biology question #955



Xinyi Chong, a 12 year old female from Beijing asks on October 10, 2002,

Q:

Do shells have nutrients in them? If so, what actually uses those nutrients?

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the answer

Peter Fankboner answered on October 22, 2002, A:

Molluscan shell is made up of two principal components: flattened, polygonal, calcium carbonate crystals (which may be organized as aragonite or calcite crystals) which are held together by a tan to dark brown organic mortar called conchiolin. Conchiolin is a complex protein which is secreted by the mollusc's outer epithelium (the mantle) to form chambers which hold and bond to the crystals of calcium carbonate which are also secreted by the mantle. Conchiolin is very resistant to chemical breakdown, but there are ocean dwelling gastropod (snail) molluscs called the Muricids and Naticids which erode holes through the shells of other molluscs during their feeding. Both of these gastropod taxa employ enzymes secreted by soft tissue boring organs to attack the conchiolin component of shell material and thus gain entry into the bivalves (clams, mussels, and oysters) and scaphopod (tooth shells) molluscs upon which they prey. The Naticids are represented on the west coast of North America by the Moon Snail Polinices lewisii which gains entry to its prey by placing a secretory structure call the Auxillary Boring Organ (ABO), which is located on the bottom of its massive foot, against the umbone, that portion of a clam's shell which covers the clam's visceral organs. The ABO secretes enzymes which attacks the conchiolin of the prey's shell, and by so doing causes the shell to fall apart much like the bricks in an old wall. Next, the ABO is withdrawn, and the snail ingests the calcium carbonate crystals freed by the breakdown of conchiolin mortar. The ABO is reapplied, and the boring process is repeated until a hole is made through the prey clam's shell. Next, P. lewisii inserts its feeding proboscis through the hole. The proboscis bares a ribbon of radular teeth and these tearing structures are used to feed upon the clam's tissues. Since the crystals of calcium and dissolved conchiolin are ingested and pass through the gut of P. lewisii, it could be argued that they comprise a small nutritional benefit to this species of predatory snail.

Molluscan shell material is, however, very important to man. Granulated oyster shell is fed to chicken's as a calcium supplement to help form the shells of their eggs. Edible oyster shell, and pearl oyster shell is formulated into tablets used as calcium supplements for man. In past, the Mikimoto Pearl Company in Japan has manufactured calcium supplement tablets produced directly from grinding up lesser grades of pearls secreted by the pearl oyster. Pearls have an organization and chemical makeup identical to the shell from the mollusc that secrets them.

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