A Comparison of Adaptive Radiations of Antarctic Fish with those of NonAntarctic Fish
19
Malawi and Victoria flocks are closely related to one of the Tanganyikan
lineages and are monophyletic and younger [71].
Although there has been behavioral and ecological differentiation, the
amount of morphological differentiation has been relatively small, with
species differences centering on head and tooth shape and body coloration.
Modifications of the pharyngeal jaw apparatus, specifically the
development of joints and the shift in muscle insertions, may have been a
key evolutionary innovation in the radiation of these flocks. The
pharyngeal jaws assumed the function of food mastication, freeing the oral
jaws for extremely diverse collecting functions which lead to ecological
differentiation [72]. Recent evaluation of the trophic diversity of the flocks
in Tanganyika lead to the conclusion that this diversity reflects historical
(phylogenetic) factors rather than results of competition [73].
Periods of climatic change probably produced fluctuations in water level
in all three lakes, and this was important in promoting microallopatric
intralacustrine speciation [69,71,74]. This was likely the most important
means of speciation in all cichlid flocks [71]. Speciation was also
extremely rapid, hundreds to a few thousand years in some small lakes and
in isolated parts of the three great lakes. This conclusion is based on the
presence of endemic species in areas of some lakes known to be dry a few
centuries ago. The youngest major flock, that in Victoria, is dated by
molecular methods at 200 000 years [71]; however seismic reflection
profiling and piston coring suggest that the lake dried completely 12 400
years ago [75]. This implies that the flock of 250 species evolved in this
short period of time or that components of the flock survived in adjacent
water and, after levels rose, repopulated Lake Victoria.
Lake Baikal Cottoids
With a maximum depth of 1620 m, Lake Baikal is the world's deepest and
possibly oldest lake. The Baikal rift has contained a deep lake since the
early Miocene (25 Ma), although the modern configuration was not
acquired until 3.2-0.8 Ma [76]. The origins of the modern fauna date to the
Miocene or Pliocene. Baikal holds the greatest animal species diversity of
any extant lake [16, 76]. Unlike the East African lakes, Baikal is
oxygenated to the bottom and therefore contains a significant deep-water
fauna including fish that approach marine species in some of their
adaptations to depth.
The fish fauna of Lake Baikal includes 56 species from 14 families
[77,78]. Of the 50 native species, 29 (58%) are endemic sculpins
(Cottoidei) belonging to the families Cottidae, Comephoridae and
Abyssocottidae. The latter two are endemic to the lake. Although cottoids
19
Malawi and Victoria flocks are closely related to one of the Tanganyikan
lineages and are monophyletic and younger [71].
Although there has been behavioral and ecological differentiation, the
amount of morphological differentiation has been relatively small, with
species differences centering on head and tooth shape and body coloration.
Modifications of the pharyngeal jaw apparatus, specifically the
development of joints and the shift in muscle insertions, may have been a
key evolutionary innovation in the radiation of these flocks. The
pharyngeal jaws assumed the function of food mastication, freeing the oral
jaws for extremely diverse collecting functions which lead to ecological
differentiation [72]. Recent evaluation of the trophic diversity of the flocks
in Tanganyika lead to the conclusion that this diversity reflects historical
(phylogenetic) factors rather than results of competition [73].
Periods of climatic change probably produced fluctuations in water level
in all three lakes, and this was important in promoting microallopatric
intralacustrine speciation [69,71,74]. This was likely the most important
means of speciation in all cichlid flocks [71]. Speciation was also
extremely rapid, hundreds to a few thousand years in some small lakes and
in isolated parts of the three great lakes. This conclusion is based on the
presence of endemic species in areas of some lakes known to be dry a few
centuries ago. The youngest major flock, that in Victoria, is dated by
molecular methods at 200 000 years [71]; however seismic reflection
profiling and piston coring suggest that the lake dried completely 12 400
years ago [75]. This implies that the flock of 250 species evolved in this
short period of time or that components of the flock survived in adjacent
water and, after levels rose, repopulated Lake Victoria.
Lake Baikal Cottoids
With a maximum depth of 1620 m, Lake Baikal is the world's deepest and
possibly oldest lake. The Baikal rift has contained a deep lake since the
early Miocene (25 Ma), although the modern configuration was not
acquired until 3.2-0.8 Ma [76]. The origins of the modern fauna date to the
Miocene or Pliocene. Baikal holds the greatest animal species diversity of
any extant lake [16, 76]. Unlike the East African lakes, Baikal is
oxygenated to the bottom and therefore contains a significant deep-water
fauna including fish that approach marine species in some of their
adaptations to depth.
The fish fauna of Lake Baikal includes 56 species from 14 families
[77,78]. Of the 50 native species, 29 (58%) are endemic sculpins
(Cottoidei) belonging to the families Cottidae, Comephoridae and
Abyssocottidae. The latter two are endemic to the lake. Although cottoids