Influence of environmental conditions on early development of the hydrothermal vent polychaete Alvinella pompejana
1 UMR CNRS 7138, Systématique, Adaptation et Evolution,
Université Pierre et Marie Curie, 7 quai Saint-Bernard, 75252 Paris
Cedex 05, France,
2 IFREMER, Direction des Recherches Océaniques, Département
Environnement Profond, BP 70, Plouzané F-29280, France
3 Oregon Institute of Marine Biology, University of Oregon, PO Box 5389,
Charleston, OR 97420, USA
* Author for correspondence at present address: Max-Planck-Institute for Marine Microbiology, Molecular Ecology Department, Celsiusstr. 1, 28 359 Bremen, Germany (e-mail: fpradill{at}mpi-bremen.de)
Accepted 2 March 2005
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Summary |
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Key words: hydrothermal vent, development, embryo, pressure vessel, polychaete, temperature tolerance, H2S, environmental condition
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Introduction |
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Collection of early life stages from deep waters has presented considerable
challenges. Larvae of probable vent origin have been collected in plankton
tows (Berg and Van Dover, 1987;
Kim et al., 1994
;
Mullineaux et al., 1995
),
pumps (Kim and Mullineaux,
1998
) and sediment traps
(Khripounoff et al., 2000
)
deployed near vent habitats, but it was often difficult to identify them and
therefore to assess their origin. As alternatives to in situ
collection and to avoid species misidentification, methods have been developed
to obtain early life stages of hydrothermal vent organisms by in
vitro fertilisation (Marsh et al.,
2001
; Pradillon et al.,
2001
). These methods still require us to overcome several
difficulties. First, adults have to be collected from vents using a
submersible and brought to the surface without being damaged. Although
biological sampling has now been conducted at vents for more than 25 years,
this remains a difficult task. Second, pressure vessels are required to
maintain many organisms retrieved from the deep sea
(Jannasch and Wirsen, 1984
;
Shillito et al., 2004
;
Wilson and Smith, 1985
;
Yayanos, 1978
,
1981
). Not only adults but
also embryos have to be returned to deep-sea pressure because they cannot
develop normally at atmospheric pressure
(Tyler and Young, 1998
;
Young and Tyler, 1993
;
Young et al., 1996
).
Incubations in pressure vessels are then required for vent animals early life
stages. Besides pressure, temperature should also be controlled. In the case
of hydrothermal vent organisms, it is a priori not known whether
embryos would need abyssal (cold) or hydrothermal (warmer and with different
chemistry) conditions to develop.
Until now, embryos of only two hydrothermal vent organisms have been
obtained by in vitro fertilisation and reared in pressure vessels:
the vestimentiferan tubeworm Riftia pachyptila Jones 1981
(Marsh et al., 2001) and the
tubicolous polychaete Alvinella pompejana Desbruyères and
Laubier 1980 (Pradillon et al.,
2001
). Embryos of R. pachyptila were successfully reared
to the trochophore stage after 34 days of incubation in abyssal conditions at
2500 m depth (2°C and pressure of 26 MPa). Larval life-span of R.
pachyptila was estimated to be 38 days by measuring oxygen consumption of
embryos during development and energy storage within the egg. These data were
coupled with measurements of local currents to estimate large dispersal
distances of up to 100 km (Marsh et al.,
2001
).
For A. pompejana, as for other species of the alvinellid family,
observation of young worms at vents suggested that embryos would develop
directly or with a lecithotrophic non-feeding mode, without a dispersal phase
(Desbruyères et al.,
1985; Desbruyères and
Laubier, 1986
; Zal et al.,
1995
). However, larvae of A. pompejana were never
identified at vent sites. This polychaete species is a pioneer in the
colonisation of the hottest areas of vent chimneys on the East Pacific Rise
(Fustec et al., 1987
;
Taylor et al., 1999
). The
hydrothermal fluid bathing alvinellid colonies differs greatly from abyssal
seawater, both in temperature and chemical composition. Hydrothermal fluids
are higher in temperature, more acidic, richer in sulphide and depleted in
oxygen relative to abyssal seawater
(Childress and Fisher, 1992
;
Johnson et al., 1986
,
1994
;
Le Bris et al., 2003
;
Luther III et al., 2001
;
Sarradin et al., 1998
). These
environments are also much more variable; steep, fluctuating gradients near
vents contrast markedly with the stability and homogeneity of abyssal
seawater. If embryos of A. pompejana develop at vent sites under
natural conditions, they probably require different conditions than embryos,
such as those of Riftia pachyptila, which develop in cold abyssal
waters.
Preliminary experiments with in vitro fertilisation of A.
pompejana suggested that embryos arrest development at low abyssal
temperature (2°C), but resume development when temperature increases to
10°C (Pradillon et al.,
2001). However, they were unable to develop when the temperature
is above 20°C. These results suggested that embryos must develop at
intermediate temperatures, lower than in the adult habitat, where temperatures
around or above 20°C have been reported
(Cary et al., 1998
;
Chevaldonné et al.,
1992
; Desbruyères et
al., 1985
), but higher than the surrounding abyssal sea. Pradillon
et al. (2001
) proposed that
embryos could either develop at the bottoms of chimneys where intermediate
temperatures are found, or be carried in an arrested state far from their
places of origin, completing development only when warmer habitats are
encountered.
Conditions in pressure vessels can be tightly controlled, permitting
determination of some of the environmental parameters that support early
development under natural conditions. However, in pressure vessels, the
complexity of the vent habitat cannot be simulated. Hydrothermal vents offer
many microhabitats with a wide range of physical and chemical conditions
(Le Bris et al., 2003;
Sarradin et al., 1998
). These
environments are highly dynamic, shifting constantly in the relative influence
of oceanic seawater and hydrothermal fluid. This environmental dynamic may
have a strong effect on developing embryos and should be taken into account
when determining the suitable conditions for early development in A.
pompejana and its dispersal capabilities.
In this regard, development experiments initially conducted in pressure
vessels (Pradillon et al.,
2001) were compared to similar experiments conducted at a vent
site in order to confirm or reject the hypotheses describing potential areas
suitable for development. During the PHARE cruise in 2002, in
parallel to pressure vessel experiments, embryos obtained by in vitro
fertilisation were returned to positions at a vent chimney that experienced
different levels of hydrothermal influence. This set of in situ
experiments allowed us to compare development in fluctuating natural
conditions to that observed in controlled thermal conditions of pressure
vessels, and to investigate further the conditions that allow development of
embryos at the vent site by considering the influence of fluctuating
temperature, pH and sulphide exposure.
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Materials and methods |
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Spermatozoa collected from the spermathecae of females yielded a higher percentage of zygotes (82%, as indicated by elevation of a fertilisation envelope) than did sperm collected directly from males (45%), so the former were used for all experiments. For each culture, oocytes and spermatozoa collected from a single female were mixed in 2.5 ml of 0.2 µm filtered seawater. Adhesive clusters of spermatozoa were broken apart by vigorous pipetting. After mixing oocytes and sperm, cultures were maintained at atmospheric pressure and at room temperature (20°C) for 30 min, then sperm concentration was reduced by dilution in a larger volume of filtered seawater.
Incubations in controlled conditions
For the experiments at atmospheric pressure, zygotes were pipetted into 10
ml glass vials filled with seawater until overflowing. In situ
pressure experiments (26 MPa), were conducted in stainless steel pressure
vessels (Autoclave, Rantigny, France). We used two types of systems. In some
experiments, embryos were incubated directly within 100 ml vessels
(Pradillon et al., 2004
); in
others, embryos in 10 ml plastic vials were enclosed in a 20 l pressure vessel
(Shillito et al., 2001
).
Zygotes were exposed to five temperature treatments (2°C, 10°C,
14°C, 20°C and 27°C) at atmospheric pressure (1 atm=
1.03x105 Pa), and three temperature treatments (2°C,
10°C and 20°C) at in situ pressure (26 MPa). Cultures
maintained at in situ pressure had to be decompressed for microscopic
observation of embryos. Embryos subsequently recompressed did not develop
further. Consequently for each observation we used a distinct batch of embryos
that was decompressed only for a single observation. For each temperature
treatment, we conducted two incubations of different durations: 24 h and 48 h
for 20°C, 48 h and 72 h for 10°C, and 72 h and 8 days for 2°C.
Each experiment was conducted on a batch of at least 100 eggs (occasionally as
many as 500), depending on the number of mature oocytes available from a
single female.
Developing embryos were observed and staged under a compound light microscope. Cultures at atmospheric pressure (0.1 MPa) were sampled every 12 h or more frequently, discarding each sample after it had been examined. We counted at least 20 embryos per sample. Embryos reared in pressure vessels were examined within 30 min of depressurisation. Light micrographs were taken either directly on board ship using a digital image capture system (Optronics, Gelota, CA, USA) coupled to an Olympus DIC microscope, or after fixation in a 3% glutaraldehyde seawater solution, using a Nikon camera coupled to the microscope. A few embryos were fixed for observation by scanning electron microscopy (SEM).
Cleavage rates of embryos were estimated using the successive observations of the developing embryos reared at 1 atm. By linearly interpolating the percentage of embryos at each developmental stage between two successive observations, we estimated the time at which 50% of the embryos had reached a given stage. This time was considered to be the average time lapse required to reach this stage in a population of embryos that were not developing synchronously.
Incubations at a vent site
Incubations in the natural environment were conducted during the
PHARE cruise on 13°N EPR in May 2002 using a simple apparatus
similar to that used for in situ rearings of Riftia
pachyptila (Marsh et al.,
2001). Three large Alvinella pompejana females, collected
by the ROV Victor, were dissected for oocytes and sperm collection. Gametes
from all three females were mixed as described above, then held at 8°C
until fertilisation membranes were observed on all oocytes (5 h after
fertilisation). The culture was then diluted and split into six 50 ml plastic
vials filled with filtered (0.2 µm mesh size) seawater at 8°C. About
1000 fertilised oocytes were introduced into each plastic vial to yield a
final density of approximately 20 embryos ml1. The vials
were covered with 30 µm nylon mesh and mounted in pairs on three weighted
moorings of 25 cm long polypropylene line. These moorings were immersed in an
insulated box filled with 8°C surface seawater, and the box was placed in
the submersible basket just before launching. This procedure allowed us to
protect the incubators and limit the temperature increase (sea surface water
temperature was around 29°C) at the beginning of each dive. Once on the
bottom, incubators were deployed at a vent site named ELSA on a 3 m high
chimney (marker HOT 3). This white smoker harboured alvinellid colonies on its
edge, and was surrounded by clumps of Riftia pachyptila. The seafloor
just at the base of the edifice was covered by rusty oxidised sulphide blocks,
with a few tubeworms between cracks. Incubators were deployed at three
selected locations (Fig. 1):
(I1) at the base of the chimney between oxidised sulphide blocks close to
tubeworms with rusty-coloured tubes, (I2) between the tubes, within the dense
Riftia clump covering most of the flanks of the chimney, (I3) at the
surface of the alvinellid colony, near the top of the chimney. Incubators were
recovered after 5 days and transported to the surface in an insulated box to
minimise thermal shock during recovery. As soon as they arrived on board (5 h
after recovery from the vent site, with depressurisation during the last 1.5
h, due to the ascent of the submersible) embryos were retrieved from the
incubators and fixed in a 3% solution of glutaraldehyde in a buffer of 0.1 mol
l1 cacodylate in 0.3 mol l1 NaCl, then
washed in a buffer of 0.2 mol l1 cacodylate in 0.35 mol
l1 NaCl. In the laboratory, a sub-sample of 100 embryos was
drawn from each incubator to estimate the proportion of embryos attaining
various developmental stages.
|
Temperature and chemical measurements
During deployment of the incubation experiments, we recorded ambient
temperatures for about 1 min at each site, at a rate of one measurement every
5 s, by positioning the ROV temperature sensor within a few centimetres below
and above incubators. These measurements were repeated twice for each
incubator during the time of the experiment. The mean temperature, standard
deviation (S.D.) and maximum temperature for each
incubator were calculated for the pooled data from each site. As part of a
general monitoring of the chimney, the temperature was also recorded
continuously over the whole duration of the experiment, at a rate of one
measurement per 10 min, using individual autonomous probes (Micrel, Hennebont,
France). Two autonomous probes were deployed in the study area: the first one
at the edge of the chimney on the alvinellid colony, about 20 cm from the
incubator I3 (Fig. 1), the
second one at the base of the chimney on oxidised sulphide rocks, about 50 cm
from the incubator I1. No probe was deployed in the tubeworm bush where was
the incubator I2. PH and sulphide measurement were taken over the alvinellid
colony, about 50 cm from the incubator I3, using an in situ probe and
a submersible flow analyser as described in Le Bris et al.
(2003). We correlated these
chemical measurements with temperature. Assuming as a first approximation that
these relationships could be extrapolated to the whole edifice, the pH and
sulphide ranges in the vicinity of each incubator were inferred from
temperature.
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Results |
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Before fertilisation, oocytes removed from the female oviducts are flattened spheres with an undulating membrane on their surface. The germinal vesicle appears as a less dense area slightly removed from the centre of the oocyte. The nucleus is markedly lighter than the cytoplasm, which is filled with dense vitelline reserves. Oocytes become spherical within a few minutes of being diluted in seawater. Sperm are not required to mediate this shape change. The germinal vesicle disappears at the time of shape change or shortly thereafter, and the oocytes at this stage appear completely homogeneous under the light microscope (Fig. 2A). Between 30 min and several hours after fertilization, the fertilisation envelope elevates progressively, beginning at a single point on the periphery of the oocyte (Fig. 2B).
|
Before first cleavage, a polar lobe is formed (Fig. 2C) and this leads to the first asymmetrical cleavage (Fig. 2D). The size ratio between the two cells is relatively constant, the large blastomere being about 1.5 times larger in diameter than the small one. However, embryos with either a larger difference in blastomere size or blastomeres of virtually identical size were also observed occasionally. As in many other spiralians, 4-cell embryos typically have one blastomere that is larger than the other three (Fig. 2E). Subsequent cleavages did not always occur in perfect synchrony; it was not uncommon to observe 5- or 6-cell stages between 4-cell and 8-cell stages (Fig. 2F) of apparently normal embryos. Twelve-cell embryos were also observed between 8-cell and 16-cell stages. We were not able to confirm the exact numbers of cells in embryos with more than 16 cells, so we refer to more advanced embryos as having >16-cells.
The cleavage pattern that we observed for Alvinella pompejana embryos was very similar to that of other polychaetes, so we regarded embryos departing from this general pattern as abnormal. Several kinds of abnormalities were observed in the general shape of the embryo, and in the integrity of the embryonic cells. Anomalies in the spatial arrangement of blastomeres (Fig. 3A) and the size ratios of cells (Fig. 3B) were observed in about 20% of embryos reared at atmospheric pressure, but rarely in embryos incubated at 250 atm. Such abnormally shaped embryos often had 1 or 2 very large blastomeres, and many very small blastomeres at one pole of the embryo. Breakdowns in cellular integrity were evidenced by the formation of bubbles or blebs on the cell membrane (Fig. 3A,C), which ultimately caused the cytoplasm to spill into the perivitelline space (Fig. 3D). This phenomenon did not seem to be linked to pressure conditions, and led to the complete degradation of the embryo within a few hours. Henceforth, we refer to embryos with shape anomalies as `abnormal embryos' and to embryos with cellular blebs as `degrading embryos'.
|
Developmental rates and embryonic survival
Experiments at 1 atm
Initial descriptions of A. pompejana embryos were based on
cultures maintained at 1 atm, as this allowed us to follow the different steps
of developmental processes. At regular intervals after fertilisation, we
sampled embryos from the main culture to determine the proportion of embryos
at various developmental stages. Abnormal embryos were discarded and not
counted in the frequency calculations. Degrading embryos were counted and used
to estimate survival rates.
As previously reported (Pradillon et
al., 2001), no embryos survived after 24 h at a temperature of
20°C, whereas more than 80% survived at all lower temperatures
(Fig. 4). Embryos developed
normally at 10°C and 14°C. At 2°C, embryos survived but did not
develop (Fig. 4). As shown in
Fig. 4, development was not
highly synchronised among embryos, and each observation showed embryos at
different developmental stages. Because of this lack of synchrony, it was
difficult to determine the typical cleavage rates by direct observation. At
14°C, all embryos remained uncleaved 7 h after fertilisation, but 56%
attained or passed the 2-cell stage by 16 h after fertilization. The two
subsequent cleavages each occurred after 3 h intervals
(Fig. 5). At 10°C, the time
to first cleavage was nearly twice as long as at 14°C; about half of the
embryos underwent first cleavage by 26 h after fertilization, and the
subsequent two cleavages occurred at 42 and 69 h after fertilization,
respectively (Fig. 5).
|
|
Experiments at 250 atm
In terms of survival, results obtained under pressure were similar to those
obtained with cultures conducted at 1 atm. At 20°C, embryos did not
survive more than 48 h, and we obtained no cleaved embryos at this temperature
(Fig. 6). However, as
observations of embryos in the pressure vessel were not made at regular
intervals, it is possible that embryos did actually cleave while they were
under pressure, but started to degrade before they were observed. At 2°C,
fertilised oocytes did not cleave, even after 8 days of incubation.
|
We obtained developing embryos at 10°C. As in 1 atm cultures, all the embryos in these cultures did not develop at the same rate. Cleavage rates were slightly slower under pressure than at 1 atm after 48 h; most of the cleaved embryos were at the 2-cell stage under pressure, whereas most embryos incubated without pressure were at the 4-cell stage after 48 h. However, the 72 h experiments at 10°C showed very comparable frequencies in developmental stages obtained with or without pressure.
Experimental incubations in situ
The prevailing local conditions at vents vary not only in temperature, but
also with respect to the chemical environment. Static incubations in pressure
vessels give only a partial picture of the developmental potential of these
embryos. In order to take into account dynamic temperature changes and the
chemistry of the hydrothermal fluids, embryos were incubated in situ,
at different locations in the vent environment.
After 5 days of incubation, only 10% of the initial input of fertilised oocytes remained in the incubator deployed on an adult alvinellid colony (I3). None of these oocytes had cleaved (Fig. 7). By contrast, embryos incubated among Riftia pachyptila (I2) or at the base of the chimney on oxidised sulphide blocks (I1) experienced nearly 100% survival, and 70% of them had cleaved (Fig. 7). As in the laboratory incubations, variable developmental stages indicated a lack of synchrony in developing embryos (Fig. 7). However, the distributions of the developmental stages at the I1 and I2 sites were very similar, with 2-cell and 4-cell stages representing 50% of the total number of embryos, and embryos at the 8-cell stage or greater representing 30% of the total number.
|
Short-term temperature records taken by the ROV temperature probe right on the incubators give an indication of the temperature ranges experienced by the embryos. Although located on oxidised rock at the bottom of the chimney where no fluid venting was visible, the temperature data at I1 displayed a weak but significant temperature increase above ambient seawater (Table 1). The environment of incubator I2 in the middle of the tubeworm clump was only slightly warmer on average than the previous one (Table 1). The mean temperature reported for I3 was a few degrees lower than 20°C. An estimation of the thermal range in the immediate environment of incubators can be defined as the difference between the maximum and mean temperatures recorded for each incubator (Table 1). In all three cases, this range was about 5°C.
|
Fig. 8 illustrates the very
different thermal patterns characterising the top and the bottom of the
chimney, during the experiment. High temperature fluctuations were recorded on
the edge of the chimney. Such fluctuations are characteristic of alvinellid
colonies (Chevaldonné et al.,
1991). This dynamic pattern results from the turbulent venting of
the hot fluid and its dilution into cold seawater, and from a daily scale
modulation by tidal and bottom current effects
(Chevaldonné et al.,
1991
). It can be noted here that while temperature mostly ranges
between 10 and 20°C, the record exhibits temperatures as high as
2527°C, sometimes in sharp peaks but often for a duration of
several hours. In contrast, the autonomous probe at the base of the chimney
did not record any significant change in temperature and stayed near the
2°C background temperature. These records are indicative of the two
extremes that can be found outside the hydrothermal plume on a single
edifice.
|
The pH and sulphide content estimated from the mean and maximum temperatures suggest that the I1 environment would remain low in sulphide with a mean value under 20 µmol l1, while the I2 environment would reach about 60 µmol l1 and the I3 value would remain above 200 µmol l1 on average (Table 1). The range of variation at these three sites is expected to be large, with maximum values of 115, 145 and 263 µmol l1, respectively. The pH of the three areas should be much less variable, with values lying around 7.5 (Table 1).
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Discussion |
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As has been observed for other embryos from these depths (e.g.
Young and Tyler, 1993;
Marsh et al., 2001
), more
embryos developed normally at the pressure of the adult colonies than at
atmospheric pressure. The abnormalities observed at 1 atm included aberrant
relative sizes of blastomeres and unusual arrangements of cells, both
suggesting alterations of cell division. Cell arrangements during cleavage are
mostly controlled by cytoskeletal dynamics, namely the assembly, movement and
disassembly of cytoskeleton proteins. Polymerisation properties of proteins
such as actin and tubulin are affected by pressure increases, causing
alterations in cell shape and cleavage processes
(Begg et al., 1983
;
Bourns et al., 1988
; Salmon,
1975a
,b
).
For organisms normally living at high hydrostatic pressure, atmospheric
pressure might also affect cytoskeleton proteins and produce abnormal cell
morphologies during development.
Although embryos of A. pompejana can sometimes develop at 1 atm,
most embryos appear to be obligately barophilic, requiring abyssal pressure to
develop normally. Similar barophilia was found in embryos of the
vestimentiferan tubeworm R. pachyptila
(Marsh et al., 2001). The
existence of physiological pressure thresholds in deep-water and shallow-water
echinoderms has previously been used to explain bathymetric distribution of
species, as well as dispersal capabilities
(Tyler and Young, 1998
;
Tyler et al., 2000
;
Young and Tyler, 1993
; Young
et al., 1997
,
1996
).
Even though embryos of Alvinella pompejana require abyssal
pressures to develop, they require temperatures higher than those typical of
the vast abyssal water column (Pradillon
et al., 2001). The only areas where temperature can rise above
24°C in the deep Pacific Ocean are near hydrothermal vents. If
embryos are transported 200 m above the ocean floor by a buoyant hydrothermal
plume, as has been suggested for other vent larvae
(Baker and Massoth, 1987
;
Kim et al., 1994
;
Mullineaux et al., 1995
), they
will disperse in a cold environment where development cannot proceed. We
suggested elsewhere that a reversible arrested development might permit very
long distance dispersal in these worms
(Pradillon et al., 2001
), but
it also seems plausible that significant numbers of alvinellid embryos might
remain near the parental colonies, developing in microhabitats with the
appropriate temperatures while experiencing very little dispersal. Indeed, in
the vast cold-water environment of the abyss, habitats with elevated
temperatures near hydrothermal vents are the only places where development of
A. pompejana is possible. Although spatially very limited, areas of
venting offer a large assortment of microhabitats even at the scale of a
single hydrothermal chimney (Sarradin et
al., 1998
). These environments reflect variable contributions of
the hydrothermal fluid to the medium, hence generating a wide range of
temperature and chemical conditions (Le
Bris et al., 2003
; Sarradin et
al., 1998
). Temperatures ranging from 20°C at tube openings
(Cary et al., 1998
;
Chevaldonné et al.,
1992
; Desbruyères et
al., 1985
), and up to 80°C inside tubes
(Cary et al., 1998
) have been
reported for A. pompejana colonies. According to our results, such
temperatures would preclude the development of embryos inside the maternal
tubes of A. pompejana, as has been hypothesized for the closely
related species Paralvinella pandorae pandorae
(McHugh, 1989
). In other
words, embryos must probably escape the high temperature portions of the adult
colonies in order to develop (Pradillon et
al., 2001
). Development in some colder part of adult colonies,
perhaps near the openings of the tubes, could be possible in some cases; the
environmental gradients in colonies are steep, and temperatures as low as
10°C have been measured at the tube openings of some colonies
(Chevaldonné et al.,
1991
; Di Meo-Savoie et al.,
2004
; Le Bris et al.,
2003
; Sarradin et al.,
1998
). Thus, from in vitro experiments at various
temperatures, we can hypothesize the places where embryos might develop: (1)
in the coolest parts of some adult colonies, (2) near the bases of chimneys or
in tubeworm aggregations where temperatures are moderate, or (3) dispersing in
the cold abyssal water column until a warm-water vent habitat is ultimately
encountered. We further tested these three hypotheses by exposing embryos
in situ to various microhabitats in the environs of a single edifice.
These experiments provide additional information on the range of thermal
conditions that permit embryonic development. Results from incubators I1 and
I2 indicate that embryos can develop at temperatures lower than 10°C; a
slight temperature increase of just a few degrees above background abyssal
temperatures is sufficient.
Analysis of the I3 incubation (in the adult colony) indicates that embryos may be killed by intermittent pulses of excessively high temperature even when the average conditions lie within the limits of thermal tolerance (Table 1). Short-term temperature records reached a maximum of 17°C close to the incubator, but longer-term continuous monitoring indicated that daily fluctuations and short spikes above 25°C are common (Fig. 8). Thus, the embryos in the colony incubators may have been exposed to temperatures exceeding 20°C during at least part of their deployment time. The effect of exposing embryos to high temperatures for brief periods has not been investigated under controlled conditions, so we cannot discount the possibility that embryos in adult colonies succumbed to physical or chemical extremes other than high temperatures.
Within the range of temperatures observed, only slight changes in pH are
expected between the three incubators and the water bathing I3 should not have
been significantly more acidic than that of I1 and I2
(Table 1). The estimated
sulphide concentration, on the other hand, distinguishes the environments of
I1 and I2 from I3. We estimated sulphide content from temperature, assuming a
similar sulphide contenttemperature relationship for the alvinellid
colony and the other habitats of the chimney. At a given temperature, however,
the siboglinid tubeworm habitat could be much lower in sulphide than
alvinellid habitats, due to various subsurface processes involving sulphide
consumption and thermal exchange (Le Bris
et al., 2003). In this case, the difference between the first two
incubators and the third one would be even larger, with sulphide
concentrations around I1 and I2 much lower than those estimated in
Table 1. Sulphide is, however,
likely to be present in the two lower areas. This experiment would suggest,
therefore, that a moderate sulphide exposure does not prevent the embryos from
developing. Second, exposure to an average sulphide content as high as 200
µmol l1 could be lethal to the embryos, providing an
alternative explanation for the differences in embryonic survival and
development observed among the different environments.
These field experiments thus complement and support our preliminary laboratory observations on the conditions required for early development of Alvinella pompejana. They show that embryos exposed to the conditions prevailing in a colony of A. pompejana do not survive, whereas ones nestled in Riftia pachyptila clumps, or in other areas of weak but significant hydrothermal influence, develop successfully. Whether temperature conditions, sulphide concentration, or a combination of these factors is responsible for high mortality rates in adult colonies remains to be discovered, but it is clear that embryos can only develop in portions of the vent environment where conditions are less extreme than those tolerated by the adults.
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Acknowledgments |
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References |
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