Pinctada radiata

Naming Problems

• Ever since Shohei Shirai's book "Pearls and Pearl Oysters of the World" (1994) came out, many species that were once believed to be different have been confirmed as actually belonging to one species. Such is the case for the Pinctada imbricata species, that now encompasses species that were once considered unique to one area. Under this name we now have the former species of "Akoya" pearl oysters: P. fucata, P. martensii, P. radiata, P. anomioides, P. vulgaris, and several others. Shirai mentions that the problem was caused by having small samples of shells, and when larger samples are used you can really see the species continuum.
• Furthermore, Southgate's book ("The Pearl Oyster") calls the Akoya pearl oyster the Pinctada fucata/martensii/radiata/imbricata species complex and -just like Shirai- considers this "group" as the most widely distributed and cosmopolitan pearl oyster species in the world, being distributed from the Mediterranean Sea, Persian Gulf, Red Sea, South Africa's Cape, India, Vietnam, China, Indonesia, Japan, Korea, Palau, Hawaii, Australia, the Gulf of Mexico and Caribbean Sea and all the way down to Brazil.

Distinguishing Characteristics

• Pinctada radiata is most notorious as one of the greatest sources of natural pearls.
• Historically, P. radiata pearls from the Ceylon region constituted the majority of pearls available. This variety has an almost entirely white nacre that produces silver and yellow pearls. The lip of the pale-yellow shells is slightly pinkish.
• Seven-eight brownish radial bands encircle the shell. The Persian variety is larger and darker with a reddish lip.

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Habitat and Ecology

1. P. radiata is characteristic of hard surfaces and has been discovered attached to rocks and hard sandy plateaus, 10-20 meters deep. The oysters attach in clusters of several to many through byssal threads (tough organic fibers created by the oysters).
2. Over years assemblages form consisting of dead pearl oyster shells, worm tubes, calcareous algal clumps, and dead coral. These structures act as a landing dock for the veliger (free-swimming) larvae to metamorphose into juvenile pearl oysters that will maintain the reproductive cycle.
3. Storms and predators are major threats to the continued health of this thriving habitat.

The Pearls of Pinctada radiata
Pinctada radiata is mostly known for producing seed pearls and some medium sized pearls. A seed pearl is a commercial term describing a pearl less than 2mm in diameter (<.25 grain). Due to the thinness of their shells, P. radiata provides the cheapest Mother of Pearl. Until the 1920's, P. radiata (mainly those from the Ceylon region of Sri Lanka) fueled the world's demand for both mother of pearl and pearls.

The Ceylon oyster is Akoya-like but is not used in pearl culture in the way that Akoya pearl oysters are. This is due to a few factors:

• Since the 1930s, Petroleum dominates the economics of countries in the Persian Gulf and Red Sea, which account for the bulk of the P. radiata shells. The discovery of oil in the 1930s ended pearl harvesting and diving is only a pastime.
• Bahrain, Sri Lanka, and India have each tried to develop a cultured pearl industry, but tradition favors natural pearls
• Through years of aquaculture, the Japanese have created farms of the fittest Akoya oysters to culture favorable pearls: large, round, lustrous and milky white.

Pearls from P. radiata were historically used for decoration and in jewelry. Most of the pearls were exported from Bahrein and then shipped to London. The shells were assigned symbolic meaning, and some have been recovered from ancient tombs of the Hellenistic-Roman period.

Originally thought to be P. radiata, P. anomioides is now designated a separate species.

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