|
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| Functional
convergence of microbes
associated with temperate
marine sponges |
Ribes, M., Jiménez E., Yahel
G., López-Sendino P., Diez B.,
Massana, R., Sharp J.H., and Coma R.
|
Environmental
Microbiology (in press)
|
Most marine sponges establish a
persistent association with wide
arrays of phylogenetically and
physiologically diverse microbes. To
date, the role of these symbiotic
microbial communities in the
metabolism and nutrient cycles of
the sponge-microbe consortium
remains largely unknown. We identify
and quantify the microbial
communities associated with three
common Mediterranean sponge species,
Dysidea avara, Agelas
oroides and Chondrosia
reniformis (Desmospongiae)
that cohabitate in the coralligenous
community. For each sponge we also
quantified its particulate diet and
the uptake and release of dissolved
organic carbon, phosphate, dissolved
organic nitrogen (DON) and carbon
(DOC), and inorganic nitrogen. Low
microbial abundance (<6% of the
30 tissue occupied by microbes) and
no evidence for DOC uptake or
nitrification was found for D.
avara. In contrast O.
oroides and C.
reniformis showed high
microbial abundance (30% and 70% of
their tissue occupied by microbes,
respectively) and both species
exhibited high nitrification and
high DOC and NH4+
uptake. Surprisingly, these unique
metabolic pathways were mediated in
each sponge species by a different,
and host specific, microbial
community. The results point to a
functional convergence of microbial
consortia in some sympatric sponge
species suggesting that these
metabolic processes may be of
special relevance to the success of
the holobiont
|
| The
sponge pump: the role of
current induced flow in the
design of the sponge body
plan |
Leys, S.P., G. Yahel, M. A.
Reidenbach, V. Tunnicliffe, U.
Shavit, and H. M. Reiswig
|
| PLOS
ONE 6(12): e27787.
doi:10.1371/journal.pone.0027787 |
| Sponges are suspension feeders
that use flagellated collar-cells
(choanocytes) to actively filter a
volume of water equivalent to many
times their body volume each hour.
Flow through sponges is thought to
be enhanced by ambient current,
which induces a pressure gradient
across the sponge wall, but the
underlying mechanism is still
unknown. Studies of sponge
filtration have estimated the
energetic cost of pumping to be
<1% of its total metabolism
implying there is little adaptive
value to reducing the cost of
pumping by using “passive” flow
induced by the ambient current. We
quantified the pumping activity and
respiration of the glass sponge Aphrocallistes
vastus at a 150 m deep reef
in situ and in a flow flume; we also
modeled the glass sponge filtration
system from measurements of the
aquiferous system. Excurrent flow
from the sponge osculum measured in
situ and in the flume were
positively correlated (r>0.75)
with the ambient current velocity.
During short bursts of high ambient
current the sponges filtered
two-thirds of the total volume of
water they processed daily. Our
model indicates that the head loss
across the sponge collar filter is
10 times higher than previously
estimated. The difference is due to
the resistance created by a fine
protein mesh that lines the collar,
which demosponges also have, but was
so far overlooked. Applying our
model to the in situ measurements
indicates that even modest pumping
rates require an energetic
expenditure of at least 28% of the
total in situ respiration. We
suggest that due to the high cost of
pumping, current-induced flow is
highly beneficial but may occur only
in thin walled sponges living in
high flow environments. Our results
call for a new look at the
mechanisms underlying
current-induced flow and for
reevaluation of the cost of
biological pumping and its
evolutionary role, especially in
sponges. |
|
Resuspension
by fish facilitates the
transport and
redistribution of coastal
sediments
|
|
Katz
T.,
Yahel G., Reidenbach M.,
Tunnicliffe T.,
Herut B., Crusius J.,
Whitney F.,
and
Lazar B.
|
|
(Limnol.
Oceanogr., in press)
|
Oxygen availability
restricts groundfish to the
oxygenated, shallow margins of
Saanich Inlet, an intermittently
anoxic fjord in British Columbia,
Canada. New and previously reported
210Pb measurements in
sediment cores compared with flux
data from sediment traps indicate
major focusing of sediments from the
oxygenated margins to the anoxic
basin seafloor. We present
environmental and experimental
evidence that groundfish activity in
the margins is the major contributor
to this focusing. Fine particles
resuspended by groundfish are
advected offshore by weak bottom
currents, eventually settling in the
anoxic basin. Transmittance and
sediment trap data from the water
column show that this transport
process maintains an intermediate
nepheloid layer (INL) in the center
of the Inlet. This INL is located
above the redox interface and
unrelated to water density shifts in
the water column. We propose that
this INL is shaped by the
distribution of groundfish (as
resuspension sources) along the
slope and hence by oxygen
availability to these fish. We
support this conclusion with a
conceptual model of the resuspension
and off-shore transport of sediment.
This fish-induced transport
mechanism for sediments is likely to
enhance organic matter decomposition
in oxygenated sediments and its
sequestration in anoxic seafloors.
|
| Glass
sponge reefs as a silicon sink |
| Chu
J.W.F., Maldonado M., Yahel G.,
Leys S.P. |
| Mar.
Ecol. Prog. Ser. (411,
1-14, 2011) |
Glass sponge reefs concentrate
large amounts of biological silicon
(Si) over relatively small areas of
the seafloor. We examined the role
of glass sponges in biological
silicon (Si) cycling by calculating
a Si budget for 3 glass sponge reefs
(Howe, Fraser, and Galiano) in the
Strait of Georgia (SOG), British
Columbia, Canada. The main
reef-forming glass sponge Aphrocallistes
vastus is heavily silicified,
with 80% of its dry weight composed
of biogenic silica (bSi). We used a
combination of field sampling and
surveys with a remote-operated
vehicle to estimate the volume,
mass, and bSi content of the reefs.
BSi content ranged from 7 to 11 kg m−2
among reefs, amounting to a total of
915 t of bSi locked in the exposed
portion of the 3 reefs. Water column
measurements of dissolved Si (dSi)
indicated that the SOG is a region
of high dSi, with average dSi
concentrations of 50 µmol l−1
in waters over the reefs. The
skeletons of glass sponges showed
very little dissolution after 8 mo
immersion in seawater, as determined
by changes in dSi in samples and
scanning electron microscopy of the
spicules. In contrast, diatom
frustules, the main source of bSi in
surface waters of the SOG, were ~200
times more soluble. Our calculations
of Si flux suggest that glass sponge
reefs can equate to 65% of the dSi
reservoir (3.6 × 109 mol
Si) in the SOG and represent a
substantial Si sink in the
continental shelf waters of the
northeastern Pacific Ocean.
|
|
In
situ
size-independent retention
of phytoplankton and
bacteria by the tropical
bivalve Lithophaga simplex
|
|
Gitai
Yahel,
Dominique Marie, Peter G.
Beninger, Shiri Eckstein, and
Amatzia Genin
|
|
Aquatic
Biology
(6,
235-246)
|
|
Few
feeding
studies have been performed on
tropical bivalves, and in
situ feeding studies are
lacking altogether. We
investigated retention
efficiencies for natural
particles in the coral-boring
tropical mytilid Lithophaga
simplex. Using the
in-situ InEx technique
(Yahel et al. L&O Methods, 3,
46-58, 2005) SCUBA
divers carefully collected
samples from the water inhaled
and exhaled by undisturbed
bivalves at the coral reef of
Eilat (Gulf of Aqaba). Particle
retention efficiencies were
determined using flow cytometry
analysis of the paired water
samples. The photosynthetic
bacterium Synechococcus (0.9±0.1
µm) and larger eukaryotic algae
(1-10 µm) were preferentially
retained by the bivalve with
removal efficiencies of up to
90% (1996-2000 average 69±14%
and 60±17%, respectively, N =74
individual bivalves). The minute
photosynthetic bacterium Prochlorococcus
(size 0.4±0.1 µm) was also
moderately retained (41±19%).
Only a small proportion of the
non-photosynthetic bacteria
(0.3±0.1 μm) were retained
(5±18%, median 9%), despite
their numerical dominance in the
plankton and considerable size
overlap with Prochlorococcus.
Size-independent preferential
retention was also observed
within particle types: (1) L.
simplex more efficiently
retained Prochlorococcus and
picoeukaryotic
algae cells with higher
chlorophyll content, and (2) the
small fraction of
non-photosynthetic bacteria
retained did not differ in size,
but had higher nucleic acid
content (compared to the inhaled
population) an indication for
viable and active bacteria. We
conclude that particle
retention is not strictly size -
dependent in L. simplex, and
probably involves other cell
attributes such as cell surface
properties and/or motility.
|
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Home
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Intense
benthic
grazing of phytoplankton
by the coral reef
community
|
|
Amatzia
Genin,
Stephen G. Monismith, Matthew A.
Reidenbach, Gitai Yahel , and Jeffrey
R.
Koseff
|
|
Limnology
and
Oceanography (54,
938–951)
|
|
This
paper
presents a study of grazing on
phytoplankton by a section of
the fringing coral reef
located
in the Gulf of Aqaba near Eilat,
Israel. To make in-situ
measurements of grazing
rates,
we used the control volume
approach common in engineering.
We sampled phytoplankton
biomass
concentrations using 4 arrays of
that defined an imaginary box extending
from
the bed to the surface, the
control volume. To compute
fluxes of materials
in
and out of the box, we made
velocity measurements made at
the center of the control
volume
using an ADCP and a set of ADVs.
While phytoplankton patchiness
and other
turbulent
variations in concentration
introduced significant
uncertainty into our results,
mass
balances of fluxes into and out
of the box indicate a
flow-dependent grazing rate
of
ca. 10 to 20 m d-1
for the Eilat reef. This value
is somewhat larger (but not statistically
different)
from a value of ca. 3 m d-1
derived from a priori estimates
of grazing
rates
given the observed benthic
community and a value of 4 m d-1
computed from
the
observed near-bed concentration
gradient. Our measurements show
the importance
of
time averaging of sampled
concentrations to reduce
uncertainty
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Home
|
|
Groundfish
overfishing,
diatom decline and the
marine silica cycle –
Lessons from Saanich
Inlet, Canada and the
Baltic Sea cod crash
|
|
Timor
Katz,
Gitai Yahel, Ruthy Yahel, Verena
Tunnicliffe, Barak Herut, Paul Snelgrove
and
Boaz Lazar
Global
Biogeochem. Cycles, 23,
GB4032
|
|
In
this
study, we link groundfish
activity to the marine silica
cycle suggesting that the
drastic mid 1980s crash of the
Baltic Sea cod population
triggered the observed decrease
in dissolved silica (DSi) and
diatom abundance in the water.
We suggest that this seemingly
unrelated sequence of events was
caused by a marked decline in
sediment resuspension due to
reduced groundfish activity
resulting from the cod crash. In
a study in Saanich Inlet, BC,
Canada, we discovered that, by
resuspending bottom sediments,
groundfish triple DSi fluxes
from the sediments and reduce
silica accumulation therein.
Using these findings and the
available oceanographic and
environmental data from the
Baltic Sea, we estimate that
overfishing and recruitment
failure of Baltic cod reduced by
20% the DSi supply from bottom
sediments to the surface water
leading to a decline in the
diatom population in the Baltic
Sea. The major importance of the
marginal ocean in the marine
silica cycle and the associated
high population density of
groundfish suggest that
groundfish play a major role in
the silica cycle. We postulate
that the dwindling groundfish
populations caused by
anthropogenic perturbations,
e.g. overfishing and bottom
water anoxia, may cause shifts
in marine phytoplankton
communities.
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Home
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Fish
activity,
a major mechanism for
nutrient and carbon
recycling from
coastal
marine sediments
|
|
Gitai
Yahel,
Ruthy Yahel, Timor
Katz, Boaz Lazar, Barak Herut
and Verena Tunnicliffe (2008)
Marine Ecology Progress
Series 372,
195-209
|
|
Sediments
underlying
deep ocean basins accumulate
carbon, nutrients and many trace
compounds and
contaminants. In a
northeast Pacific basin, we find
that organic mater
remineralization and the
redistribution of bottom
sediments are substantially
affected by the resuspension
activity of benthic fish.
Fish rework >40% of the
seabed daily with over 100
disturbances m-2 day-1.
This resuspension activity
reduces organic carbon
sequestration by 230 mg C m-2
day-1, equivalent to
~1/2 of its downward flux and
enhances near-bottom turbidity,
oxygen demand, and nutrients
regeneration. To date,
these processes are missing from
geochemical models.
Exploitation of ground fish
stocks is likely to have a
significant effect on global
geochemical cycles.
|
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Home
|
|
Size
independent
selective filtration of
ultraplankton by
hexactinellid glass
sponges
|
|
Gitai
Yahel,
Dafne
I.
Eerkes-Medrano, and Sally P.
Leys (2006)
|
|
Aquatic
Microbial
Ecology 52,428-440
|
|
Selective
feeding
is an important mechanism by
which animals can maximize their
energy gain, avoid harmful or
toxic food items, and optimized
their diet. Selective predation
by flagellates and ciliated
protists is a major mechanism
shaping the structure of
microbial communities in pelagic
habitats. Microbial communities
at the benthic boundary layer
take advantage of enhanced flux
of organic materials but at the
same time are exposed to
enhanced predation by benthic
suspension feeders. Our
understanding of the functioning
of this "benthic-pelagic loop"
is yet limited but sponges are
considered to be key grazers in
many benthic habitats. We
studied the feeding preferences
of two of the most common glass
sponges of the North-East
Pacific coast, Rhabdocalyptus
dawsoni and Aphrocallistes
vastus. Sponges were
maintained in large darkened
tanks supplied with running sea
water from the nearby fjord. The
water inhaled and exhaled by the
sponges was simultaneously
sampled and analyzed using a
flow cytometer. Both sponges
showed a similar (but not
identical) feeding pattern,
efficiently removing up to 99%
of the most abundant bacteria
cells whereas clays, silts and
debris particles were expelled
into the exhaled water.
Surprisingly, filtration
efficiencies were maximal for
the relatively large and rare
eukaryotic algae (3-5 µm, 86 ±
9%) and for small non
photosynthetic bacteria (<0.4
µm, 89 ± 10%) while intermediate
size non-photosynthetic bacteria
characterized with higher
nucleic acid content were
efficiently removed in February
(92±3%) when overall plankton
concentration was low but not in
July (28 ± 16%). The
intermediate size photosynthetic
prokaryote Synechococcus
(1.1-1.5 µm) was also less
preferred. Based on detailed
analysis of the ultra-structure
of the glass sponge filtration
apparatus we argue that the
selective filtration we observed
involved individual processing,
recognition, sorting, and
transport of each particle
through the sponge syncytial
tissue. Selective grazing by
glass sponges, like their
protozoans pelagic counterparts
can potentially be an important
mechanism shaping the microbial
communities in the deep water of
North-East Pacific fjords.
|
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Home
|
|
In
situ
feeding
and
metabolism of glass
sponges (Hexactinellida,
Porifera) studied in a
deep temperate fjord with
a remotely operated
submersible.
|
|
Yahel
G.,
Whitney F., Reiswig H.M.,
Eerkes-Medrano D.I., and Leys
S.P.
(2007)
|
|
Limnol.
Oceanogr.
52,428-440
|
|
Glass
sponges
are conspicuous inhabitants of
the deep benthic community of
temperate Northeast Pacific
fjords. Using the Canadian
remotely operated submersible
ROPOS and a new device, the SIP,
for clean sampling of the water
inhaled (incurrent) and exhaled
(excurrent) by the sponges, we
measured in situ the
removal of total organic carbon
(TOC)
and living cells, as well as
the excretion of nitrogenous
waste, by two species of glass
sponge at 120-160 m. At the
deep fjord habitat, ambient
water resembled true deep-sea
water with low oxygen
concentrations (<2 mL L-1),
high
dissolved silica (>55 µmol
L-1), and low
organic content (TOC<53±6
µmol L-1). The
turbid water (<35% m-1
transmissivity) contained high
suspended sediment load
(7.1±0.9 mg L-1)
and the organic content of the
seston was low (<2%). Both
Aphrocallistes vastus and
Rhabdocalyptus dawsoni
were effective bacteriovores,
removing up to 95% of the
bacteria (median removal was
79% for both species). Nano
heterotrophic protists (4-10
µm) were also removed at
similar efficiencies and
contributed a significant
proportion (~30%) of carbon to
the sponges’ nutrition. Ultra
plankton (<10 µm) removal
(2.2±1.3 µmol L-1)
accounted for the entire TOC
uptake and ammonia excretion
(0.20±0.13 µmol L-1)
by the sponge with no evidence
for dissolved organic uptake.
Despite the massive siliceous
skeleton of both sponges,
silica uptake was below
detection levels (0.28 µmol L‑1).
This low silica uptake
supports previous suggestions
of low growth rates in
Hexactinellida. Our results
suggest that glass sponges are
efficient filter feeders,
selectively removing the
scarce microbial cells from a
'soup' of suspended clay and
detritus particles. Reported
sponge abundances of up to 250
individuals 10 m-3
suggest that the filtering
activity of these organisms
could significantly impact
benthic-pelagic mass exchange
in NE Pacific fjords.
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Home
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Environmental
Impacts
of air-gun surveys on
Glass Sponges
|
|
PI:
Tunnicliffe
V. and Chapman R. | Draft
prepared by Yahel G. and Wimut
M. (2007)
|
|
Ministry
of
Energy and Mines and University
of Victoria, Social Science and
Science Partnerships
|
|
Air-gun
surveys
associated with the oil and gas
exploration in the Queen
Charlotte Basin will insonify
the seafloor with broadband,
high intensity noise, thereby
exposing the unique glass sponge
reef systems of that area to
acoustic impacts. We used the
opportunity offered by an NSERC
funded research cruise to the
Fraser Ridge glass sponge reef
to conduct a brief, and
preliminary, study of the
immediate biological response of
one of the major reef building
glass sponges (Aphrocallistes
vastus) to acoustic
vibrations. The measure of
response in these animals was
reduction in their feeding
currents. These currents were
measured in the exit orifice by
deployed instruments that had no
physical contact with the
animals. The sound source was a
small, surface air-gun (164 cm3)
that generated average sound
exposure levels of 151 dB re μPa2s-1
at the sponge location. The
air-gun signal was distinct from
background noise below 300 Hz.
The question of whether the
sponge’s excurrent flow
responded to the pressures from
the series of air-gun shots was
addressed statistically. The
sponge response to ambient
conditions was compared to the
excurrents measured at the
air-gun shot times. For this
single sample of 16 air-gun
shots, the statistical analysis
indicated that there was little
or no evidence that the acoustic
pressure from the air-gun
influenced the physiological
functions. The experimental work
carried out here was very
challenging, and many factors
were difficult to control.
First, since the ambient flow
speed accounted for ~ 70% of the
observed variation in the
excurrent flow, it was
impossible to separate this
factor from the analysis.
Second, the complexity of the
approach constrained the sample
size and duration of the
observations. Daylight hours,
marine mammal intrusion,
preparation time for the ROV
(Remote Operated Vehicle) and
instrument malfunction all
became confounding factors.
Nonetheless, the experimental
study is novel and the
statistical approach is a
benchmark for future studies.
Further laboratory work and
field studies are necessary
before meaningful conclusions
can be drawn. New research
should make use of more
realistic sound levels, and
study recovery times,
habituation and longer term
effects on tissue and skeletal
integrity.
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Home
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Boundary
layer
turbulence and flow
structure over a fringing
coral reef
|
|
Reidenbach
M.A.,
Monismith S.G., Koseff J.R., Yahel
G., and Genin A. (2006)
|
|
Limnol.
Oceanogr.
51,
1956-1968
|
|
Hydrodynamics
control
key processes in coral reef
ecology, from advective
transport of food and larvae,
through turbulence-dependent
mass fluxes across the boundary
layer over single organisms.
Here we report on the flow
structure and its interaction
with the bed roughness over a
fringing coral reef in the Gulf
of Aqaba,
Red
Sea.
Measurements were made at two
fore-reef sites and a nearby
sandy slope. The shear
velocity, u* and
drag coefficient, CD
were determined directly
from turbulent Reynolds
stresses measured using
Acoustic Doppler Velocimeters.
Values of CD for
the coral substrates ranged
from 0.009 to 0.015, three to
five times greater than the
sandy bottom. The turbulence
dissipation rate, ε, was
determined by fitting spectra
of vertical velocity to the
theoretical "5/3" law expected
for the inertial subrange
of turbulence. A local balance
was found between production
and dissipation of turbulent
kinetic energy (TKE). This
equilibrium signified that we
could estimate u*
from the either the mean
velocity profile, turbulence,
or dissipation rate of TKE.
Estimates from all three
measures agreed well,
indicating that existing
turbulent boundary layer flow
theory can be applied to flows
over the rough topography of
coral reefs. This has
important implications for
both numerical modeling of the
flow and estimating dispersion
rates over reef systems. The
results also suggest that the
bottom topography, by
enhancing both reef scale and
local drag and mixing levels,
allows reef biota to more
effectively exchange dissolved
and particulate matter with
oceanic waters.
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Home
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Phytoplankton
grazing
by epi- and in-fauna
inhabiting exposed rocks
in coral reefs
|
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Yahel
G.,
Zalogin T., Yahel R., and
Genin A. (2005)
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Coral
Reefs
125, 153-163
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|
Exposed
rocks
with no visible macro-fauna are
abundant in all coral reefs.
Depletion of phytoplankton cells
and pigments by the minute
crypto fauna inhabiting the
outer few centimeters of such
rocks was experimentally studied
over an annual cycle in the Gulf
of Aqaba,
Red
Sea.
Different substrata were
introduced into small (3.6 L),
well mixed, tanks that were
fed by running seawater pumped
directly from the reef at a
rate of 11±1 L hr-1.
A steady-state reduction in
phytoplankton abundance and
chlorophyll a
concentration of 38±26% (mean
± 1 SD) was found for
untreated rocks but not for
sand, gravel, or killed
controls. Average areal
clearance rate by untreated
rocks was 17.3±8.0 ml cm-2
hr-1. Conservative
extrapolation of this rate to
the whole reef community,
suggests that the fauna
inhabiting exposed rocks
clears 2.1± 0.9 m3
m-2 d-1
at Eilat. Phytoplankton
removal by untreated rocks
varied from 1.5 ng chlorophyll
a cm-2 hr-1
during the oligotrophic summer
conditions to 6 ng chlorophyll
a cm-2 hr‑1
during the spring bloom. These
values correspond to a
potential nitrogen gain of 1.3
and 5.2 mmol
N m-2 day-1,
respectively. Cryptic
reef-rock fauna can have a key
role in the biogeochemical
functioning of coral reef
communities.
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Home
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Thermally
driven
exchanges between a coral
reef and the adjoining
ocean
|
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Monismith
S.G.,
Genin A., Reidenbach, M.A.,
Yahel G., and Koseff J.R.
(2006)
|
|
Journal
of
Physical Oceanography 36,
1332-1347
|
|
In
this
paper hydrographic observations
made over a fringing coral reef
at the northern end of the Gulf
of Aqaba near Eilat, Israel, are
discussed. These data show
exchange flows driven by the
onshore–offshore temperature
gradients that develop because
shallow regions near shore
experience larger temperature
changes than do deeper regions
offshore when subjected to the
same rate of heating or cooling.
Under heating conditions, the
resulting vertically sheared
exchange flow is offshore at the
surface and onshore at depth,
whereas when cooling dominates,
the pattern is reversed. For
summer conditions, heating and
cooling are both important and a
diurnally reversing exchange
flow is observed. During winter
conditions, heating occupies a
relatively small fraction of the
day, and only the cooling flow
is observed. When scaled by ΔV,
the
observed profiles of the
cross-shore during cooling
velocity collapse onto a single
curve. The value of ΔV
depends on the convective
velocity scale uf
and the bottom slope
through the inertial scaling, ΔV
~β1/3uf
first proposed by Phillips
in the 1960s as a model of
buoyancy-driven flow in the Red
Sea. However, it is found that
turbulent stresses associated
with the long shore tidal flows
and unsteadiness due to the
periodic nature of the buoyancy
forcing can act to weaken the
sheared exchange flow.
Nonetheless, the measured
exchange flow transport agrees
well with previous field and
laboratory work. The paper is
concluded by noting that the
“thermal siphon” observed on the
Eilat reef may be a relatively
generic feature of the nearshore
physical oceanography of reefs
and coastal oceans in general.
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Home
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Fouling
reefal
communities on artificial
reefs: Does age matter?
|
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Perkol-Finkel
S.,
Shashar N., Barneah O.,
Ben-David-Zaslow R., Oren U.,
Reichart T., Yacobovich T.,
Yahel G., Yahel R., and Benayahu
Y. (2005)
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Biofouling
21,
127-140
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Man-made
submerged
structures, including
shipwrecks, offering substrata
for fouling organisms and fish,
have been classified secondarily
as artificial reefs (ARs). The
current approach in AR design is
that of low-profile structures
placed on the seabed and
attempting to mimic natural reef
(NR) communities with the aim of
mitigating degraded marine
ecosystems. To examine the
validity of this concept, a
long-term comparison of the
developing AR fouling
communities to those of nearby
NRs is required. A survey of the
fouling reefal organisms was
conducted on seven shipwrecks
(Red Sea, Egypt), comprising
three young (ca 20 years old)
and four old (4100 years old)
unplanned ARs, in comparison to
nearby NR communities. The
hypothesis tested was that the
age of the ARs shapes the
structure of their fouling coral
communities. The results
demonstrated distinct
differences between ARs and NRs
and between young and old ARs.
While the species composition on
ARs may resemble that of NRs
after approximately 20 years,
obtaining a similar extent of
coral cover may require a full
century. Moreover, differences
in structural features between
ARs and NRs may lead to
differences in species
composition that persist even
after 100 years.
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“InEx”
–
a direct in situ method to
measure filtration rates,
nutrition and metabolism
of active suspension
feeders
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Yahel
G.,
Marie D., Hadas
E., and Genin A.
(2005)
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Limnol.
Oceanogr.
Methods
3, 46-58
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Sponges,
bivalves
and tunicates play an important
role in the trophic dynamics of
many benthic communities.
However, accurate in situ
measurements of their filtration
and excretion rates are lacking.
Our knowledge of these rates is
based on indirect,
artifact-prone, mostly in
vitro measurements.
This paper presents and
evaluates an in situ,
non-intrusive technique to
directly measure the rate and
efficiency by which an active
suspension feeder removes (or
discharges) substances from (to)
the water it filters. The
technique, termed “InEx”, is
based on the simultaneous,
pair-wise collection of the
water Inhaled and Exhaled
by the animal. The
difference in the concentrations
of a substance among a pair of
samples provides a measure of
the retention (or excretion) of
the substance by the animal.
Calculations of feeding (or
excretion) rates are obtained by
multiplying the concentration
difference by pumping rate. The
latter is concurrently measured
by recording the movement of a
dye front in a transparent tube
positioned within the ex-current
jet. An important quality of the
InEx technique is the lack of
any manipulation of the studied
organisms thus allowing
realistic estimates of the
organism’s performance under
natural conditions.
Preliminary results showing the
diet composition, feeding rates
and removal efficiencies of some
coral reef sponges, bivalves and
tunicates are presented and
discussed.
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Diel
pattern
with abrupt crepuscular
changes of zooplankton
over a coral reef
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Yahel
R.,
Yahel G., and Genin (2005)
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Limnol.
Oceanogr.
50, 930-944
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Zooplankton
abundance
and emergence patterns were
studied over coral reefs in the
Gulf of Aqaba using high
resolution acoustics (multi-beam
imaging sonar, FTV, 1.6MHz and
WH600 ADCP, 614 KHz), emergence
traps, net tows, and underwater
pump arrays. These measurements
were supplemented with field
observations on feeding behavior
of zooplanktivorous fish. The
zooplanktonic community over
coral reefs is a complex and
highly dynamic mixture of
pelagic plankton advected into
the reef, larvae and eggs
released by benthic animals, and
demersal plankton. At sunset (±4
min, 95% confidence interval)
acoustic back-scattering
intensity and zooplankton
biomass started to increase
rapidly. In contrast to this
ascent, the pre-dawn decline
began earlier during the dark
(82±5 min before sunrise,
mean±95% confidence interval)
and terminated in complete
darkness 43±8 min before
sunrise. This light dependent
diel behavior was highly
consistent throughout the year,
regardless of seasonal and
environmental changes. Direct
sampling indicated that smaller
zooplankters (500-700 µm)
ascended first and that demersal
zooplankton accounted for most
of the increase in zooplankton
soon after sunset. Surprisingly,
the emergence of zooplankton
occurred while large schools of
daytime zooplanktivorous fish
were still foraging in the water
column. However, at that time
the fish’ predatory efficiency
is greatly reduced and corals do
not yet expand their tentacles.
We suggest that the early
emergence of reef zooplankton
utilizes a “window of
opportunity” when nocturnal
predatory risk by sessile
zooplanktivores (e.g., corals)
is still low and the efficiency
of visual predators is already
reduced.
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Home
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Near-bottom
depletion
of zooplankton over coral
reefs: I.
Diurnal dynamics and size
distribution
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Yahel
R.,
Yahel G., and Genin A.
(2005)
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Coral
Reefs
24,
75-85
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Changes
in
the near-bottom abundance of
zooplankton on scales of
centimeters to meters and hours
to seasons are of great
importance to corals and other
benthic zooplanktivores. Our
objective was to characterize
such spatio-temporal
changes over several coral reefs
in the Gulf
of Aqaba,
Red
Sea.
Using arrays of underwater
pumps, we found a substantial
depletion of zooplankton near
the bottom. Vertical gradients
in zooplankton abundance were
steeper during the night than
day, mostly due to a greater
nocturnal increase in
zooplankton biomass higher in
the water column. On average,
the layer <1 m above bottom
(mab) was depleted by 2.6±2.2
mg m-3 (46±35%)
and 1.4±1.4 mg m-3
(37±43%) during night and day,
respectively. A long time
series of bi-weekly samples at
0.5 mab, lasting 1.5 yrs,
indicated a doubling of the
biomass during the night with
no apparent seasonality. The
diel change was due to an
increase in the abundance of
only large (>200 µm)
zooplankters around dusk and
their disappearance in the
morning. Diurnal predation by
zooplanktivorous fish,
sediment resuspension by
benthivorous fish and
zooplankton behavior appear to
control the dynamics of
suspended particles over the
reef, creating sharp vertical
gradients and a remarkable
diel cycle in the ratio
between nutritious plankton
and inorganic particles.
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In
situ
feeding and element
removal in the
symbiont-bearing sponge
Theonella swinhoei: Bulk DOC
is
the major source for carbon
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Yahel
G.,
Sharp J.H., Marie D., Häse
C., and Genin A. (2003)
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Limnol.
Oceanogr.
48 141-149.
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The
vast
majority of organic matter in
the world ocean is found in the
dissolved pool. However,
no evidence has been
demonstrated for direct uptake
of bulk dissolved organic matter
(DOM)
by organisms other than
bacteria and some
invertebrate larvae. The total
organic carbon (TOC)
is 10-30% higher in coral
reefs than in adjacent open
waters. The dissolved organic
carbon (DOC)
accounts for > 90% of the TOC.
Using a new in situ
technique for clean sampling
of the seawater inhaled and
exhaled by benthic suspension
feeders we measured directly
the removal of DOC
in the symbiont-bearing reef
sponge Theonella swinhoei.
The sponge removed
up to 26% (12±8%, mean±1SD) of
the TOC
(dissolved and particulate)
from the water it filtered
during a single passage
through its filtration
system. Most of the
carbon gained by the sponge
was from the dissolved pool
(10±7 µmol C liter-1),
an order of magnitude greater
than the carbon gained from
the total living cells
(phytoplankton and bacteria)
the sponge removed (2±1 µmol C
liter-1). In
T. swinhoei, over two
thirds of the sponge biomass
consists of symbiotic
bacteria, likely having an
important role in the DOC
uptake. Our findings indicate
that the role of DOC
in metazoan nutrition as well
as the role of metazoans in DOC
cycling may have been grossly
underestimated.
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Daily
Cycles
of Suspended Sand at Coral
Reefs: A Biological
Control
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Yahel
R.,
Yahel G., and Genin A. (2002)
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Limnol.
Oceanogr.
47: 1071-1083.
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Sediment
resuspension
has a major ecological role in
many coastal habitats, lakes and
marine environments. Waves and
currents are generally thought
to be the major control on
sediment resuspension. In this
study, the temporal and spatial
distributions of Suspended Sand
Concentrations (SSAC) were
measured at five coral reefs in
the northern Gulf
of Aqaba,
Red
Sea,
by directly filtering seawater
through 100 µm mesh nets.
Levels of near-bottom SSAC
were twice as high during the
day (4.8 – 9.7 mg m-3)
than at night (2.3 – 5.2 mg m-3).
Vertical profiles showed a
sharp increase of SSAC (2-6
folds) toward the bottom
during the daytime, but not at
night. The contributions of
wind, waves and currents to
variations in SSAC were
non-significant except during
rare southern storms. Temporal
and spatial changes in the
level of SSAC during the day
corresponded with the
abundance of benthivorous fish
and their activity pattern.
Resuspension activities by
fish numbered >1.5
resuspension events m-2
h-1 and were
confined to daytime. An in
situ experiment in which
fish were excluded from a
large section (250 m2)
of the reef resulted in
elimination of the day - night
differences and a significant
decrease in daytime SSAC over
the treated reef section in
comparison to the open,
control sites. This study is
the first report of a daily
pattern of suspended sediment
at coral reefs with fish
activity as it’s
causative agent.
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Home
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Intense
benthic
grazing on phytoplankton
in coral reefs revealed
using the control volume
approach
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Genin
A.,
Yahel G., Reidenbach
M.A., Monismith S.G., Koseff
J.R. (2002)
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Oceanography
15, 90-96.
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A
major objective of
biogeochemical studies of coral
reefs is to quantify fluxes of
particulate and dissolved matter
between the reef and overlying
waters. However, direct
measurements of these fluxes are
hard to obtain due to the
typically small concentration
changes as the water flows over
the bottom and due to shear,
turbulent mixing and
concentration gradients
characterizing benthic boundary
layers. Using state-of-the-art
underwater technology, we were
able to apply the “Control
Volume” approach to measure in
situ phytoplankton grazing on a
scale of a whole coral-reef
community. The results indicate
that the import of carbon and
nutrients via this grazing is a
major ,
previously underestimated,
trophic pathway in coral reefs.
The amount of phytoplankton
grazed by 1 m 2 of
reef is similar to the total
phytoplankton produced in the
entire water column of the
surrounding sea under 1 m 2
of sea-surface. The import of
allochthonous nutrients into the
reef via this grazing balances
the downstream leak of dissolved
nutrients. Physically, the flow
over the rough topography of the
reef produces enhanced
turbulence, enabling high
grazing rates to be sustained,
while on larger scales, the
exchange between the offshore
ocean and the reef is supported
by buoyancy- driven flows. With
the advent of underwater
technology, the control volume
technique is no longer limited
to unique situations (e.g.
closed lagoons, shallow flats),
but should be generally
applicable for measurements of
benthic-pelagic fluxes in oceans
and lakes.
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Phytoplankton
distribution
and grazing near coral
reefs
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Yahel
G.,
Post A.F., Fabricius
K.E., Marie D., Vaulot
D., and Genin A. (1998)
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Limnol.
Oceanogr.
43, 551-563.
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Depletion
of
phytoplankton cells and pigments
over coral reefs were studied in
the Gulf
of Aqaba,
Red
Sea,
during 1994-1996.
Phytoplankton abundance and
chlorophyll a
concentration were 15-65%
lower near the reefs than in
the adjacent open waters. The
decrease in chlorophyll near
the reef was typically
associated with an increase in
the concentration of its
degradation products, the
phaeopigments. The
steepest slope of these
cross-shore gradients occurred
within 1-3 m above bottom.
Over 50% of the variation in
the extent of the chlorophyll
gradients, but not of
phaeopigments, could be
explained by the advection of
water during 2 hrs preceding the transect
and by the concentration of
chlorophyll a in the
open water. No cross-shore
gradients were observed at a
sandy-bottom site without
reef. Eukaryotic phytoplankton
(<5 mm) contributed >70%
of the total depleted carbon
near the reef during winter,
while the cyanobacterium Synechococcus
(1 mm) contributed the largest
share in summer. The
proportions of different taxa
in depleted fractions were
similar to those in ambient
waters, indicating no size
selectivity. Direct
measurements of phytoplankton
removal rates were made in
water passing through a unique
5 m-long perforated reef,
dominated by herbivorous soft
corals. The waters downstream
of that reef were strongly
depleted of phytoplankton (10
to >36%, or 32 to >100
ng chlorophyll a l-1).
When
converted to carbon fluxes,
these rates greatly exceeded
reported values of carbon
input to coral reefs via
zooplankton predation.
Phytoplankton grazing is an
important component of
benthic-pelagic coupling in
coral reefs.
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In
situ
depletion of phytoplankton
by an azooxanthellate
soft coral
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Fabricius
K.E., Yahel G., and Genin A.
(1997)
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Limnol.
Oceanogr.
43, 354-356.
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Abstract
-
The
in
situ removal of phytoplankton by
the soft coral Dendronephthya
hemprichi
was investigated by taking
small-scale measurements of
chlorophyll concentrations
around colonies on a reef of the
Northern
Red Sea.
The chlorophyll concentration
downstream of a 0.75 m
deep
colony thicket was
depleted by
6.4% (± 1.4% SE)
compared with the water
upstream. Neighboring
organisms are thus exposed to
water, which is significantly
depleted of phytoplankton. A
0.75 x 0.1 x 0.1 m
passage of actively feeding
colonies removed 34 mg carbon
per day from the water,
equivalent to ~ 1.3 times
their respiratory carbon
demand. Rates of algae intake
were also estimated by
determining the decrease in
gut fluorescence in starved
colonies. The in situ
depletion technique showed a
three to six-fold greater
sensitivity compared with the
gut fluorescence technique,
and should be preferred as a
technique for estimating
feeding rates.
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Suspension
feeding
in coral reefs - what
about dissolved matter?
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Yahel
G.
(1998)
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Isr.
J. Zool.
44, 90-91.
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Benthic
grazing
on phytoplankton is a principal
trophic pathway in shallow,
temperate coastal habitats. In
tropical reefs, on the other
hand, studies of benthic-pelagic
coupling have focused on
zooplankton, rather than
phytoplankton, as the principal
source of allochthonous prey.
Strong phytoplankton grazing is
to be expected at the reef, as
numerous members of the
coral-reef community are known
to feed on particles within the
size range of phytoplankton.
Conceptual considerations also
support expectations for intense
phytoplanktivory at the reef, as
phytoplankton biomass in coral
reefs commonly exceeds that of
zooplankton by an order of
magnitude. Furthermore, the
bottom topography in coral reefs
is typically rough, with
numerous living and non-living
objects having a high
“slenderness ratio” (height over
width), which is an optimal
morphology for feeding on small
suspended particles. Direct
evidence showing that reef
dwellers actually feed on
phytoplankton are nevertheless
scarce and the role of
phytoplankton as a source for
allochthonous carbon and
nutrients in coral reefs is
poorly understood. Moreover, the
role of Dissolved Organic Carbon
(DOC)
as food for the guild of
benthic suspension feeders is
virtually unknown. Recently we
have found that the boundary
layer over coral reefs in the
Red Sea, a few meters in
thickness, is significantly
depleted (up to 60%) of
phytoplankton, compared with
off-shore waters, and that DOC
is substantially higher near
the reef. In order to identify
the major phytoplankton
grazers at the reef, we have
developed a technique to
sample, in situ,
uncontaminated exhaled waters
and to measure pumping rates
of active suspension feeders.
Using this method we have
measured the diet composition,
pumping rates and clearance
efficiencies for seven most
conspicuous members of the
active suspension feeders
guild in the coral reefs of Eilat,
Israel.
A flow cytometer was to
used measure
concentrations of
phytoplankton and bacteria, a
Flow Injection Analyzer was
used to measure the
concentrations of NH3
and NO2, and a high
temperature TOC
analyzer was used to measure
concentrations of DOC.
Diet composition, filtration
efficiency and pumping rates
were nearly uniform within
taxa but vary considerably
between taxa. Synechococcus
was the most efficiently
removed prey by all taxa
(55-95% removal efficiency).
Heterotrophic bacteria were
efficiently removed by sponges
(60-88%) but not by bivalve or
ascidians (<15%). During
the spring bloom eukaryotic
algae (2-5mm) were the most
abundant phytoplankton
(biomass wise) at the study
site, and although being less
efficiently removed by
suspension feeders, the carbon
gained from these relatively
large cells exceeded that of
smaller prey. NH3,
but not NO2, was
significantly elevated in
exhaled waters of sponges and
bivalves, but not of
ascidians. Preliminary results
indicate a substantial removal
of DOC
by sponges and ascidians (but
not by bivalves). If
corroborated, these
findings suggests
that the carbon gained via DOC
removal exceeds that from
particulate matter
(phytoplankton, bacteria and
detritus) by an order of
magnitude. The contribution of
carbon and nutrients via
consumption of DOC,
phytoplankton and bacteria to
the individual feeders and the
reef community need to be
re-evaluated.
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Herbivory
in
Soft Corals: Correction
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Fabricius
K.E., Yahel G., and Genin A.
(1997)
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Science
273, 293b-297
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We
have
demonstrated the ability of
several azooxanthellate
soft corals to feed on
phytoplankton (Reports,
7 Apr. 1995, p. 90) (1).
The
conversion of gut fluorescence
values to chlorophyll
concentrations yields nanograms
of chlorophyll in the gastrovascular
system of Dendronephthya hemprichi,
not micrograms, as we
erroneously stated. Hence the
data presented in our "third
line of evidence" for herbivory,
and in figure 1B, should have
read "Chlorophyll a content (nanograms
per polyp)" instead of
microgram. Gut chlorophyll
contents of well-fed D. hemprichi
are thus comparable to those of
herbivorous copepods with
similar body weight (2).
As
these corrections could change
our conclusion with regard to
herbivory in D. hemprichi,
we have estimated the coral's
feeding rate using a new
approach, based on in situ
measurements of chlorophyll
removal from natural seawater.
Chlorophyll a concentrations in
seawater upstream of large D. hemprichi
colonies were compared with
those downstream of the corals
with a time lag between pairs of
samples equal to the water
passage time through the coral
thicket. On average, D. hemprichi
removed 0.035 microgram per
liter ± 0.064 (SD)
or 6.4% ± 11.6 of
chlorophyll at a mean flow speed
through the colonies of
3.5 centimeters per second.
This removal rate is equivalent
to an uptake of
16.34 milligrams of
phytoplankton carbon per gram of
ash-free dry weight (AFDW) of
the coral per day, which is
2.5 to 3.6 times the
corals' daily respiratory carbon
demand (3).
The
chlorophyll depletion of the
water and recent electron
microscopic documentation of
digested algal cells within the
endodermal
tissue of the polyps suggest
that our finding of herbivory in
these soft corals is valid.
However, our initial method of
analyzing gut fluorescence
underestimated the actual rates
of intake and digestion of
phytoplankton by D. hemprichi.
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Selective,
size
independent, filtration of
pico-plankton by three
tropical suspension
feeders: a sponge, an
ascidian, and a bivalve
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Yahel
G.,
Marie
D., Beninger
P., and Genin A.
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Diet
composition
and prey preference patterns
were studied in three benthic
coral-reef suspension feeders: a
sponge (Theonella swinhoei),
a mollusk (the boring bivalve Lithophaga
simplex), and a tunicate
(the solitary ascidian Halocynthia
gangelion).
The
water inhaled and exhaled by
undisturbed specimens was
cleanly collected in situ.
Flow cytometric comparison of
these water samples provided a
direct measure of the grazer's
filtration efficiencies over the
course of four years (1996-2000)
covering a wide range of
environmental conditions.
Ultraplankton (<8 µm), which
dominated the planktonic biomass
in the oligotrophic waters
overlying the reef, was removed
efficiently by each of the three
grazers exhibiting a unique
species-specific selectivity
patterns. Synechococcus
removal efficiencies were
70±14%, 76±16%, and 92±8%
(mean±1SD), for the bivalve,
ascidian and sponge,
respectively. Coccoid
photosynthetic bacteria were
preferred by all three taxa over
both the larger eukaryotic algae
and the somewhat smaller
non-photosynthetic bacteria. The
ascidian and the bivalve
efficiently removed the minute
photosynthetic bacteria Prochlorococcus
(~0.6 µm, 65±19% and 41±19%,
respectively) but not non
photosynthetic bacteria (8±7%
and 5±19%, respectively). The
negative selection against non
photosynthetic bacteria was
surprising considering their
dominance in the planktonic
community and the considerable
size overlap on non
photosynthetic bacteria with the
Prochlorococcus population.
The small fraction of
non-photosynthetic bacteria
retained by the bivalve and the
ascidian did not differ in size
but had higher apparent nucleic
acid content in comparison with
the ambient (inhaled)
population. Considerable
retention of non photosynthetic
bacteria was exhibited only by
the sponge (84±8%) which was the
most efficient and least
selective suspension feeder (T.
swinhoei removal
efficiencies ranged from 73±27%
for eukaryotic algae to 95±7%
for Prochlorococcus).
Selectivity for cell attributes
within prey taxon was evident
for the bivalve which preferred
Prochlorococcus and
eukaryotic algae with higher
chlorophyll content. The sponge
preferred the smaller
photosynthetic cells but showed
no preferential size selection
for non-photosynthetic bacteria.
We suggest that for benthic
tropical picoplankton grazers,
selectivity is not size
dependent and probably relies on
other cell attributes such as
cell surface properties and/or
motility. The mechanisms
underlying the observed
selectivity patterns are still
unresolved and - as each phylum
relies on a unique filtration
mechanism – should vary between
the three phyla. As the
selectivity did not appear to
maximize carbon or energy gain,
it is suggested that at the
reef, where carbon is not a rare
commodity, suspension feeders
have evolved to optimize the
gain of other nutrients or to
avoid harmful prey taxa.
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