ENVIRONMENTAL AND PHYSIOLOGICAL CONTROLS ON SHELL
MICROGROWTH PATTERN OF RUDITAPES PHILIPPINARUM
(BIVALVIA: VENERIDAE) FROM JAPAN
TAKU KANAZAWA 1,3 AND SHIN’ICHI SATO 2
1
Institute of Geology and Paleontology, Graduate School of Science, Tohoku University, Aoba 6-3, Aramaki, Aoba-ku, Sendai 980-8578, Japan;
2
The Tohoku University Museum, Aoba 6-3, Aramaki, Aoba-ku, Sendai 980-8578, Japan;
3
Present address: Nippon Paint Co. Ltd, Oyodokita 2-1-2, Kita-ku, Osaka 531-8511, Japan
(Received 4 April 2007; accepted 7 November 2007)
ABSTRACT
The reproductive cycle and shell microgrowth patterns of the venerid bivalve Ruditapes philippinarum
from Matsukawa-ura, a small inlet facing the Pacific Ocean, northeastern Japan were examined. Histological examination of the gonads revealed that spawning in this species occurred twice in 2005, once
between late June and early August and the other between late September and early October. Comparison of the shell microgrowth patterns with the developmental stages of the gonad in each individual
revealed that in spawning individuals (mature and spawning stages), the mean lunar-day growth rates
were significantly smaller than those in individuals which were not in spawning condition (spent, recovery and growing stages). In non-spawning individuals, the mean lunar-day growth rates were positively
correlated with seawater temperature up to 208C. However, in spawning individuals, no correlation
was observed between shell growth and seawater temperatures .168C. These facts suggest that physiological stress during reproduction has a negative influence on shell growth and results in spawning
breaks. The present study indicate that spawning breaks can be used to identify the timing of sexual
maturity and to count the number of spawning events occurring per year in extant and fossil bivalves.
INTRODUCTION
In organisms that secrete accretionary skeletons, such as
bivalves, environmental and physiological information is
recorded within the shell microstructure (Pannella & MacClintock, 1968; Rhoads & Pannella, 1970; Kennish, 1980; Lutz &
Roads, 1980; Jones, 1983; Richardson, 2001). Recently, many
geochemical studies have reconstructed environmental changes
with a high resolution based on the shell microgrowth pattern
of bivalves (e.g. Dettman, Reische & Lohmann, 1999;
Goodwin et al., 2001; Surge, Lohmann & Dettman, 2001;
Schöne et al., 2003, 2004; Dunca, Schöne & Mutvei, 2005).
Further, some sclerochronological studies have reconstructed
the daily water temperature based on the daily shell growth
rates (e.g. Schöne et al., 2002, 2006).
However, shell microgrowth patterns are disrupted by irregular
growth breaks that reflect periods of environmental or physiological stresses such as the cold in winter, the heat in summer, rapid
changes in the water temperature, shell-margin abrasions, spawning, neap tides, and storms (Kennish, 1980). In particular, spawning breaks are observed in many bivalve species, and daily growth
ceases for a few days to several weeks during spawning ( Jones,
Thompson & Ambrose, 1978; Kennish, 1980; Thompson, Jones
& Dreibelbis, 1980; Sato, 1995; Schöne et al., 2005b). Therefore,
in the reconstruction of daily environmental conditions by sclerochronological analysis, it is necessary to clarify the influence of
both environmental and physiological factors on the shell microgrowth pattern of each bivalve species.
Spawning breaks can be distinguished from other types of
growth breaks by the characteristic microstructure pattern.
They appear annually following the onset of sexual maturity;
therefore, they are expected to represent a good indicator of
sexual maturation in extant and fossil bivalve species (Pannella
& MacClintock, 1968; Kennish, 1980; Sato, 1995, 1999).
Correspondence: S. Sato; e-mail: [email protected]
However, only a few studies have delineated the relationship
between the shell microgrowth pattern and the annual reproductive cycle using histological examination of the gonad in
each individual (Sato, 1995). If spawning breaks and other
types of growth breaks can be distinguished in fossil shells, it
becomes possible to estimate the age of sexual maturity and
the number of spawning events occurring each year in fossil
bivalves based on the sclerochronological analysis (Sato, 1999).
The venerid bivalve Ruditapes philippinarum (Adams & Reeve,
1850) is a common intertidal to subtidal species, inhabiting originally the coasts of Japan, Korea, China and Far East Russia
(Higo, Callomon & Goto, 1999). Fossils of this species are also
abundantly found in the Pleistocene – Holocene marine deposits
of the Japanese Islands (Oyama, 1980; Matsushima, 1984). The
ease of access to a large number of samples, for both extant and
fossil materials, as well as the recent accumulation of knowledge
on its life history make this species suitable for investigation. Previous studies have revealed that the shell microgrowth patterns
of this species mainly reflect tidal periodicity (Richardson,
1987, 1988), and winter breaks can be used to determine the
age and growth rate (Yamamoto & Iwata, 1956).
Further, latitudinal variations have been found in the
maximum shell size, size at sexual maturity and annual reproductive cycle of this species. For example, spawning in this
species occurs once per year in Hokkaido, northern Japan, and
twice per year in Tokyo Bay and southern Japan (Fig. 1;
Toba, Natsume & Yamakawa, 1992). Moreover, individuals
from Hokkaido are characterized by a larger maximum shell
size and later sexual maturation in their life than individuals
from southern Japan (Kakino & Toba, 1990; Toba &
Miyama, 1994; Goshima et al., 1996). Matsukawa-ura, a small
inlet facing the Pacific coast of northeast Japan, where sampling
was conducted during the present study, is considered to be
located around the geographical boundary of the life-history
variations in this species, and there may be cases of spawning
once on twice each year (Fig. 1; Toba et al., 1992).
Journal of Molluscan Studies (2008) 74: 89 –95
# The Author 2008. Published by Oxford University Press on behalf of The Malacological Society of London, all rights reserved.
doi:10.1093/mollus/eym049
T. KANAZAWA AND S. SATO
Figure 1. A. Map of the sampling site for Ruditapes philippinarum in Matsukawa-ura, northeastern Japan. The white and black circles indicate sites at
which R. philippinarum spawned once and twice per year, respectively (see Introduction; Data from Ponurovsky & Yakovlev, 1992; Toba et al., 1992).
B. Map of Matsukawa-ura showing the sampling area for R. philippinarum (white circle) and the locality in which daily measurement of the seawater
temperature was conducted by the Fukushima Prefectural Fisheries Research Station (black triangle).
prepared, stained with hematoxylin-eosin, and examined using
an optical microscope with magnification of 400– 1,000 times.
Based on histological examination of the gonad sections, each
individual was assigned to one of the specific gonad developmental stages (recovery, growing, mature, spawning, spent) as previously defined by Ko (1957) and Goshima et al. (1996). The
frequency of occurrence of each gonad developmental stage in
monthly samples provided data on the temporal progression of
the reproductive cycle.
The shell microgrowth patterns of R. philippinarum were examined using the individuals of age 3 from specific size classes
between 24 and 35 mm in shell length (Table 1). Individuals exhibiting irregular growth breaks on the shell surface between the
last winter break and the shell margin were excluded from analysis. A single valve of each specimen was sectioned from the umbo
to the ventral margin along the maximum growth axis. The 3-mm
thick section of each valve was mounted and polished on a glass
slide, and was then etched and stained with Mutvei’s solution
(0.5% acetic acid, 12.5% glutaraldehyde and ca. 5 g alcian
blue/litre of solution) for 21 min at 36–408C (Schöne et al.,
2005a). The etched surface of each valve was observed using a
stereoscopic microscope at 100 times and photographed. The
maximum width of each microgrowth increment (lunar-day
growth increment, as defined later) was successively measured
from the shell margin to 15 or more than 60 increments by
using image analysis software (Win Roof, ver. 1; Mitani
We investigated whether spawning breaks of R. philippinarum
can be used to identify the timing of sexual maturity and to
count the number of spawning events in each year. Thus, the
objectives of this study were (1) to examine annual reproductive
cycle and shell microgrowth pattern of this species, (2) to delineate the relationship between the shell microgrowth pattern and
spawning condition in each individual and (3) to describe
spawning breaks in the shell microstructure of this species.
MATERIAL AND METHODS
Living individuals of Ruditapes philippinarum were collected from
the intertidal sand flat of Unoo Coast, Matsukawa-ura, Fukushima Prefecture, northeastern Japan (Fig. 1; 378490 07.600 N,
1408590 16.200 E). More than 30 individuals were sampled on a
monthly or semi-monthly basis between March 2005 and
October 2005 (Table 1). The bivalves were dug by hand at
low tide during spring tides in an area approximately 10 m
long and 1 m wide, at a sea level of approximately 50 cm.
The reproductive cycle of R. philippinarum was examined using
sexually mature individuals (shell length .17 mm, age .1;
Table 1). Dissected gonadal tissue of each individual was fixed
for 24 h in FAA solution (5% formalin, 5% acetic acid, 90%
ethanol), followed by dehydration through a graded series of
ethanol and butyl alcohol, and then embedded in paraffin
(melting point 548C). 10-mm sections of the gonad tissue were
Table 1. Number and range of shell length of individuals collected and examined for reproductive cycle and shell microgrowth pattern at each
sampling date.
Date (2005)
Individuals collected
Individuals examined for reproductive cycle
Individuals examined for shell microgrowth pattern
No. individuals
Range of shell length (mm)
No. individuals
Range of shell length (mm)
No. individuals
Mar. 14
36
17 –36
35
18 –36
0
–
Apr. 15
37
23 –39
37
23 –39
10
24–31
May 13
34
12 –39
32
18 –39
9
26–32
May 27
38
17 –36
37
18 –36
14
26–34
June 10
30
26 –39
30
26 –39
10
26–33
June 24
31
24 –40
31
24 –40
11
24–31
July 8
33
13 –40
30
18 –40
8
26–30
Range of shell length (mm)
July 22
39
11 –37
30
18 –37
14
27–32
Aug. 6
40
12 –37
33
18 –37
11
27–32
Aug. 19
34
14 –36
29
18 –36
12
24–35
Sep. 16
36
17 –35
35
18 –35
8
24–35
Oct. 15
33
18 –41
33
18 –41
7
25–32
90
SHELL MICROGROWTH OF RUDITAPES PHILIPPINARUM
Corporation, Tokyo). However, microgrowth increments near
the shell margin of the individuals collected in March were too
narrow (,5 mm) to identify them individually; therefore, the
analysis of shell microgrowth patterns considered individuals collected from April to October (Table 1).
To compare the relationship between the shell microgrowth
patterns and the seawater temperatures tidal data of the sampling
site were obtained with the freeware TIDE for WIN (ver. 2.11),
and the daily surface seawater temperatures (taken at 10 am) at
approximately 1 km west from the sampling area (Fig. 1) were
obtained from the unpublished data of the Fukushima Prefectural
Fisheries Research Station. Further, to delineate the relationship
between the shell microgrowth pattern and spawning conditions,
the individuals examined were classified into two groups, namely,
a growing group (spent, recovery and growing gonad stages) and
a spawning group (mature and spawning stages), based on the
state of gonad development. The mean microincrement width
of the last 15 increments from the shell margin was compared
between the two groups on each sampling date.
Figure 3. A. Local tidal periodicity, as determined using TIDE for
WIN. The bold grey line indicates the height above sea level of the
sampling site. B. Shell microgrowth pattern formed from 7 to 27 May
2005 for the individual (shell length, 29 mm; age, 3) of Ruditapes philippinarum collected from Matsukawa-ura on 27 May 2005. Each microgrowth line is comparable to emersion event in each day. The grey
area indicates shell microgrowth increments in 1.5 lunar days during a
neap tide, from 18 to 20 May 2005. Scale bar ¼ 100 mm.
RESULTS
Histological examinations of the gonads revealed that spawning
periods of Ruditapes philippinarum in Matsukawa-ura occurred
twice in 2005, the first between late June and early August
and the second between late September and early October
(Fig. 2). The gonads recovered and grew from March to May,
and ripening continued throughout the interval of June to
August. Some individuals began to spawn in late May, and
most individuals were either at the mature or spawning stage
between late June and early August. Further, some individuals
were in the recovery and growing stages between late August
and September, while all individuals were at the spawning
and spent stages in October (Fig. 2).
In the sampling area, R. philippinarum is emersed once on each
lunar day, except during neap tides; thus, each microgrowth
increment usually corresponds to one lunar day (Fig. 3).
During neap tides, the clams are emersed twice each lunar day,
e.g. May 19 (Fig. 3). Therefore, to measure the growth rate on
each lunar day, the increment during neap tides, i.e. 1.5 lunar
days of shell growth, was not considered. The mean lunar-day
growth rate during the 15 lunar days before the sampling date
(except for 1.5 lunar days during neap tides) rapidly increased
from approximately 40 mm in April to .80 mm in early June,
and then remained constant (80–90 mm) between June and
October (Fig. 4A). Each lunar-day shell growth rate was positively correlated with seawater temperature when seawater temperature was ,208C; however, this correlation became
Figure 4. A. Seasonal changes in the mean lunar-day growth rate
measured from the shell margin to 15 increments in individuals of
approximately 30 mm in shell length (age 3) of Ruditapes philippinarum
collected from Matsukawa-ura and the mean surface seawater temperature. The mean and range of one standard deviation (vertical bar) are
indicated. B. Scatter plot of the lunar-day growth rate measured from
the shell margin to 15 increments in individuals of approximately
30 mm in shell length (age 3) of Ruditapes philippinarum collected from
Matsukawa-ura in April–October 2005 versus daily data on the
surface seawater temperature.
Figure 2. Frequency of occurrence of each gonad developmental stage in
samples of Ruditapes philippinarum collected on a monthly/semimonthly
basis from Matsukawa-ura from March to October 2005. A. Recovery
stage. B. Growing stage. C. Mature stage. D. Spawning stage. E.
Spent stage.
91
T. KANAZAWA AND S. SATO
temperature until it exceeded 208C while, in the latter, shell
growth was not correlated with a seawater temperature
.168C (Fig. 6).
Among the individuals of R. philippinarum of age 3 that were
collected in September 2005, the shell microgrowth patterns differed between the individuals of the growing and spawning
groups (Fig. 7A– E). The individual microincremental widths
were usually more than 100 mm in individuals of the growing
group (Fig. 7A, B), while they were typically less than 60 mm
in those of the spawning group (Fig. 7C –E). Moreover, in
some individuals of the spawning group, the shell microincrement width suddenly decreased from .100 to ,60 mm and
then slowly increased within 15 increments near the shell
margin (Fig. 7C, E– G). Such a temporary cessation of shell
growth creates a spawning break in the shell microstructure.
In a male individual of spent stage that was collected in
October 2005, a set of rapid decreases and subsequent slow
increases in the microincrement width occurred twice between
August and October (Fig. 7G). Thus, the sudden decrease in
the lunar-day growth rate corresponded to the spawning break
that occurred during the two spawning seasons, i.e. from late
June to early August and from late September to early
October (Fig. 2).
Figure 5. Seasonal changes in the mean lunar-day growth rate of the
two groups in Ruditapes philippinarum collected from Matsukawa-ura in
relation to the mean surface seawater temperature. Based on its gonad
developmental state, each individual was classified under either the
growing group (spent, recovery and growing stages) or the spawning
group (mature and spawning stages). The mean and the range of one
standard deviation (vertical bar) are indicated.
DISCUSSION
In Ruditapes philippinarum from Matsukawa-ura, spawning
occurred twice in 2005, first between late June and early
August and second between late September and early October
(Fig. 2). In late May, more than 80% of the individuals were
in the recovery and growing stages and, by late June, more
than 90% of the individuals were in the mature and spawning
stages. These facts suggest that most individuals spawned
during the first spawning season in this year. However,
between late August and late September, approximately 50%
of the individuals were in the recovery and growing stages
and, in late October, almost all the individuals were in the
spawning and spent stages (Fig. 2). Therefore, it is unclear
whether the other 50% of the individuals spawned during the
second spawning season in this year. Toba et al. (1992) reported
that in R. philippinarum from Tokyo Bay, a few mature gametes
are released several times during one spawning event; thus
considerably weaker at .208C (Fig. 4B). Further, the range of
variation in the lunar-day growth rate became considerably
greater (10 to 250 mm) at .208C (Fig. 4B).
Comparison of the shell microgrowth patterns with the gonad
development stages revealed that the mean lunar-day growth
rates in individuals of the spawning group (mature and
spawning stages) were significantly lower than those in individuals of the growing group (spent, recovery and growing stages)
(Fig. 5). Both growing and spawning groups were found
among the samples obtained on 27 May, 10 June, 8 July, 16 September and 15 October. On these days, the mean lunar-day
growth rate of individuals in the growing group was always significantly higher than that of the individuals in the spawning
group (Table 2). Moreover, in the former, the lunar-day
growth rate was positively correlated with the seawater
Table 2. Mean lunar-day growth rate of individuals belonging to the growing group (spent, recovery and growing stages) and the spawning group
(mature and spawning stages) at each sampling date.
Date
Mean surface
Growing group (spent, recovery and growing
(2005)
seawater
stages)
Spawning group (mature and spawning stages)
t test
temperature (8C)
Mean
Standard
No. measured
Mean lunar-day
Standard
No. measured
lunar-day
deviation
microincrements
growth rate (mm)
deviation
microincrements
growth rate
(No. individuals)
t value
P
–
(No. individuals)
(mm)
Apr. 15
8.72
38.44
13.86
150 (10)
–
–
0 (0)
–
May 13
11.95
44.66
15.02
135 (9)
–
–
0 (0)
–
–
May 27
13.91
62.07
16.92
195 (13)
41.85
15 (1)
4.583
,0.0001
June 10
16.29
95.42
30.09
90 (6)
71.65
27.77
60 (4)
4.886
,0.0001
June 24
19.05
–
–
0 (0)
80.71
26.32
165 (11)
–
–
July 8
20.41
112.07
28.75
15 (1)
81.90
21.39
105 (7)
4.882
,0.0001
7.95
July 22
21.19
–
–
0 (0)
81.14
24.11
210 (14)
–
–
Aug. 6
22.83
–
–
0 (0)
68.99
29.32
165 (11)
–
–
Aug. 19
24.85
–
–
0 (0)
91.34
36.26
180 (12)
–
–
Sep. 16
23.68
115.96
55.22
60 (4)
72.48
51.25
60 (4)
4.450
,0.0001
Oct. 15
20.97
106.03
33.82
60 (4)
47.72
20.63
45 (3)
92
10.22
,0.0001
SHELL MICROGROWTH OF RUDITAPES PHILIPPINARUM
Figure 7. Photomicrographs of the shell microgrowth patterns near the
shell margin in five individuals of Ruditapes philippinarum collected from
Matsukawa-ura on 16 September (A– E), 5 August (F), and 15
October (G) 2005. Scale bar ¼ 100 mm. A. Female, recovery stage.
B. Female, growing stage. C. Male, mature stage. D, E. Female, spawning stage. F. Female, spawning stage. G. Male, spent stage. The arrows
indicate the areas at which the shell microincrement width suddenly
decreased to less than 60 mm (i.e. spawning breaks).
Figure 6. Scatter plot of the lunar-day growth rate measured from the
shell margin to 15 increments in individuals of approximately 30 mm
in shell length (age 3) of Ruditapes philippinarum versus the daily surface
seawater temperature. Individuals were divided into (A) the growing
group (spent, recovery and growing stages) and (B) the spawning
group (mature and spawning stages).
some individuals whose gonads are in the mature and spawning
stages are considered to have maintained a state of partial
spawning during the two spawning seasons in this year.
et al., 2005b), Mercenaria mercenaria (see Pannella & MacClintock, 1968; Kennish, 1980) and Phacosoma japonicum (see Sato,
1995, 1997). According to Kennish (1980), at the time of spawning the bivalve suddenly stops feeding and its growth rate
decreases substantially.
In R. philippinarum, the mean lunar-day growth rates of individuals in the growing group were always significantly higher
than those of individuals in the spawning group (Fig. 5,
Table 2). Moreover, among individuals in the growing group,
the lunar-day growth rate was positively correlated with the seawater temperature until it exceeded 208C; however, among individuals in the spawning group, it was not correlated with a
seawater temperature of .168C (Fig. 6). Further, since all the
individuals in the growing and spawning groups were mixed,
the range of variations in the lunar-day growth rate became considerably greater (10 to 250 mm) at a seawater temperature of
.208C (Fig. 4B). These facts suggest that spawning negatively
influenced the lunar-day growth rate of individuals in the
spawning group. Therefore, when reconstructing the daily
water temperature by sclerochronological analysis, it is necessary to consider the physiological factors involved.
Environmental and physiological factors controlling shell
microgrowth patterns
The shell microgrowth patterns in R. philippinarum were positively correlated with the seawater temperature until it exceeded
208C; however, this correlation became extremely weak at
.208C (Fig. 4B). Isono, Kita & Kishida (1998) have confirmed
that the seawater temperature affects the respiratory activities,
heartbeat and ciliary movement of the gills in this species and
that the shell growth rate attains a maximum value at approximately 258C. Further, it has been demonstrated that the
optimum seawater temperature for oxygen consumption by
this species ranges from 20 to 258C (Ebihara & Murata, 1967;
Akiyama, 1988). The results of the present study are consistent
with these previous observations. Therefore, at least when the
seawater temperature is less than 208C, lunar-day growth rate
of this species is controlled by seawater temperature, because
feeding and other metabolic activities increase with the seawater
temperature. However, when the seawater temperature exceeds
208C, the lunar-day growth rate is affected by other factors.
The shell microgrowth patterns of bivalves are also controlled
by physiological processes such as spawning, and result in shell
breaks (Kennish, 1980). For example, spawning breaks are
recognized in Cryptopecten vesiculosus (see Takenaka, 1999),
Pecten irradians (see Gutsell, 1930), Spisula solidissima (see Jones
et al., 1978), Arctica islandica (see Thompson et al., 1980; Schöne
Spawning breaks in Ruditapes philippinarum shells
The microstructural features of spawning breaks in the shell
microgrowth pattern have been documented in detail in M. mercenaria (see Pannella & MacClintock, 1968; Kennish, 1980) and
P. japonicum (see Sato, 1995). These studies revealed that manner
in which spawning breaks develop is very similar between the
93
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then immediately followed by a sequence of narrow microincrements. Thus, spawning breaks in these species can be distinguished from other types of growth breaks based on the
characteristic shell microgrowth patterns (Kennish, 1980;
Sato, 1995).
The shell microgrowth patterns during spawning in R. philippinarum also occur in a manner similar to those in M. mercenaria
and P. japonicum, i.e. the sequence of wide (usually .100 mm)
microincrements is abruptly interrupted by a spawning break
and immediately followed by a sequence of narrow (usually
,60 mm) microincrements (Fig. 7C, E – G). However, in this
species it is difficult to distinguish spawning breaks from other
types of growth breaks because spawning breaks do not always
occur during summer. Further, the rapid decrease in the microincrement thickness following a spawning break (from .100 to
,60 mm) is not as prominent in this species as in M. mercenaria
and P. japonicum (usually from .200 to ,50 mm).
The present study revealed that in R. philippinarum from Matukawa-ura, two spawning seasons occurred in 2005 (Fig. 2).
Further, this species released a few matured gametes several
times at a spawning event (Toba et al., 1992). In contrast, in
P. japonicum from Tokyo Bay, the spawning season occurs only
once a year (late June to early August), and all the gametes
are released in only one or two episodes at one spawning event
(Sato, 1995). Therefore, spawning breaks in P. japonicum are
more prominent than those in R. philippinarum and can be precisely distinguished from other types of growth breaks.
Nevertheless, spawning breaks of some specimens of R. philippinarum can be recognized distinguished from the shell microgrowth patterns (Fig. 7). In R. philippinarum, latitudinal
variations have been observed in the annual reproductive
cycle, and Matsukawa-ura is considered to be located at the geographical boundary of the life-history variation (Toba et al.,
1992). Spawning breaks can be used to identify the timing of
sexual maturity, to count the number of spawning events occurring each year, and to determine whether the reproductive cycle
of this species varies from year to year. Moreover, by observing
spawning breaks, it may be possible to estimate the age of sexual
maturity and the number of spawning events that occurred per
year in fossil bivalves.
ACKNOWLEDGEMENTS
We would like to thank Kazushige Tanabe (University of
Tokyo) for his critical review of the manuscript and valuable
comments, Masanori Shimamoto (Tohoku University
Museum) and Tsuzumi Miyaji (University of Tokyo) for their
valuable advice and suggestions, and Kazutaka Kobayashi
(Tohoku University) for instruction of histological techniques.
We also thank to the staff of the Souma Futaba Fisheries cooperative for their help in collecting samples and of the Fukushima
Prefectural Fisheries Research Station for providing daily
measured environmental data of seawater in Matsukawa-ura.
This work was partly supported by the Environmental research
grant of the Sumitomo Science Foundation in 2005, and Grantsin-Aid for Scientific Research from the Japan Society for the
Promotion of Science (No. 15740308 for S.S.).
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