Like other bivalves, the pearl oyster is a filter feeder. Minute food organisms in the water, enter inside the mantle cavity along with water current passing though the narrow slit formed by the inwardly directed edges of the pallial lobes. These are carried towards the branchiae which act as fine strainers arresting every particle in the water current. The food particles collected thus are carried by the cilia to the crest of the branchial lamellae and from there they are directed by the labial palps into the mouth. The labial palps have the ability to reject unwanted materials like mud particles. Unicellular organisms including infusorians, foraminifers and radiolarians have been found in the stomach of pearl oyster. Minute embryos and larvae of various organisms, algal filaments, spicules of alcyonarians and sponges were also observed. The presence of diatoms, flagellates, larvae of lamellibranchs, gastropods, heteropods, crustacean nauplii, appendages and frustules of copepods, spicules of sponges and unidentified spores, algal filaments, detritus and sand particles were also noted in the stomachs and intestines of cultured P. fucata collected off the coast of Tuticorin. Oysters from natural beds were also found to contain the same organisms in their stomach and intestine.
The age and growth of pearl oysters in the Gulf of Mannar have been studied in detail. Observations made on cultured pearl oysters collected near Krusadai Island and at Tuticorin show that the oysters can grow to a height of about 35–45 mm at the end of one year, 50–55 mm at the end of the second year, 55–60 mm at the end of the third year, 60–65 mm at the end of the fourth year and 65–70 mm at the end of the fifth year. The weight of the oysters was 10, 30, 45, 60 and 70 g at the end of the first, second, third, fourth and fifth years, respectively. Tracing the growth history of P. fucata produced in the hatchery and grown in the farm at Tuticorin Harbour during 1983 revealed that the species attains a modal size of 47.0 mm at the end of first year, 64.5 mm at the end of the second year and 75.0 mm at the end of the third year. The corresponding weights at ages 1 to 3 years were 8.3, 31.6 and 45.4 g, respectively.
The pearl oysters have been estimated to have a longevity of 5–5.5 years in natural beds, but have been observed to live up to seven years when reared in the farm.
In pearl oysters, the sexes are separate although hermaphrodite conditions have been observed in some individuals. Change of sex takes place in some oyster towards the end of spawning. Based on the external appearance, microscopic examination of smears and histological studies, five developmental stages have been distinguished in the gonads of P. fucata off Tuticorin coast. The five stages of sexual maturity described below are based on the gonad development in female oysters.
- Stage 1: Inactive/spent/resting
The gonad is completely shrunken and translucent. In some cases it is pale orange in colour. Large vacuolated yellow (fat) cells are seen in the interfollicular spaces. The sex at this stage can hardly be distinguished.
- Stage 2: Developing/maturing
The transparent nature of the resting gonad is lost and it becomes distinguished from other visceral masses. Gametogenic materials begin to appear in the gonad. As the stage advances, the gonad begins to branch along the posterior side of the retractor muscle and advances to the anterio-dorsal region. The gametes begin to proliferate along the follicle wall. In advanced stages, the inter-follicular spaces become reduced and the lumen of the follicle may contain some free oocytes. The majority of the oocytes are irregular in shape and the germinal vesicle (nucleus) is not distinctly seen. The average size of the oocytes is 60.0 × 47.5 μm and the germinal vesicle, if present, is 20.0 μm.
- Stage 3: Mature
The gonad spreads on to most of the visceral tissues. It is mostly yellowish cream. The lumen of the follicle is filled with free oocytes. Some of them are attached to the follicular wall by means of slender stalks. The majority of the oocytes are pyriform in shape. The average size of the oocyte is 68.0 x 50 μm with a well defined germinal vesicle. The mean diameter of the nucleus is 25 μm.
- Stage 4: Partially spawned
The gonads become loose in consistency and the visceral epithelium becomes dull. The follicles shrink with the reduction of gametes in the lumen. The oocytes are free and found along the follicular wall. Most of the oocytes are spherical and nucleated. The average size of the oocyte is 51.7 μm.
- Stage 5: Spent
The gonads shrink further with a few left over gametes in the lumen of the follicles. Ruptured follicles are seen in some cases and the lumen sometimes contains ruptured cells. Oocytes, if present are few and spherical. The average size of the oocytes is 54.4 μm. The description of the spent stages applies to the oysters which have recently undergone oogenesis. Otherwise they transform to the spent resting stage quickly.
Males show the same pattern of reproductive activity. However, in stages 2 and 3, the colour of the gonad is pale cream. In other stages of gametogenesis, the gonads of males and females appear similar when observed externally.
Pinctada fucata from the Gulf of Mannar has two peak spawning seasons in a year: June-September and November-December, coinciding with the southwest and northeast monsoons, respectively. A slight rise in water temperature may be considered as the stimulating factor for the onset of the gametogenic cycle and a slight reduction in water temperature stimulates the oysters to spawn. However a few inactive, maturing and ripe gonads are present in almost all of the months of the year. Individual oysters spawn more than once in the same spawning season as the gonads are not emptied at one stretch.
Pearl oysters are found from the low tide level to depths of about 75 m, therefore they can adapt and live in varying environmental conditions within this range. Environmental factors such as bottom topography, wind, waves, water movement, current, light, temperature, salinity, dissolved oxygen, pH, nutrient salts, and primary production play a crucial role in the settlement, growth and reproductive pattern of oysters, both in the natural beds and farms. In the Gulf of Mannar, the pearl oysters live on rocky or other hard substrata which lie roughly in a line parallel with and at a distance of 10–16 km from the coast. A rich fauna, comprising members of various groups like sponges, hydroids, polychaetes, lamellibranchs, amphipods, decapods, echinoderms, fishes etc. are usually associated with pearl oyster beds. The oysters obtained from the beds are successfully reared in shallow coastal waters with depths ranging from 4–8 m, where the sea does not become rough. In farming the pearl oysters, the preferred depth of culture is about 10 m where silting should be minimal. Unlike in Japan, the variation in temperature and salinity is not much pronounced in the Gulf of Mannar. The temperature of seawater in the natural beds varies from 27.0 °C (January) to 32.5 °C (May) whereas in the oyster farm at Tuticorin, it fluctuates between 24.0–32.5 °C. Similarly, salinity in the natural beds fluctuates between 27.4 ‰ and 35.90 ‰ with an annual range of 8.5 ‰. The salinity values in the oyster culture site at Veppalodai, ranged between 32.15 ‰ and 33.50 ‰ during 1974–76. An unusual dilution of seawater to 15.69 ‰ for short durations at the Veppalodai farm in November 1977 did not affect the oysters. If salinity level falls below 15 ‰, and if such condition is prolonged, it may lead to mortality. This may happen during unusual heavy rain and heavy discharge of fresh water from rivers in the vicinity. It is well known that the benthic ecology of the pearl culture grounds plays a vital role in the rate of production as well as quality of pearls. Rocky or gravelly bottoms are more suitable. A high amount of silt in the farm water may affect the filtration efficiency of pearl oysters. In Japan it has been observed that some culture grounds yielded pearls of good quality, while others did not. Some grounds yielded pink or white pearls while others produced only yellow and golden pearls. Repeated culture on the same ground sometimes affects the quality of pearls. Areas rich in phytoplankton which is consumed by oyster, are good but there should not be noxious blooms. A mild current of two knots per hour is necessary not only as a source of oxygen-rich seawater, but also to bring in fresh plankton as well as for the removal of metabolic products, and faecal matter. If the water current is strong, the formation of the nacreous layer is faster, but the quality of pearls is inferior. The rich nutrients discharged by the rivers into the sea increase the productivity of the water. The oysters can also directly remove the organic matter and calcium dissolved in the water. If oyster culture farms are located in places such as the vicinity of a river mouth, which is often exposed to prolonged dilution of seawater due to flooding, oyster growth will be affected.