342
John D. GAGE
where dynamic flow regimes often prevail. The colonial, cold-water coral Lophelia pertusa is able to take in
particles up to the size of live zooplankton (Frederiksen
et al., 1992), while other large epifaunal suspension
feeders, such as demospongids are limited to smaller
particles from bacterial size up to a few microns
(Witte et al., 1997). However, study of the gut contents
of the abyssal sea anemone Sicyonis tuberculata,
with a tentacle spread of c. 28 cm, indicates that
this suspension feeder is able to take particles in
a wide size-range, from less than 4 mm across to
motile megafauna of similar size to itself (Lampitt and
Paterson, 1987). This gradation by sessile suspension
feeders into carnivory is characteristic of other groups
in the abyss, such as benthic tunicates (Monniot and
Monniot, 1978). Lampitt and Paterson (1987) also indicated that a similar microphagous and macrophagous
diet applied to the deep-sea anemone Actinoscyphia
aurelia, previously thought to be an obligate detrivore
(Aldred et al., 1979). Values for d
13 N in stable isotope
studies undertaken by Iken et al. (2001) on megafauna
from the BENGAL site on the Porcupine Abyssal Plain
have confirmed that sessile microphagous organisms
such as the sea pen Umbellula sp. show varying
values indicative of feeding on higher trophic levels
as well. In deep-sea sponges Iken et al. (2001) found
even higher d
13 N values in an hexactinellid sponge,
but this possibly may result from symbiotic bacteria
as well as small particles serving as a source of
nutrition, as has been described for some shallow-water
sponges (Arillo et al., 1993). Iken et al. speculated
that such bacteria may be able to metabolize highly
refractory material, which can then be assimilated by
the sponge. Little else seems to be known of the
nature or size spectra of the particles taken by deepsea suspension feeders. However, seabed photographs
commonly show the tentacle crowns of such organisms
actively orientated upstream to the direction of flow.
Lampitt and Paterson (1987) were able to monitor
the upstream orientation of Sicyonis tuberculata in
relation to changing flow direction and speed using
the in situ time-lapse camera system Bathysnap, and
from this estimated that the anemone feeds about
150 times per day on a wide range of material. The
disc-shaped filter facing into the prevailing current is
particularly efficient at intercepting suspended particles
(Leversee, 1976; Warner, 1977). This explains why
feeding is influenced by the fan shape of sessile
suspension feeders, such as gorgonians. However, deepsea gorgonians frequently also have a low, bushy
structure which will be equally efficient in all directions
of flow.
Importance of bacteria attached to suspended
particles as food
It has been observed that erosion/deposition cycles
that are characteristic of areas subjected to very strong
flow stimulate the growth of sedimentary bacteria
attached to lithogenic sediment particles (Yingst and
Rhoads, 1980; Wainright, 1987). These particles will
then be of some nutritive value to bottom biota that
are able to intercept them in suspension or when
redeposited. However, many suspension feeders are
not abundant in the most energetic sites, perhaps
because such high densities of suspended particles act
to block filtering mechanisms (Thistle et al., 1985).
They may, however, be more abundant at slightly less
energetic sites (Gage et al., 1983), along with a higherthan-expected macrofaunal biomass (e.g., Gage, 1979).
Shimeta and Jumars (1991) have reviewed the complex
suite of factors affecting efficiency in capturing and
ingesting particles by suspension feeders.
Interface feeders
Studies of some smaller macrofauna, such as spionid
polychaetes and Foraminifera, have revealed a response
where the organism, by means of a relatively simple
change in behaviour, may change to feeding on
suspended particles rather than on particles lying on the
sediment surface. These ‘interface’ feeders can switch
to suspension feeding, depending on flow energy,
by a simple re-orientation of feeding appendages
into the overlying water (Taghon et al., 1980; Dauer
et al., 1981). Switching modes may be associated
with markedly differing rates of particle ingestion and
somatic growth (Taghon and Greene, 1992). Thomsen
et al. (1995) provided experimental proof that interface
feeding may be very effective in removing suspended
material from near-bed flow (Fig. 11.14). Flach and
Heip (1996) and Flach et al. (1998) came to the
startling conclusion that interface feeders (mainly
branching, arborescent agglutinating foraminiferans
able to use a pseudopodial net to catch particles)
may be the predominant feeding mode in smaller
macrobenthos at intermediate depths on the continental
slope. This, perhaps as much as any measurement
of physical conditions, underlines the short temporal
scales of change in flow conditions that characterize
conditions in the deep-sea benthic boundary layer on
the slope.
John D. GAGE
where dynamic flow regimes often prevail. The colonial, cold-water coral Lophelia pertusa is able to take in
particles up to the size of live zooplankton (Frederiksen
et al., 1992), while other large epifaunal suspension
feeders, such as demospongids are limited to smaller
particles from bacterial size up to a few microns
(Witte et al., 1997). However, study of the gut contents
of the abyssal sea anemone Sicyonis tuberculata,
with a tentacle spread of c. 28 cm, indicates that
this suspension feeder is able to take particles in
a wide size-range, from less than 4 mm across to
motile megafauna of similar size to itself (Lampitt and
Paterson, 1987). This gradation by sessile suspension
feeders into carnivory is characteristic of other groups
in the abyss, such as benthic tunicates (Monniot and
Monniot, 1978). Lampitt and Paterson (1987) also indicated that a similar microphagous and macrophagous
diet applied to the deep-sea anemone Actinoscyphia
aurelia, previously thought to be an obligate detrivore
(Aldred et al., 1979). Values for d
13 N in stable isotope
studies undertaken by Iken et al. (2001) on megafauna
from the BENGAL site on the Porcupine Abyssal Plain
have confirmed that sessile microphagous organisms
such as the sea pen Umbellula sp. show varying
values indicative of feeding on higher trophic levels
as well. In deep-sea sponges Iken et al. (2001) found
even higher d
13 N values in an hexactinellid sponge,
but this possibly may result from symbiotic bacteria
as well as small particles serving as a source of
nutrition, as has been described for some shallow-water
sponges (Arillo et al., 1993). Iken et al. speculated
that such bacteria may be able to metabolize highly
refractory material, which can then be assimilated by
the sponge. Little else seems to be known of the
nature or size spectra of the particles taken by deepsea suspension feeders. However, seabed photographs
commonly show the tentacle crowns of such organisms
actively orientated upstream to the direction of flow.
Lampitt and Paterson (1987) were able to monitor
the upstream orientation of Sicyonis tuberculata in
relation to changing flow direction and speed using
the in situ time-lapse camera system Bathysnap, and
from this estimated that the anemone feeds about
150 times per day on a wide range of material. The
disc-shaped filter facing into the prevailing current is
particularly efficient at intercepting suspended particles
(Leversee, 1976; Warner, 1977). This explains why
feeding is influenced by the fan shape of sessile
suspension feeders, such as gorgonians. However, deepsea gorgonians frequently also have a low, bushy
structure which will be equally efficient in all directions
of flow.
Importance of bacteria attached to suspended
particles as food
It has been observed that erosion/deposition cycles
that are characteristic of areas subjected to very strong
flow stimulate the growth of sedimentary bacteria
attached to lithogenic sediment particles (Yingst and
Rhoads, 1980; Wainright, 1987). These particles will
then be of some nutritive value to bottom biota that
are able to intercept them in suspension or when
redeposited. However, many suspension feeders are
not abundant in the most energetic sites, perhaps
because such high densities of suspended particles act
to block filtering mechanisms (Thistle et al., 1985).
They may, however, be more abundant at slightly less
energetic sites (Gage et al., 1983), along with a higherthan-expected macrofaunal biomass (e.g., Gage, 1979).
Shimeta and Jumars (1991) have reviewed the complex
suite of factors affecting efficiency in capturing and
ingesting particles by suspension feeders.
Interface feeders
Studies of some smaller macrofauna, such as spionid
polychaetes and Foraminifera, have revealed a response
where the organism, by means of a relatively simple
change in behaviour, may change to feeding on
suspended particles rather than on particles lying on the
sediment surface. These ‘interface’ feeders can switch
to suspension feeding, depending on flow energy,
by a simple re-orientation of feeding appendages
into the overlying water (Taghon et al., 1980; Dauer
et al., 1981). Switching modes may be associated
with markedly differing rates of particle ingestion and
somatic growth (Taghon and Greene, 1992). Thomsen
et al. (1995) provided experimental proof that interface
feeding may be very effective in removing suspended
material from near-bed flow (Fig. 11.14). Flach and
Heip (1996) and Flach et al. (1998) came to the
startling conclusion that interface feeders (mainly
branching, arborescent agglutinating foraminiferans
able to use a pseudopodial net to catch particles)
may be the predominant feeding mode in smaller
macrobenthos at intermediate depths on the continental
slope. This, perhaps as much as any measurement
of physical conditions, underlines the short temporal
scales of change in flow conditions that characterize
conditions in the deep-sea benthic boundary layer on
the slope.