CDS (II) 2020 General Knowledge · Question 93 of 120

Correct — D, Oyster. A pearl is made by the animal itself, not found lying in the shell. When a grain of sand, a parasite or a fragment of tissue lodges between the mantle and the shell, the mollusc walls the intruder off: the mantle forms a pearl sac around it and lays down coat after coat of the same material it uses to line its own shell — calcium carbonate as aragonite, cemented by a protein called conchiolin. That layered material is nacre, or mother-of-pearl, and enough concentric coats of it make a pearl. Only an animal that secretes nacre can do this, which rules out every option except the bivalve. Commercially the work is done by pearl oysters of the genus Pinctada in the sea and by freshwater mussels in rivers and lakes; India's classical pearl fishery was in the Gulf of Mannar off the Tamil Nadu coast.

• (a)Prawn — A prawn is a crustacean with a chitinous exoskeleton that it moults as it grows. It has no mantle and secretes no nacre, so it has no mechanism for laying concentric coats around an irritant.
• (b)Pila — The tempting one, because Pila — the apple snail — really is a mollusc with a shell. But it is a gastropod whose shell is not lined with the thick nacre that pearl-making needs; gastropods almost never yield gem pearls, and none of commercial value comes from Pila.
• (c)Tuna — Tuna is a bony marine fish. Fish have no shell and no shell-secreting mantle; the only hard mineral deposits they carry are bones, scales and ear stones.

Pearls are a product of the mollusc mantle, the sheet of tissue that lines the shell and builds it. Nacre is calcium carbonate in the aragonite form laid down in microscopic plates and glued by conchiolin; the same layering that gives the inside of the shell its sheen gives the pearl its lustre. Cultured pearls are made by deliberately inserting a bead nucleus with a piece of mantle tissue and returning the animal to the water for a few years, a technique perfected in Japan by Kokichi Mikimoto in the early twentieth century.

The four options are chosen from four different animal groups — a crustacean, a gastropod mollusc, a fish and a bivalve mollusc — so the item is really asking which body plan can make nacre. Reason from the mechanism and Pila stops being tempting: having a shell is not enough, the shell has to be nacre-lined and the mantle has to be able to wrap an intruder. One nuance worth carrying: the pearl oyster of family Pteriidae is not the same animal as the edible oyster of family Ostreidae, and freshwater mussels are also major pearl producers. The stem's 'oyster' is the everyday label for the pearl-bearing bivalve, and that is how the key reads it.

• A pearl is concentric layers of nacre — aragonite crystals of calcium carbonate bound by the protein conchiolin — secreted by the mollusc's mantle around an irritant.
• Commercial marine pearls come from pearl oysters of the genus Pinctada, family Pteriidae; freshwater pearls come from mussels.
• Pearl oysters are not the same family as edible oysters, which are Ostreidae.
• The Gulf of Mannar between India and Sri Lanka is the classical Indian pearl-fishing ground, worked since Sangam times.
• Cultured pearling — inserting a nucleus and mantle tissue and returning the animal to the water — was commercialised by Kokichi Mikimoto in Japan from the 1890s onwards.

• Choosing Pila because it is a mollusc with a shell — having a shell is not the same as having a nacre-lined shell.
• Assuming pearls are only marine; freshwater mussels are a major source.
• Equating the pearl oyster with the oyster served as food — different families.

As a one-line source-of-product item, or inside a mollusc question testing which animal is a bivalve, a filter feeder or a shell secretor.

• practice — not a real PYQ

  The material of which a natural pearl is chiefly composed is

  • (a)silica
  • (b)calcium carbonate
  • (c)calcium phosphate
  • (d)chitin

  Answer(b) calcium carbonate — deposited as aragonite in concentric layers with the protein conchiolin, the combination known as nacre.

• practice — not a real PYQ

  Which one of the following coastal stretches of India is historically associated with pearl fishing?

  • (a)Gulf of Mannar
  • (b)Gulf of Khambhat
  • (c)Sundarbans coast
  • (d)Konkan coast

  Answer(a) Gulf of Mannar — the classical Indian pearl-fishing ground, between the Tamil Nadu coast and Sri Lanka.