18
David THISTLE
(Jumars and Fauchald, 1977; Sokolova, 1997). Deepsea workers have frequently grouped species by feeding
mode. This approach has led to interesting results,
but few direct observations of the feeding of deepsea species have been made. Although some gutcontent studies have been done (Sokolova, 1994), most
inferences about how a deep-sea species feeds have
been based on knowledge of the feeding of its shallowwater relatives.
Deposit feeders
A deposit feeder ingests sediment. During gut
passage, the animal converts a portion of the organic
material contained in the sediment into a form that can
be assimilated. Deposit feeding is the dominant feeding
mode in the deep sea (Thiel, 1979). For example, at
an oligotrophic site in the abyssal Pacific, 93% of the
macrofauna were deposit feeders (Hessler and Jumars,
1974; see also Flach and Heip, 1996). The dominance
of deposit feeding may arise because the rain of organic
material into the deep sea consists primarily of small
particles of little food value. Deposit feeders apparently
can collect and process this material profitably despite
the costs of manipulating the mineral grains that they
simultaneously ingest.
Adaptations to deep-sea deposit feeding include an
increase in gut volume (Allen and Sanders, 1966).
The larger volume is thought to allow the rate of
sediment processing to increase without a decrease in
gut residence time or to allow gut residence time to
increase without a decrease in the rate of sediment
processing (Jumars and Wheatcroft, 1989). Either
adjustment would increase the rate of food assimilation
by organisms feeding on the relatively food-poor deepsea sediment as compared with that which could be
achieved with the gut morphology of a closely related
shallow-water species.
Deposit feeders can be grouped by the sediment
horizon at which they feed and by their mobility
(Jumars and Fauchald, 1977). Sessile surface-deposit
feeders remain in a fixed location and feed from
the sediment surface. Discretely motile surface-deposit
feeders move infrequently but must be stationary to
feed efficiently (echiuran worms: Ohta, 1984; Bett and
Rice, 1993). Both sessile and discretely motile surface
deposit feeders extend structures (a proboscis, palps,
tentacles) over the sediment surface to collect material.
Motile surface-deposit feeders (holothurians such as
Scotoplanes globosa) ingest sediment as they move
over the sediment surface. Subsurface deposit feeders
tend to be motile and feed as they burrow through the
sediment.
Among deposit feeders, some ecologically interesting patterns have been observed. The decrease in
the average size of macrofaunal deposit feeders as
depth increases (and the rate at which food reaches
the deep-sea floor decreases) was described above.
In addition, as depth increases from about 400 m to
that of the abyss, the proportion of sessile forms
among deposit-feeding polychaetes decreases (Jumars
and Fauchald, 1977; see also Rowe et al., 1982).
Jumars and Fauchald (1977) suggested that this pattern
could arise if the maximum feeding radius of sessile
surface-deposit feeders were fixed (e.g., because of
mechanical limitations to the length of polychaete
tentacles). Therefore, as food flux decreases, fewer
sessile deposit feeders are able to reach a large enough
area to survive. Because the foraging areas of motile
polychaete deposit feeders do not have such mechanical
limits, they would not be as much affected by the
decrease in food flux.
The rules can be different in areas that experience
strong near-bottom flows. For example, at such a site
at a depth at which sessile deposit-feeding polychaetes
should be rare, the dominant polychaete is a sessile
deposit feeder (Thistle et al., 1985). This species digs
a pit around itself approximately 1 cm deep and 4 cm
in diameter. As the near-bottom flow encounters the
pit, the streamlines of the flow expand and its speed
decreases (by the principle of continuity: Vogel, 1981).
When the speed of the flow decreases, its capacity to
transport particles (including food particles) is reduced,
which increases the flux of food particles to the bed.
The worm harvests these particles (Nowell et al., 1984)
and thus can occur in large numbers at a depth where
sessile feeding on deposits would not be expected to
function well.
Exploiters of large food parcels
Not all of the food that enters the deep sea does
so as small particles of little food value. For example,
the carcasses of fishes and whales reach the sea
floor. These high-quality food parcels are rare (Smith
et al., 1989) but attract a subset of the fauna. These
“parcel-attending species” include necrophages, which
consume the carcass directly, and species that benefit
indirectly from the food fall. The parcel attenders
include certain species of demersal fishes (Dayton and
Hessler, 1972; Smith, 1985), amphipods of the family
Lysianassidae (Hessler et al., 1978; Thurston, 1979),
David THISTLE
(Jumars and Fauchald, 1977; Sokolova, 1997). Deepsea workers have frequently grouped species by feeding
mode. This approach has led to interesting results,
but few direct observations of the feeding of deepsea species have been made. Although some gutcontent studies have been done (Sokolova, 1994), most
inferences about how a deep-sea species feeds have
been based on knowledge of the feeding of its shallowwater relatives.
Deposit feeders
A deposit feeder ingests sediment. During gut
passage, the animal converts a portion of the organic
material contained in the sediment into a form that can
be assimilated. Deposit feeding is the dominant feeding
mode in the deep sea (Thiel, 1979). For example, at
an oligotrophic site in the abyssal Pacific, 93% of the
macrofauna were deposit feeders (Hessler and Jumars,
1974; see also Flach and Heip, 1996). The dominance
of deposit feeding may arise because the rain of organic
material into the deep sea consists primarily of small
particles of little food value. Deposit feeders apparently
can collect and process this material profitably despite
the costs of manipulating the mineral grains that they
simultaneously ingest.
Adaptations to deep-sea deposit feeding include an
increase in gut volume (Allen and Sanders, 1966).
The larger volume is thought to allow the rate of
sediment processing to increase without a decrease in
gut residence time or to allow gut residence time to
increase without a decrease in the rate of sediment
processing (Jumars and Wheatcroft, 1989). Either
adjustment would increase the rate of food assimilation
by organisms feeding on the relatively food-poor deepsea sediment as compared with that which could be
achieved with the gut morphology of a closely related
shallow-water species.
Deposit feeders can be grouped by the sediment
horizon at which they feed and by their mobility
(Jumars and Fauchald, 1977). Sessile surface-deposit
feeders remain in a fixed location and feed from
the sediment surface. Discretely motile surface-deposit
feeders move infrequently but must be stationary to
feed efficiently (echiuran worms: Ohta, 1984; Bett and
Rice, 1993). Both sessile and discretely motile surface
deposit feeders extend structures (a proboscis, palps,
tentacles) over the sediment surface to collect material.
Motile surface-deposit feeders (holothurians such as
Scotoplanes globosa) ingest sediment as they move
over the sediment surface. Subsurface deposit feeders
tend to be motile and feed as they burrow through the
sediment.
Among deposit feeders, some ecologically interesting patterns have been observed. The decrease in
the average size of macrofaunal deposit feeders as
depth increases (and the rate at which food reaches
the deep-sea floor decreases) was described above.
In addition, as depth increases from about 400 m to
that of the abyss, the proportion of sessile forms
among deposit-feeding polychaetes decreases (Jumars
and Fauchald, 1977; see also Rowe et al., 1982).
Jumars and Fauchald (1977) suggested that this pattern
could arise if the maximum feeding radius of sessile
surface-deposit feeders were fixed (e.g., because of
mechanical limitations to the length of polychaete
tentacles). Therefore, as food flux decreases, fewer
sessile deposit feeders are able to reach a large enough
area to survive. Because the foraging areas of motile
polychaete deposit feeders do not have such mechanical
limits, they would not be as much affected by the
decrease in food flux.
The rules can be different in areas that experience
strong near-bottom flows. For example, at such a site
at a depth at which sessile deposit-feeding polychaetes
should be rare, the dominant polychaete is a sessile
deposit feeder (Thistle et al., 1985). This species digs
a pit around itself approximately 1 cm deep and 4 cm
in diameter. As the near-bottom flow encounters the
pit, the streamlines of the flow expand and its speed
decreases (by the principle of continuity: Vogel, 1981).
When the speed of the flow decreases, its capacity to
transport particles (including food particles) is reduced,
which increases the flux of food particles to the bed.
The worm harvests these particles (Nowell et al., 1984)
and thus can occur in large numbers at a depth where
sessile feeding on deposits would not be expected to
function well.
Exploiters of large food parcels
Not all of the food that enters the deep sea does
so as small particles of little food value. For example,
the carcasses of fishes and whales reach the sea
floor. These high-quality food parcels are rare (Smith
et al., 1989) but attract a subset of the fauna. These
“parcel-attending species” include necrophages, which
consume the carcass directly, and species that benefit
indirectly from the food fall. The parcel attenders
include certain species of demersal fishes (Dayton and
Hessler, 1972; Smith, 1985), amphipods of the family
Lysianassidae (Hessler et al., 1978; Thurston, 1979),