Energy metabolism and valve closure behaviour in the Asian clam Corbicula fluminea
Institute of Zoophysiology, Heinrich-Heine-University, Düsseldorf 40225, Germany
* Author for correspondence (e-mail: ortmann{at}uni-duesseldorf.de)
Accepted 11 August 2003
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Summary |
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Simultaneous measurements of heat dissipation and oxygen consumption (calorespirometry) revealed an intermittent metabolism in the clam. With the onset of valve closure, C. fluminea reduced its metabolic rate to 10% of the standard metabolic rate (SMR) measured when the valves were open. Nevertheless, this depressed metabolism remained aerobic for several hours, enabling the clam to save energy and substrates compared to the requirements of the tenfold higher SMR. Only during long-lasting periods of valve closure (more than 5-10 h) did the clams become anaerobic and accumulate succinate within their tissues (2 µmol g-1 fresh mass). Succinate is transported into the mantle cavity fluid, where it reaches concentrations of 4-6 mmol l-1. Because this succinate-enriched fluid must pass the gills when the valves open again, we suggest that this anaerobic end product is at least partly reabsorbed, thus reducing the loss of valuable substrates during anaerobiosis. Propionate was also produced, but only during experimental N2-incubation, under near-anoxic conditions.
The intermittent metabolism of C. fluminea is discussed as an adaption to efficiently exploit the rare food supply, saving substrates by the pronounced metabolic depression during valve closure.
Key words: anaerobiosis, bivalve, Asian clam, Corbicula fluminea, calorimetry, circadian rhythm, metabolic depression, energy metabolism
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Introduction |
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The life history of C. fluminea is characterized by typical
r-strategic features such as high fecundity, along with short generation times
and high growth rates (McMahon,
1983; Meister,
1997
). Thus, C. fluminea occurs in high abundance
wherever it is found, frequently more than 1000 individuals m2
(McMahon, 2000
). Meister
(1997
) reported mean densities
of 350-600 individuals m2 in the Rhine, corresponding to a biomass
of 1 kg m-2. Thus, C. fluminea is one of the most
important biomass producers of the Rhine
(Kinzelbach, 1995
).
Because of its economic impact on artificial water currents such as
irrigation channels or cooling water systems of power stations, there have
been many investigations into the clam's reactions to abiotic parameters (for
reviews, see Doherty and Cherry,
1988; McMahon,
2000
). However, although the clam's valve movements are used to
monitor water-borne stressors (Doherty et
al., 1987
; Allen et al.,
1996
), little is known about the animal's energy metabolism during
periods when the shells are closed, nor about the clams' natural valve closure
behaviour throughout the year.
It is generally thought that bivalves with opened valves rely on an aerobic
metabolism, mostly fuelled by glycogen. After closing the valves, however,
most authors suggest that the enclosed oxygen is spent within a few minutes
(Widdows, 1987), since the
partial pressure of oxygen seems to decrease rapidly in the mantle cavity, as
measured in Arctica islandica
(Taylor, 1976
) and Mytilus
edulis (Davenport and Woolmington,
1982
). Anaerobic energy provision commences as soon as the partial
pressure of oxygen falls below between 20 and 50 mmHg
(Pörtner et al., 1985
).
As environmental anaerobiosis commences, ATP is replenished by degradation of
a phosphagen (usually phospho-L-arginine in molluscs) and
glycolytic substrate phosphorylations, with lactate and opines (guanidoamino
acids) accumulating as electron-accepting end products. Later on,
mitochondrial fumarate reduction with concomitant formation of succinate, and
the subsequent synthesis of propionate and acetate, provides most of the ATP.
The anaerobic ATP yield, which comprises at most 20% of the aerobic energy
provision, is not compensated by a Pasteur effect; instead, the energy
expenditure of anaerobic molluscs is usually markedly reduced
(Grieshaber et al., 1994
). A
functional anaerobiosis is only demonstrated by some mobile species such as
some Pectinidae and Cardiidae, which are capable of vigorous swimming and
jumping movements, and is characterized by rapid degradation of a phosphagen,
and an increased glycolytic flux, i.e. a Pasteur effect, leading to the
accumulation of lactate and/or opines
(Grieshaber et al., 1994
).
If bivalves become anaerobic during valve closure, even without ambient
stressors, as suggested by some authors
(Higgins, 1980;
Williams et al., 1993
;
Holopainen and Penttinen,
1993
; Sobral and Widdows,
1997
), then one must question how these animals fuel inefficient
anaerobic pathways with enough substrates to endure long-lasting periods
within closed valves. Obviously rapid exhaustion of substrates can only be
prevented if the metabolic rate is lowered during anaerobiosis. But even when
reduced down to 10% of the standard metabolic rate (SMR), the amount of
substrates required to sustain the metabolism anaerobically still equals the
amount used during aerobic conditions
(Hand and Hardewig, 1996
).
Thus, the following questions arise. Does C. fluminea reduce its
metabolism during valve closure and to what extent? And does this species
really become anaerobic during valve closure?
In an attempt to answer these questions we continuously monitored the
metabolic rate of C. fluminea by simultaneously measuring heat
dissipation and oxygen consumption of clams maintained in well aerated as well
as hypoxic conditions, including numerous consecutive periods of valve
closure. Anaerobic end products such as acetate, propionate and the transient
intermediate succinate (Grieshaber et al.,
1994) were analysed in specimens incubated in aerated artificial
freshwater (therein retaining their natural valve-closing behaviour) and, in
addition, in specimens incubated in N2-saturated water. Finally, we
recorded the valve movements of C. fluminea directly in situ
in the Rhine over more than 2 years to compare its natural behaviour with that
seen in the laboratory.
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Materials and methods |
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Valve closure behaviour
From August 1997 until August 1999 we continuously recorded the valve
movements of C. fluminea in the Rhine using two different bivalve
monitors. The Dreissena-Monitor® (Envicontrol, Köln, Germany) at Bad
Honnef (640 km) was supplied with 34 specimens; the Mosselmonitor® (Delta
Consult, Kapelle, Netherlands) at Koblenz (590 km) carried only eight
specimens. Both monitors were situated in a bypass that permanently received
water directly pumped out of the river. Specimens of C. fluminea were
attached onto holders with one valve, so that the movement of the other valve
could be recorded. Whereas the Dreissena-Monitor® uses the interaction of
a Reed Switch and a small magnet, which is glued onto the free moving valve,
the Mosselmonitor® measures the distance between two electric coils that
are also glued onto the valves (Fig.
1). The signals from each single specimen were recorded using a PC
with monitor-specific software. Valve movement data were processed and
illustrated with Excel and SigmaPlot software. The use of both measuring
systems is well established in water surveillance projects and toxicological
behaviour studies (Jenner et al.,
1989; Hoffmann et al.,
1994
; Borcherding and Wolf,
2001
). Another Mosselmonitor® was used in a thermostatted
chamber inside the laboratory, operated with circulated AFW at
15±0.5°C. After various time intervals, specimens with known
valve-closure periods were collected and analysed for anaerobic end products
(see below).
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Determination of metabolic rates
Heat dissipation and oxygen consumption rates of unfed C. fluminea
were measured simultaneously between 3 and 27 days in an open-flow system
(Fig. 2). A single clam was
placed inside the reaction chamber of the perfusion systems (`2250 111': 4 ml
or `2254 001': 20 ml) within the microcalorimeter [Thermal activity Monitor
(TAM) LKB 2277, Thermometric, Järfälla, Sweden], which was kept at
precisely 15°C (±0.001°C) and perfused with AFW at flow speeds
of 14-20 ml h-1 and 30-40 ml h-1 for the 4 ml and 20 ml
chamber, respectively. The oxygen concentration of the inflowing and the
outflowing water was determined using polarographic oxygen sensors (POS:
Orbisphere, Geneva, Switzerland), which are part of a respirometer (Twin-Flow
2, Cyclobios, Innsbruck, Austria) situated inside a water bath (at
15±0.5°C). The oxygen concentration and heat signal were
continuously recorded via the Twin-Flow-Monitor, which also
compressed and converted the analogue data. These data were recorded and
processed by Cyclobios Software (`Datgraf Acquisition' and `Datgraf 2.1
Analysis' © 1993 Michael Reck). Prior to and following each experiment,
baseline heat dissipation and oxygen consumption were measured in a blank run
without an animal and used to correct the experimental data. At the end of an
experiment the clam's soft body tissue was removed and either used for
extracting metabolites or dried at 80°C for 2 days.
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Metabolic rate is expressed in J h-1 g-1 dry mass
(heat dissipation) and µmol h-1 g-1 dry mass (oxygen
consumption). Using an oxycaloric equivalent of 450 kJ mol-1
O2 (Gnaiger et al.,
1983a), it is possible to compare both rates and thus decide
whether anaerobic pathways contribute to the clams' energy provision. If the
heat dissipation exceeds the converted value of the oxygen consumption, the
difference in energy expenditure can only be the result of anaerobiosis
(Gnaiger, 1983b
;
Widdows, 1987
).
Analysis of anaerobic end products
Levels of the main anaerobic end products, acetate, propionate and
succinate, were estimated in the adductor muscles, foot, gills and mantle
cavity fluid of C. fluminea. During near-anoxic incubations, clams
were transferred into flasks containing water that had previously been bubbled
with nitrogen to guarantee a PO2 of less than
0.4 kPa. Later the nitrogen was bubbled continuously through the incubation
water until the clams were removed for preparation of their tissues. To
analyse the onset of anaerobic metabolism as well as the occurrence of a
prolonged anaerobiosis, incubations lasted 1, 2 and 4 h or 1, 2 and 3 days,
respectively. When a clam was removed from the flask, the water enclosed by
the valves was collected first, and is referred to below as mantle cavity
fluid because it represents the greatest fraction of the collected fluid. Then
foot, adductor muscles and gills were dissected and freeze-clamped
(Wollenberger et al., 1960).
All tissues and the mantle cavity fluid were quickly transferred into liquid
nitrogen and stored therein until extracted with perchloric acid (PCA) (see
below). Specimens in the laboratory taken out of the Mosselmonitor® after
known time periods with closed valves under normoxic conditions were prepared
in the same manner.
Metabolites were extracted according to Beis and Newsholme
(1975). Tissues were ground
with a mortar and pestle under liquid nitrogen and added to 200 µl (foot)
or 150 µl (adductors and gills) of 0.6 mol l-1 PCA, which was
approximately three times the volume of the tissues. Mantle cavity fluid was
diluted by a factor of two with 0.6 mol l-1 PCA. Then the extracts
were homogenized and centrifuged at 14 000 g, and the
supernatant neutralised with 5 mol l-1 KOH and centrifuged again.
The remaining supernatant was stored at -20°C until analysed.
Succinate, which indicates the onset of anaerobiosis in mitochondria
(Pörtner et al., 1985),
was determined enzymatically according to Beutler (1985).
Succinate-CoA-synthetase (SCS), an enzyme needed for this assay, is no longer
available commercially, but can be purified according to methods described by
Buck et al. (1985
) and Wolodko
et al. (1994
). The SCS that we
used in this study was generously provided by Dr William Wolodko (University
of Alberta, Edmonton, Canada).
Acetate and propionate were measured by gas chromatography using a WCOT-column heated at 100°C (fused silica, coating CP-wax 58, FFAP-CB, 25 m x 0.53 mm i.d.; Varian, Darmstadt, Germany). To separate acetate and propionate, the gas chromatograph (GC; CP-9001, Varian) was operated with a split-splitless injector (270°C) and a flame ionisation detector (FID, 300°C). Prior to the injections, the extracts were diluted with an equivalent amount of 1% HCl to convert the acids into their undissociated form. Samples (1 µl) were injected manually using a 10 µl gas-tight syringe (type #1801, RNE; Hamilton, Bonaduz, Switzerland).
Statistics
For normally distributed data we used one-way analysis of variance (ANOVA);
otherwise we used the Kruskal-Wallis one-way ANOVA on ranks, to identify
significant differences (P<0.05) between samples and controls,
marked by an asterisk. Statistical analyses were performed with SigmaStat 1.0
(Jandel Scientific, Erkrath, Germany).
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Results |
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During the warmer months, the clams showed a circadian rhythm of valve opening, with valves open in the afternoon and predominantly closed at night. Some specimens exhibited a strikingly regular pattern with one period of closed and one period of opened valves each day, each period lasting for about 10-12 h. If the clams were disturbed, for example during the maintenance of the monitor (Fig. 3A), a break in this rhythm could occur. The clams did not always close their valves in such a synchronous pattern (Fig. 3A), but for most of the clams monitored in the Rhine this rhythmic pattern persisted from the end of April until mid-October, with a daily minimum at night (most animals closed at around 1:00 h to 4:00 h) and a maximum at late afternoon (most animals open between 16:00 h to 18:00 h). Fig. 3B shows the magnitude of valve opening of the eight clams at Koblenz during a week in June 1998. Each value is the mean of a total of 420 readings taken every minute during each hour for each of the eight specimens for 7 successive days (7x60 readings per hour per specimen). This figure clearly demonstrates that despite some differences in behaviour during the week (e.g. after maintenance of the monitor) and between the specimens, the mean magnitude of valve closure of these eight animals (white diamonds) followed an impressive circadian rhythm. Moreover, C. fluminea also showed circadian valve movements in the laboratory, despite being held under constant conditions without any light. But in contrast to the valve closure behaviour recorded in situ in the Rhine, clams inside the laboratory did not move synchronously at all. Rather, each clam followed its own rhythm for up to several weeks, with different periods and phases, until the rhythm was completely lost.
During winter the valve movements of C. fluminea almost ceased and the clams stood open for much longer periods, frequently up to several days. Nevertheless, the periods during which the valves remained closed lengthened too, especially on very cold days. At water temperatures below 5°C some clams remained closed inside their valves for more than a week without any movement at all (Fig. 4).
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Energy metabolism of C. fluminea
Heat dissipation and rate of
oxygen consumption
O2 mirror the
valve closure behaviour of C. fluminea. Its intermittent metabolism
changed rapidly between periods of high and low metabolic rate
(Fig. 5). Unfortunately, a
visual control of valve opening inside the calorimeter was impossible, but
because of the great similarity in the patterns of valve movement recordings
and metabolic rates, it is highly likely that the two are closely correlated.
The valve closing behaviour in the Mosselmonitor®
(Fig. 3) was similar to that in
the calorimeter (Fig. 5) with
alternating periods of about 10-12 h. Also, a circadian rhythm frequently
occurred (e.g. first 5 days in Fig.
5) in the metabolic rates, with one period at a high metabolic
rate and one period of depressed metabolism within a 24 h period. During such
periods the metabolic rate was at least ten times higher (8.74±0.44 J
h-1 g-1 dry mass for the specimen in
Fig. 5) than during the
depressed periods (0.78±0.12 J h-1 g-1 dry
mass).
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The separate integration of metabolic rates during periods with open and
closed valves (Fig. 6) clearly
demonstrates that during valve closure C. fluminea depressed its
energy metabolism to less than 10% of the standard metabolic rate (SMR)
measured when the valves are open. For the 18 specimens investigated by
calorespirometry the mean metabolic rate with closed valves accounted for
9.3±2.7% (measured as heat dissipation) or 9.1±3.5% (measured as
oxygen consumption) of the rate with opened valves. This relative reduction of
metabolism during valve closure was independent of mass, whereas the
dissipated heat at both levels followed an allometric relationship and could
be calculated as:
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Comparing heat dissipation and oxygen consumption via the
oxycaloric equivalent (450 kJ mol-1 O2;
Gnaiger et al., 1989), there
is no doubt that the clams' metabolism was completely aerobic during periods
when the valves were open (Figs
5,
6A). Although in most cases the
differences were still very small during periods with closed valves,
occasionally the heat dissipated exceeded the corresponding value of oxygen
consumption (Figs 5,
6B). Thus, it is likely that
anaerobiosis contributed to the clams' metabolism during these periods.
Anaerobic end products
When incubated in near-anoxic conditions, C. fluminea expressed a
kind of anaerobiosis frequently described for other bivalves (De Zwaan et al.,
1976; Isani et al., 1995).
Propionate levels in both the foot (0.13±0.09 µmol g-1
fresh mass) and the adductor muscles (0.25±0.11 µmol g-1
fresh mass) were already significantly increased after 1 h of N2
incubation compared to the controls (0.04±0.04 µmol g-1
fresh mass and 0.05±0.05 µmol g-1 fresh mass,
respectively). After more than 24 h of incubation the concentration in all
tissues as well as in the mantle cavity fluid was significantly elevated,
reaching a maximum plateau after the second day of incubation of approximately
1 µmol g-1 fresh mass for the tissues and 1 µmol
ml-1 in the mantle cavity fluid
(Fig. 7A; foot,
1.19±0.87 µmol g-1 fresh mass; adductors,
1.07±0.66 µmol g-1 fresh mass; gills, 0.78±0.18
µmol g-1 fresh mass; mantle cavity fluid, 0.97±0.87
µmol ml-1). Acetate, in contrast, did not accumulate in the
tissues; after at least 24 h of N2 incubation only the
concentrations inside the mantle cavity fluid were significantly elevated (at
24 h, 0.28±0.19 µmol ml-1; at 48 h, 0.55±0.43
µmol ml-1; controls, 0.11±0.04 µmol ml-1).
Concentrations of succinate, on the other hand, reached a significantly
elevated level in all tissues within the first 2 h of N2 incubation
(foot, 1.13±0.51 µmol g-1 fresh mass; adductors,
2.90±0.97 µmol g-1 fresh mass; gills, 0.75±0.28
µmol g-1 fresh mass) and remained at this level during prolonged
incubations (Fig. 7B). However,
succinate also accumulates inside the mantle cavity fluid, and this has not
been previously demonstrated in clams. During the first hour of hypoxic
incubation, succinate concentrations in the mantle cavity fluid had already
increased to 1.19±0.91 µmol ml-1 and remained
significantly increased thereafter. In fact, it appeared as if the succinate
concentration in the mantle cavity fluid had not reached maximum levels, even
after the longest (72 h) incubations (Fig.
7B).
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Unlike the N2-incubated animals, clams removed from the Mosselmonitor® (laboratory constant conditions, see above) without hypoxic stress, did not produce propionate, even after prolonged periods of valve closure of up to several days. In contrast to propionate, during `voluntary' valve closure the amounts of succinate accumulated by C. fluminea in its tissues were equal to those accumulated in the N2-incubation experiments (Fig. 8), e.g. after 2-3 days of valve closure, succinate levels were 1.85±0.49 µmol g-1 fresh mass (foot), 2.26±0.58 µmol g-1 fresh mass (adductors) and 1.18±0.53 µmol g-1 fresh mass (gills). But the onset was delayed; there were no elevated concentrations of succinate in the tissues after 2-5 h of `voluntary' valve closure: 0.56±0.18 µmol g-1 fresh mass (foot), 1.48±0.47 µmol g-1 fresh mass (adductors) and 0.52±0.25 µmol g-1 fresh mass (gills). As for the hypoxic incubated specimens, clams with `voluntarily' closed valves transported succinate from the mitochondria into the mantle cavity, where the concentrations steadily increased up to 5.66 µmol ml-1 after more than 4 days of valve closure (Fig. 8).
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Discussion |
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Previous experiments suggested that bivalves remain predominantly open
within a suitable environment. However, consistent with some earlier studies
on C. fluminea (Doherty et al.,
1987; Ham and Peterson,
1994
; Allen et al.,
1996
; Tran et al.,
2003
), this study clearly confirms that extended periods of valve
closure, frequently up to 10-12 h, are part of the natural behaviour pattern
of this clam (Fig. 3A).
Furthermore, in the Rhine its valve movements followed a circadian rhythm from
April until October (Fig. 3),
accompanied by a 90% decrease in metabolic rate during valve closure.
Moreover, the almost exactly matching rates of heat dissipation and oxygen
consumption (Fig. 5), as well
as the integrated energy demands of the corresponding intervals
(Fig. 6) suggested that the
clams' energy metabolism remained aerobic almost all the time. However, under
near-anoxic conditions, C. fluminea did accumulate typical anaerobic
end products such as succinate and propionate in its tissues, as well as in
the mantle cavity fluid, and the concentrations of these metabolites were
already significantly elevated within the first hours of incubation
(Fig. 7). But, in contrast to
the nitrogen-incubated animals, aerobically incubated specimens removed from
the Mosselmonitor® while their shells were closed did not produce
propionate, and the onset of succinate accumulation was delayed for several
hours (Fig. 8). After more than
2 days of valve closure, however, the concentrations of succinate in the
tissues (2-3 µmol g-1 fresh mass) and mantle cavity fluid (4-6
µmol ml-1) in the specimens also reached similar values to those
measured under conditions of hypoxic stress, even though the animals were
still in aerobic water.
To confirm our assumption that C. fluminea saves energy during valve closure in normoxic water, we calculated how long the enclosed oxygen could fuel energy metabolism during voluntary valve closure. Assuming approximately normoxic conditions at the moment of valve closure and knowing the volume of the mantle cavity fluid as well as the fresh mass of the soft body, it is possible to estimate the amount of oxygen trapped inside the closed valves. From the oxycaloric equivalent and the known metabolic rate occurring during valve closure, it can be calculated that C. fluminea remains aerobic for approximately 4-9 h after valve closure, depending on the size of the individual (Table 1). This result confirms our assumption that larger specimens hardly become anaerobic at all during the usual daily period of valve closure.
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The metabolic rate of C. fluminea declines to 10% during valve
closure, which is similar to the 90% reduction in heat dissipation observed
for Pisidium amnicum and Sphaerium corneum by Holopainen and
Penttinen (1993). Because
these authors could not demonstrate any significant difference in the
metabolic rates of specimens with closed valves under normoxic and anoxic
conditions, they concluded that these Pisiid clams quickly became anaerobic
after valve closure. By contrast, our measurements for C. fluminea
provide clear evidence that this clam is able to remain aerobic for several
hours during valve closure. The tremendous reduction in the clams' metabolism
after closing the valves saves a great amount of energy and substrates
compared to the tenfold higher metabolic rate when the valves are open. Such a
large decrease in metabolic rate, down to 10% or less of the standard
metabolic rate (SMR), which is defined as the metabolic rate at rest, constant
temperature, normoxia and without food
(Grieshaber et al., 1994
;
Hulbert and Else, 2000
), has
frequently been described among invertebrates facing severe stress (for a
review, see Guppy and Withers,
1999
). A reduced metabolism needs less energy and fewer
substrates, which is especially important if an animal has to rely on
inefficient anaerobic pathways (Hochachka
and Guppy, 1987
; Grieshaber et
al., 1994
). However, the ability to close the valves and thus
protect the animal from any major environmental stress, such as predators,
water pollution or contamination of the gills, along with the concomitant
reduction in the metabolic rate in an aerobically respiring animal, saves an
enormous amount of energy and can be particularly beneficial during periods of
starvation.
In addition to C. fluminea
(Allen et al., 1996;
Tran et al., 2003
) and the
Pisiid clams Sphaerium corneum and Pisidium amnicum
(Holopainen and Penttinen,
1993
), long periods of valve closure in conditions that are not
obviously stressful, lasting at least several hours, have also been reported
for several other bivalve species, e.g. Anodonta cygnea
(Salanki, 1965
),
Crassostrea virginica (Higgins,
1980
), Mytilus edulis
(Kramer et al., 1989
) and
Dreissena polymorpha (Borcherding,
1992
). Unfortunately, continuous and simultaneous direct
recordings of heat dissipation, oxygen consumption and biochemical studies
during such `voluntary' valve closures are lacking in these species.
As revealed by the analysis of anaerobic end products, C. fluminea
nevertheless does become anaerobic during extended periods of valve closure
(Fig. 8), presumably after the
enclosed oxygen is depleted. To our surprise, however, succinate produced
anaerobically is only processed into propionate under conditions of artificial
hypoxia (Fig. 7). The
production of propionate, via methylmalonyl-CoA, is the most
efficient anaerobic pathway, yielding more ATP than during anerobic glycolysis
and removing most protons from the given storage substrate
(Hochachka and Guppy, 1987;
Grieshaber et al., 1994
).
Moreover, during `voluntary' valve closure of more than a day, C.
fluminea transports the accumulating succinate out of the tissues,
instead of processing it further into propionate. Perhaps protons are
discharged by cotransport with succinate into the mantle cavity. Regardless of
the mechanism, succinate levels accumulated in the mantle cavity fluid clearly
exceed the concentration inside the tissues (Figs
7 and
8). We think this is another
strategy for saving valuable substrates under anaerobic conditions, because
some of this succinate may be reabsorbed when the valves open again. The
morphology of C. fluminea ensures that the succinate-enriched pallial
water has to pass the gills before leaving the clam. Its mantle is closely
connated, with only narrow slits for the foot and the adductor muscles
(Kraemer, 1979
). In addition,
the typical gill structure of the Eulamellibranchia ensures that the whole
water current has to pass the gill tissues via the mantle cavity.
But what are these energy and substrate savings needed for? So far all the
data portend to some kinds of unfavourable conditions underlying these
energy-saving strategies. First of all, the need to save energy depends on the
energy balance of the metabolism, characterised by the available food and
stored substrates on the one hand and the costs of growth, reproduction and
sustenance on the other. There is only one reason that could explain the need
for saving substrates: the clams are unable to get enough food out of the
water to fulfil their requirements. But how is this possible while they
exhibit such high productivity? A closer look at their main food supply, the
phytoplankton, reveals that it has declined during recent years. Data from the
North Rhine-Westphalia State Environment Agency reveal that chlorophyll
a concentrations at Bad Honnef were mostly less than 10 µg
l-1 (Table 2),
especially after the spring bloom had disappeared. According to Foe and Knight
(1985), C. fluminea
is already food-limited at chlorophyll a concentrations below 20
µg l-1. Since 1993, such high amounts are only found during the
annual algal bloom in spring. However, these chlorophyll concentrations
(Table 2) derive from weekly
samples taken around 10:00 h and they may be higher in the afternoon. In the
absence of better data on chlorophyll a concentration, the hourly
available oxygen concentration may be used to indicate the diurnal character
of the phytoplankton concentrations inside the river. Moreover, the
O2-maxima fit perfectly with the times that C.
fluminea kept its valves opened, indicating that the clams only
filter feed during those hours of a day where the highest amount of food is
available (Fig. 9).
Nevertheless, the diurnally changing oxygen tension might be involved in
sustaining their circadian rhythm, although ambient oxygen concentrations
seemed to be too high (always above 60% O2 saturation) to impose
any serious stress on the bivalves.
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|
Further support for a feeding-dependent rhythm comes from the work of
Borcherding (1992) and Williams
et al. (1993
), who reported
similar circadian rhythms in another freshwater species, Dreissena
polymorpha, and an Australian intertidal clam, Austrovenus
stutchburyi, respectively. Feeding experiments at a constant oxygen
supply revealed that both bivalves altered their rhythm of valve closure
according to the shift of food supply
(Borcherding, 1992
;
Williams and Pilditch, 1997
).
So it seems that the available food rather than the ambient oxygen tension
controls the rhythm. It is also well known that bivalves lengthen their
periods of valve closure under poor nutritional conditions
(Higgins, 1980
;
Williams et al., 1993
).
Finally, Borcherding (1992
)
demonstrated that the rhythm in Dreissena polymorpha vanished if
sufficient food was supplied. Hence, the difference between the chlorophyll
a maxima in the afternoon and minima during the nights seems to be
high enough to induce the circadian oscillation in the valve closure behaviour
of C. fluminea. Especially at relatively high temperatures during the
summer, the daily chlorophyll maxima alone deliver enough food for the clams
to sustain their expensive metabolism, and it seems to be more economic to
shut it down at night. Nevertheless, there must be components of endogenous
rhythm as well, because those rhythms persisted, for at least some days, under
the constant conditions in the laboratory
(Williams and Pilditch, 1997
;
this study). However, further work is needed to confirm the influence of food
and/or oxygen concentrations on circadian rhythms of bivalves, as well as the
mechanisms involved in the control of endogenous, circadian rhythms.
But what is so expensive in the clam's SMR that it is worthwhile depressing
metabolism and isolating itself from the environment instead of increasing the
amount of food obtained from the water current? Without considering
controversial opinions on bivalves' ability to control their filtration rate
or to sort food particles of different nutritional values
(Jørgensen, 1996;
Ward et al., 1997
;
Bayne, 1998
), there is no doubt
that the gills dissipate a great part of the metabolic energy because they are
extremely densely covered by different kinds of cilia
(Clemmesen and Jørgensen,
1987
; Riisgård and
Larsen, 2000
). Thus, in addition to the most important energy
consumers such as protein turnover, ion regulation and the proton leak over
the inner mitochondrial membrane, each comprising approximately 20% of the
oxygen consumption of the SMR (Hochachka
and Guppy, 1987
; Brand et al.,
2000
; Pakay et al.,
2002
), the gills are another important target for saving energy
during a metabolic depression. Bivalve gills are highly developed organs whose
main function is the filtration of food particles, with gas exchange now
regarded as less important
(Jørgensen, 1990
).
Especially under conditions of low food supply it seems reasonable to cut down
the costs of filter feeding by valve closure, because inside closed valves the
cilia are thought to cease beating (Newell
and Branch, 1980
;
Jørgensen, 1990
). This
is presumably relevant for every bivalve species, but C. fluminea
seems to coordinate the times of valve closure with those of low food
availability and vice versa through its circadian rhythm, so that the
concomitantly depressed metabolism can be sustained aerobically for about 4-9
h (Table 1).
We conclude that by adopting an intermittent metabolism, rhythmically coordinated during the summer and accompanied by an approximately tenfold reduced aerobic metabolic rate during valve closure, efficient exploitation of the available food resources is guaranteed. Thus, saving substrates during periods of low food supply and ongoing aerobic energy metabolism during normal periods of valve closure may support C. fluminea's high productivity, namely its rapid growth and high fecundity.
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