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and Beukema, 1997; Honkoop and van der Meer, 1997, 1998. A higher temperature in winter results in a higher energy demand metabolism whereby the weight-index
decreases and the gonadal tissues may partly or completely be resorbed, and subsequent spawning and recruitment may then be reduced or completely absent.
Temperature is therefore a major factor for the occurrence or disappearance of a species near its distribution limit. Our working hypothesis was that the limiting condition
for Baltic clams is related to temperature through its impact on the energy budget. We would expect that animals translocated South of their area of distribution, confronted
with temperatures higher than they are able to adapt to, show a higher respiration rate, lower weight and lower level of glycogen, than within their area of distribution.
To test the above hypothesis, we carried out a translocation experiment of animals from the Netherlands to areas South of their distribution limit, where they have not been
regularly observed during the last decades, i.e., the Bay of Arcachon and the Bidasoa estuary at the French–Spanish border. The respiration rate, weight index and glycogen
reserves were followed in these animals.
Additionally, some animals of the same stock were translocated to the Grevelingen, a brackish lake in the Netherlands. In this lake, Baltic clams disappeared in the last two
decades after some changes of the tidal regime, whereas mussels and cockles are still present in the same territory. A change of temperature and salinity in this lake did not
occur, and thus the clams must have disappeared from this region due to still another reason, which may help to explain the disappearance of clams near the Southern limit.
Before starting these translocation experiments a preliminary test was carried out to assess the performance of clams when translocated over longer distances between the
Netherlands and France, i.e., within the distribution area. If the translocated clams showed a strongly dissimilar ecophysiological performance weight index, glycogen
reserves, etc. from local specimens, then the translocation experiments would not be valid. These preliminary experiments showed that clams adapted within 1 month to the
local situation.
In addition, two different types of laboratory experiments were performed. Firstly, each season two sub-groups of clams from the Netherlands were kept for 4 weeks at two
different temperatures in the laboratory in order to assess the potential of these animals to acclimatize their respiration rate in a relatively short period to a changing
temperature. Secondly, on the basis of the first results of the translocation experiments a second type of laboratory experiments was performed to assess the effect of local
substrate- or water-bound factors on the respiration and weight index of clams. To this end, clams from the Netherlands Paulina were exposed to sediment and unfiltered
water from Arcachon, Bidasoa and Grevelingen.
2. Material and methods
In the preliminary translocation experiment, animals from the Netherlands Dortsman were brought on 17 April, 1991 to several estuaries in France Fig. 1. One month later,
on 15 May, 1991, at least 20 translocated animals were recovered at each station as well as 20 of the local animals nearby. The animals were purged for 6 h to empty the
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Fig. 1. Location of the sampling stations open squares 5 used in preliminary experiment with clams from Dortsman, the Netherlands, translocated to France within their area of distribution 1991; filled circles 5 used
in experiment with clams from Paulina, the Netherlands, translocated to France South of their distribution limit and to Grevelingen.
digestive tract, frozen in liquid nitrogen, lyophilized during 3 days, and further treated for analysis of the weight index as explained below, the copper content according to
Amiard et al., 1987, glycogen according to Keppler and Decker, 1974, and free amino acids content according to Hummel et al., 1996a.
In May 1996, 2000 Baltic clams of rather uniform shell-length average shell-length 5 14.7 mm, std 5 1.4 mm from the Westerschelde Paulina, Netherlands: 518 21.69 N, 38
42.79 E, salinity 5 15–23 were transported to South-West France over 3 days wrapped
H . Hummel et al. J. Exp. Mar. Biol. Ecol. 251 2000 85 –102
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in dry cloths on top of melting ice blocks in a cooling box. The animals 1000 specimens at each location were distributed just above mean low water level over an
2
area of 16 m at the Banc d’Arguin at the entrance of the Bay of Arcachon 448 35.29 N, 18 14.19 W; salinity 5 26–32, and temperature 5 12–238C, during sampling, and in the
Bidasoa estuary 438 22.09 N, 18 46.39 E; salinity 5 9–28, and temperature 5 10–248C, during sampling, see Fig. 1.
At intervals of 3 months some translocated specimens were collected. They were transported in water taken at the sampling location and kept at constant temperature in a
Dewar isolation vessel. Within 4 h after sampling the respiration rate of ten clams was measured in respiration chambers of 0.13 l over 1–2 h at two temperatures 10 and
208C; in winter at 3 and 108C; simultaneously, one set of ten animals per temperature. For each measurement a simultaneous blank determination of oxygen consumption was
made in respiration chambers with the same water at the same temperature, but without the animals. The decrease in oxygen tension was measured at intervals of 10 min by YSI
5331 Oxygen Probes Clark type polarographic electrodes. Subsequently, the animals were frozen and lyophilized during 3 days for further analysis of the soft tissue weight,
the weight index and energy reserves glycogen. The respiration rate was calculated from the tangent of the line along the series of 6–12 measurements on the decrease of
the oxygen concentration in the respiration chamber with time, and corrected for the oxygen consumption in the blank. One series of measurements with ten animals gave
only one value, and thus no standard errors could be calculated.
The weight index was measured for all animals individually as the dry tissue weight
3
after lyophilizing per volume volume calculated from length ; Hummel et al., 1996b. Glycogen was analyzed in the groups of ten animals together, according to Keppler and
Decker 1974. Additional samples were translocated from the Westerschelde Paulina to the brackish
lake Grevelingen 518 42.19 N, 48 07.89 E, salinity 5 9–28. The animals were sampled from the same stock, and translocated and treated at the same dates and according the
same methods as those translocated to Arcachon and Bidasoa. To test the acclimatization potential of clams to temperature, two sub-groups of clams
were kept in the laboratory in a layer of 10 cm sediment and filtered Oosterschelde water salinity 5 23–34; with a continuous flow of algal culture, Phaeodactylum tricornutum,
into the water to give a nominal average seston concentration of 2.2 mg l over 4 weeks at a constant temperature of 10 and 208C in winter 3 and 108C. After this period the
respiration, weight index and glycogen was measured as described above. This experiment was repeated each season, parallel to the measurements in the translocation
experiment.
A second type of laboratory experiment was performed in May and June 1997 on the basis of the first results of the translocation experiments, indicating that not only
temperature caused an increased respiration. Clams from the Westerschelde Paulina were exposed at a temperature of 158C to unfiltered water and sediment from Arcachon,
Bidasoa and Grevelingen [sediments mix from top 2 cm and water were transported in jerrycans of 5 l and stored at 48C]. This in order to assess the effect of local
‘substrate’-bound factors on the respiration, glycogen and weight index of clams.
The data on differences in respiration, weight index or glycogen, of the animals in the
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translocation and exposure experiments were statistically analysed with ANOVA Sokal and Rohlf, 1995 by the Systat programme Wilkinson, 1988.
3. Results
