As
first described by Claude Bernard in 1854, for animals to survive, they must
maintain a constant internal environment, including a balance between the gain
and loss of water and salts. If excessive water is gained, cells swell and
burst; if water loss is too great, cells shrivel up and die. The processes
animals use to maintain this balance, osmoregulation, involve two different
approaches. Osmoconformers include most marine invertebrates whose internal
salt and water concentrations are equivalent to their external environment.
They “go with the flow” and have no need to actively control their salt and
water balance. By contrast, many marine vertebrates, such as fish, have
internal salt concentrations different from their external aquatic surroundings
and must actively control their salt concentration; they are osmoregulators.
Freshwater
fish live in water that is far more dilute than their body fluids and face the
problem of salt loss and excessive water gain. They deal with this by drinking
almost no water and excreting large volumes of highly dilute urine. Salt stores
are built up by eating and by the active uptake of chloride ions across the
gills into the body, followed by sodium ions. Marine fish are faced with the
reverse problem—living in water that is far more concentrated than their body
fluids—and, therefore, they face the loss of body water and the excessive
movement of chloride and sodium ions into their body. Their strategy involves
drinking large volumes of water and actively transporting chloride ions across
the gills and out of the body; once again, sodium ions follow. In 1930, Homer
Smith, working at New York University and the Mt. Desert Island Biological
Laboratory, determined many patterns of marine fish osmoregulation.
The
survival challenges are even greater for anadromous salmon, which, after
spending most of their lives in the ocean, then breed in freshwater. After
using the osmoregulatory strategies noted above, their acclimation from
freshwater to saltwater and back again is not immediate. Salmon remain at the
interphase of freshwater and saltwater for days to weeks before moving forward.
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