86 H
. Hummel et al. J. Exp. Mar. Biol. Ecol. 251 2000 85 –102
clam population disappears. It was concluded that factors other than temperature influenced the respiration and weight-index of clams, and hence their presence or absence, e.g., food con-
centration, innate seasonal cycles, and possible pollutants in the water.
2000 Elsevier Science B.V. All rights reserved.
Keywords : Macoma balthica; Distribution limit; Performance; Survival; Respiration; Weight; Glycogen
1. Introduction
The Baltic clam Macoma balthica L. is in NW Europe a widespread and often dominant species in the coastal benthic zone with highest densities in muddy or silty
intertidal estuarine and coastal areas. Its current Southern distribution limit is located near the Gironde Bachelet, 1980. In the 1960s the clams were still found more to the
South in Galicia, Spain Otero and Millan, 1970. Genetically all clam populations along the Atlantic coast line from South South-West France to North Murmansk in Russia,
¨ ¨ ¨
belong to one and the same race or species Vainola and Varvio, 1989; Hummel et al., 1997.
Near the Southern distribution limit, the Baltic clam shows a poor performance. A reduced genetic diversity was observed, due to differential selection of some genotypes,
as well as a reduced ecophysiological performance as was measured by means of a reduced weight, energy reserves, bio-indicators and stress resistance Hummel et al.,
1996a,b, 1997. This corresponds to the general theorem that marginal populations are living at the limits of their adaptation capacities Conover, 1978; Hoffmann and Parsons,
1991. Yet, for the Baltic clam it is not clear which environmental and or innate ecophysiological factors are of decisive importance.
For the Baltic clam the major ecophysiological and environmental factors involved might be summarized in two groups. The first set of factors is based on temperature
interrelated effects on the energy budget in many marine species Newell, 1979; Bayne et al., 1985. A high temperature may result in a high metabolic rate, thus high
respiration, and consequently high food demand, whereas this will lead, if food is limiting, to a low weight-index and low level of energy reserves. This will minimize the
chances of survival during harsh periods, e.g., periods of low food availability.
The second set of factors is derived from seasonal cycles in gonadal development or from the coincidence of a species Southern limit and latitudinal isotherms. For Baltic
clams, inhibition of the proliferation of gonads was found when temperatures were above 108C, whereas on the other hand to induce spawning a rising temperature to a
threshold level of 10–128C was needed Caddy, 1967; Lammens, 1967; de Wilde, 1975; de Wilde and Berghuis, 1978. Thus, in areas where temperatures never fall below 108C,
clams would not reproduce. In this respect, for Baltic clams the Southern limit of distribution coincides with the 108C winter isotherm Beukema and Meehan, 1985. Yet,
as observed before in other species Bayne et al., 1985, recent studies in Baltic clams on the relation between temperature and gonadal development or recruitment do show
that also these relationships can be explained on basis of the energy budget Honkoop
H . Hummel et al. J. Exp. Mar. Biol. Ecol. 251 2000 85 –102
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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
