CHAPTER 11
ECOLOGY OF DISTRIBUTION
Some species of euphausiids are widely distributed throughout the
oceans of the world while others seem to be restricted to very small
geographical areas. The factors which affect their vertical position in
the three-dimensional environment of the seas have already been
discussed (Chapter 5) and the characteristics of the geographical or
horizontal distributions of the species must now be examined. The
distribution of each species is briefly described and shown on the series
of maps in Chapter 3 and reference should be made to these throughout
this chapter. This discussion is concerned not only with the descriptive
aspects of the distribution of individual species but also with patterns
of distribution within genera, with factors limiting the extents of the
areas of distribution of species, with mechanisms for maintenance of
populations occurring in small geographical areas, with the relationships, in the horizontal and vertical planes, of the different species living
in the same sea area, and with " patchiness " and '' swarming " of
individuals and populations within the general area of occurrence of a
species.
First, however, the general topographical and other characteristics
of the oceans which will influence the distribution of the different
species must be examined and borne in mind when reading the rest of
this chapter, The areas, volumes and mean depths of the different
oceans and seas are shown in Table XXXIII and the areas have been
broadly divided into the areas of the different depth zones in Table
XXXIV. There are only two extensive areas of shallower water, mostly
less than 2 000 m depth (Fig. 118) ; one is in the North Atlantic north
of about 50"N, the other extends from the Bering Sea southwards via
the Sea of Okhotsk, Sea of Japan, East and South China Seas, East
Indian Archipelago, to Eastern Australia. The shallower areas in the
Pacific and Indian Oceans have depths between 2000 and 4000m,
as have most of the shallower areas in the tropical and subtropical
regions of the Atlantic. The isotherms a t depths of 200 and 400 m are
shown in Fig. 119; it is temperatures a t these depths rather than at
the surface which influence the distribution of euphausiids. Finally,
the main current systems of the oceans are illustrated in Fig. 120.
313
ECOLOGY OF DISTRIBUTION
Some species of euphausiids are widely distributed throughout the
oceans of the world while others seem to be restricted to very small
geographical areas. The factors which affect their vertical position in
the three-dimensional environment of the seas have already been
discussed (Chapter 5) and the characteristics of the geographical or
horizontal distributions of the species must now be examined. The
distribution of each species is briefly described and shown on the series
of maps in Chapter 3 and reference should be made to these throughout
this chapter. This discussion is concerned not only with the descriptive
aspects of the distribution of individual species but also with patterns
of distribution within genera, with factors limiting the extents of the
areas of distribution of species, with mechanisms for maintenance of
populations occurring in small geographical areas, with the relationships, in the horizontal and vertical planes, of the different species living
in the same sea area, and with " patchiness " and '' swarming " of
individuals and populations within the general area of occurrence of a
species.
First, however, the general topographical and other characteristics
of the oceans which will influence the distribution of the different
species must be examined and borne in mind when reading the rest of
this chapter, The areas, volumes and mean depths of the different
oceans and seas are shown in Table XXXIII and the areas have been
broadly divided into the areas of the different depth zones in Table
XXXIV. There are only two extensive areas of shallower water, mostly
less than 2 000 m depth (Fig. 118) ; one is in the North Atlantic north
of about 50"N, the other extends from the Bering Sea southwards via
the Sea of Okhotsk, Sea of Japan, East and South China Seas, East
Indian Archipelago, to Eastern Australia. The shallower areas in the
Pacific and Indian Oceans have depths between 2000 and 4000m,
as have most of the shallower areas in the tropical and subtropical
regions of the Atlantic. The isotherms a t depths of 200 and 400 m are
shown in Fig. 119; it is temperatures a t these depths rather than at
the surface which influence the distribution of euphausiids. Finally,
the main current systems of the oceans are illustrated in Fig. 120.
313