THE DEEP-SEA FLOOR: AN OVERVIEW
27
Table 2.2
Comparison of soft-bottom and hard-bottom faunas in the deep sea
Feature
Soft-bottom
habitat
Manganesenodule
habitat
Other
hard-bottom
habitats
Dominant taxa polychaetes,
nematodes
foraminifers
sponges,
black corals,
horny corals
Dominant size macrofaunal
and
meiofaunal
macrofaunal and
meiofaunal
megafaunal
Dominant
mobility mode
mobile
sessile
sessile
Dominant
feeding mode
deposit
suspension and
surface deposit
suspension
organisms are essentially absent. One exception to
this generalization is “cauliflower”-type manganese
nodules, where sediment accumulates in the crevices
and harbors a more or less conventional infauna (Thiel
et al., 1993).
The dominant taxa of deep-sea soft bottoms are
polychaetes, nematodes, and foraminifers. The dominant taxon on manganese nodules is the agglutinating
foraminifers (Mullineaux, 1987). On other hard bottoms, sponges, horny corals (gorgonians), and black
corals (antipatharians) dominate (Genin et al., 1986,
1992; Tyler and Zibrowius, 1992), and xenophyophores
may be present (Levin and Thomas, 1988). Soft
bottoms are dominated by macrofauna and meiofauna.
The dominant hard-bottom taxa on manganese nodules
and mollusk shells are macrofauna and meiofauna, but
on other hard bottoms megafauna appear to dominate.
Soft-bottom animals tend to be mobile and to depositfeed; whereas on manganese nodules and mollusk
shells, most organisms are sessile (Mullineaux, 1987;
Voight and Walker, 1995). They suspension-feed or
collect particles from the surface of the substratum
(Mullineaux, 1987, 1989), a process analogous to
surface-deposit feeding in soft bottoms. On other hardbottom habitats, the dominant animals are sessile or
sedentary, and they suspension-feed (Genin et al., 1986,
1992; Tyler and Zibrowius, 1992).
Distribution
Regularities in the distribution of hard-bottom organisms are apparent at several scales. For example, on
the Bahamian slope, sponges occur in greater numbers
on vertically oriented surfaces than on horizontal
surfaces, creating marked patchiness on scales of
meters. The difference in exposure to siltation, which
adversely affects sponges, may underlie the difference
(Maldonado and Young, 1996).
At scales of hundreds of meters along a depth
transect, there are changes in species composition, and
there may be changes in the general characteristics
of the fauna. For example, sponge species diversity is
high on the shelf and upper slope of the Bahamas. As
depth increases, the diversity decreases as the species
of the lower reefs disappear. Deeper on the slope, deepsea sponges occur, resulting in a secondary peak in
diversity and abundance. Branched erect sponges are
common in shallow but not in deep water, so there is
also a change in the morphological composition of the
fauna with depth (Maldonado and Young, 1996).
The importance of near-bottom flow
The hard-bottom fauna includes many sessile suspension feeders. These animals depend on near-bottom
flow to transport their propagules to settlement sites.
To a first approximation, the flux (flow velocity x
propagule concentration) of propagules determines the
initial distribution of these species. These animals
also depend on the near-bottom flow to supply them
with food. To a first approximation, the flux of food
determines survivorship.
Evidence of such effects on the hard-bottom fauna
has been found at several scales. For example, on a
wide peak on the summit of Jaspar Seamount (Pacific
Ocean), individuals of a species of black coral are
more abundant on the edges of the peak than in the
center. This pattern matches that of variation in the
velocity field (Genin et al., 1986). Also, where the
Western Boundary Undercurrent encounters the Blake
Spur (Atlantic Ocean), the cliffs are bathed by flows of
30 cm s
−1 (much faster than the 3 cm s
−1 of typical
deep-sea flows). There, massive sponges and horny
corals are unusually abundant (Genin et al., 1992).
In these examples, Genin et al. assumed that the
concentration of propagules (or food) was the same
in the water that impinged on the high- and lowflow environments. This assumption may not always
be valid, particularly near the seabed. For example,
Mullineaux (1988) compared the settlement rate of
sessile organisms (predominantly foraminifers) on
manganese nodules suspended 20 cm above the seabed
to the rate on nodules at the seabed. She found that the
27
Table 2.2
Comparison of soft-bottom and hard-bottom faunas in the deep sea
Feature
Soft-bottom
habitat
Manganesenodule
habitat
Other
hard-bottom
habitats
Dominant taxa polychaetes,
nematodes
foraminifers
sponges,
black corals,
horny corals
Dominant size macrofaunal
and
meiofaunal
macrofaunal and
meiofaunal
megafaunal
Dominant
mobility mode
mobile
sessile
sessile
Dominant
feeding mode
deposit
suspension and
surface deposit
suspension
organisms are essentially absent. One exception to
this generalization is “cauliflower”-type manganese
nodules, where sediment accumulates in the crevices
and harbors a more or less conventional infauna (Thiel
et al., 1993).
The dominant taxa of deep-sea soft bottoms are
polychaetes, nematodes, and foraminifers. The dominant taxon on manganese nodules is the agglutinating
foraminifers (Mullineaux, 1987). On other hard bottoms, sponges, horny corals (gorgonians), and black
corals (antipatharians) dominate (Genin et al., 1986,
1992; Tyler and Zibrowius, 1992), and xenophyophores
may be present (Levin and Thomas, 1988). Soft
bottoms are dominated by macrofauna and meiofauna.
The dominant hard-bottom taxa on manganese nodules
and mollusk shells are macrofauna and meiofauna, but
on other hard bottoms megafauna appear to dominate.
Soft-bottom animals tend to be mobile and to depositfeed; whereas on manganese nodules and mollusk
shells, most organisms are sessile (Mullineaux, 1987;
Voight and Walker, 1995). They suspension-feed or
collect particles from the surface of the substratum
(Mullineaux, 1987, 1989), a process analogous to
surface-deposit feeding in soft bottoms. On other hardbottom habitats, the dominant animals are sessile or
sedentary, and they suspension-feed (Genin et al., 1986,
1992; Tyler and Zibrowius, 1992).
Distribution
Regularities in the distribution of hard-bottom organisms are apparent at several scales. For example, on
the Bahamian slope, sponges occur in greater numbers
on vertically oriented surfaces than on horizontal
surfaces, creating marked patchiness on scales of
meters. The difference in exposure to siltation, which
adversely affects sponges, may underlie the difference
(Maldonado and Young, 1996).
At scales of hundreds of meters along a depth
transect, there are changes in species composition, and
there may be changes in the general characteristics
of the fauna. For example, sponge species diversity is
high on the shelf and upper slope of the Bahamas. As
depth increases, the diversity decreases as the species
of the lower reefs disappear. Deeper on the slope, deepsea sponges occur, resulting in a secondary peak in
diversity and abundance. Branched erect sponges are
common in shallow but not in deep water, so there is
also a change in the morphological composition of the
fauna with depth (Maldonado and Young, 1996).
The importance of near-bottom flow
The hard-bottom fauna includes many sessile suspension feeders. These animals depend on near-bottom
flow to transport their propagules to settlement sites.
To a first approximation, the flux (flow velocity x
propagule concentration) of propagules determines the
initial distribution of these species. These animals
also depend on the near-bottom flow to supply them
with food. To a first approximation, the flux of food
determines survivorship.
Evidence of such effects on the hard-bottom fauna
has been found at several scales. For example, on a
wide peak on the summit of Jaspar Seamount (Pacific
Ocean), individuals of a species of black coral are
more abundant on the edges of the peak than in the
center. This pattern matches that of variation in the
velocity field (Genin et al., 1986). Also, where the
Western Boundary Undercurrent encounters the Blake
Spur (Atlantic Ocean), the cliffs are bathed by flows of
30 cm s
−1 (much faster than the 3 cm s
−1 of typical
deep-sea flows). There, massive sponges and horny
corals are unusually abundant (Genin et al., 1992).
In these examples, Genin et al. assumed that the
concentration of propagules (or food) was the same
in the water that impinged on the high- and lowflow environments. This assumption may not always
be valid, particularly near the seabed. For example,
Mullineaux (1988) compared the settlement rate of
sessile organisms (predominantly foraminifers) on
manganese nodules suspended 20 cm above the seabed
to the rate on nodules at the seabed. She found that the