THE DEEP-SEA FLOOR: AN OVERVIEW
19
decapod shrimps (Thurston et al., 1995), gastropods
(Tamburri and Barry, 1999), and brittle stars (Smith,
1985). Whether any species depends exclusively on carcasses has not yet been shown (Jumars and Gallagher,
1982; Ingram and Hessler, 1983), but some omnivorous
species include carcasses adventitiously in their diets
(Smith, 1985; Priede et al., 1991).
The response of the parcel attenders to carcasses
placed on the seafloor has revealed much about their
ecology. Minutes to hours after a bait parcel is
placed on the seafloor, swimming parcel attenders
begin to arrive; nonswimmers arrive more slowly. Both
approach predominantly from down current (Dayton
and Hessler, 1972; Thurston, 1979; Smith, 1985),
attracted by a current-borne cue, probably odor (SainteMarie, 1992). These animals feed voraciously until
their guts are full. Satiated individuals leave the carcass
but remain in the vicinity, perhaps to optimize digestive
efficiency (Smith and Baldwin, 1982) or to return to
the carcass after the gut is partially emptied (Smith,
1985). At peak abundance around a fish carcass, tens
of fishes, hundreds of amphipods, and hundreds of
brittle stars may be present (although these peaks are
not simultaneous) (Smith, 1985). These abundances are
many times greater than abundances in the background
community, so carcasses cause local concentrations
of individuals. As the amount of flesh decreases, the
parcel attenders disperse. Some species depart while
some flesh remains; others remain weeks after the
flesh has been consumed (Smith, 1985). Dispersal
distances may be a few meters for walkers, such as
brittle stars; but Priede et al. (1990) have shown that
food-parcel-attending fishes disperse more than 500 m.
Of the parcel attenders, amphipods are best known
biologically (but see Tamburri and Barry, 1999, for
other taxa). According to Smith and Baldwin (1982),
these crustaceans survive the long periods between
food parcels by greatly reducing their metabolic rate
while retaining an acute sensitivity to the arrival of
carcasses at the seafloor. When they detect the odor
from a carcass, they rapidly increase their metabolic
rate and begin a period of sustained swimming toward
the bait. To maximize consumption at the food parcel,
they feed rapidly, filling their extensible guts. At
satiation, the gut fills most of the exoskeleton, which
can be greatly distended (Shulenberger and Hessler,
1974; Dahl, 1979). The ingested material is rapidly
digested (95% in 1–10 days), making space in the gut
for more food and increasing the flexibility of the body
for swimming (Hargrave et al., 1995). Younger stages
need to feed more frequently than adults, but all can
survive for months between meals (Hargrave et al.,
1994).
Differences in behavior and morphology suggest that
groups of parcel attenders have different strategies.
For example, some parcel-attending amphipods have
shearing mandibles. They consume bait rapidly and
probably combine scavenging and carnivory in their
feeding strategy. Other parcel-attending amphipods
have triturating mandibles and combine scavenging
with detritivory (Sainte-Marie, 1992). Jones et al.
(1998) have reported that the former arrive first at the
carcass and are replaced by the latter over time. Ingram
and Hessler (1983) found that the populations of three
species of small-bodied, parcel-attending amphipods
were concentrated about 1 m above the bottom and
that the population of a larger-bodied species was
concentrated about 50 m above the bottom. Turbulent
mixing in the bottom boundary layer causes the
chemical signal from a carcass to widen and to
increase in vertical extent with increasing distance
from a carcass, while it simultaneously decreases in
concentration. Ingram and Hessler (1983) therefore
suggested that the two groups of species exploited
the carcass resource differently. The high-hovering
species surveys a wide area and detects primarily
large carcasses. The low-hovering species detect the
full range of carcass sizes but from a smaller area.
These ideas are suggestive, but depend on the untested
assumptions that carcasses produce chemical signals
in proportion to their sizes, and that the threshold
concentrations at which a signal can be detected are
approximately the same for the two guilds (Jumars and
Gallagher, 1982). Also, differences between guilds in
swimming speed and ability to sequester food are likely
to be necessary to explain why the optimal foraging
height for the small-bodied species is lower than that
for the large-bodied species (see also Sainte-Marie,
1992).
After leaving the carcass, necrophages transfer
calories and nutrients to other deep-sea soft-bottom
organisms by defecating (Dayton and Hessler, 1972).
Smith (1985) estimated that about 3% of the energy
required by a bathyal benthic community can be
provided in this way (see also Stockton and DeLaca,
1982).
The concentration of potential prey that a carcass
attracts may itself be a resource. Jones et al. (1998)
reported that none of the fish species attending cetacean
carcasses that they placed in the abyssal Atlantic
19
decapod shrimps (Thurston et al., 1995), gastropods
(Tamburri and Barry, 1999), and brittle stars (Smith,
1985). Whether any species depends exclusively on carcasses has not yet been shown (Jumars and Gallagher,
1982; Ingram and Hessler, 1983), but some omnivorous
species include carcasses adventitiously in their diets
(Smith, 1985; Priede et al., 1991).
The response of the parcel attenders to carcasses
placed on the seafloor has revealed much about their
ecology. Minutes to hours after a bait parcel is
placed on the seafloor, swimming parcel attenders
begin to arrive; nonswimmers arrive more slowly. Both
approach predominantly from down current (Dayton
and Hessler, 1972; Thurston, 1979; Smith, 1985),
attracted by a current-borne cue, probably odor (SainteMarie, 1992). These animals feed voraciously until
their guts are full. Satiated individuals leave the carcass
but remain in the vicinity, perhaps to optimize digestive
efficiency (Smith and Baldwin, 1982) or to return to
the carcass after the gut is partially emptied (Smith,
1985). At peak abundance around a fish carcass, tens
of fishes, hundreds of amphipods, and hundreds of
brittle stars may be present (although these peaks are
not simultaneous) (Smith, 1985). These abundances are
many times greater than abundances in the background
community, so carcasses cause local concentrations
of individuals. As the amount of flesh decreases, the
parcel attenders disperse. Some species depart while
some flesh remains; others remain weeks after the
flesh has been consumed (Smith, 1985). Dispersal
distances may be a few meters for walkers, such as
brittle stars; but Priede et al. (1990) have shown that
food-parcel-attending fishes disperse more than 500 m.
Of the parcel attenders, amphipods are best known
biologically (but see Tamburri and Barry, 1999, for
other taxa). According to Smith and Baldwin (1982),
these crustaceans survive the long periods between
food parcels by greatly reducing their metabolic rate
while retaining an acute sensitivity to the arrival of
carcasses at the seafloor. When they detect the odor
from a carcass, they rapidly increase their metabolic
rate and begin a period of sustained swimming toward
the bait. To maximize consumption at the food parcel,
they feed rapidly, filling their extensible guts. At
satiation, the gut fills most of the exoskeleton, which
can be greatly distended (Shulenberger and Hessler,
1974; Dahl, 1979). The ingested material is rapidly
digested (95% in 1–10 days), making space in the gut
for more food and increasing the flexibility of the body
for swimming (Hargrave et al., 1995). Younger stages
need to feed more frequently than adults, but all can
survive for months between meals (Hargrave et al.,
1994).
Differences in behavior and morphology suggest that
groups of parcel attenders have different strategies.
For example, some parcel-attending amphipods have
shearing mandibles. They consume bait rapidly and
probably combine scavenging and carnivory in their
feeding strategy. Other parcel-attending amphipods
have triturating mandibles and combine scavenging
with detritivory (Sainte-Marie, 1992). Jones et al.
(1998) have reported that the former arrive first at the
carcass and are replaced by the latter over time. Ingram
and Hessler (1983) found that the populations of three
species of small-bodied, parcel-attending amphipods
were concentrated about 1 m above the bottom and
that the population of a larger-bodied species was
concentrated about 50 m above the bottom. Turbulent
mixing in the bottom boundary layer causes the
chemical signal from a carcass to widen and to
increase in vertical extent with increasing distance
from a carcass, while it simultaneously decreases in
concentration. Ingram and Hessler (1983) therefore
suggested that the two groups of species exploited
the carcass resource differently. The high-hovering
species surveys a wide area and detects primarily
large carcasses. The low-hovering species detect the
full range of carcass sizes but from a smaller area.
These ideas are suggestive, but depend on the untested
assumptions that carcasses produce chemical signals
in proportion to their sizes, and that the threshold
concentrations at which a signal can be detected are
approximately the same for the two guilds (Jumars and
Gallagher, 1982). Also, differences between guilds in
swimming speed and ability to sequester food are likely
to be necessary to explain why the optimal foraging
height for the small-bodied species is lower than that
for the large-bodied species (see also Sainte-Marie,
1992).
After leaving the carcass, necrophages transfer
calories and nutrients to other deep-sea soft-bottom
organisms by defecating (Dayton and Hessler, 1972).
Smith (1985) estimated that about 3% of the energy
required by a bathyal benthic community can be
provided in this way (see also Stockton and DeLaca,
1982).
The concentration of potential prey that a carcass
attracts may itself be a resource. Jones et al. (1998)
reported that none of the fish species attending cetacean
carcasses that they placed in the abyssal Atlantic