THE DEEP PACIFIC OCEAN FLOOR
209
important. Oligotrophic scavenger aggregations differ
markedly from those from eutrophic slope habitats
in a number of intriguing ways. Oligotrophic aggregations are comprised of relatively few species
of fast swimmers (mostly amphipods and rattails),
and do not include the epibenthic ophiuroids, crabs
and polychaetes that are attracted to bait-falls on the
Californian slope (Hessler, 1974; Smith, 1985). The
body sizes of oligotrophic scavengers also tend to
be larger, the amphipod Eurythenes gryllus attaining
a length of 14 cm (Ingram and Hessler, 1983) and
Alicella gigantea reaching the “supergiant” size of
34 cm (Barnard and Ingram, 1986), whereas scavenging amphipods from depths of 1000–1700 m on
the Californian slope are less than 1 cm in length
(Smith, 1985; C.R. Smith, unpublished data). The
abyssal lysianassids also exhibit dramatic adaptations
for scavenging, including mouthparts capable of tearing
off and ingesting large chunks of flesh, and capacious
guts designed to store enormous quantities of food
(Dahl, 1979; Barnard and Ingram, 1986; Hargrave
et al., 1994). Adult E. gryllus, for example, can fill
their guts in less than thirty minutes, and apparently
can survive on one such meal for some 300 days
(Hargrave et al., 1994). The exploitation of carrion by
fewer species of more specialized necrophages in the
oligotrophic abyss suggests that large food falls provide
a higher proportion of the energy requirements for the
scavengers than on the California slope.
The epibenthic megafauna of the oligotrophic abyss
is dominated by deposit feeders in the form of
xenophyophores and the holothurian Amperima sp.,
although apparently suspension-feeding cnidarians are
also important (K.L. Smith, 1992). It should be noted,
however, that the xenophyophores could conceivably
occupy a number of trophic levels, because they may
have the potential to take up dissolved organic matter,
to prey on small metazoans, and to garden bacteria
(Levin and Gooday, 1992). Overall, megafaunal trophic
structure in the oligotrophic abyss appears to be fairly
similar to that in the eutrophic equatorial Pacific (see
discussion above), although the oligotrophic data base
is very slim. The infaunal macrobenthos is overwhelmingly dominated (93%) by deposit feeders, with
potential suspension feeders and carnivores/omnivores
constituting just 7% of abundance (Hessler and Jumars,
1974). Among the polychaetes, the deposit feeders
are divided roughly equally between surface- and
subsurface-deposit feeders, and nearly all are motile
(Hessler and Jumars, 1974; Jumars and Gallagher,
1982). Based on analogies with other deep-sea settings,
the infaunal meiobenthos of the oligotrophic abyss
is dominated (95%) by taxa (the Foraminiferida and
Nematoda) thought to indulge primarily in detritivory,
which may include scavenging, deposit-feeding, and
uptake of dissolved organic matter (Gooday et al.,
1992; Brown, 1998). Because of the extreme nature
of oligotrophic habitats, the feeding biology of the
oligotrophic meiobenthos merits greater direct study
before drawing strong conclusions about trophic composition.
In contrast to the infaunal macrofauna and meiofauna, a large proportion (>44%) of the noduleencrusting fauna in the oligotrophic abyss appear to
be suspension-feeders (Mullineaux, 1987). The trophic
(as well as taxonomic) composition of the oligotrophic
nodule fauna closely resembles that on nodules in
mesotrophic equatorial habitats (Mullineaux, 1987).
Rates of key ecological processes
The rates of very few key ecological processes have
been measured in the oligotrophic abyssal benthos.
Reliable data exist for sediment community respiration,
and respiration rates of the epibenthic megafauna
and benthopelagic fauna (specifically, rattails and
E. gryllus) also have been roughly estimated (e.g.,
K.L. Smith, 1992). Sediment-community respiration
rates, as estimated by seafloor respirometers, range
from 0.25 to 1.02 g C m
−2 y
−1 in the MPG-I area (K.L.
Smith, 1992). Because the microbiota and meiofauna
dominate the sediment community biomass (see above,
pp. 207–208), these size classes are likely to control
sediment-community respiration in the oligotrophic
abyss. These rates overlap the lower half of the range
for the mesotrophic equatorial Pacific, and are roughly
one-third to one-tenth of those for California-slope
habitats (Table 6.1). Respiration rates estimated for
the epibenthic metazoan megafauna (Amperima sp. and
cnidarians) and the benthopelagic fauna (scavenging
amphipods and rattails) are ~0.05 g C m
−2 y
−1 and
~0.001 g C m
−2 y
−1 , respectively (K.L. Smith, 1992).
Thus, the seemingly sparse epibenthic megafauna
appear to account for roughly 5–15% of total benthic
community respiration, while benthopelagic amphipods
and rattails account for no more than about 0.3%.
It should also be noted that sediment-community
respiration in the MPG−I area shows significant
temporal variability, which could be related to seasonal
or aperiodic pulses in the flux of particulate organic
carbon resulting from phytoplankton blooms (Smith
209
important. Oligotrophic scavenger aggregations differ
markedly from those from eutrophic slope habitats
in a number of intriguing ways. Oligotrophic aggregations are comprised of relatively few species
of fast swimmers (mostly amphipods and rattails),
and do not include the epibenthic ophiuroids, crabs
and polychaetes that are attracted to bait-falls on the
Californian slope (Hessler, 1974; Smith, 1985). The
body sizes of oligotrophic scavengers also tend to
be larger, the amphipod Eurythenes gryllus attaining
a length of 14 cm (Ingram and Hessler, 1983) and
Alicella gigantea reaching the “supergiant” size of
34 cm (Barnard and Ingram, 1986), whereas scavenging amphipods from depths of 1000–1700 m on
the Californian slope are less than 1 cm in length
(Smith, 1985; C.R. Smith, unpublished data). The
abyssal lysianassids also exhibit dramatic adaptations
for scavenging, including mouthparts capable of tearing
off and ingesting large chunks of flesh, and capacious
guts designed to store enormous quantities of food
(Dahl, 1979; Barnard and Ingram, 1986; Hargrave
et al., 1994). Adult E. gryllus, for example, can fill
their guts in less than thirty minutes, and apparently
can survive on one such meal for some 300 days
(Hargrave et al., 1994). The exploitation of carrion by
fewer species of more specialized necrophages in the
oligotrophic abyss suggests that large food falls provide
a higher proportion of the energy requirements for the
scavengers than on the California slope.
The epibenthic megafauna of the oligotrophic abyss
is dominated by deposit feeders in the form of
xenophyophores and the holothurian Amperima sp.,
although apparently suspension-feeding cnidarians are
also important (K.L. Smith, 1992). It should be noted,
however, that the xenophyophores could conceivably
occupy a number of trophic levels, because they may
have the potential to take up dissolved organic matter,
to prey on small metazoans, and to garden bacteria
(Levin and Gooday, 1992). Overall, megafaunal trophic
structure in the oligotrophic abyss appears to be fairly
similar to that in the eutrophic equatorial Pacific (see
discussion above), although the oligotrophic data base
is very slim. The infaunal macrobenthos is overwhelmingly dominated (93%) by deposit feeders, with
potential suspension feeders and carnivores/omnivores
constituting just 7% of abundance (Hessler and Jumars,
1974). Among the polychaetes, the deposit feeders
are divided roughly equally between surface- and
subsurface-deposit feeders, and nearly all are motile
(Hessler and Jumars, 1974; Jumars and Gallagher,
1982). Based on analogies with other deep-sea settings,
the infaunal meiobenthos of the oligotrophic abyss
is dominated (95%) by taxa (the Foraminiferida and
Nematoda) thought to indulge primarily in detritivory,
which may include scavenging, deposit-feeding, and
uptake of dissolved organic matter (Gooday et al.,
1992; Brown, 1998). Because of the extreme nature
of oligotrophic habitats, the feeding biology of the
oligotrophic meiobenthos merits greater direct study
before drawing strong conclusions about trophic composition.
In contrast to the infaunal macrofauna and meiofauna, a large proportion (>44%) of the noduleencrusting fauna in the oligotrophic abyss appear to
be suspension-feeders (Mullineaux, 1987). The trophic
(as well as taxonomic) composition of the oligotrophic
nodule fauna closely resembles that on nodules in
mesotrophic equatorial habitats (Mullineaux, 1987).
Rates of key ecological processes
The rates of very few key ecological processes have
been measured in the oligotrophic abyssal benthos.
Reliable data exist for sediment community respiration,
and respiration rates of the epibenthic megafauna
and benthopelagic fauna (specifically, rattails and
E. gryllus) also have been roughly estimated (e.g.,
K.L. Smith, 1992). Sediment-community respiration
rates, as estimated by seafloor respirometers, range
from 0.25 to 1.02 g C m
−2 y
−1 in the MPG-I area (K.L.
Smith, 1992). Because the microbiota and meiofauna
dominate the sediment community biomass (see above,
pp. 207–208), these size classes are likely to control
sediment-community respiration in the oligotrophic
abyss. These rates overlap the lower half of the range
for the mesotrophic equatorial Pacific, and are roughly
one-third to one-tenth of those for California-slope
habitats (Table 6.1). Respiration rates estimated for
the epibenthic metazoan megafauna (Amperima sp. and
cnidarians) and the benthopelagic fauna (scavenging
amphipods and rattails) are ~0.05 g C m
−2 y
−1 and
~0.001 g C m
−2 y
−1 , respectively (K.L. Smith, 1992).
Thus, the seemingly sparse epibenthic megafauna
appear to account for roughly 5–15% of total benthic
community respiration, while benthopelagic amphipods
and rattails account for no more than about 0.3%.
It should also be noted that sediment-community
respiration in the MPG−I area shows significant
temporal variability, which could be related to seasonal
or aperiodic pulses in the flux of particulate organic
carbon resulting from phytoplankton blooms (Smith