346
John D. GAGE
is too simplistic. These authors studied the elasipod
holothurian, Oneirophanta mutabilis, in the abyssal
Northeast Atlantic where it is the dominant component
of the motile invertebrate megafauna. Simulations
based on estimated population density and movement
data from time-lapse photographs showed very different outcomes in terms of time required for 50%
coverage of the sea bed, ranging from 17 years for a
random-ranging strategy to 12 days for a systematic
search. A simulation based on observed speeds and
turning angles gives a 50% coverage in 9.6 years.
Evidence for particle selectivity comes from particleselection studies showing deposit feeders very commonly show strong selection for particles low in specific gravity and those covered with organic coatings.
This particularly applies to surface deposit feeders, but
also occurs in subsurface deposit feeders (literature
reviewed by Wheatcroft et al., 1990). For the deep
sea, Wheatcroft (1992) has provided experimental
evidence of size-selective vertical transport in the
sediment which is presumably mediated by deposit
feeders. There is a rapidly growing body of data
directly showing particle selectivity in deep-sea deposit
feeders. In epifaunal holothurians, which often make
up the overridingly dominant component of the abyssal
megabenthos, resource partitioning classically has been
related to tentacle morphology (Roberts and Moore,
1997). Particles are selected in terms of size or organic
quality (Khripounoff and Sibuet, 1980; Billett et al.,
1988). However, other morphological and behavioural
characteristics, such as body extension, motility and
buoyancy/swimming capability of surface deposit feeders, also need to be taken into account (Billett, 1991).
Other studies have demonstrated the presence of
algal cells, Cyanobacteria and chlorophyll pigments
(both serving as biomarkers) from recently deposited
phytodetritus and copepod faecal pellets in the guts,
faeces and burrows of deposit-feeding holothurians,
asteroids, echinoids, amphipods and sipunculans (Billett et al., 1988; Thiel et al., 1988/89; Graf, 1989;
Billett, 1991; Pfannkuche and Lochte, 1993; CamposCreasey et al., 1994). The observation of Billett
et al. (1988) that the detrital content of the foregut
of deposit-feeding holothurians closely matches the
overlying phytodetritus, even when this material has
a patchy distribution on the seabed, shows that the
foraging behaviour of these relatively slow-moving
animals is effective in feeding on patchily dispersed
resources. It also indicates that this may involve
active behaviour concentrating animals in such patches.
The small elasipod species Kolga hyalina has been
photographed in dense congregations, which may result
from attraction to patches of detrital accumulation
(Billett and Hansen, 1982). The deposit-feeding regular
echinoid Echinus affinis also seems to be attracted
preferentially to patches of phytodetritus lying on the
bed (Grassle et al., 1975; Billett et al., 1983; Grassle
and Morse-Porteous, 1987; Lampitt and Billett, 1984;
Campos-Creasey et al., 1994).
Stable isotope analyses are now revealing new
evidence for particle selection. Excess
234 Th activity
and
234 Th/
210 Pb activity ratios have been used in studies
of bioturbation and particle selectivity. The isotopes
are scavenged by particles settling through the water
column.
234 Th, with a short half-life of 24.1 days is
produced throughout the water column by natural decay
of
238 U, while
210 Pb, with a half-life of 22.3 yrs, is supplied by in situ production from
236 Ra and atmospheric
input. Their ratio thus provides a useful indication of
the relative age, as well as the source, of material in
the deep sea. Sediment-mixing coefficients measured
using
234 Th are two orders of magnitude greater than
those measured using
210 Pb (C.R. Smith et al., 1993).
These authors demonstrated a negative relationship
between estimates of sediment mixing and the halflives of radiotracers used in studies of bioturbation, and
ascribed this to age-dependent mixing of particles. The
preference by deposit-feeding organisms responsible
for bioturbation for recently deposited material has also
been demonstrated in studies comparing gut contents
with superficial sediment, recently deposited phytodetrital material, and material collected in sediment traps
moored near the bottom in the abyssal Northeast Pacific
(Lauerman et al., 1997). These authors found that
values of excess
234 Th activity and
234 Th/
210 Pb activity
ratios in the guts of two motile epibenthic holothurians,
Abyssocucumis abyssorum and Oneirophanta mutabilis, and in detrital aggregates from the sea floor
were similar. They calculated that 91% of the gutcontent material of A. abyssorum must have come from
material similar to that in the sediment traps.
Miller et al. (2000) also found very high levels of
excess
234 Th in two species of motile surface-deposit
feeding elasipod holothurians, Pannychia moseleyi and
Scotoplanes globosa, in the bathyal Santa Catalina
Basin off California. The gut of Pannychia moseleyi
also showed a 500-fold enrichment in chlorophyll-a relative to surface sediment, further indicating that these
animals are indeed actively selecting for fresh organic
matter, as would be expected from the age-dependent
John D. GAGE
is too simplistic. These authors studied the elasipod
holothurian, Oneirophanta mutabilis, in the abyssal
Northeast Atlantic where it is the dominant component
of the motile invertebrate megafauna. Simulations
based on estimated population density and movement
data from time-lapse photographs showed very different outcomes in terms of time required for 50%
coverage of the sea bed, ranging from 17 years for a
random-ranging strategy to 12 days for a systematic
search. A simulation based on observed speeds and
turning angles gives a 50% coverage in 9.6 years.
Evidence for particle selectivity comes from particleselection studies showing deposit feeders very commonly show strong selection for particles low in specific gravity and those covered with organic coatings.
This particularly applies to surface deposit feeders, but
also occurs in subsurface deposit feeders (literature
reviewed by Wheatcroft et al., 1990). For the deep
sea, Wheatcroft (1992) has provided experimental
evidence of size-selective vertical transport in the
sediment which is presumably mediated by deposit
feeders. There is a rapidly growing body of data
directly showing particle selectivity in deep-sea deposit
feeders. In epifaunal holothurians, which often make
up the overridingly dominant component of the abyssal
megabenthos, resource partitioning classically has been
related to tentacle morphology (Roberts and Moore,
1997). Particles are selected in terms of size or organic
quality (Khripounoff and Sibuet, 1980; Billett et al.,
1988). However, other morphological and behavioural
characteristics, such as body extension, motility and
buoyancy/swimming capability of surface deposit feeders, also need to be taken into account (Billett, 1991).
Other studies have demonstrated the presence of
algal cells, Cyanobacteria and chlorophyll pigments
(both serving as biomarkers) from recently deposited
phytodetritus and copepod faecal pellets in the guts,
faeces and burrows of deposit-feeding holothurians,
asteroids, echinoids, amphipods and sipunculans (Billett et al., 1988; Thiel et al., 1988/89; Graf, 1989;
Billett, 1991; Pfannkuche and Lochte, 1993; CamposCreasey et al., 1994). The observation of Billett
et al. (1988) that the detrital content of the foregut
of deposit-feeding holothurians closely matches the
overlying phytodetritus, even when this material has
a patchy distribution on the seabed, shows that the
foraging behaviour of these relatively slow-moving
animals is effective in feeding on patchily dispersed
resources. It also indicates that this may involve
active behaviour concentrating animals in such patches.
The small elasipod species Kolga hyalina has been
photographed in dense congregations, which may result
from attraction to patches of detrital accumulation
(Billett and Hansen, 1982). The deposit-feeding regular
echinoid Echinus affinis also seems to be attracted
preferentially to patches of phytodetritus lying on the
bed (Grassle et al., 1975; Billett et al., 1983; Grassle
and Morse-Porteous, 1987; Lampitt and Billett, 1984;
Campos-Creasey et al., 1994).
Stable isotope analyses are now revealing new
evidence for particle selection. Excess
234 Th activity
and
234 Th/
210 Pb activity ratios have been used in studies
of bioturbation and particle selectivity. The isotopes
are scavenged by particles settling through the water
column.
234 Th, with a short half-life of 24.1 days is
produced throughout the water column by natural decay
of
238 U, while
210 Pb, with a half-life of 22.3 yrs, is supplied by in situ production from
236 Ra and atmospheric
input. Their ratio thus provides a useful indication of
the relative age, as well as the source, of material in
the deep sea. Sediment-mixing coefficients measured
using
234 Th are two orders of magnitude greater than
those measured using
210 Pb (C.R. Smith et al., 1993).
These authors demonstrated a negative relationship
between estimates of sediment mixing and the halflives of radiotracers used in studies of bioturbation, and
ascribed this to age-dependent mixing of particles. The
preference by deposit-feeding organisms responsible
for bioturbation for recently deposited material has also
been demonstrated in studies comparing gut contents
with superficial sediment, recently deposited phytodetrital material, and material collected in sediment traps
moored near the bottom in the abyssal Northeast Pacific
(Lauerman et al., 1997). These authors found that
values of excess
234 Th activity and
234 Th/
210 Pb activity
ratios in the guts of two motile epibenthic holothurians,
Abyssocucumis abyssorum and Oneirophanta mutabilis, and in detrital aggregates from the sea floor
were similar. They calculated that 91% of the gutcontent material of A. abyssorum must have come from
material similar to that in the sediment traps.
Miller et al. (2000) also found very high levels of
excess
234 Th in two species of motile surface-deposit
feeding elasipod holothurians, Pannychia moseleyi and
Scotoplanes globosa, in the bathyal Santa Catalina
Basin off California. The gut of Pannychia moseleyi
also showed a 500-fold enrichment in chlorophyll-a relative to surface sediment, further indicating that these
animals are indeed actively selecting for fresh organic
matter, as would be expected from the age-dependent