318
THE BIOLOGY O F EUPHAUSIIDS
It has become clear in recent years that many planktonic organisms
have their main centres of distribution contained within one specific
water mass. A water mass is a body of water, in a coastal region or in
the open ocean, which can be identified over a geographical area, often
extensive, by characteristic relationships of temperature to salinity.
There are boundary areas or transition zones between water masses and
these zones are frequently characterized by sharp horizontal gradients
of temperature. It is, therefore, possible t o divide the oceans latitudinally into the following ecological zones, north to south ; arctic,
subarctic, transition zone, central, equatorial, central, transition zone,
subantarctic, antarctic. This division is not completely regular ; for
example, in the Pacific a transition zone exists between subarctic water
forming the California Current and equatorial water. The distribution
of coastal neritic species is often localized and hydrographic conditions
in these small geographical areas must be such as to enable maintenance
of these populations and prevent them being dispersed to unsuitable
areas. Maintenance of populations of an oceanic species restricted to a
water mass must depend on a gyral system-there is an eastern and a
western gyral system in the northern Central Pacific region (Fig. 120)
-or on a current/counter current system-such as is found in Pacific
equatorial regions (Fig. 120)-which maintain the populations in that
area and prevent fatal dispersion into adjacent unsuitable water masses.
The ocean is a three-dimensional fluid medium and all euphausiids,
except the bathypelagic species, live in the upper 1000 m and most
frequently in the upper 700 m or so of the water column. The water
masses extend down to depths of 200-1 000 m. I n tropical and subtropical seas, the water above a depth of about 700 m has a temperature
of more than 10°C and the majority of euphausiids living in these areas
live within this depth zone. Species in subarctic or subantarctic regions
live a t shallower depths than those in subtropical or tropical regions
and presumably do not penetrate deeper because of the lower temperatures prevailing in the deeper layers. I n equatorial regions of the
Pacific and Atlantic, the warm surface layer only extends down to
about 300 m and it is interesting that some of the equatorial species
are restricted to depths between the surface and 300 m. Recourse to
Fig. 119 shows that there are two deep basins of warm water in the
western and central areas of the Pacific and one in the northwestern
central area of the Atlantic ; temperatures in the northwestern Atlantic
basin at 400 m depth are higher than the corresponding temperatures
in the Pacific basins. The effects of temperature regimes a t different
depths on the vertical distribution of these animals have already been
considered (Chapter 5) and it is shown that species, maintaining them-
THE BIOLOGY O F EUPHAUSIIDS
It has become clear in recent years that many planktonic organisms
have their main centres of distribution contained within one specific
water mass. A water mass is a body of water, in a coastal region or in
the open ocean, which can be identified over a geographical area, often
extensive, by characteristic relationships of temperature to salinity.
There are boundary areas or transition zones between water masses and
these zones are frequently characterized by sharp horizontal gradients
of temperature. It is, therefore, possible t o divide the oceans latitudinally into the following ecological zones, north to south ; arctic,
subarctic, transition zone, central, equatorial, central, transition zone,
subantarctic, antarctic. This division is not completely regular ; for
example, in the Pacific a transition zone exists between subarctic water
forming the California Current and equatorial water. The distribution
of coastal neritic species is often localized and hydrographic conditions
in these small geographical areas must be such as to enable maintenance
of these populations and prevent them being dispersed to unsuitable
areas. Maintenance of populations of an oceanic species restricted to a
water mass must depend on a gyral system-there is an eastern and a
western gyral system in the northern Central Pacific region (Fig. 120)
-or on a current/counter current system-such as is found in Pacific
equatorial regions (Fig. 120)-which maintain the populations in that
area and prevent fatal dispersion into adjacent unsuitable water masses.
The ocean is a three-dimensional fluid medium and all euphausiids,
except the bathypelagic species, live in the upper 1000 m and most
frequently in the upper 700 m or so of the water column. The water
masses extend down to depths of 200-1 000 m. I n tropical and subtropical seas, the water above a depth of about 700 m has a temperature
of more than 10°C and the majority of euphausiids living in these areas
live within this depth zone. Species in subarctic or subantarctic regions
live a t shallower depths than those in subtropical or tropical regions
and presumably do not penetrate deeper because of the lower temperatures prevailing in the deeper layers. I n equatorial regions of the
Pacific and Atlantic, the warm surface layer only extends down to
about 300 m and it is interesting that some of the equatorial species
are restricted to depths between the surface and 300 m. Recourse to
Fig. 119 shows that there are two deep basins of warm water in the
western and central areas of the Pacific and one in the northwestern
central area of the Atlantic ; temperatures in the northwestern Atlantic
basin at 400 m depth are higher than the corresponding temperatures
in the Pacific basins. The effects of temperature regimes a t different
depths on the vertical distribution of these animals have already been
considered (Chapter 5) and it is shown that species, maintaining them-