26
David THISTLE
discrete regions of high abundance within the range of
a species which extend both along and across isobaths
(Maynou et al., 1996). Therefore, the distribution of
a species within its depth range may be a series of
patches (see Blake and Hilbig, 1994).
These results have been based on the distribution
of macrofauna. Theory suggests (Fenchel, 1993) that
animals of smaller size (meiofauna, microfauna) may
have larger species ranges, so many species may be
cosmopolitan. Relatively little information is available
with which to evaluate this idea in the deep sea, but
many common deep-sea formaminiferal species appear
to be cosmopolitan (Gooday et al., 1998).
Finally, deep-sea biogeography is based almost entirely on morphological species, which is not surprising
given the difficulty of working in this environment.
Substantial genetic variation has been discovered
within some nominal species (Etter et al., 1999), raising
the possibility that some morphological species may
be complexes of cryptic species, a situation that would
make deep-sea biogeography even more difficult.
Factors controlling the depth range of a species
The physiological limits of a species set the ultimate
bounds of its range; shallow-water species could therefore be excluded from the deep sea and vice versa, and
the ranges of deep-sea species restricted. For example,
mitosis is inhibited when shallow-water sea urchins
are exposed to deep-sea pressures (Marsland, 1938,
1950). Also, larvae of a bathyal sea urchin require
bathyal temperatures to develop properly, setting a
physiological limit to the minimum and maximum
depths at which the larvae can develop (Young and
Cameron, 1989). Similarly, when eggs of an asteroid
that lives between 1000 and 2500 m were exposed to
pressures corresponding to 0 and 3000 m, virtually no
normal development occurred (Young et al., 1996).
Ecological limitations could be imposed in a variety
of ways. As depth increases, sediment composition
can change. Because some species are restricted to
particular sediment types, sediment changes can limit
their ranges. For example, on a depth transect off
North Carolina, sands give way to sandy silts and
then to clayey silts. Nematode species were restricted
to these sediment-defined depth bands – 17, 5, and
49 species, respectively (Tietjen, 1976). The depth
distribution of a species may also be controlled by food
availability. For example, the decrease in suspended
food concentration with depth limits the penetration of
certain suspension-feeding species into the deep sea
(see above, pp. 20–21). In contrast, on the Carolina
slope unusual conditions occur such that food is
supplied to the slope at rates comparable to those on
the shelf, and some species ordinarily confined to the
shelf are present on the slope in large numbers (Blake
and Hilbig, 1994). Examples of other environmental
variables that can affect the ranges of species include
low oxygen concentration in the near-bottom water
(Wishner et al., 1990), strong near-bottom flows (Rice
et al., 1990; Paterson and Lambshead, 1995), and
correlates of the permanent thermocline (Gage, 1986).
The depth distribution of a species may also be
controlled by ecological interactions with other species.
For example, Rex (1977) found that groups made up
largely of croppers and predators such as the epibenthic
macrofauna and gastropods had smaller species ranges
on average than did infaunal deposit feeders. To explain
the difference, he noted that the former were at a higher
trophic level than the latter and that studies in other
environments have found that the higher the trophic
level the greater the competitive interactions among
species. He suggested that the increased probability of
competition among species of epibenthic macrofauna
and gastropods resulted in lower average range sizes
than in the infauna, whose populations were much less
likely to interact competitively because of the intense
predation upon them (but see Carney et al., 1983).
THE HARD-BOTTOM HABITAT
Examples of deep-sea hard-bottom habitats include the
exposed portions of rocks and manganese nodules,
mollusk shells, new oceanic crust, regions of steep
topography where sediment does not accumulate, and
locations where the sediment has been washed away
by currents (e.g., portions of some seamounts). Despite
this variety, two generalizations can be made. The hardbottom fauna differs in taxonomic composition and
life-style from that of deep-sea soft bottoms (Table 2.2),
and near-bottom flow tends to be more important to this
fauna than to that of the deep-sea soft bottoms.
The fauna
Soft bottoms are three-dimensional, and although the
sediment surface has great ecological importance to
many of the species present, few animals live on it.
Hard bottoms are two-dimensional. Most of the animals
live on the surface of the substratum, and infaunal
David THISTLE
discrete regions of high abundance within the range of
a species which extend both along and across isobaths
(Maynou et al., 1996). Therefore, the distribution of
a species within its depth range may be a series of
patches (see Blake and Hilbig, 1994).
These results have been based on the distribution
of macrofauna. Theory suggests (Fenchel, 1993) that
animals of smaller size (meiofauna, microfauna) may
have larger species ranges, so many species may be
cosmopolitan. Relatively little information is available
with which to evaluate this idea in the deep sea, but
many common deep-sea formaminiferal species appear
to be cosmopolitan (Gooday et al., 1998).
Finally, deep-sea biogeography is based almost entirely on morphological species, which is not surprising
given the difficulty of working in this environment.
Substantial genetic variation has been discovered
within some nominal species (Etter et al., 1999), raising
the possibility that some morphological species may
be complexes of cryptic species, a situation that would
make deep-sea biogeography even more difficult.
Factors controlling the depth range of a species
The physiological limits of a species set the ultimate
bounds of its range; shallow-water species could therefore be excluded from the deep sea and vice versa, and
the ranges of deep-sea species restricted. For example,
mitosis is inhibited when shallow-water sea urchins
are exposed to deep-sea pressures (Marsland, 1938,
1950). Also, larvae of a bathyal sea urchin require
bathyal temperatures to develop properly, setting a
physiological limit to the minimum and maximum
depths at which the larvae can develop (Young and
Cameron, 1989). Similarly, when eggs of an asteroid
that lives between 1000 and 2500 m were exposed to
pressures corresponding to 0 and 3000 m, virtually no
normal development occurred (Young et al., 1996).
Ecological limitations could be imposed in a variety
of ways. As depth increases, sediment composition
can change. Because some species are restricted to
particular sediment types, sediment changes can limit
their ranges. For example, on a depth transect off
North Carolina, sands give way to sandy silts and
then to clayey silts. Nematode species were restricted
to these sediment-defined depth bands – 17, 5, and
49 species, respectively (Tietjen, 1976). The depth
distribution of a species may also be controlled by food
availability. For example, the decrease in suspended
food concentration with depth limits the penetration of
certain suspension-feeding species into the deep sea
(see above, pp. 20–21). In contrast, on the Carolina
slope unusual conditions occur such that food is
supplied to the slope at rates comparable to those on
the shelf, and some species ordinarily confined to the
shelf are present on the slope in large numbers (Blake
and Hilbig, 1994). Examples of other environmental
variables that can affect the ranges of species include
low oxygen concentration in the near-bottom water
(Wishner et al., 1990), strong near-bottom flows (Rice
et al., 1990; Paterson and Lambshead, 1995), and
correlates of the permanent thermocline (Gage, 1986).
The depth distribution of a species may also be
controlled by ecological interactions with other species.
For example, Rex (1977) found that groups made up
largely of croppers and predators such as the epibenthic
macrofauna and gastropods had smaller species ranges
on average than did infaunal deposit feeders. To explain
the difference, he noted that the former were at a higher
trophic level than the latter and that studies in other
environments have found that the higher the trophic
level the greater the competitive interactions among
species. He suggested that the increased probability of
competition among species of epibenthic macrofauna
and gastropods resulted in lower average range sizes
than in the infauna, whose populations were much less
likely to interact competitively because of the intense
predation upon them (but see Carney et al., 1983).
THE HARD-BOTTOM HABITAT
Examples of deep-sea hard-bottom habitats include the
exposed portions of rocks and manganese nodules,
mollusk shells, new oceanic crust, regions of steep
topography where sediment does not accumulate, and
locations where the sediment has been washed away
by currents (e.g., portions of some seamounts). Despite
this variety, two generalizations can be made. The hardbottom fauna differs in taxonomic composition and
life-style from that of deep-sea soft bottoms (Table 2.2),
and near-bottom flow tends to be more important to this
fauna than to that of the deep-sea soft bottoms.
The fauna
Soft bottoms are three-dimensional, and although the
sediment surface has great ecological importance to
many of the species present, few animals live on it.
Hard bottoms are two-dimensional. Most of the animals
live on the surface of the substratum, and infaunal