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Joseph
T. Eastman, Ph.D.
Professor of Anatomy
Department of Biomedical Sciences
eastman@ohiou.edu
119 Life Sciences Building
740-593-2350
Fax: 740-593-2400 |
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Introduction
My field and laboratory work deals with a group of
perch-like Antarctic fishes called notothenioids,
the dominant and most diverse fish group in the
Southern Ocean. The objectives are to gain insight
into the evolution and diversification of this
group, and to understand the role of notothenioids
in the Antarctic marine ecosystem. I study the
biology of these fishes in a historical and
phylogenetic context. Like much evolutionary
research, my work is concerned with historical
events and the approach is frequently retrospective,
investigating the modern results of historical
processes. I am attempting to decipher the results
of a series of evolutionary events that have
proceeded for 40 million years under the unusual
conditions found in Antarctic waters. I seek to
answer a series of questions relating to the nature
of Antarctic fish diversity. Why did the fish fauna
evolve the way it did? Why is the modern fauna
unlike the preceding fossil faunas as well as the
shelf faunas of other southern continents? Why do
notothenioids contribute so heavily to Antarctic
fish diversity at both the organismal and ecological
levels? When did the fauna become "modern" in taxonomic
composition? How did neutrally buoyant fish
evolve from ancestors who were heavy bottom
dwellers? Are there correlations between changes in
the morphology of various body systems and the
ecological changes that accompanied the notothenioid
radiation? Is novel morphology required for
perciform fish to live in the subzero waters of the
Antarctic shelf?
Are
notothenioids an example of a species flock? Is
Antarctica an evolutionary center of origin for
marine organisms? Some of these questions will have
no clear answers. The sections below provide some
background information on my work. |
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Antarctica and the Southern Ocean
Antarctica is a continental island, about twice the
size of Australia, with the dominant fauna
inhabiting the water rather than the ice-covered
landmass. The sea is the largest living space on
earth and the Southern Ocean surrounding Antarctica
is 10% of the world's ocean. Antarctica and its fish
fauna are commanding increased attention in a world
attuned to loss of biological diversity, depletion
of marine fisheries and the encroachment of human
activities into isolated and incompletely studied
ecosystems such as the Southern Ocean. For example,
the popular gourmet delicacy known as “Chilean sea
bass” is actually a heavily exploited Subantarctic
species (shown in Figure 1k). |
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Antarctica as an evolutionary center
Until recently Antarctica had been under appreciated
as an evolutionary site because most Antarctic
biological research had focused on adaptations to
the “extreme” environment rather than on
similarities between the Antarctic fauna and faunas
in other isolated habitats such as islands and
ancient rift lakes. Unlike other large marine
ecosystems, the waters of the continental shelf
around Antarctica resemble a closed basin, isolated
from other shelf areas in the Southern Hemisphere by
distance, current patterns, bathymetry and subzero
water temperatures. As these isolating conditions
developed over the past 10-30 million years, the
marine biota became adapted to new shelf habitat and
their ranges became highly circumscribed. For
example, the level of endemism reaches 97% in the
case of fishes. Antarctica has the world’s most
distinctive marine fauna. The attention of
evolutionary biologists is drawn to these isolated
habitats because of their unusual faunas. In this
sense the waters of the Antarctic shelf are
comparable to, but less well known than, classic
evolutionary sites such as the Galápagos, Hawaii and
Lake Baikal. Research on Antarctic fishes provides
insight into evolutionary processes and
macroevolutionary events in the marine realm where
these processes are not as well documented as in
terrestrial and freshwater habitats. The Antarctic
fauna occupies an extreme in the spectrum of
habitats where fishes are found – it provides a
glimpse of the wide scope of adaptation and
evolution in a habitat once thought to be
incompatible with marine life. |
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The Antarctic fish fauna
Over the past 40 million years there has been a
complete replacement of the fish fauna on the
Antarctic shelf. The highly endemic, cold adapted
modern fauna succeeded a diverse, cosmopolitan
temperate fauna dating from the late Eocene about 38
million years ago. Today 221 species of fishes
inhabit the waters of the Antarctic shelf. The fauna
is dominated by a suborder of 101 species of
notothenioids. Notothenioids are phyletically
derived among bony fish and are related to walleyes
and perch as well as to coral reef fishes. A variety
of interesting specializations distinguish Antarctic
notothenioids. Among these are: glycoprotein
antifreeze compounds that prevent freezing in
subzero water; extreme stenothermia that allows
survival only in the range of -2.5 to +6.0ºC;
reduced blood viscosity through absence or decreased
numbers of erythrocytes; and, in some species,
neutral buoyancy without benefit of a swim bladder.
The Antarctic fish fauna lacks the higher taxonomic
diversity typical of all other inshore marine
habitats. On the Antarctic shelf notothenioids
dominate the fauna in terms of species diversity,
abundance and biomass, the latter two at levels of
90-95%. The grounding of the ice sheet on the
continental shelf and changing trophic conditions
may have eliminated the taxonomically diverse late
Eocene fauna and initiated the original
diversification of notothenioids. In a habitat with
few other fishes, notothenioids underwent a
depth-related diversification directed away from the
ancestral benthic habitat toward pelagic or
partially pelagic zooplanktivory and piscivory.
Notothenioids were able to fill these niches as well
as remaining the dominant benthic group.
Figure
1a-1
provides an example of the range of morphological
diversity within one family of notothenioids.
The diversification of notothenioids centered on the
evolutionary alteration of buoyancy and the
morphology associated with swimming and feeding in
the water column. Although they lack swim bladders,
in some species density reduction to neutral
buoyancy has been achieved through a combination of
reduced skeletal mineralization and lipid
deposition. Pedomorphic changes in the skeleton are
also associated with reduced density. Panel l in
Figure 1 depicts a streamlined pelagic species that
is neutrally buoyant. In the dominant family
Nototheniidae, about 50% of the Antarctic
species temporarily or permanently inhabit the water
column rather than the ancestral benthic habitat.
Referred to as pelagization, this evolutionary
tailoring of morphology for life in the water column
is the hallmark of the notothenioid radiation and
probably arose independently in different
notothenioid clades. |
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Notothenioids on the high Antarctic shelf – a
species flock?
The notothenioid diversification has produced
different life history or ecological types similar
in magnitude to those displayed by taxonomically
unrelated shelf fishes elsewhere in the world. This
is unique in the marine realm and raises the
possibility that notothenioids are one of the few
known examples of a species flock of marine fishes.
A species flock is an assemblage of a
disproportionately high number of closely related
species which evolved rapidly within a circumscribed
area where most species are endemic. Classic
examples include Darwin’s finches in the Galápagos,
Drosophila fruit flies in Hawaii, cichlid fishes in
the East African Great Lakes and sculpin fishes in
Siberian Lake Baikal. Notothenioids on the high
Antarctic shelf possess all the characteristics of a
species flock – disproportionate speciosity,
morphological and ecological diversification,
habitat dominance, high endemicity and monophyly.
The presence of this flock, and other marine flocks
(lobodontine seals, isopods, amphipods, octopodids),
reinforces the status of Antarctica as a unique
evolutionary site. |
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See NSF Press Release
on this research |
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