(via Wahroongai News, Volume 33, Number 8, August 1999) Joseph Taylor (Project Manager, Atlas Pacific Ltd) writes:
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 favor development of females, while adverse conditions tend to favor males. In the wild the sex ratio of male to female is roughly equal in oysters larger than 15cm (greater than two years of age); however, on the farm there considerably more males than males—probably as a result of regular disturbance during cleaning and other farm activities.
In southern Indonesia and Australia (e.g. Kupang and Broome) the breeding season commences in September and continues through to late April or early May. The pearl breeding period is October to February. At Alyui Bay, Waigeo, the season is extended and we have been able to induce spawning (the release of eggs and sperm) in mid-June, and spawning has been observed into July.
Pearl oysters spawn as 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. Unfertilized 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 fertilization. Fertilization 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 fertilization are small. In the hatchery fertilization can be controlled due to small water volume and the close proximity of spawning males and females.
The division of cells after fertilization 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 0.1mm 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’. The ‘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 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’ secrets 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 ‘pediveliger’ 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 ‘plantigrade’, and within a few days of settlement is already nearing a millimeter in size. The ‘plantigrade’ stage only lasts a few days before the small oyster becomes a ‘spat’. The ‘spat’ look much like the adult oyster but come in a multitude of colors that include yellow, brown, black, green and white. A prominent feature of young spat is ‘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 10cm sizes during their first year of life.
Between 18 months and two years the silver or gold-lipped pearl oyster reaches maturity… and the cycle of reproduction and growth begin once more.
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