls are formed inside the shell of certain molluscs as a defence mechanism against a potentially threatening irritant. Their colour, shape and size vary, depending on what material acted as stimuli. Most pearls are only a few millimetres in diameter; those reaching tens of centimetres are much rarer to find.

There is a legend, according to which a young man driven by traditions went to the seashore. He looked for a pearl as a present to his bride. He never came back. His lifeless body was eventually found lying on a coral reef, with his hand trapped inside the shell of a giant Tridacna mollusc. This mineral-bearing marine species can grow to enormous sizes — up to 1,5 m in length and 300 kg in weight. The death story of the unfortunate groom may seem unreal, but the closing sharp-edged shell of tridacna clams is. If the valves of this large mollusc are tightly closed, even a crowbar may be of little help. The only way out is to use a knife and cut through the muscles closing the shell.

Missing content item.

Tridacna may pose a fatal danger, yet it produced the world's second-largest pearl — 'the Pearl of Lao Tzu', also 'the Pearl of Allah' (it resembled the turbaned head of the Islamic prophet, Muhammad). Lao Tzu, a Chinese philosopher, placed an amulet onto which the faces of Buddha and Confucius were carved inside the pearl. Over time, the decoration item accumulated nacre, whilst Tzu's successors kept moving it into more giant shells. When the time had come to place the ornament into tridacna shell, a storm broke out, and the mollusc, with the pearl inside it, transported by ship was lost. It was retrieved in 1934 when found near Palawan Island and raised from the bottom of the South China Sea. The find measures 238 millimetres in diameter and weighs 1,280 carats. Its estimated price, according to the Guinness Book of World Records, is $40 million.

The almost one-metre-long 'aggressive clam', which once inhabited the Indian Ocean, is one of the key exhibits in the St. Petersburg Mining Museum palaeontological collection.