Oysters, Beds, and Pearls Edible oysters, such as Ostrea virginica, are plentiful on European and North American coasts. Oysters were a major food of coastal Native American peoples. Colonists ate them too, and natural oyster supplies were severely depleted by the late 1800's. Cultivated oyster beds were then developed to enhance nature's yield. In the twenty-first century, many food oysters come from cultivated beds. The beds begin with the millions of eggs spawned yearly by every female oyster. They are started by finding places holding many newly hatched oyster larvae. Bricks, flowerpots, old shells, and other objects are placed on shallow sea bottoms beneath them. Then, a week after hatching, larvae produce tiny shells and, weighed down, drop to the sea bed. They attach to the objects and remain attached for the rest of their lives. After attachment, oyster-laden objects are dredged up and moved to the new beds. Some oysters are invaded by foreign bodies that lodge between mantle and shell. These irritants cause the mollusks to produce mother-of-pearl to seal them off. In time, an irritant encased inside many layers of mother-of-pearl becomes a pearl. Most pearls are irregularly shaped and much less valuable than a few perfectly spherical pearls used in pearl necklaces. Pearl color depends on oyster diet and bed temperature. The pearls sought most are white, rose, steel blue, or black. They arise naturally in small numbers, or are cultured by the insertion of mother-of-pearl beads into oysters.
Salt Loss Preformed water is present in any food. Even the driest seeds contain a small amount of water. The nutrients also always include salts. The presence of great quantities of salts may require urinary loss of water in excess of the preformed water found in the food. Although feces may appear to be solid, they contain some water that was not absorbed in the gut. The presence of salts and other solutes in the digesta may also draw water from the hypoosmotic body fluids into the gut. One of the reasons that humans cannot drink seawater, in fact, is that the magnesium ions in ocean water increase the permeability of the gut and increase water loss, because the seawater is hyperosmotic to body fluids.More water is lost than can be gained. Salts can be lost from the body by means other than urine formation and defecation. Marine reptiles and birds have salt glands located on the head. Since neither can produce hyperosmotic urine, these glands allow the elimination of salt with aminimumloss of water as the secretionmay be four to five times as concentrated as body fluids. The cloaca of birds and the rectal glands of sharks also have the capacity to excrete salts. One of the most fascinating mechanisms of osmoregulation is found in elasmobranch fish- the sharks, skates, and rays. Their body fluids are hyperosmotic but hypoionic to seawater. Blood salt concentrations are below those of seawater. Excess osmotic pressure is supplied by two molecules: urea and trimethylaminoxide (TMAO). Urea is toxic to most organs, but some organs resist its deleterious effects. Others would be harmed but are apparently protected by the TMAO. Retention of both urea and TMAO minimizes enzymatic disturbances by urea and allows the elasmobranchs to avoid the salt gain associated with hypoosmotic body fluids. Organisms exposed to environmental variations have two choices: they can maintain internal constancy or homeostasis at the expense of metabolic energy, or they can allow their internal conditions to follow that of the environment. Organisms that maintain their internal osmotic pressure despite changes in external osmotic pressure are called osmoregulators. These euryosmotic organisms are protected from environmental changes. Their metabolism can continue to function, but much of the energy will be used to maintain their body fluids at the appropriate osmotic pressure. Organisms that allow their osmotic pressure to follow that of the environment are called osmoconformers. These poikilosmotic organisms often have a limited tolerance for such changes. They are stenohaline. They may be less vigorous at salinities other than their optimal levels. The adults of such groups (for example, mollusks such as oysters and mussels) may be found in salinity extremes not tolerated by their young. These populations must be maintained by immigration of young spawned in more favorable salinity conditions.
