4
J.T. Eastman and A. Clarke
reasonably well established, there remain critical areas of uncertainty.
These include the nature of the parent stock, and the timing of key
evolutionary events in relation to tectonic and climatic changes.
Species Flocks or Radiations of Fish
Evolutionary biologists have long been fascinated by the species flocks or
adaptive radiations of organisms in isolated aquatic habitats, especially
ancient lakes [10-16]. The distinguishing characteristics of ancient lakes
are these: considerable age (5-20 Ma), great depth (permanence) and
isolation. They also contain speciose and highly endemic faunas,
sometimes including species flocks, and have been centers of evolution for
a variety of organisms [15,16].
Species flocks usually exhibit monophyly, endemism and speciosity [15].
With respect to fish species flocks, the importance of monophyly has been
considered either essential [17] or not a general criterion [18]. We follow
Ribbink [18 p.24] in recognizing a species flock as "an assemblage of a
disproportionately high number of closely related species which evolved
rapidly within a narrowly circumscribed area to which all or almost all the
member species are endemic."
The shelf and upper slope waters around Antarctica share many of the
features of isolated aquatic habitats. Although large in size, the Antarctic
shelf and upper slope is isolated from other shelf areas in the Southern
Hemisphere by large areas of deep water, a major oceanographic feature
(the Polar Front), and by being at subzero temperatures. In these respects
the fish fauna of the Antarctic continental shelf is as isolated as a lacustrine
fauna. The Antarctic continental shelf also resembles a lacustrine habitat in
supporting a highly endemic fish fauna, although the species richness
might be considered to be somewhat small for the size of the area. This
fauna contains two groups which match Ribbink's [18] criteria for a species
flock, the notothenioids and the liparids. In Table 1 we summarize the
features of these groups with flocks of fish from ancient lakes.
The Fish Fauna of the Antarctic Continental Shelf and Upper
Slope
The fish fauna of the Southern Ocean is small and limited in taxonomic
diversity, containing only 274 species in 49 families [19]. Thus although
representing 10% of the world's ocean, the Southern Ocean accommodates
only about 1 % of the world's fish fauna of 25 000 species. As will be seen
below, however, a count of taxa leads to an underappreciation of the
morphological and ecological diversity of the fauna. The fauna is highly
J.T. Eastman and A. Clarke
reasonably well established, there remain critical areas of uncertainty.
These include the nature of the parent stock, and the timing of key
evolutionary events in relation to tectonic and climatic changes.
Species Flocks or Radiations of Fish
Evolutionary biologists have long been fascinated by the species flocks or
adaptive radiations of organisms in isolated aquatic habitats, especially
ancient lakes [10-16]. The distinguishing characteristics of ancient lakes
are these: considerable age (5-20 Ma), great depth (permanence) and
isolation. They also contain speciose and highly endemic faunas,
sometimes including species flocks, and have been centers of evolution for
a variety of organisms [15,16].
Species flocks usually exhibit monophyly, endemism and speciosity [15].
With respect to fish species flocks, the importance of monophyly has been
considered either essential [17] or not a general criterion [18]. We follow
Ribbink [18 p.24] in recognizing a species flock as "an assemblage of a
disproportionately high number of closely related species which evolved
rapidly within a narrowly circumscribed area to which all or almost all the
member species are endemic."
The shelf and upper slope waters around Antarctica share many of the
features of isolated aquatic habitats. Although large in size, the Antarctic
shelf and upper slope is isolated from other shelf areas in the Southern
Hemisphere by large areas of deep water, a major oceanographic feature
(the Polar Front), and by being at subzero temperatures. In these respects
the fish fauna of the Antarctic continental shelf is as isolated as a lacustrine
fauna. The Antarctic continental shelf also resembles a lacustrine habitat in
supporting a highly endemic fish fauna, although the species richness
might be considered to be somewhat small for the size of the area. This
fauna contains two groups which match Ribbink's [18] criteria for a species
flock, the notothenioids and the liparids. In Table 1 we summarize the
features of these groups with flocks of fish from ancient lakes.
The Fish Fauna of the Antarctic Continental Shelf and Upper
Slope
The fish fauna of the Southern Ocean is small and limited in taxonomic
diversity, containing only 274 species in 49 families [19]. Thus although
representing 10% of the world's ocean, the Southern Ocean accommodates
only about 1 % of the world's fish fauna of 25 000 species. As will be seen
below, however, a count of taxa leads to an underappreciation of the
morphological and ecological diversity of the fauna. The fauna is highly