THE DEEP PACIFIC OCEAN FLOOR
207
on surface deposits, and an unidentified, suspensionfeeding cnidarian (K.L. Smith, 1992). The combined
densities of these metazoans is ~0.15 m
−2 , which is
comparable to the abundance of metazoan megafauna
in the mesotrophic abyss (Table 6.1). Xenophyophores
appear to be common relative to metazoan megabenthos (K.L. Smith, 1992), but their identification and
abundance remain unknown.
The scavenging megafauna have been well studied
in the oligotrophic abyss, in part because of their
potential to disperse radioactive wastes spilled on the
seafloor. In the absence of food falls, scavenging
megafauna rarely appear in photographs and are
likely to be very sparsely distributed. Nonetheless,
baited-trap and camera deployments rapidly attract a
voracious assemblage of highly mobile necrophages
(Dayton and Hessler, 1972; Hessler, 1974; Ingram and
Hessler, 1983). Scavengers include two species of giant
lysianassid amphipods reaching lengths greater than
10 cm (Alicella gigantea and Eurythenes gryllus), a
suite of smaller lysiannassids a few centimeters in
length (Orchomene gerulicorbis, Paralicella caperesca
and P. tenuipes), rattail fish (Coryphaenoides armatus)
and natantian decapods (Hessler et al., 1972; Hessler,
1974; Ingram and Hessler, 1983, 1987; Barnard and
Ingram, 1986; Priede et al., 1991). All these scavengers
are very good swimmers, typically arriving at bait-falls
within minutes to hours and achieving concentrations
of tens to hundreds of individuals at single bait-falls
(Dayton and Hessler, 1972; Priede et al., 1994). Bait
parcels are consumed very rapidly, tens of kilograms
of fish flesh being eaten within 12 to 24 hr (Dayton
and Hessler, 1972; Hessler, 1974). The abundance
and biomass of mobile scavengers is very difficult
to evaluate, but Priede et al. (1990, 1994) and
K.L. Smith (1992) have used arrival times at baits
to estimate roughly the abundance and biomass of
Eurythenes gryllus (3.5 to 47.2 individuals km
−2 and
0.5 to 6.2 g wet weight km
−2 ) and Coryphaenoides
armatus (330 individuals km
−2 and ~150 kg wet
weight km
−2 ) in the oligotrophic abyss.
The macrofauna of the oligotrophic abyss is very
sparse, diminutive in body size, and yet highly diverse.
Densities of infaunal metazoan macrobenthos at the
CLIMAX II site range from 64 to 160 individuals m
−2 , that is, they are roughly one-hundredth
as numerous as those in oxygenated slope habitats. Macrofaunal abundance is dominated by polychaetes (55%), tanaids (18%), bivalves (7%) and
isopods (6%) (Hessler and Jumars, 1974); thus, the
polychaetes are somewhat less important, and the
tanaids substantially more important, than on the slope
(Hessler, 1974). At the familial level, the macrofauna
has substantial proportions of cirratulid (25%), capitellid (14%), fauveliopsid (11%), and paraonid (>6%)
polychaetes (Hessler and Jumars, 1974); these families
are also prominent in equatorial and California-slope
sediments (Kukert and Smith, 1992; Borowski and
Thiel, 1998). Mean macrofaunal body size is very
small at ~0.07 mg (Table 6.1) – that is, nearly an
order of magnitude lower than in the equatorial abyss.
Total macrofaunal biomass (0.02–0.12 mg m
−2 : K.L.
Smith, 1992) is roughly two orders of magnitude lower
than in slope settings, and somewhat lower than in
the mesotrophic abyss (Table 6.1). Species diversity
in the oligotrophic macrofauna is very high, even by
deep-sea standards, with more than 45 species found
among 100 polychaete individuals from pooled boxcore samples (Fig. 6.7; Hessler and Jumars, 1974).
Because of the low standing crop of macrobenthos,
however, the number of macrofaunal species in any unit
area of seafloor is relatively low.
The macrofaunal size class in the oligotrophic abyss
also includes the relatively abundant Komokiacea, a
group of agglutinating protozoans (Foraminifera) in
which the test consists of systems of fine tubules
(Tendal and Hessler, 1977). The tests of these protists
frequently reach several centimeters in diameter, but
their standing crop is difficult to evaluate because they
usually fragment, and because their diffuse protoplasm
occupies only a small proportion of their test volume
(Tendal and Hessler, 1977).
The meiobenthos have been studied at one oligotrophic abyssal site (MPG-I) and appear to constitute
a major component of the infaunal benthos. Snider
et al. (1984) found 202 000 meiofaunal individuals m
−2 ,
with 90% of them concentrated in the top 3 cm of
sediment. Foraminifera and nematodes accounted for
the bulk of meiofaunal abundance (50% and 45%,
respectively), with harpacticoids (5%) also occurring
frequently. Tardigrades, ostracods, kinorhynchs and
gastrotrichs constituted less than 1% of the meiofauna
(Snider et al., 1984). Meiofaunal biomass (0.24 mg wet
weight m
−2 ) was dominated by the Foraminifera (87%)
nematodes (7%) and harpacticoids (6%).
The sediment microbes (or nanobenthos) larger than
10 mm in diameter were also studied at MPG-I by
Snider et al. (1984). In decreasing order of numerical
importance, these consisted of prokaryotes (e.g., large
bacteria), “yeast-like” cells, flagellates, and amoebae.
207
on surface deposits, and an unidentified, suspensionfeeding cnidarian (K.L. Smith, 1992). The combined
densities of these metazoans is ~0.15 m
−2 , which is
comparable to the abundance of metazoan megafauna
in the mesotrophic abyss (Table 6.1). Xenophyophores
appear to be common relative to metazoan megabenthos (K.L. Smith, 1992), but their identification and
abundance remain unknown.
The scavenging megafauna have been well studied
in the oligotrophic abyss, in part because of their
potential to disperse radioactive wastes spilled on the
seafloor. In the absence of food falls, scavenging
megafauna rarely appear in photographs and are
likely to be very sparsely distributed. Nonetheless,
baited-trap and camera deployments rapidly attract a
voracious assemblage of highly mobile necrophages
(Dayton and Hessler, 1972; Hessler, 1974; Ingram and
Hessler, 1983). Scavengers include two species of giant
lysianassid amphipods reaching lengths greater than
10 cm (Alicella gigantea and Eurythenes gryllus), a
suite of smaller lysiannassids a few centimeters in
length (Orchomene gerulicorbis, Paralicella caperesca
and P. tenuipes), rattail fish (Coryphaenoides armatus)
and natantian decapods (Hessler et al., 1972; Hessler,
1974; Ingram and Hessler, 1983, 1987; Barnard and
Ingram, 1986; Priede et al., 1991). All these scavengers
are very good swimmers, typically arriving at bait-falls
within minutes to hours and achieving concentrations
of tens to hundreds of individuals at single bait-falls
(Dayton and Hessler, 1972; Priede et al., 1994). Bait
parcels are consumed very rapidly, tens of kilograms
of fish flesh being eaten within 12 to 24 hr (Dayton
and Hessler, 1972; Hessler, 1974). The abundance
and biomass of mobile scavengers is very difficult
to evaluate, but Priede et al. (1990, 1994) and
K.L. Smith (1992) have used arrival times at baits
to estimate roughly the abundance and biomass of
Eurythenes gryllus (3.5 to 47.2 individuals km
−2 and
0.5 to 6.2 g wet weight km
−2 ) and Coryphaenoides
armatus (330 individuals km
−2 and ~150 kg wet
weight km
−2 ) in the oligotrophic abyss.
The macrofauna of the oligotrophic abyss is very
sparse, diminutive in body size, and yet highly diverse.
Densities of infaunal metazoan macrobenthos at the
CLIMAX II site range from 64 to 160 individuals m
−2 , that is, they are roughly one-hundredth
as numerous as those in oxygenated slope habitats. Macrofaunal abundance is dominated by polychaetes (55%), tanaids (18%), bivalves (7%) and
isopods (6%) (Hessler and Jumars, 1974); thus, the
polychaetes are somewhat less important, and the
tanaids substantially more important, than on the slope
(Hessler, 1974). At the familial level, the macrofauna
has substantial proportions of cirratulid (25%), capitellid (14%), fauveliopsid (11%), and paraonid (>6%)
polychaetes (Hessler and Jumars, 1974); these families
are also prominent in equatorial and California-slope
sediments (Kukert and Smith, 1992; Borowski and
Thiel, 1998). Mean macrofaunal body size is very
small at ~0.07 mg (Table 6.1) – that is, nearly an
order of magnitude lower than in the equatorial abyss.
Total macrofaunal biomass (0.02–0.12 mg m
−2 : K.L.
Smith, 1992) is roughly two orders of magnitude lower
than in slope settings, and somewhat lower than in
the mesotrophic abyss (Table 6.1). Species diversity
in the oligotrophic macrofauna is very high, even by
deep-sea standards, with more than 45 species found
among 100 polychaete individuals from pooled boxcore samples (Fig. 6.7; Hessler and Jumars, 1974).
Because of the low standing crop of macrobenthos,
however, the number of macrofaunal species in any unit
area of seafloor is relatively low.
The macrofaunal size class in the oligotrophic abyss
also includes the relatively abundant Komokiacea, a
group of agglutinating protozoans (Foraminifera) in
which the test consists of systems of fine tubules
(Tendal and Hessler, 1977). The tests of these protists
frequently reach several centimeters in diameter, but
their standing crop is difficult to evaluate because they
usually fragment, and because their diffuse protoplasm
occupies only a small proportion of their test volume
(Tendal and Hessler, 1977).
The meiobenthos have been studied at one oligotrophic abyssal site (MPG-I) and appear to constitute
a major component of the infaunal benthos. Snider
et al. (1984) found 202 000 meiofaunal individuals m
−2 ,
with 90% of them concentrated in the top 3 cm of
sediment. Foraminifera and nematodes accounted for
the bulk of meiofaunal abundance (50% and 45%,
respectively), with harpacticoids (5%) also occurring
frequently. Tardigrades, ostracods, kinorhynchs and
gastrotrichs constituted less than 1% of the meiofauna
(Snider et al., 1984). Meiofaunal biomass (0.24 mg wet
weight m
−2 ) was dominated by the Foraminifera (87%)
nematodes (7%) and harpacticoids (6%).
The sediment microbes (or nanobenthos) larger than
10 mm in diameter were also studied at MPG-I by
Snider et al. (1984). In decreasing order of numerical
importance, these consisted of prokaryotes (e.g., large
bacteria), “yeast-like” cells, flagellates, and amoebae.