Queensland South Sea Pearls

Australia is the world’s major producer of white South Sea pearls, for during the 1998 season the tropical waters off northern Australia yielded ~ 60 per cent of total world production of these much desired and very valuable bead-nucleated cultured pearls.

White South Sea pearls are large (>10 mm diameter) rounded, lustrous, white, silvery, cream, or golden pearls that are cultivated by man in the gold- and silver-lipped pearl oyster (Pinctada maxima) that is indigenous to the off-shore waters of northern Australia. Other white South Sea pearls produced by the Australian pearling industry include ‘accidental’ whole baroque keshi pearls, and cultured bead nucleated half pearls that are commonly termed mabés.

The white South Sea pearl is a comparative newcomer to Australia’s pearling industry, for the history of this primary industry stretches back for well over a century to the time when Australia was one of the major supplier of natural pearls to European and American markets.

Artificial spat culture in Australia

In Australia, the success of hatchery-bred shell has been limited; but is improving with refinements of hatchery technology. Basic steps in the hatchery breeding of P. maxima spat are:

Selection and growth of a suitable algal culture to support optimal growth of spat.
Selection of suitable breeding stock.
Controlled spawning.
Laval culturing.
Laval settling.
Planting and growout

The basis of the lucrative South-sea pearl industry the silver- or gold-lipped pearl oyster, Pinctada maxima, begins life with the odds well stacked against its survival. As the oyster matures it generally begins its reproductive life as a male and may change, sex to female later in life.

The switch from male to female, and even back again, is triggered by environmental conditions. Excellent conditions in terms of food availability and water quality will favour development of females, whilst adverse conditions tend to favour males. In the wild the sex ratio of male to female is roughly equal in oysters larger than 15 cm (greater than two years of age); however, on the farm there are considerably more males than females — probably as a result of regular disturbance during cleaning and other farm activities.

Pearl oysters spawn as a result of external stimuli such as rising water temperature or changes in salinity. In the hatchery spawning may be induced by increasing the water temperature in the holding tanks. Generally, males spawn before females. The release of sperm into the water stimulates spawning in ‘ripe’ females. Unfertilised eggs or ova are released in enormous numbers, a single female may release up to 50 million eggs! The eggs are initially pear shaped and become spherical following fertilisation. Fertilisation in the wild is haphazard and will only occur where sperm and egg are united. In the vast bays and oceans that silver-lipped pearl oysters populate, the chances of successful fertilisation are small. In the hatchery fertilisation can be controlled due to small water volume and the close proximity of spawning males and females.

The division of cells after fertilisation is rapid; and within 24 hours the newly developed larvae have a functional stomach and are able to swim. At this early stage they are called "D" or straight hinge larvae. A week later, the larvae begin to change shape and become more rounded. They are now at the umbo stage of life. At this time they are only 0.1 mm in size, but appear very much like a cockle or pipi when viewed under a microscope. At between 16 and 20 days of age they will develop two red pigment spots called ‘eye-spots’. These eye-spots are light sensitive. Within a few days the larvae will begin to develop a foot which is used to crawl snail-like on surfaces in order to search out appropriate place to settle. At this stage the larvae are called pediveligers and are about of 0.2 - 0.3 mm size. In the hatchery specially prepared rope panels, or collectors, are placed in the culture tanks to ‘catch’ settling larvae.

The first stage of settlement occurs when the pediveliger secretes hair like fibres (the byssus) from its fool The byssal fibres adhere to the surfaces of collectors or other objects in the water. Once firmly attached, the pediveligers will begin to metamorphose. This is a traumatic time and many of these larvae will not survive. During the three or four days following settlement larvae lose the ability to swim, and many of the organs that have served them during the early part of their lives are resorbed … and new organs, such as gills, rapidly develop. The larval shell takes on a new shape and growth is very rapid. With the development of its new shell, the ‘oyster’ is now called a plantigrade, and within a few days of settlement is already nearing a millimetre in size. The plantigrade stage only lasts a few days before the small oyster becomes a spat. The spat look much lace the adult oyster but come in a multitude of colours that include yellow, brown, black, green and white. A prominent feature of young spat are the ‘finger-like’ growth processes that they have along the edge of their shells. Over the next twelve months growth of spat is rapid and most oysters will have reached 10 cm sizes during their first year of life.

Between 18 months and two years the silver- or gold-lipped pearl oysters reach maturity - and the cycle of reproduction and growth can begin once more.

To date the greatest problem experienced with Australian-produced hatchery shell occurs when the spat are transferred from the hatchery to the ocean based pearl farms for growout.
To Australian producers, the downside of increasing reliability on hatchery-bred stock is:

Farmsites suitable for hatchery grow-out are limited.
The overall costs of utilising hatchery-bred stock are much greater.
Hatcheries must be licensed and all oysters must be inspected by government marine biologists before they are released for grow-out.
The financial return from hatchery-bred stock is much longer — >5 years for smaller pearls and up to 7 years for larger diameter pearls. Most of the additional cost comes from the 2 year grow-out period required before these oysters are ready for implantation.
The size of pearls recovered are usually smaller, as it is much more economic to utilise younger and therefore smaller oysters for implantation. The largest pearls obtained from hatchery-bred stock are 14 mm. Overall, sizes range from 10-11 mm.
The yield of round pearls from hatchery-bred stock is lower (~ 20 per cent) than with wild oysters, for most pearls are off-round and baroque. Drop shapes are a little more common than round pearls.
Yield of valuable colours (particularly golden pearls) tends to be reduced.

Although each Australian producer is allowed to seed 20,000 hatchery-bred oysters annually, only 70,000 shells were seeded during 1997. By 2002 this agreement will cease, with producers not establishing spat-breeding technology loosing their quota. It has been estimated that it will take a further 10 years before all Australian producers adopt hatchery-bred stock.

Factually, over the last decade many Australian pearl cultivators who have implemented hatchery shell development are experiencing mixed success with the development of the necessary ocean based husbandry skills essential for successful growout to maturity. Although the future use of hatchery bred shell undoubtedly will increase in the Australian industry, from its current low levels of utilisation

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