FOOD INPUTS, UTILIZATION, CARBON FLOW AND ENERGETICS
339
phytodetritus (Lambshead and Hodda, 1994; Rice and
Lambshead, 1994).
Response of larger metazoans to particle flux:
body-size scaling, particle selection and choice of
feeding method
The majority of multicellular organisms of macrofaunal
size and above inhabiting deep-sea sediments utilize
sedimented detrital particles as food. I have mentioned
above that dietary response is closely dependent
on body-size scaling. Whereas the smallest microorganisms and metazoans can rapidly expand their
populations in response to the availability of labile
particles, those of macrofaunal size and above have
life-spans extending well beyond the times of plenty.
It is not known whether this has proved to be
an important factor in the trend towards body-size
miniaturization observed in traditionally ‘macrofaunal’
taxa compared to shallow water. As a consequence
macrofaunal taxa are of meiofaunal size in the deepsea miniaturization (Thiel, 1975; Gage and Tyler,
1991; Rex and Etter, 1998). But little is known, for
example, of the potential abilities of metazoans to
select individual particles actively, despite presumably
powerful evolutionary pressures on deep-sea sedimentdwelling fauna towards adaptation to cope with scarcity
of food. One argument is that by reducing body size
and increasing motility organisms are able to forage
more efficiently for the few labile particles available
(Jumars et al., 1990). These authors cite observations
of relative enlargement of feeding palps in protobranch
bivalves in relation to their much diminished body size,
with gut contents packed with diatom remains (Allen
and Sanders, 1973), perhaps suggesting an ability to
evaluate individual particles before ingestion. Another
advantage of size is a relatively greater gut volume
compared to smaller organisms. In the deep sea,
organisms seem to have relatively large gut volumes
in relation to their size. Comparisons of closely related
species pairs conclusively show a greater gut volume
than in shallow water (e.g., Allen and Sanders, 1966).
However, because rates of deposit ingestion lag behind
the linear increase in gut volume in relation to body
size, residence times of material within the gut will be
longer in larger deposit feeders (Cammen, 1980), and
will favour more complete digestion and absorption of
particulate material (Jumars et al., 1990).
Simple calculations of probable individual feeding
rates of surface deposit-feeding animals from the
geometry of foraging radius in relation to estimates of
particle flux arriving on the bottom suggest that each
sediment particle will be repeatedly reingested before
its final burial. For larger metazoans of macrofaunal
size, or larger, the only exception to a constraint
to unselective, bulk particle processing will apply to
their early postlarval stages. Their small size will
enable them to feed selectively on individual particles
in the same way as other metazoans of meiofaunal
size (Jumars et al., 1990). However, as adults their
strategies for intercepting and ingesting these particles
are usually separated between suspension and deposit
feeders, depending on adaptations to either intercepting
somewhat unselectively particles suspended in the
water, or ingesting them in bulk as settled particles
forming part of the sediment. In shallow water attempts
to categorize particular fauna are often complicated
by the flexibility in feeding habits shown by many
species.
Feeding type and hydrodynamics on the
continental margin: localized populations of
suspension feeders
There are strong associations between the feeding types
in the benthic community and hydrodynamic conditions
at the continental margin. Typically, suspension-feeding
organisms dominate on the upper slope, while interface
feeders (able to feed on organic particles either
when lying on the surface or when resuspended by
currents) mainly occur in the higher-energy midslope zone. On the Celtic Sea continental slope off
Ireland, Flach and Thomsen (1998) found a peak
in density and biomass of benthic interface feeders
between depths of 1000 and 1500 m where flow is
most vigorous. Further north on the slope off Scotland,
dense populations of macrobenthic organisms, such as
the brittle star Ophiocten gracilis and cerianthid sea
anemones (Fig. 11.9), probably subsist on particles
moving over the sediment between depths of about 700
and 1000 m (Lamont and Gage, 1998). Below this zone
deposit feeders dominate at both sites (Flach and Heip,
1996; Flach et al., 1998, and personal data from the
Hebridean margin).
On the upper part of the continental slope, highly
depth-localized bands of suspension-feeding epifaunal
organsims may occur. Examples include cold-water
coral banks (Frederiksen et al., 1992), dense fields of
large species of demospongids (Klitgaard et al., 1996),
and, at greater depth on the slope, aggregations of
339
phytodetritus (Lambshead and Hodda, 1994; Rice and
Lambshead, 1994).
Response of larger metazoans to particle flux:
body-size scaling, particle selection and choice of
feeding method
The majority of multicellular organisms of macrofaunal
size and above inhabiting deep-sea sediments utilize
sedimented detrital particles as food. I have mentioned
above that dietary response is closely dependent
on body-size scaling. Whereas the smallest microorganisms and metazoans can rapidly expand their
populations in response to the availability of labile
particles, those of macrofaunal size and above have
life-spans extending well beyond the times of plenty.
It is not known whether this has proved to be
an important factor in the trend towards body-size
miniaturization observed in traditionally ‘macrofaunal’
taxa compared to shallow water. As a consequence
macrofaunal taxa are of meiofaunal size in the deepsea miniaturization (Thiel, 1975; Gage and Tyler,
1991; Rex and Etter, 1998). But little is known, for
example, of the potential abilities of metazoans to
select individual particles actively, despite presumably
powerful evolutionary pressures on deep-sea sedimentdwelling fauna towards adaptation to cope with scarcity
of food. One argument is that by reducing body size
and increasing motility organisms are able to forage
more efficiently for the few labile particles available
(Jumars et al., 1990). These authors cite observations
of relative enlargement of feeding palps in protobranch
bivalves in relation to their much diminished body size,
with gut contents packed with diatom remains (Allen
and Sanders, 1973), perhaps suggesting an ability to
evaluate individual particles before ingestion. Another
advantage of size is a relatively greater gut volume
compared to smaller organisms. In the deep sea,
organisms seem to have relatively large gut volumes
in relation to their size. Comparisons of closely related
species pairs conclusively show a greater gut volume
than in shallow water (e.g., Allen and Sanders, 1966).
However, because rates of deposit ingestion lag behind
the linear increase in gut volume in relation to body
size, residence times of material within the gut will be
longer in larger deposit feeders (Cammen, 1980), and
will favour more complete digestion and absorption of
particulate material (Jumars et al., 1990).
Simple calculations of probable individual feeding
rates of surface deposit-feeding animals from the
geometry of foraging radius in relation to estimates of
particle flux arriving on the bottom suggest that each
sediment particle will be repeatedly reingested before
its final burial. For larger metazoans of macrofaunal
size, or larger, the only exception to a constraint
to unselective, bulk particle processing will apply to
their early postlarval stages. Their small size will
enable them to feed selectively on individual particles
in the same way as other metazoans of meiofaunal
size (Jumars et al., 1990). However, as adults their
strategies for intercepting and ingesting these particles
are usually separated between suspension and deposit
feeders, depending on adaptations to either intercepting
somewhat unselectively particles suspended in the
water, or ingesting them in bulk as settled particles
forming part of the sediment. In shallow water attempts
to categorize particular fauna are often complicated
by the flexibility in feeding habits shown by many
species.
Feeding type and hydrodynamics on the
continental margin: localized populations of
suspension feeders
There are strong associations between the feeding types
in the benthic community and hydrodynamic conditions
at the continental margin. Typically, suspension-feeding
organisms dominate on the upper slope, while interface
feeders (able to feed on organic particles either
when lying on the surface or when resuspended by
currents) mainly occur in the higher-energy midslope zone. On the Celtic Sea continental slope off
Ireland, Flach and Thomsen (1998) found a peak
in density and biomass of benthic interface feeders
between depths of 1000 and 1500 m where flow is
most vigorous. Further north on the slope off Scotland,
dense populations of macrobenthic organisms, such as
the brittle star Ophiocten gracilis and cerianthid sea
anemones (Fig. 11.9), probably subsist on particles
moving over the sediment between depths of about 700
and 1000 m (Lamont and Gage, 1998). Below this zone
deposit feeders dominate at both sites (Flach and Heip,
1996; Flach et al., 1998, and personal data from the
Hebridean margin).
On the upper part of the continental slope, highly
depth-localized bands of suspension-feeding epifaunal
organsims may occur. Examples include cold-water
coral banks (Frederiksen et al., 1992), dense fields of
large species of demospongids (Klitgaard et al., 1996),
and, at greater depth on the slope, aggregations of