24
David THISTLE
relatives) occurs in the deep sea. Among the giants
are species of scavenging amphipods (Hessler et al.,
1972), surface-deposit-feeding holothurians (Gage and
Tyler, 1991), and isopods (Wolff, 1956). The giants are
not closely related evolutionarily and have different life
styles, so it is unlikely that gigantism arises for a single
reason.
Some abyssal ascidians are three to ten times larger
than other abyssal members of their families. These
giants have greatly modified filtering structures and
appear to have abandoned active suspension feeding
for passive suspension feeding. Monniot (1979) argued
that the elimination of the energetic costs of pumping
allowed individuals of greater mass to be supported
from a given concentration of suspended particles than
could be supported by active suspension feeding; but
these giants can occur only in locations where the flux
of food particles is relatively high.
The changes in size with depth reported above were
for entities that contained many species, such as macrofauna and nematodes. Rex and Etter (1998) argued that
such patterns should be investigated within individual
species, because only within species will it be possible
to understand the ecological and evolutionary forces
that created the patterns. When they did so for several
gastropod species, they found that the trend was for
individuals to increase rather than decrease in size with
depth (see also Wilson, 1983, and Macpherson and
Duarte, 1991).
Biogeography
Introduction
Geographical patterns in the distribution of species
(or higher taxa) and the causes of those patterns are
not well known for animals of the deep-sea floor.
This situation arises, in part, because of the great
mismatch between the vastness of the habitat and the
small amount of sampling that has been done. In
addition, large numbers of species are present in the
deep sea, most of which are undescribed. Further, the
number of specialists who can provide identifications
or taxonomic descriptions is small and is decreasing.
The shape of the ocean floor sets the stage for deepsea biogeography. Briefly, within major ocean basins
at slope depths, the habitat is more or less continuous
along isobaths, interrupted by relatively small features,
such as submarine canyons. The major oceans and
most of the secondary seas are connected at these
depths. Below about 2500 m, the mid-ocean ridges and
submarine mountain ranges divide the major oceans
into regions, for instance, the eastern Atlantic. Below
about 3500 m, the deep-sea floor consists of isolated
basins (Fig. 2.20).
Fig. 2.20. The Atlantic Ocean showing the location of the deepocean (>3500 m) basins (black) and the mid-Atlantic Ridge (dashes).
Modified from Allen and Sanders (1996). Copyright: Elsevier
Science.
Patterns along isobaths
The distribution of higher taxa is unusually homogeneous in the deep sea. For example, of the 143 genera
of asellote isopods known from the World Ocean,
all but nine are found in the Atlantic. This level of
similarity is much greater than expected on the basis of
shallow-water isopods (Hessler and Wilson, 1983).
Some deep-sea species are widespread, but many
more have restricted distributions. For example, along
180 km of the 2100-m isobath off the northeastern coast
of the United States, 10 abundant macrofaunal species
occurred at all stations (Grassle and Maciolek, 1992),
but 43% of the peracarid crustacean species, 34% of
the polychaete species, and 21% of the bivalve species
occurred at only one. The constraints (ecological or
historical) that cause large numbers of species to
be endemics in this environment are unknown, but
the differences among higher taxa in the proportion
of species with localized distributions may provide
a point of departure for further research. Because
the widespread species constitute the bulk of the
individuals at each station, the faunas at either end
David THISTLE
relatives) occurs in the deep sea. Among the giants
are species of scavenging amphipods (Hessler et al.,
1972), surface-deposit-feeding holothurians (Gage and
Tyler, 1991), and isopods (Wolff, 1956). The giants are
not closely related evolutionarily and have different life
styles, so it is unlikely that gigantism arises for a single
reason.
Some abyssal ascidians are three to ten times larger
than other abyssal members of their families. These
giants have greatly modified filtering structures and
appear to have abandoned active suspension feeding
for passive suspension feeding. Monniot (1979) argued
that the elimination of the energetic costs of pumping
allowed individuals of greater mass to be supported
from a given concentration of suspended particles than
could be supported by active suspension feeding; but
these giants can occur only in locations where the flux
of food particles is relatively high.
The changes in size with depth reported above were
for entities that contained many species, such as macrofauna and nematodes. Rex and Etter (1998) argued that
such patterns should be investigated within individual
species, because only within species will it be possible
to understand the ecological and evolutionary forces
that created the patterns. When they did so for several
gastropod species, they found that the trend was for
individuals to increase rather than decrease in size with
depth (see also Wilson, 1983, and Macpherson and
Duarte, 1991).
Biogeography
Introduction
Geographical patterns in the distribution of species
(or higher taxa) and the causes of those patterns are
not well known for animals of the deep-sea floor.
This situation arises, in part, because of the great
mismatch between the vastness of the habitat and the
small amount of sampling that has been done. In
addition, large numbers of species are present in the
deep sea, most of which are undescribed. Further, the
number of specialists who can provide identifications
or taxonomic descriptions is small and is decreasing.
The shape of the ocean floor sets the stage for deepsea biogeography. Briefly, within major ocean basins
at slope depths, the habitat is more or less continuous
along isobaths, interrupted by relatively small features,
such as submarine canyons. The major oceans and
most of the secondary seas are connected at these
depths. Below about 2500 m, the mid-ocean ridges and
submarine mountain ranges divide the major oceans
into regions, for instance, the eastern Atlantic. Below
about 3500 m, the deep-sea floor consists of isolated
basins (Fig. 2.20).
Fig. 2.20. The Atlantic Ocean showing the location of the deepocean (>3500 m) basins (black) and the mid-Atlantic Ridge (dashes).
Modified from Allen and Sanders (1996). Copyright: Elsevier
Science.
Patterns along isobaths
The distribution of higher taxa is unusually homogeneous in the deep sea. For example, of the 143 genera
of asellote isopods known from the World Ocean,
all but nine are found in the Atlantic. This level of
similarity is much greater than expected on the basis of
shallow-water isopods (Hessler and Wilson, 1983).
Some deep-sea species are widespread, but many
more have restricted distributions. For example, along
180 km of the 2100-m isobath off the northeastern coast
of the United States, 10 abundant macrofaunal species
occurred at all stations (Grassle and Maciolek, 1992),
but 43% of the peracarid crustacean species, 34% of
the polychaete species, and 21% of the bivalve species
occurred at only one. The constraints (ecological or
historical) that cause large numbers of species to
be endemics in this environment are unknown, but
the differences among higher taxa in the proportion
of species with localized distributions may provide
a point of departure for further research. Because
the widespread species constitute the bulk of the
individuals at each station, the faunas at either end