A Comparison of Adaptive Radiations of Antarctic Fish with those of NonAntarctic Fish
15
The Origin of the Present Fauna
The modem Antarctic fauna of continental shelf and slope fishes is
dominated in terms of species by liparids and notothenioids.
The liparids (snailfishes) appear to have their center of origin in the
north Pacific. It is likely that they reached the Southern Ocean by range
extension along the Pacific continental shelf and slope of the Americas,
probably at a time when glacial maxima had pushed colder waters to lower
latitudes. Once in Antarctica the group has radiated to produce over 60
species, though nowhere do liparids appear to be abundant [4]. The timing
of the radiation is not known but if it was assumed to have taken place at
the last glacial maximum then the radiation of liparids in the Southern
Ocean must be a relatively recent affair.
The notothenioids have both radiated and become the dominant taxa of
the Southern Ocean continental shelf fauna [1]. The notothenioids appear
to be a monophyletic clade, though the ancestral stock cannot be identified
with certainty [1]. Neither can we rely on fossil history, for as yet no
unequivocal fossil notothenioids have been identified.
We must assume that the notothenioids evolved from a demersal
perciform stock, and probably one which was already living in the shallow
waters of Gondwana and probably in the WeddellianiTethyan rather than
the Pacific faunal province. Once the early Tertiary fauna became extinct
the notothenioids were able to undergo a radiation to produce the fauna we
see today. Diversification of the emerging notothenioid fauna was
facilitated by the oceanographic and thermal isolation of Antarctica, by the
increasing productivity of the Southern Ocean beginning about 22 Ma [64]
and by the absence of competition from nonnotothenioids. This ecological
space was filled by a diversifying notothenioid fauna tolerant of deeper
and colder habitats, which poses the questions as to why it was the
notothenioids that radiated, rather than some other group, and when did
the radiation take place?
Why Notothenioids?
This is an impossible question to answer at present. It may be, however,
that through an accident of history the notothenioids were simply the
group that survived whatever caused the demise of much of the early
Tertiary fauna. Certainly a number of features of their biology suggest that
the notothenioids were in no way "pre-adapted" to radiate in the Southern
Ocean. In particular the secondary loss of a functional swimbladder in the
ancestral stock has limited their ability to move into the water column and
15
The Origin of the Present Fauna
The modem Antarctic fauna of continental shelf and slope fishes is
dominated in terms of species by liparids and notothenioids.
The liparids (snailfishes) appear to have their center of origin in the
north Pacific. It is likely that they reached the Southern Ocean by range
extension along the Pacific continental shelf and slope of the Americas,
probably at a time when glacial maxima had pushed colder waters to lower
latitudes. Once in Antarctica the group has radiated to produce over 60
species, though nowhere do liparids appear to be abundant [4]. The timing
of the radiation is not known but if it was assumed to have taken place at
the last glacial maximum then the radiation of liparids in the Southern
Ocean must be a relatively recent affair.
The notothenioids have both radiated and become the dominant taxa of
the Southern Ocean continental shelf fauna [1]. The notothenioids appear
to be a monophyletic clade, though the ancestral stock cannot be identified
with certainty [1]. Neither can we rely on fossil history, for as yet no
unequivocal fossil notothenioids have been identified.
We must assume that the notothenioids evolved from a demersal
perciform stock, and probably one which was already living in the shallow
waters of Gondwana and probably in the WeddellianiTethyan rather than
the Pacific faunal province. Once the early Tertiary fauna became extinct
the notothenioids were able to undergo a radiation to produce the fauna we
see today. Diversification of the emerging notothenioid fauna was
facilitated by the oceanographic and thermal isolation of Antarctica, by the
increasing productivity of the Southern Ocean beginning about 22 Ma [64]
and by the absence of competition from nonnotothenioids. This ecological
space was filled by a diversifying notothenioid fauna tolerant of deeper
and colder habitats, which poses the questions as to why it was the
notothenioids that radiated, rather than some other group, and when did
the radiation take place?
Why Notothenioids?
This is an impossible question to answer at present. It may be, however,
that through an accident of history the notothenioids were simply the
group that survived whatever caused the demise of much of the early
Tertiary fauna. Certainly a number of features of their biology suggest that
the notothenioids were in no way "pre-adapted" to radiate in the Southern
Ocean. In particular the secondary loss of a functional swimbladder in the
ancestral stock has limited their ability to move into the water column and