ARTICLE

Memoir of the Fukui Prefecturel Dinosaur Museum 13: 25–35 (2014)© by the Fukui Prefectural Dinosaur Museum

AN AUTOECOLOGICAL STUDY OF ANNUAL SHELL GROWTH INCREMENTS
IN MARGARITIFERA MARGARITIFERA FROM LAPLAND,SUBARCTIC FINLAND
Samuli HELAMA 1* and Ilmari VALOVIRTA 2
1
2

Finnish Forest Research Institute, Northern Unit, P.O. Box 16, 96301 Rovaniemi, Finland
Finnish Museum of Natural History, P.O. Box 17, 00014 University of Helsinki, Finland
ABSTRACT

Three shells of the freshwater pearl mussel (Margaritifera margaritifera (Linnaeus 1758)) were studied using
sclerochronological methods. The long-lived shells were retrieved from a modern death assemblage beside a subarctic
river in Lapland. An autoecological assessment of these bivalves was carried out by analyzing their annual shell
growth increments. Subsequent to sclerochronological cross-dating, the incremental series were compared to eachother and historical summer temperature records as observed at the local weather station between the years 1876 and1979. Wavelet coherence, providing local correlation between the studied series in time frequency space, guided us
to demonstrate that the shell growth of different bivalves has responded differently to climate ﬂuctuations on separate
time scales through the study period. Despite the incremental series showed distinct and positively high correlation
with each other and climate (summer temperature) on sub-decadal scales, the shell growth variations of longer period
pinpoint deviating growth fluctuations especially over the later half of the study period, since the 1930s. Over thisperiod, the shell growth of one studied specimen started to follow the 18.6 year lunar nodal cycle. Interestingly, thisbivalve experienced a shell damage/repair episode that perturbed its growth during the 1930s. Yet, the shell exhibiting
the highest summer temperature correlation was also the shell with the highest growth level. Although strongest
scleroclimatic associations can generally be found for local/regional increment chronology, our results demonstrate theinsight of autoecological approach in studying archetypal sets of incremental series from a relatively small collection ofmolluscan fossil shell samples.
Key words : paleoecology, paleoclimatology, sclerochronology, malacology, freshwater pearl mussel

al. 2000; Strom et al., 2004, 2005; Butler et al., 2009a, b, 2013),
freshwater bivalves of several species inhabiting rivers and lakes
produce annual increments in their shells that can be reliable
recognized and analyzed for building chronologies of multiple
individual specimens (Rypel et al., 2008, 2009; Soldati et al.,2009; Black et al., 2010). Before calculating such chronology, theincremental series of individual shell specimens are compared
to one another and to the mean series of other samples, to ensure
correctness of the series in terms of their chronological dating.
This process brings the methods of incremental growth research,sclerochronology (Hudson et al., 1976; Jones, 1983), close to treering research, dendrochronology (Fritts, 1976), where the tree-ringseries are routinely cross-dated before growth and environmental
interpretations are to made.
Recent analyses of annual shell growth increments of different
freshwater bivalve species have emphasized the complexity of
their responses to a variety of environmental factors both on local

INTRODUCTION
When bivalve shell growth is periodically interrupted, thegrowth history of an organism can be traced back in time atresolution of that period. A pervasive feature of this type isformed in the bivalve shells as annual increments and theanalyses of their widths reveal shell growth variations and theirsynchroneity (Marchitto et al., 2000; Rypel et al., 2008; Butler
et al., 2009a). Moreover, the growth information from several
individuals can be used to detail the environmental and climatic
factors controlling the shell growth at level of studied population(Jones, 1981; Witbaard et al., 1997, 2003; Epplé et al., 2006).
Apart from commonly studied marine bivalve shells (Marchitto etReceived April 14, 2014. Accepted October 7, 2014.
Corresponding author—Samuli HELAMA
E-mail : samuli.helama＊metla.fi
25

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SAMULI HELAMA AND ILMARI VALOVIRTA

and regional level (Valdovinos and Pedreros, 2007; Black et al.,2010; Sansom et al., 2013). Moreover, an interesting approach
could be seen arising from the ﬁeld of tree-ring research, whererecent analyses have concentrated on individualistic signals
and responses of trees to external factors (Carrer, 2011; Rozas
and Olano, 2013; Zhang et al., 2013). This approach could bear
importance particularly on paleontological collections of shellmaterials. Compared to live-collected bivalves, the fossil bivalve
shells as excavated from sedimentological sections may notexhibit overlapping lifespans. Thus, the construction of local orregional chronologies via cross-dating of growth increment series
from several individual bivalves may not simply be possible indeep time.
In order to investigate the individualistic approach (Carrer,2011) in more details, an autoecological analysis of annual shell
growth increment series was conducted. To do so, we delve into
the slerochronological data of Margaritifera margaritifera, afreshwater bivalve species from northern Europe (Helama andValovirta, 2008a). There were several reasons to concentrateon this material. First, the shell collection used in this study
represents an archetypal set of molluscan fossil shell samples
and their incremental series (Helama et al., 2009a). The
incremental series of this collection have previously been shownto cross-date systematically between the centennia-long sample
series (Helama and Valovirta, 2008a; Helama, 2011) and the
local chronology, built from a modern death assemblage, has
successfully been cross-dated against the tree-ring chronologies
from the same region (Helama et al., 2009b). Subsequently,
the temporal variations of the produced annual shell growthincrement chronology were shown to correlate strongly withvariations in the historical summer temperatures in the region(Helama and Valovirta, 2008a; Helama et al., 2010; Helama,2011). In addition, it has also been demonstrated that one of thebivalves has experienced a marked shell damage/repair episode(Helama and Valovirta, 2008a) during which the shell growth
exhibited drastic fluctuations not recorded in other shells. This
event could be specifically interesting in the autoecologicalcontext, in order to investigate the shell damage/repair impacts
on correlativity between climate and growth. In this shellcollection, the individualistic growth signals and their responses
to environmental and climatic variations have, however, remained
unanalyzed.
Here we compare the annual incremental series of threeshells of M. margaritifera to one another, using both thelinear correlations and the wavelet coherence (Torrence andWebster, 1999; Grinsted et al., 2004). These comparisons werecomplemented by analyzing the sclerochronological data inclimatic context. In addition to summer temperature influenceon the shell growth , an important driver of Arctic and thus ofclimate in the study region may originate from the lunar nodal
cycle (Yndestad, 2006). Consequently, an attempt was made to
additionally connect the incremental series with the lunar nodal
time-series. Using the autoecological approach, the purpose of thisstudy was to throw more light on the credibility of individual shell
growth increments as recorders of their past aquatic environment.

FIGURE 1. A map showing the collection point of specimens (a), a photoshowing the cutting line from umbonal shell region towards the ventral
shell margin (b), and microscopic view to prepared cross-section of a shell
showing the periostracum, prismatic, and nacreous shell layers, with annual
growth increment widths (bars), measured perpendicularly to winter-lines.
Direction of growth is from right to left (c).

In so doing, the study demonstrates the beneﬁts and pitfalls that
may arise when individualistic data are used, instead of local
or regional chronologies, for paleoecological or paleoclimaticinterpretations.
MATERIAL
Description of the shells
The studied material comprises three shells of the freshwater
pearl mussel (Margaritifera margaritifera (Linnaeus 1758)),
collected from beside a tributary of River Luttojoki in northeastern
Finnish Lapland (latitude 68 36 N, longitude 28 14 E) (Fig. 1a).
The mollusc species involved in this study, M. margaritifera, is ananthropogenically threatened invertebrate listed in Annexes II and
V of the European Habitats Directive and Appendix III of the Bern
Convention. M. margaritifera was protected by law in Finland in
1955, and Finland s Nature Conservation Act requires permission
for any disturbance on living organisms and even for thecollection of empty shells or fragments of dead mussels (Valovirta
1998). The shells of these dead mussels have previously beencollected under license from The Lapland Regional Environment
Centre and the material deposited in the collections of the Finnish
Museum of Natural History (University of Helsinki). The lengthsof the studied shells ranged between 120 and 126 mm (Helama
and Valovirta, 2008a, b).

AN AUTOECOLOGICAL STUDY OF BIVALVE SHELL GROWTH

27

Production of the measurement series
Previous studies have demonstrated that the shells of M.
margaritifera contain clear growth pattern of annual periodicity
in the resulting increments (Mutvei et al., 1994; Dunca, 1999;
Dunca et al., 2005). Later, Helama et al. (2007a) determined thespeciﬁc characteristics of these increments from cross-sections of
M. margaritifera shells. Following these instructions, the annual
increments were measured from cross-sectioned shells. Shell
specimens were cut along the axis of minimum growth from theumbo to the ventral margin (Fig. 1b). The sections were ground,polished and etched in Mutvei s solution, that comprise a 1:1
mixture of 25% glutaraldehyde and 1% acetic acid with added
alcian blue, at 37–40℃ for approximately 25 min (Mutvei et al.,1994). Following previous recommendations (Mutvei et al., 1994,
Dunca, 1999), the increment widths between successive winterlines were measured from photographic enlargements of known
scale from the outer pristimatic shell layer, perpendicular tointernal winter-lines (Fig. 1c).
ANALYTICAL METHOD
Cross-dating of the annual incremental series
Annual shell growth increment series were first cross-dated
using sclerochronological methods (Helama et al., 2006). The
three series were compared to one another for synchroneityof their wide and narrow increments. During the cross-datingprocess, any discrepancy in temporal placement between theconspicuous annual increments is suspected as a possible
chronological error in that series. The suspected series are remeasured to pinpoint the measurement errors and to correct thechronology. Cross-dating ensures the correct temporal alignmentof all annual increments to exact years in absolute calendricscale (Helama et al., 2006, 2007a). Thus, the sclerochronologicalcross-dating closely mimics the routinely applied cross-dating oftreerings in the context of dendrochronology (Fritts, 1976). The
cross-dating of the annual incremental series used here has been
demonstrated elsewhere (Helama and Valovirta, 2008a) and it has
also been confirmed that these shells exhibit super-centenarian
ages as the number of annual increments in the shells A, B, andC were as high as 99, 162, and 137, respectively (Helama andValovirta, 2008a). Such long annual incremental growth records
further enabled a chronological comparison between the obtainedsclerochronology and the tree-ring chronologies from the same
region (Helama et al., 2009b). This comparison made it possible
to statistically demonstrate that the increments under this study
represent the calendar years 1819–1980 (Helama, 2011). The
growth of the last year (1980) was not used here because ofevidently incompleted growth in the middle of the season (Helama
and Valovirta, 2008a).
Trend in the annual incremental series
Apart from year-to-year variations in the annual shell growth,

FIGURE 2. Squared wavelet coherence between the annual incremental
series of the shells A and B (a), A and C (b), and B and C (c). In- and antiphase relationships are shown using rightward and leftward arrows, thelagging and leading phases as upward and downward arrows. Areas withsignificant (p<0.05) coherence against red noise are denoted by a thickcontour.

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SAMULI HELAMA AND ILMARI VALOVIRTA

FIGURE 3. Visual and statistical comparison between the sub-decadal growth variations in the annual incremental series of the shells A and B (a), A and C(b), and B and C (c), and between the variations of longer term in the annual incremental series of the shells A and B (d), A and C (e), and B and C (f). Pearson
correlations (r) were computed over the early (1876–1927) and late (1928–1979) periods.

the annual incremental series are known to contain an ontogenetic
trend in their widths. Because of this trend, the biologically young
increments (near the umbo) become considerably more wider
than the older increments, with a growth decline until the ventral
margin of the shell. For the studied M. margaritifera shells, thisconcave trend could be quantified using a modified negative
exponential function (Fritts, 1976) as follows,yx = 2.979e-0.0307x + 0.553
where yx is the annual increment width (expressed in mm x10) for annual increment number x (Helama et al., 2010). Using
this function, the cross-dated annual incremental series weredetrended and sclerochronological indices derived as ratios
between the measured annual increment width and the width
value as expected by the ontogenetic trend model. This method oftrend removal follows the dendrochronological theory whereby
the longtime trend present in the tree-ring series (here, annual

shell growth increment series) can be defined empirically for a
restricted sample of trees (here, bivalves) and that the same trend
can be subsequently removed from the series under investigation(Melvin and Briffa, 2014). Again, the applicability of the tree-ring
methods for incremental data has been demonstrated through theprevious sclerochronological analyses of shell growth (Strom etal., 2004, 2005; Helama et al., 2007b, 2009a, 2010; Black, 2009;Black et al., 2009; Rypel et al., 2008, 2009; Helama, 2011).
Evaluating environmental effects on shell growth
Annual incremental series of the shells A, B, and C werecompared with the regionally representative summer temperaturerecord and the lunar nodal time-series. Temperature record
originated from the weather station of Karasjok (latitude 69 28
N, longitude 25 31 E). Previous sclerochronological studies,correlating the summer temperature records and the mean andmedian chronologies of annual shell growth increments, have

AN AUTOECOLOGICAL STUDY OF BIVALVE SHELL GROWTH

29

evidenced a consistent connection between the shell growth andthe mean temperature of June through August season over various
temporal scales (Helama et al., 2009a, 2010; Helama, 2011). Here
we build upon these ﬁndings and seek for the linkages between
the individual series of the shell growth and the June-August
temperature. Moreover, the annual incremental series werecompared with the time-series of lunar nodal cycle amplitudes,
as deﬁned in a previous study detailing the linkages between thiscycle and Arctic climate (Yndestad, 2006).
Annual incremental series were analyzed using wavelet
coherence (Torrence and Webster, 1999; Grinsted et al., 2004),
which represents the local correlation between the series underinvestigation in time frequency space. This means that thecorrelativity between the analyzed series can be calculated on
multiple time-scales and for sliding intervals of time. Wavelet
coherence belongs to the family of wavelet based time-series
analysis (Torrence and Compo, 1998) and can be used to deﬁne
the locally phase locked behavior of two series. In this study, thewavelet coherence was calculated between the incremental series,
between these series and the temperature record, and between
the incremental series and the lunar nodal time-series using the
published algorithms (Grinsted et al., 2004).
In addition to wavelet coherence, the incremental series wereplotted for visual comparisons with temperature and lunar nodal
time-series. Moreover, the relationships were quantified using
the Pearson product-moment correlation coefﬁcient (r). Pearson
correlation measures the linear relationship between two timeseries. Following the results from the wavelet coherence, thecorrelation analyses were targeted separately on different timescales. That is, the series were correlated for their short- (highfrequency) and long-term (low-frequency) variations. Hence, theincremental, temperature, and lunar nodal time-series were band
pass filtered (Christiano and Fitzgerald, 2003) to depict theirvariations on sub-decadal (wavelengths less than 10 years) anddecadal and longer time-scales (wavelengths more than 10 years).
These components of the total variations thus represented thehigh- and low-frequency bands of the studied time-series.
RESULTS
Comparison of annual incremental series
Annual incremental series of the three shells were comparedpair-wise using the wavelet coherence. Common to all of thecomparisons was that the high-frequency (short-term and -period)
variability in the series showed strong in-phase variations
and therefore a consistent phase relationships (Fig. 2). Thesevariations occurred on sub-decadal and characteristically on very
short time-scales. In addition to their high-frequency agreement,
the incremental series indicated growth fluctuations of longer
term that did not show such in-phase variations. Instead, thesevariations, occurring on roughly decadal and longer scales,
evidenced highly inconsistent phase relationships. That is,
the low-frequency (long-term and -period) growth variability
appeared poorly synchronous in comparison to the high-frequency

FIGURE 4. Squared wavelet coherence between the summer temperatureand incremental variations in the shells A (a), B (b), and C (c). For further
explanations, see Fig. 2.

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SAMULI HELAMA AND ILMARI VALOVIRTA

FIGURE 5. Visual and statistical comparison between the sub-decadal variations in summer temperatures and in the annual incremental series of the shells A(a), B (b), C (c), and between the variations of longer term in summer temperatures and the sample series A (d), B (e) and C (f). Pearson correlations (r) werecomputed over the early (1876–1927) and late (1928–1979) periods.

growth variability.
Guided by the outcomes of the wavelet coherence, the full
spectrum of incremental variations was divided into its highand low-frequency components for visual and further statisticalcomparisons (Fig. 3). On sub-decadal time-scales, the incremental
series correlations averaged r = 0.449 and r = 0.311 over the early(1876–1927) and late (1928–1979) periods, respectively (Fig.
3a–c). These ﬁgures could be compared with the low-frequency
correlations, those averaging r = 0.322 over both halves of thestudy period (Fig. 3d–f). It is also notable that the growth of theshell C exhibited considerably higher growth level over the fullstudy period, in comparison to the shells A and B. With theseregards, the incremental series of specimen C is also correlatedwith substantially high coefficients (r ~ 0.45) with both of theother series, A and B. In comparison the incremental series of thespecimens A and B are correlated with a grand mean of r = 0.130.

Summer temperature and shell growth
Comparison between the annual incremental series and theclimatic records evidenced similarly strong high-frequency
coherence (Fig. 4). Namely, all of the three incremental series
were seen to exhibit rather consistent in-phase relationships with
the June-August temperature record especially on sub-decadalscales. In addition to these relationships, the shell A indicatedlow-frequency growth variations on multi-decadal scales throughthe study period. On the other hand, the specimen B showed
highly inconsistent phase relationships on decadal and especiallylonger scales, suggesting that among the three shells studied, thegrowth variations in this specimen B were poorly connected to
low-frequency temperature variations. Moreover, the specimen
C showed strong in-phase variations with decadal and longer
temperature variations, at least over the first half of the study
period, until the 1930s.
The analyses of the wavelet coherence suggest us to isolate theAN AUTOECOLOGICAL STUDY OF BIVALVE SHELL GROWTH

31

high- and low-frequency components of the annual incremental
and temperature records for further visual comparisons andcorrelation analyses (Fig. 5). On high-frequency band ofvariations, the incremental series is correlated well with thesummer temperature record with a mean correlation r = 0.561(Fig. 5a–c). In this context, the annual incremental series of thespecimen B is correlated rather poorly with climate over the lateperiod (r = 0.259), although its climatic correlation over the early
period is considerably high (r = 0.621). In comparison to these
correlations on sub-decadal scales, the low-frequency correlations
evidenced more complex relationships between the shell growthand climate variability, thus conﬁrming the view obtained throughwavelet coherence. While the annual incremental series of theshell A is correlated positively with temperature variations overthe early period but negatively over the late period (Fig. 5d), thatof the specimen B is correlated with low coefficients ( r ~ 0.2)
over the both halves of the study period (Fig. 5e). In contrast tothese observations, the annual incremental series of the shell C
shows markedly high correlation with the temperature record
through the study period (Fig. 5f).
Seek for lunar signals
Wavelet coherence between the annual incremental series of theshells A and C did not exhibit statistically signiﬁcant connections
to the lunar nodal time-series (Fig. 6). Instead, the annual
incremental series of the specimen B evidenced statisticallysignificant coherence with this record over the later half of thestudy period. These results suggest that the growth variations of
this specimen only followed the lunar fluctuation, at least sincethe 1930s. Further visual and statistical comparisons between theincremental series and the lunar nodal time-series on decadal tobi-decadal time-scales demonstrate that all the three series show
positive correlations between the shell growth and the lunar
nodal time-series, over the later half of the study period (Fig. 7).
However, only the annual incremental series of the shell B could
be linked with the lunar nodal time-series with clear in-phaselinkage (Fig. 7b).
DISCUSSION
Comparisons between the annual incremental series of M.
margaritifera evidenced similar shell growth variations between
individuals, although the growth characteristics of different
specimens exhibited also deviations. Following the wavelet
coherence, the similarity of annual growth variations among theshells were most evident in their high-frequency band. Namely,
the areas of signiﬁcant in-phase relationships between the annual
shell growth variations occurred robustly at time-scales less thanten years (Fig. 2). In fact, these are the time-scales on which thecross-dating can successfully operate by comparing the temporal
growth synchrony between the consecutive increments (Helama etal., 2006). Similarly to these ﬁndings, the isolation of short-term
variations were seen beneﬁcial for the sclerochronological crossdating of M. margaritifera and Arctica islandica annual increment

FIGURE 6. Squared wavelet coherence between the lunar nodal time-series
and incremental variations in sample A (a), B (b), and C (c). For further
explanations, see Fig. 2.

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SAMULI HELAMA AND ILMARI VALOVIRTA

FIGURE 7. Visual and statistical comparison between the lunar nodal timeseries and growth variations between the sample series A (a), B (b), andC (c) on decadal to bi-decadal time-scales. Pearson correlations (r) werecomputed over the early (1876–1927) and late (1928–1979) periods.

series (Helama et al., 2006). In fact, these delineations follow thecommonly agreed approaches in the field of dendrochronology,
in which the importance of high-frequency variations for crossdating has been frequently emphasized (Yamaguchi, 1986; Wigley
et al., 1987; Grissino-Mayer et al., 2010).
Highest correlations among the incremental series wereobtained between the annual incremental series of the shells A
and C. As previously suggested, the shell B experienced shelldamage in the middle of its life (Helama and Valovirta, 2008a).
Consequently, the increment that should have formed in 1930
was missing in this shell, with following shell growth anomalies
characterized by markedly and moderately wide increments overthe post-disturbance period, that continued for 5 and 8 years,
respectively. Notably, an influence of this anomaly became
noticeable also in the wavelet coherence, appearing as a nonsignificant area in corresponding time frequency plots over the
1930s (Fig. 2a and 2c). While no similar event was observed inother shells (Helama and Valovirta, 2008a), it could be postulated
that the correlations between the sample B and other series

were possibly deteriorated owing to this event on similar subdecadal scales, particularly over the late period (1928 –1979).
Simply omitting the most anomalous growth increment values
in connection to the mentioned shell damage/repair event(1930–1935) from calculations actually elevated the correlations
between the shell B and other series considerably (Fig. 8). Theseobservations evidence the inﬂuence of disturbance, that only an
individual organism may have encountered in the past, on theestimation of common signals on population level.
The shell damage/repair event could be detected also in theconnections between the incremental and climatic variations.
Clearly, there appeared an area with non-significant coherence
between the shell B and summer temperature particularly over the
1930s (Fig. 4b), whereas no similar lack of signiﬁcance could beobserved in the case of the other two shells. Instead, the annual
incremental series of the shells A and C both showed strong inphase relationships with sub-decadal temperature variations withcorresponding areas of statistically signiﬁcant coherence over the
1920s and 1930s (Fig. 4a and 4c). Thereafter, it was the shell B
that showed a linkage between shell growth and lunar nodal timeseries, as evidenced both using wavelet coherence (Fig. 6b) andcorrelations (Fig. 7b).
Possible influences of the 18.6-year lunar nodal cycle(Yndestad, 2006) have previously been noted in the context ofNorth American (Cook et al., 1997) and North European treering chronologies (Linderholm, 2001; Edvardsson et al., 2011;
Edvardsson, 2013). Moreover, late Holocene chronology using
the time-series analysis of cross-dated M. margaritifera implied
a periodicity that mimicked the lunar nodal cycle (Helama
and Nielsen, 2008). Typical to this signal has been its weak
appearance as well as its presence in treerings of trees growing
on wet peat soils or bogs (Linderholm, 2001; Edvardsson etal., 2011; Edvardsson, 2013). Likewise, the lunar influence has
been postulated to impact the growth through hydroclimate
variations (Cook et al., 1997) that may consistently alter the
groundwater levels particularly in habitats with wet substrate(Linderholm, 2001; Edvardsson et al., 2011; Edvardsson, 2013).
In mountainous regions of Sweden, where summer temperature
had strongest influence on tree-ring growth, with a relatively
weaker implication of lunar influence, the lunar tidal maxima
coincided with growth peaks (Linderholm, 2001). This setting
was notably similar to our sclerochronological context, where theshell growth of M. margaritifera correlated well with summer
temperatures with an in-phase relationship with lunar cycle.
While the lunar signal remains a speculative signal, relative tothe well-demonstrated temperature inﬂuence on M. margaritifera
shell growth (Dunca, 1999; Helama et al., 2009a, 2010; Helama,2011), the obtained in-phase relationship could indeed indicate apossible linkage through food supply. The studied river meanders
through marshy landscape and the lunar nodal cycle could modify
the concentrations of microbial biomass and algae from the riversurroundings through changes in the water-table levels. Elevationof such concentrations could be seen benefitting the diet of thesuspension feeding bivalves (Rypel et al., 2009). Putatively, therepairing of the shell B damage may have resulted in lingering

AN AUTOECOLOGICAL STUDY OF BIVALVE SHELL GROWTH

33

FIGURE 8. Correlativity change between the annual incremental series of the damaged (shell B) and undamaged shells (A and C) when calculating thecorrelation over the full late period (1928–1979) and after omitting the years (1930–1935) with damage/repair anomalies.

effects of higher food demand over the subsequent decades.
Alternatively, the ecological position of this individual could havechanged at the sampling site during the damage, with concomitanteffects on its post-damage growth ﬂuctuations.
It was found out that the annual incremental series of the shell
C correlated markedly well with those of the other two shells.
In comparison to the other two shells, this shell also portrayedconsiderably higher growth level over the full study period (Fig.
3). This was also the shell that correlated with generally highest
coefficients with summer temperatures (Fig. 5c), in comparison
to the other two shells examined. These lines of evidence suggestthat although climate variability evidently controls the annualand decadal growth fluctuations, the climatic factor does notappear correspondingly limiting the overall growth rate of theshells. Instead, a set of additional factors may actually play a role
behind the life-long shell growth levels. Again, these observations
are suggestive of intrinsic (i.e., reproduction, sex, genetics) or
ecological inﬂuence on absolute growth rate of individual shells,
rather than straightforward influence of climate, that clearly, on
the other hand, influences the relative growth variations. This
interpretation would follow a global analysis of tree-ring data inthat the absolute growth level of a limited sample set may actually
be a poor paleoclimate indication (Falcon-Lang, 2005). Whilethe efforts to cross-date larger assemblages of fossil shells from
Holocene marine (Butler et al., 2009b, 2013) and freshwater(Helama and Nielsen, 2008) archives are in progress, the smaller
assemblages of incremental data may, if interpreted with thegreatest care, contribute to achieve the information of highresolution past climatic and environmental variations. Although
the most reliable scleroclimatic correlations can be retrieved
using local/regional increment chronology, the autoecologicalapproaches has their beneﬁt for augmenting our understanding ofprocesses that may have signiﬁcantly inﬂuenced the shell growthof the modern and fossil specimens during their life history.
ACKNOWLEDGMENTS
WWF-Finland has supported the Margaritifera project. This
study was also supported by the Academy of Finland. The shellsamples were collected under the license from The Lapland
Regional Environment Centre. Three reviewers are thanked fortheir valuable and constructive comments.
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