20
J.T. Eastman and A. Clarke
are a marine group of North Pacific origin, they have colonized freshwaters
in Eurasia and North America. Cottoids from the freshwaters of Eastern
Siberia may have been a source of the ancestral stock for the Baikal
cottoids [77]. Recent molecular research has provided divergence times for
a few species of the family Cottidae. Merged sequences from the
cytochrome b and ATPase regions of mtDNA suggest that three species of
Cottus and Cottocomephorus diverged 1-2 Ma [79].
Although lacking swim bladders, some Baikal cottoids diversified away
from the ancestral coastal benthic habitat into water column habitats and
deeper (abyssal) waters [77,80]. Species diversity peaks at 400-500 m [78].
In an interesting ecological and morphological parallel with the
nototheniids Dissostichus and Pleuragramma [1], the two species of
Comephorus have become secondarily pelagic. They are close to neutral
buoyancy through a combination of reduced skeletal mineralization and
increased levels of lipid [78]. Adaptation to deep water habitats in Baikal
has involved modification of sensory systems similar to those seen in deep
dwelling marine species. For example, in some Baikal cottoids the
photoreceptors of the retina consist exclusively of rods [81,82], and canal
neuromasts have been replaced by free neuromasts [78].
Final Remarks
Radiations or species flocks of fish are found in isolated aquatic habitats
throughout the world, especially in geologically old lakes. The Antarctic
shelf and upper slope, an isolated evolutionary site similar to ancient lakes,
may also hold flocks of fish. The appearance of some species flocks is
coincident with periods of climatic change and habitat instability that may
have served as vicariant events. There is good documentation for the rise
and fall of water levels in Lakes Malawi and Victoria. On the other hand,
periods of habitat instability cannot be associated with the diversification
of the deep living Antarctic liparids or Baikal cottoids. While flocks are a
repeatable evolutionary response among phyletically diverse groups of
fish, the size, age and nature of the diversifications are quite different
(Table 1). In the Antarctic, notothenioids dominate the fish biomass of the
shelf and their morphological and ecological diversity parallels their
phyletic diversity. The deeper living liparids, on the other hand, are
speciose but are not present in great numbers and are not morphologically
diverse. Diversification in both groups has been keyed to underutilized
water column habitats.
A key evolutionary innovation is a novel feature characterizing a clade
(synapomorphy) and correlated with the adaptive radiation of the clade [29,
p.181). Innovations are the exception rather than the rule among flocks of
J.T. Eastman and A. Clarke
are a marine group of North Pacific origin, they have colonized freshwaters
in Eurasia and North America. Cottoids from the freshwaters of Eastern
Siberia may have been a source of the ancestral stock for the Baikal
cottoids [77]. Recent molecular research has provided divergence times for
a few species of the family Cottidae. Merged sequences from the
cytochrome b and ATPase regions of mtDNA suggest that three species of
Cottus and Cottocomephorus diverged 1-2 Ma [79].
Although lacking swim bladders, some Baikal cottoids diversified away
from the ancestral coastal benthic habitat into water column habitats and
deeper (abyssal) waters [77,80]. Species diversity peaks at 400-500 m [78].
In an interesting ecological and morphological parallel with the
nototheniids Dissostichus and Pleuragramma [1], the two species of
Comephorus have become secondarily pelagic. They are close to neutral
buoyancy through a combination of reduced skeletal mineralization and
increased levels of lipid [78]. Adaptation to deep water habitats in Baikal
has involved modification of sensory systems similar to those seen in deep
dwelling marine species. For example, in some Baikal cottoids the
photoreceptors of the retina consist exclusively of rods [81,82], and canal
neuromasts have been replaced by free neuromasts [78].
Final Remarks
Radiations or species flocks of fish are found in isolated aquatic habitats
throughout the world, especially in geologically old lakes. The Antarctic
shelf and upper slope, an isolated evolutionary site similar to ancient lakes,
may also hold flocks of fish. The appearance of some species flocks is
coincident with periods of climatic change and habitat instability that may
have served as vicariant events. There is good documentation for the rise
and fall of water levels in Lakes Malawi and Victoria. On the other hand,
periods of habitat instability cannot be associated with the diversification
of the deep living Antarctic liparids or Baikal cottoids. While flocks are a
repeatable evolutionary response among phyletically diverse groups of
fish, the size, age and nature of the diversifications are quite different
(Table 1). In the Antarctic, notothenioids dominate the fish biomass of the
shelf and their morphological and ecological diversity parallels their
phyletic diversity. The deeper living liparids, on the other hand, are
speciose but are not present in great numbers and are not morphologically
diverse. Diversification in both groups has been keyed to underutilized
water column habitats.
A key evolutionary innovation is a novel feature characterizing a clade
(synapomorphy) and correlated with the adaptive radiation of the clade [29,
p.181). Innovations are the exception rather than the rule among flocks of