Environmental Factors

Pearl oysters in their natural environment experience the simultaneous effects of a broad range of environmental factors. Some of these aspects, such as wavelength of ambient light may never have a major influence, although it seems that blue light promotes nacre secretion on cultured pearls. Other factors, both biological and physical, may continually influence the pearl oyster, such as water temperature, while others may be significant only at particular times, such as salinity. Additionally, the effects of environmental elements on a pearl oyster vary according to the developmental stage.

Temperature

Temperature is the main influence on distribution. The temperature range of Pinctada margaritifera is from 19°C to 32° C. Cold water reduces the heart rate, slows growth rates, hinders reproductive development and renders pearl oysters more vulnerable to infection. Temperature is also the most important factor relating to gonad development and spawning seasonality. The temperature also determines the rate of deposition of nacre (Cahn 1949) and therefore limits the pearl culture sites to areas within the optimum temperature ranges.

Pinctada maragritifera, like other oysters is an ectotherm, and so their body temperature is variable and similar to the temperature of its surrounding environment. Within their normal ranges, metabolic rate is greatly influences by temperature. Studies have shown that there is usually an optimum temperature or narrow temperature range, at which there is maximum metabolic rate, growth rate and survival (Yukihira et al. 2000). The optimum temperature for pearl oysters is roughly around 28°C. Metabolic rate and associated biochemical reaction rates increase with temperature until a maximum is reached, at which point they begin to decrease. This decrease is due to the opposing function of enzyme denaturation rates increasing with temperature.

Salinity

As in other bivalves, water constitutes a high proportion of the soft tissues of Pinctada margaritifera. They are osmoconformers and thus maintain their internal salinity so that it is always in equilibrium with their surrounding environment. While unable to regulate their body fluid osmolality, they use sustained valve closure to buffer rapid salinity changes. Pinctada margaritifera has a preference for full salinity seawater around 35.069 - 35.290pps but can tolerate a wide range of salinities. This is a common trait in organisms that inhibit the intertidal zone and for species that occur in coastal waters bordering large landmasses in at least part of their geographic distribution. There is potential lowering of salinity during periods of rainfall and runoff from rivers and estuaries in these coastal waters. Function relating to metabolic rate however, declines in low salinities and hypersaline conditions (Southgate & Lucas 2008).