In comparison with the northern hemisphere, the major
continents of the southern hemisphere have a relatively small
freshwater fish fauna. This is probably due to the fact
that the predominant portions of the main southern hemisphere
land-masses lie in tropical, subtropical or temperate zones
and except for the southern-most extremities are not
overendowed with rivers and lakes. Consequently the
potential for invasion of fish into freshwater areas where
they can become established is limited. Most freshwater
fishes living today are considered to be derived from marine
ancestors which have subsequently invaded freshwater.
Evidence for this can be found in the fact that many groups
of fishes which are considered to be predominately freshwater
species periodically descend to the sea or to the estuaries
to spawn or carry out some other phase of the life cycle