PEARLS – Seawater vs Freshwater

Now that you know how a pearl is formed from our last Blog it is time to explore the provenance and difference between saltwater pearls and freshwater pearls.

Saltwater pearls have their provenance in the PINTADINES

Seawater pearl producing shellfish are not in fact oysters. Although for ease everyone has and will continue to call pearl bearing shellfish oysters, for the most part, seawater pearl bearing molluscs belong to the Pintadine family. Within the Pintadine family there are seven pearl producing shellfish; unlike their edible sedentary namesakes, the Pintadines are not edible and are mobile from one generation to another.

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The mobility of the Pintadine shellfish is due to their reproductive cycle, when conditions are right one shellfish releases spermatozoa into the water; this act begins a chain reaction on all other pearl producing Pintadines in the area. They release eggs and spermatozoa into the water; which are mixed at the mercy of the currents and larvae is formed. The larva propels itself with a small foot in the water and grows into spat. At 45 days the spat is ½ inch long or approximately the size of your thumb nail, with the appearance of a very thin and transparent oyster.  It will, at this early stage, make the biggest decision of its existence: once this small spat finds a suitable spot in which to attach itself and grow, surrounded with plenty of light, food and warmth,  it can no longer move.

Once the spat is attached and has become a baby oyster much of its energy will go into growing mother of pearl layers to cover its shell. These nacre layers are in effect the oyster’s protection against hungry predators.

It is a miracle of nature that we have pearl bearing oysters at all! The existence of pearls rely on chance fertilisation, the avoidance of being eaten by predators and then the precarious decision of where to settle for life. If a life threatening piece of coral or shell is lodged in the flesh of the oyster before it is 3 years old or weak it will die.

Looking on the bright side if the oyster is alive and healthy at 3 years of age it is strong enough to withstand the introduction of a foreign body into its organism. As the intruder slices its way into the depths of the oysters ‘body’ it carries with it epithelial cells from the mantle, these cells form a pearl sac around the intruder and the epithelial cells start doing what they do best they deposit concentric layers of nacre that surround the offending object and slowly form the pearl, layer by layer, a miracle of nature. It is not surprising that this rarity is reflected in the value of pearls.

Freshwater pearls have their provenance in UNIONIDES

Freshwater mussels kept by Alfred, 1938

The Unionides produce the majority of the freshwater pearls that we know. These are bivalve shellfish, normally referred to as mussels or mulettes; they too are mobile and mainly inedible. The mobility of pearl producing mussels is also due to their reproductive cycle; in this case the fertilised egg enters the gills of a fish and feeds off its blood turning into larva. When the larva has been in the host fish for about two months and the fish reaches a particularly suitable stretch of water, the larva disengages from the fish and settles. It will usually choose a stretch of slow moving waters in a river or a suitable spot in a lake, the depth at which these mussels are found is between 1 and 1.5 metres from the surface. Hence when fishing for freshwater pearls they can be spotted by looking into a glass bottomed jar, which will give clarity to the water and enable the fisher to see if mussels have unusually protruding areas in the smooth outer shell. Pearls are formed when a small stone or a calcareous concretion lodges in the pearl bearing mussel and starts the formation of a pearl; these pearls have rounded surfaces although they can be of many different shapes. Their colours can be among others white, soft pink, mauve, heather, brown and pale grey.

How do you fish for Natural freshwater pearls?  Take your lead from Chrissie Douglas’s ancestor Alfred Smith who regularly sought pearls in the river Tay, a glass bottomed viewer and a staff to move things around in the river bed, as seen below, but be warned it is illegal nowadays to fish for pearls in Scotland!

Alfred pearl fishing 1, Inverurie

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