337
viable count
vores (comprising members of many phyla) and carnivores, including
pelagic fish. With echosounders, but also with plankton nets that can sample different depth layers, it has been demonstrated that planktonic animals
move upwards over distances of sometimes hundreds of meters at sunset, to
spend the night in surface layers. At sunrise, the opposite vertical movement
brings them back to the depth, where they stay during the day (-7scattering
layer). These vertical movements must require considerable energy, and must
therefore have a clear function. However, it has not yet been possible to
determine which of a number of hypothetical explanations is the correct
one, though the changes in light intensity in the morning and evening are
generally accepted as the stimulus governing this behavior. Explanations
suggested are, for instance, that the animals can reduce their energy requirements by staying part of the day in colder water layers in the depth, or that
vertical migration leads to an optimal use of phytoplankton resources which
can grow during the day unhampered by grazing. It is often thought that
evading visually hunting predators in surface waters during the day might
be a functional reason as well.
vertical mixing of water is caused by wind at the surface, by shear at the bottom
or between different layers, and by convection at the surface. In shallow
waters, wind and (tidal) current are sufficient to keep the water column well
mixed, i.e., homogeneous in temperature and salinity. In deep waters, a
wind-induced mixed layer is found over only a fraction of the total depth,
the remainder being stably stratified. In temperate regions, surface cooling
in autumn and winter renders the surface layer gravitationally unstable
leading to convective overturning, which deepens the upper mixed layer,
down to the permanent thermocline.
vestigial organ organ, of which the size, structure and function have diminished
and simplified in the course of evolution until only a trace remains. Such
vestiges may still have an important function in organizing embryonic
development, even if their adult function has disappeared.
Vestimentifera class of the phylum Pogonophora. These marine tube-living worms
lack a digestive system, but obtain their nutriments through their gills. They
contain an enormous number of symbiotic (-7symbiosis) sulfur-oxidizing,
carbon-fixing bacteria, which can contribute up to half the weight of the
animal, and supply most of the worm's metabolic needs. They live near cold
submarine seepage areas or hydrothermal vents. Example: Riftia, which can
grow over 1 m long.
viable count is the estimation of the number of live cells in a microbial population and thus differing from the total count, i.e., the total amount of bacteria seen by microscopic methods, in which also dead cells are included.
viable count
vores (comprising members of many phyla) and carnivores, including
pelagic fish. With echosounders, but also with plankton nets that can sample different depth layers, it has been demonstrated that planktonic animals
move upwards over distances of sometimes hundreds of meters at sunset, to
spend the night in surface layers. At sunrise, the opposite vertical movement
brings them back to the depth, where they stay during the day (-7scattering
layer). These vertical movements must require considerable energy, and must
therefore have a clear function. However, it has not yet been possible to
determine which of a number of hypothetical explanations is the correct
one, though the changes in light intensity in the morning and evening are
generally accepted as the stimulus governing this behavior. Explanations
suggested are, for instance, that the animals can reduce their energy requirements by staying part of the day in colder water layers in the depth, or that
vertical migration leads to an optimal use of phytoplankton resources which
can grow during the day unhampered by grazing. It is often thought that
evading visually hunting predators in surface waters during the day might
be a functional reason as well.
vertical mixing of water is caused by wind at the surface, by shear at the bottom
or between different layers, and by convection at the surface. In shallow
waters, wind and (tidal) current are sufficient to keep the water column well
mixed, i.e., homogeneous in temperature and salinity. In deep waters, a
wind-induced mixed layer is found over only a fraction of the total depth,
the remainder being stably stratified. In temperate regions, surface cooling
in autumn and winter renders the surface layer gravitationally unstable
leading to convective overturning, which deepens the upper mixed layer,
down to the permanent thermocline.
vestigial organ organ, of which the size, structure and function have diminished
and simplified in the course of evolution until only a trace remains. Such
vestiges may still have an important function in organizing embryonic
development, even if their adult function has disappeared.
Vestimentifera class of the phylum Pogonophora. These marine tube-living worms
lack a digestive system, but obtain their nutriments through their gills. They
contain an enormous number of symbiotic (-7symbiosis) sulfur-oxidizing,
carbon-fixing bacteria, which can contribute up to half the weight of the
animal, and supply most of the worm's metabolic needs. They live near cold
submarine seepage areas or hydrothermal vents. Example: Riftia, which can
grow over 1 m long.
viable count is the estimation of the number of live cells in a microbial population and thus differing from the total count, i.e., the total amount of bacteria seen by microscopic methods, in which also dead cells are included.