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2.1 Taxonomy

The true pearl oyster belongs to the genus Pinctada (Roding) under the family Pteriidae, order Dysodonta. Members belonging to the Pteriidae family are characterized by a straight hinge with 1–2 small tooth-like thickening, a cavity below the anterior angle for the byssus, and usually a scaly surface of the outer shell valves. The family includes the pearl oysters belonging to the genus Pinctada and the winged oyster shells of the Pteria genus. In Pteria spp. the shell width is much longer than the height and the hinge angle is prominent and pronounced.

In Pinctada spp. the hinge is rather long and straight, the long axis of the shell is at a right angle to the hinge, the left valve is slightly deeper than the right and there is a byssal notch on each valve at the base of the anterior ear.

Six species of pearl oysters, Pinctada fucata (Gould), P. margaritifera (Linnaeus), P. chemnitzii (Philippi), P. sugillata (Reeve), P. anomioides (Reeve) and P. atropurpurea (Dunker) occur along the Indian coasts. Their morphological characteristics are as follows:

Pinctada fucata (Gould)

The hinge is fairly long and its ratio to the broadest width of the shell is about 0.85 and that to the dorsoventral measurement is about 0.76. The left valve is deeper than the right. Hinge teeth are present in both valves, one each at the anterior and posterior ends of the ligament. The anterior ear is larger than in the other species, and the byssal notch, at the junction of the body of the shell and the ear, is slit-like. The posterior ear is fairly well developed. The outer surface of the shell valves is reddish or yellowish-brown with radiating rays of lighter colour. The nacreous layer is thick and has a bright golden-yellow metallic lustre (Plate I A).

Pinctada margaritifera (Linnaeus)

The hinge is shorter than the width of the shell and has no teeth. The anterior border of the shell extends in front of the anterior lobe. The byssal notch is broad. The anterior ear is well developed while the posterior ear and sinus are absent. The posterior end of the shell meets the hinge almost at a right angle. Shell valves are moderately convex. Externally, the shell is dark grayish-brown with radially disposed white spots. The nacreous layer is iridescent with a silvery lustre except distally where it is black. This pearl oyster is also known as the Black-lip pearl oyster due to the dark marginal colouration of the shell. The width of the nacreous region at the hinge is about 2/3 that of the broadest part of the valves (Plate I B).

Pinctada chemnitzii (Philippi)

The hinge is almost as long as the antero-posterior measurement of the valves. The posterior ear is well developed and the convexity of the valves is less than in P. fucata. The anterior and posterior hinge teeth are present, the former is small and rounded and the latter prominent and ridge-like. The posterior ear and the posterior sinus are well developed. The shell valves are yellowish externally with about four or more light brownish radial markings from the umbo to the margin of the shell. The growth lines of the shell are broad. The nacreous layer is thin and bright, while the non-nacreous layer is yellowish-brown (Plate I C).

Pinctada sugillata (Reeve)

The hinge is considerably shorter than the antero-posterior axis of the shell with a ratio of 1:1.3. The anteroposterior measurement is almost equal to the dorso-ventral measurement. The anterior ear in both valves is small and the byssal notch is a moderately wide slit. The anterior ears are slightly bent towards the right. The posterior ear and sinus are poorly developed. The convexity of the valves is not prominent, especially that of the right valve. The hinge teeth are small and the posterior one is slightly elongated. The shell valves are reddish-brown with six yellowish radial markings (Plate I D).

Pinctada anomioides (Reeve)

The hinge is shorter than the width of the broadest region of the antero-posterior axis of the shell with a ratio of 1:1.2–1.5. The hinge and dorso-ventral axis have a ratio of 1:1.4. Hinge teeth are absent or poorly developed. The anterior ear is moderately developed and the byssal notch at its base is deep. The posterior ear and sinus are absent. The shell valves are translucent and externally yellowish or grayish. Some shells have faint radial markings. The nacreous layer is slightly iridescent (Plate I E).


PLATE I. (A) Pinctada fucata and (B) Pinctada margaritifera.


PLATE I. Cont'd. (C) Pinctada chemnitzii and (D) Pinctada sugillata.

Pinctada atropurpurea (Dunker)

The shell is roundish and its hinge narrow. The valves are thin, translucent and moderately convex. A poorly developed anterior hinge tooth is present in some oysters. The shell valves are copper coloured (Plate I F).

2.2 Distribution

Pearl oysters of the genus Pinctada are widely distributed in the world. They occur in several seas of the tropical belt and in the sub-tropical region. Although a number of species of pearl oysters have been identified, only a few have been found to produce pearls of good quality and commercial value. Of these, P. maxima, P. margaritifera and P. fucata stand out. The gold/silver-lip pearl oyster P. maxima occurs along the north coast of Australia, Burma, Thailand, Indonesia, Philippines and Papua New Guinea at depths ranging from low tide level to 80 m. The black-lip pearl oyster, P. margaritifera is widely distributed in the Persian Gulf, Red Sea, Sudan, Papua New Guinea, Australia, French Polynesia, Indonesia, Andaman and Nicobar Islands, Southwestern part of the Indian Ocean, Japan and the Pacific Ocean. The occurrence of this species is sporadic along the coasts of mainland India. The pearl oyster P. fucata is distributed in the Red Sea, Persian Gulf, India, China, Korea, Japan, Venezuela and Western Pacific Ocean (Fig. 1).

In the Indian waters six species of pearl oysters occur but only P. fucata has contributed to the pearl fisheries in the Gulf of Mannar and Gulf of Kutch. In the Gulf of Mannar, the pearl oysters occur in large numbers on the submerged rocky or hard substrata known as paars. The paars lie at depths of 12 to 25 m off the Tuticorin coast along a stretch of 70 km. In the Palk Bay, P. fucata occurs sporadically on loose sandy substratum attached to submerged objects in littoral waters. In the Gulf of Kutch, the pearl oysters are found as stray individuals on the intertidal reefs known as khaddas. In the southwest coast of India at Vizhinjam, Kerala coast, large numbers of spat of P. fucata have been collected from mussel culture ropes. The blacklip pearl oyster, P. margaritifera is confined mostly to the Andaman Islands where it is common in some places. From Lakshadweep, settlement of spat of P. anomioides has been observed on the ridges of rocks and corals.


PLATE I. Cont'd. (E) Pinctada anomioides and (F) Pinctada atropurpurea.


FIGURE 1. World distribution of pearl oysters.

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