Seasonal changes in the shell microstructure of the bloody clam

Palaeogeography, Palaeoclimatology, Palaeoecology 363–364 (2012) 99–108
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Palaeogeography, Palaeoclimatology, Palaeoecology
journal homepage: www.elsevier.com/locate/palaeo
Seasonal changes in the shell microstructure of the bloody clam, Scapharca
broughtonii (Mollusca: Bivalvia: Arcidae)
Kozue Nishida a,⁎, Toyoho Ishimura b, 1, Atsushi Suzuki b, 1, Takenori Sasaki c, 2
a
b
c
Department of Earth and Planetary Science, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo,113-0033, Japan
Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Higashi 1-1-1, Tsukuba, Ibaragi, 305‐8567, Japan
The University Museum, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo,113-0033, Japan
a r t i c l e
i n f o
Article history:
Received 9 March 2012
Received in revised form 1 August 2012
Accepted 28 August 2012
Available online 8 September 2012
Keywords:
Scapharca broughtonii
Stable oxygen isotopes
Shell microstructure
Seasonal change
a b s t r a c t
In this study, seasonally controlled changes in shell microstructures in Scapharca broughtonii (S. broughtonii,
Mollusca: Bivalvia) were demonstrated. We observed shell microstructures and analyzed stable oxygen
isotope ratios to reveal the factors controlling cyclical microstructural changes in S. broughtonii. The specimens examined were collected alive from three localities in Japan. The outer layer of S. broughtonii was
subdivided into a composite prismatic structure on the exterior side and a crossed lamellar structure on
the interior side. Relative thickness of these two structures in the outer layer changed cyclically with ontogeny. Major growth breaks were formed immediately after thickening of the crossed lamellar layer in most
specimens. Growth breaks were also marked by thickening of the composite prismatic structure. Fluctuations in the relative thickness of the two microstructures were synchronized with those of the shell oxygen
isotope ratios that indicated the seasonality of water temperature. The crossed lamellar structure thickened
at high water temperatures in summer. Shell oxygen isotope records indicated that the clams can form
their shells at temperature higher than approximately 12 °C with no record of lower winter temperatures.
Growth breaks observed after the peaks of high water temperatures may have formed as a result of
spawning, because the breaks corresponded to the spawning season. The proportion of the thickness of
the shell microstructures can directly indicate fluctuations in the water temperature. The methods used
in this study can contribute to age determination, the characterization of seasonal shell growth, and an
understanding paleotemperature changes in coastal regions.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Shell microstructure is one of the primary characteristic in classifying mollusk shells. Various structures have been categorized
according to the criteria of orientation, aggregation, size and shape
of crystallites or their structural units, constituent minerals, and
the presence of organic matrix (Taylor et al., 1969; Kobayashi,
1971; Taylor et al., 1973; Carter, 1980a; Carter and Clark, 1985;
Carter, 1990a). Shell microstructure has been studied mainly regarding taxonomy and phylogeny (Taylor, 1963; Taylor et al., 1969;
Kobayashi, 1971; Taylor et al., 1973; Uozumi and Suzuki, 1981;
Shimamoto, 1986; Carter, 1990b; Hikida, 1996). Shell microstructure
formation is also affected by environmental factors (Carter, 1980b;
Kennish, 1980; Lutz and Clark, 1984). If environmental signatures
⁎ Corresponding author. Tel.: +81 3 5841 2824; fax: +81 3 5841 8451.
E-mail addresses: [email protected] (K. Nishida), [email protected]
(T. Ishimura), [email protected] (A. Suzuki), [email protected] (T. Sasaki).
1
Tel.: +81 29 861 3918.
2
Tel.: +81 3 5841 2820.
0031-0182/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.palaeo.2012.08.017
can be detected from the microstructure of fossil specimens, it is possible to develop new paleoenvironmental or paleoecological proxies.
Scapharca broughtonii (S. broughtonii) is distributed in Asia covering China, Korea, Japan, and the Far Eastern part of Russia (Habe,
1965; Evseev and Lutaenko, 1998; Matsukuma and Okutani, 2000).
S. broughtonii burrows shallowly in sandy mud or muddy bottoms
at 5–50 m depths (Matsukuma and Okutani, 2000). The name
“bloody clam” originates from the presence of hemoglobin in the hemocoel (Oliver and Holmes, 2006). This species is used in sushi;
therefore, a large amount of these shells are cultured and collected
in Japan. Scapharca shells have been present in the fossil record
since the Pleistocene (Noda, 1966; Matsushima, 1984; Lutaenko,
1993) and have been excavated in shell mounds (Sakazume, 1952;
Matsushima, 1984; Lutaenko, 1993; Rakov, 2004). Therefore, this species is a model species for paleoenvironmental studies. Kobayashi
(1976a) reported that the outer layer of S. broughtonii changes its
shell microstructures cyclically during its lifetime. However, the reason for and mechanism of these microstructural changes remain unknown. This study performed a detailed analysis of the growth
cycles of shell microstructures. In addition, stable oxygen isotopic
analysis is used to demonstrate that the cycles are controlled primarily by ambient water temperature.
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2. Materials and methods
2.1. Materials
We used recent specimens collected from three localities in Japan
(Figs. 1 and 2). All specimens are registered at the Department of
Historical Geology and Paleontology, University Museum, University of Tokyo (UMUT). Specimen 1 (UMUT RM 31012) was cultured
in a hanging net at an approximate depth of 5–10 m in Mutsu Bay,
Aomori Prefecture, (locality 1) and was collected on September 20,
2010. Specimens 2a, (UMUT RM 31013) and 2b (UMUT RM 31014)
were dredged on December 22, 2010 at an approximate depth of
22–23 m off Yuriage city, Miyagi Prefecture, in the Pacific Ocean (locality 2). Specimens 2a and 2b were collected at 38°05′N, 140°58′E
(locality 2‐1) and 38°09′N, 140°59′E (locality 2‐2), respectively.
Specimens 3a (UMUT RM 31015) and 3b (UMUT RM 31016) were
cultured at 33°58′N, 131°50′E off Kudamatsu city, Yamaguchi
Prefecture, in Seto Island Sea (locality 3) and collected on December
28, 2010. These specimens were cultured in nets near the surface
until the shell length reached 25–30 mm after the development of
the sessile stage with byssi. The clams were then transferred to a
coarse-meshed cage at the sea bottom at a depth of 10 m. For this
type of cultivation, cages are half-buried in sediments, which allow
the clams to grow in a natural environment. The shells of specimens
1, 3a, and 3b were slightly flattened because of cultivation in cages
or nets.
prepared by using Bio-den film (Ohkenshoji Co. Ltd.) for optical
microscopy.
Microstructures were observed with a scanning electron microscope (SEM) on fractured and polished planes in radial, transverse,
and horizontal sections. Polished sections were treated by etching
in 0.2% HCl for 10–120 min. Shell pieces were Pt-coated and examined with a SEM (Hitachi S-2250, The University Museum, The University of Tokyo). In this study, we used the terminology of Carter
and Clark (1985), Popov (1986), and Carter (1990a) in describing
shell microstructure.
The thickness of the two microstructures in the outer layer was
measured from acetate peels as shown in schematic illustrations of
Fig. 3. In specimens 1, 2a, and 3a, we measured the thickness of the
composite prismatic structure (shown as A in Fig. 3) and outer
layer at approximately 1 mm intervals with ImageJ/NIH version
1.45 image analysis software (available as freeware at http://
rsbweb.nih.gov/ij/). The positions indicating the relative thickness
of the composite prismatic structure are represented by the distance
from the umbo to the external shell surface (shown as B in Fig. 3). To
compare among shell growth of three specimens in each year that
differs shell thicknesses, we divide the thickness of the composite
prismatic structure by the outer layer to get the relative thickness
of the composite prismatic structure. The maximum thickness of
the outer layer was used to calculate the relative thickness in the
thin marginal region outside of pallial myostracum (e.g. more than
12 cm from the umbo, Fig. 3).
2.2. Acetate peel method and SEM observation
2.3. X-ray diffraction analysis
Shell structure was examined by the acetate peel method
(Kennish et al., 1980) and scanning electron microscopy. Each specimen was radially sectioned along a radial rib (Fig. 2) and etched in
0.2% hydrochloric acid (HCl) for 5–10 min. Acetate peels were
Identification of shell minerals was conducted by X-ray diffraction (XRD) analysis (RINT-2500, Rigaku). Powder samples were collected from each layer of specimen 3a.
Fig. 1. Map of Japan indicating the sampling localities (localities 1–3, solid stars) of five specimens.
K. Nishida et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 363–364 (2012) 99–108
101
Fig. 3. Schematic illustrations of shell microstructure distribution. The thicknesses of
composite prismatic structure (A) and the outer layer were measured. The position
of the measurement (white arrow) is represented by the distance from the umbo at
the external shell surface (B).
throughout the year in 2008–10 by the Yamaguchi Institute of Fisheries Science at depths of 0 m and 10 m, which is close to depth of locality 3, off Kudamatsu city, Yamaguchi Prefecture (Fig. 4C). Because
S. broughtonii lives below the tidal zone, the animals experience only
small fluctuations in seawater salinity (Fig. 4B, D, and F). Thus, the
calculation of water temperature from shell oxygen isotopes is less
affected by salinity in this species; this topic is subsequently
discussed in Section 4. Monthly precipitation data are available at
the Japanese Meteorological Agency website (http://www.data.
jma.go.jp/obd/stats/etrn/index.php). Thus, we used the data of
Wakinosawamura in Aomori Prefecture, Natori city in Miyagi Prefecture, and Kudamatsu city in Yamaguchi Prefecture (Fig. 4A and C).
2.5. Stable oxygen isotope analysis
Fig. 2. Photographs of the outer surfaces of three specimens. (A) Specimen 1 at locality
1. (B) Specimen 2a at locality 2. (C) Specimen 3a at locality 3. The white line indicates a
section of acetate peel; arrow indicates the position of growth break.
2.4. Environmental data
Seawater temperature, salinity, and precipitation data from the
three localities are shown in Fig. 4. We obtained the seawater temperature and salinity statistics from a database at the Japan Oceanographic Data Center (JODC, http://www.data.jma.go.jp/obd/stats/
etrn/index.php) (1° × 1° box). The data from northern Mutsu Bay
at depths of 10 m in Aomori Prefecture (41–42°N, 140–141°E)
and 20 m in Miyagi Prefecture in the Pacific Ocean (38–39°N,
141–142°E) are shown in Fig. 4A–B, respectively. Water temperature
and salinity were measured monthly by the Miyagi Prefecture Fisheries Technology Institute between May and November in 2009–10
at a depth of 20 m in Miyagi Prefecture near locality 2‐1 (Fig. 4B).
In addition, water temperature and salinity were measured monthly
Stable oxygen isotope compositions were analyzed for specimens
2a and 3a. After removing the periostracum, microsamples of approximately 70–100 μg were obtained for isotope analysis with a
dental drill at a low rotational speed on the outer shell surface
(Fig. 2B–C). Sampling lines were along the radial rib near the section
of acetate peel (Fig. 2B–C). Carbonate powder was reacted with
phosphoric acid at 25 °C for 4 h. A detailed description of the CO2
purification procedure is described by Ishimura et al. (2004). The
oxygen isotope (δ 18O) of S. broughtonii shells was determined by a
customized isotope ratio mass spectrometer (Micromass ISOPRIME,
Manchester, UK) at the National Institute of Advanced Industrial
Science and Technology (AIST). All isotopic data are reported in standard δ notations (δ 18O; ‰) relative to the Vienna‐Pee Dee belemnite
(VPDB) standard scale. The NBS-19 carbonate standard was used
for calibration of the VPDB scale. External analytical precision was
within ± 0.1‰ for δ 18O.
We calculated the seasonal fluctuations in temperature (T) by the
Grossman and Ku (1986) equation for molluscan shell aragonite
(Fig. 8). The equation is
h
i
18
18
Tð CÞ ¼ 20:6−4:34 δ Oshell −ðδ Ow −0:27Þ ;
where T is the calculated temperature, δ 18Oshell is the oxygen isotope ratio of aragonite in the shell, and δ 18Ow is the oxygen isotope
ratio of seawater. We calculated δ 18Ow from Oba (1990) using the
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Fig. 4. Monthly seawater temperature, salinity, and precipitation data at the three localities. Precipitation data was obtained by Japanese Meteorological Agency (http://
www.data.jma.go.jp/obd/stats/etrn/index.php). Statistics of temperature and salinity were obtained by the Japan Oceanographic Data Center (JODC, http://www.data.
jma.go.jp/obd/stats/etrn/index.php). Error bars represent standard deviation (2σ) of the monthly observed values. (A) Statistics of temperature at 41–42°N, 140–141°E
(N = 5672) near locality 1. (B) Statistics of salinity (N = 4038) and observed precipitation in 2010 at locality 1. The area of statistics of salinity is the same as that in A. Precipitation data was provided by Wakinosawamura in Aomori Prefecture. (C) Observed measurements and statistics of temperatures at locality 2. Monthly observed values
for water temperature in 2009–10 (May–November) were measured by the Miyagi Prefecture Fisheries Technology Institute at a depth of 20 m near locality 2‐1 in Miyagi
Prefecture. Statistics of temperature were obtained by JODC (N = 3146) at a depth of 20 m at 38–39°N, 141–142°E in the Pacific Ocean near locality 2‐1. (D) Salinity and
observed precipitation in 2010 at locality 1. The area of observed and statistics of salinity (N = 1154) is the same that in C. Precipitation data was obtained by the city of
Natori in Miyagi Prefecture in 2009–10. (E) Observed temperature at locality 3. Monthly observed values for water temperature in 2008–10 were measured by the
Yamaguchi Institute of Fisheries Science at depths of 0 m and 10 m in close proximity to the sample locality off the city of Kudamatsu in Yamaguchi Prefecture. (F) Observed
salinity and precipitation at locality 3. Salinity data were measured at the same locality as that in E. Precipitation data was obtained by the city of Kudamatsu in Yamaguchi
Prefecture in 2008–10.
average seasonal salinity data (33.5 psu at locality 2; 33.1 psu at
locality 3). The equation is
18
δ Ow ¼ 0:203 S−6:76;
where S denotes salinity. δ 18Ow are +0.04‰ at locality 2 and −0.05‰
at locality 3. Because the fluctuations in salinity were up to
33.0–34.1 psu at locality 2 and 32.0–33.7 psu at locality 3, we regarded
the fluctuation of δ18O data of seawater as low (within ±0.1‰ at locality 2; within ±0.2‰ at locality 3).
K. Nishida et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 363–364 (2012) 99–108
3. Results
3.1. Shell microstructures of S. broughtonii
The shell microstructure of S. broughtonii was examined by the
acetate peel method (Fig. 5) and SEM (Fig. 6) in each shell layer.
This species has outer and inner layers that are divided by the
103
myostracum (Fig. 5A–B). Each layer consists of aragonite, as confirmed by XRD analysis.
The outer layer is subdivided into a composite prismatic structure on the exterior side and a crossed lamellar structure on the
interior side (Figs. 5B–E and 6E). A transitional zone between the
two structures (up to 10 μm) is formed of acicular prisms with
longitudinal axes generally perpendicular to the depositional
Fig. 5. Optical micrographs of acetate peels. Growth direction is toward the right side of the micrographs. Gray arrow indicates growth break. (A) Radial section of specimen 3a.
(B) Close-up of A showing outer and inner layers near the umbonal region. (C) Close-up of A with outer layer that shows composite prismatic and crossed lamellar structures. The
composite prismatic structure has thinned with growth. (D) The position of the growth break of specimen 2a in 2006, obtained from the shell isotopic profile of that in Fig. 8. The
composite prismatic structure has thickened after the growth break. (E) The marginal region of specimen 1 at approximately 67–86 mm from the umbo. This specimen has a
strong growth break, and the proportion of the thickness of the two structures shows partially irregular fluctuations. Abbreviations: CL = crossed lamellar layer; CP = composite
prismatic layer; i-CCL = irregular complex crossed lamellar layer.
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Fig. 6. Scanning electron micrographs of shell microstructures. (A) Composite prismatic structure of specimen 3a showing radial, polished, and acid etched section. (B) Composite prismatic structure of specimen 2b on the shell outer surface. (C) Crossed lamellar structure of specimen 2b showing radial and fractured section. (D) Crossed lamellar structure of specimen 3b showing transverse, polished, and acid etched section. (E) Composite prismatic structure on the exterior side and crossed lamellar structure on the interior
side in the outer layer of specimen 3b showing radial and fractured surfaces. White dot line indicates a transitional zone between the two structures. (F) Radial polished and
etched section of specimen 1. A thick growth line in the outer layer appears at the position of a growth break. A thick organic sheet appears within the crossed lamellar structure.
surface. Optical microscopy of the acetate peels shows columnar
(prismatic) and branching (crossed-lamellar) patterns (Fig. 5B–E).
The thickness of the two structures changed cyclically, and the composite prismatic structure is not evident at several growth stages.
The composite prismatic structure shows aggregations of elongated
first-order structural units that consist of acicular second-order
prisms (Fig 6A–B and E). Popov (1986) categorized this structure
as a compound prismatic-type group. No organic sheet appears
among the first-order units; however, the outer surface of these
units is less acid-etched than the inside of the units (Fig. 6A). The
crossed lamellar structure is composed of aggregations of parallel,
elongated, and highly branched structural units (first-order lamellae) that consist of laths (third-order lamellae) (Fig. 6C–D and F).
Longitudinal axes of first order lamellae dip in two dominant directions relative to the depositional surface. The inner layer appears as
an irregular complex crossed lamellar structure formed by irregular
K. Nishida et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 363–364 (2012) 99–108
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aggregations of parallel, elongated, and irregular-sized structural
units of acicular prisms.
3.2. Growth breaks
In all specimens, distinct notches known as growth breaks were
observed on the outer shell surfaces, as indicated in Figs. 2 and 5D–
E. Growth break is formed by a decrease or cessation of shell growth
in response to environmental or physiological stress (Kennish, 1980;
Lutz and Rhoads, 1980). Fourteen, nine, and four growth breaks were
observed in specimens 1, 2a, and 3a, respectively (Fig. 2). Thick
growth lines in the outer layer were formed at the position of growth
breaks with thick organic sheets (Fig. 6F).
3.3. Cyclical change in the shell microstructures of the outer layer
The relative thickness of the two structures in the outer layer was
measured in specimens 1, 2a, and 3a (Fig. 7), and was determined to
change cyclically with ontogeny. The crossed lamellar structure always appeared below the composite prismatic structure. The thickness ratio of the composite prismatic structure in the outer layer
ranged from 0 to 69%, 0 to 67%, and 0 to 60% in specimens 1, 2a,
and 3a, respectively. In all specimens, intervals of prism-bearing
stages (dark gray shading in Fig. 7) decreased ontogenetically. The
range of fluctuations in the relative thickness of the composite prismatic structure decreased with ontogeny in specimen 2a. Specimen
1 showed partially irregular fluctuations near the marginal region
at 60–86 mm from the umbo (Figs. 5E and 7).
At the positive peaks of thickness of the crossed lamellar structure,
major growth breaks were observed in most cases. In addition, other
growth breaks appeared near the peaks where the composite prismatic structure is predominant.
3.4. Shell oxygen isotopic records
Annual δ 18O profiles from specimens 2a and 3a were analyzed in
conjunction with changes in the shell microstructures of the outer
layers (Fig. 8). The δ 18O values of specimen 2a and 3a ranged from
− 0.5‰ to + 2.1‰ and from − 2.0‰ to + 1.8‰, respectively. Calculated water temperature from δ 18O values is also shown in Fig. 8. Because the highest inferred water temperature recorded in δ 18O of the
shell was nearly the same as that observed in the field, the shells precipitated aragonite slightly near the isotopic equilibrium (Fig. 8). The
δ 18O profiles showed seasonal fluctuations in water temperature in
both the specimens. Both the specimens contain records above approximately 12 °C without records of lower winter temperatures
(7.0–12 °C in specimen 2a; 9.5–12 °C in specimen 3a). The range of
isotopic fluctuations remained almost unchanged with ontogeny.
In both the specimens, the fluctuations in relative thickness of the
two microstructures were synchronized with seasonal changes in
water temperatures. At cooler temperatures, the outer layer was
multilayered, and the relative thickness of the composite prismatic
structure was increased. The crossed lamellar structure thickened
at high water temperatures in summer. Covariance of calculated
temperature from δ 18O data and relative thickness of the composite
prismatic structure in the outer layer are shown in Fig. 9. Specimens
2a and 3a show correlations between these data (specimen 2a: R =
0.85, p b 0.001; specimen 3a: R = 0.69, p b 0.001). In specimen 2a,
the relative thickness of the composite prismatic structure was
decreased, especially in winter, as the specimen aged (Fig. 9B–C).
The summer peaks of the δ 18O values showed a bell curve at the
age of 1 year in specimen 2a and 1–2 years in specimen 3a; sawtoothed patterns appeared at the age of 2–7 years in specimen 2a
and 3 years in specimen 3a. Similar patterns were also recorded in
the relative thickness of the two structures. Fig. 10 shows growth
curves of three specimens estimated by summer growth breaks and
Fig. 7. Profiles of the relative thickness of the composite prismatic structure in the
outer layer in specimens 1, 2a, and 3a. Maximum value of the thickness of the outer
layer was substituted for calculating of relative thickness in the marginal region
that is the outer side of the pallial myostracum and decreases the thickness of the
outer layer (white arrow). Black arrow indicates growth break, dark gray area indicates composite prismatic structure, light gray area indicates crossed lamellar structure, and white area indicates lost section of the outer layer.
the positive peaks of relative thickness of the composite prismatic
structure. Annual shell growth rate was decreased in the three-year
age period in both specimens (Figs. 7, 8, 9 and 10).
A growth break was formed after the negative or near the positive
δ 18O peak values. In specimen 2a, water temperature calculated from
δ 18O values decreased rapidly from the peaks of high water temperature, and the relative thicknesses of the composite prismatic structure increased rapidly from the negative peaks of the thicknesses in
later growth stage (in 2005–10). Fig. 5E shows the growth break of
specimen 2a in 2006 at a distance of 61.5 mm from the shell margin
(Figs. 7 and 8). The composite prismatic structure thickened after the
growth break (Figs. 5E, 7, and 8).
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Fig. 8. Relationship of the shell microstructures in the outer layer, δ18O of shell and calculated temperatures of specimens 2a and 3a. (A) Specimen 2a at locality 2. (B) Specimen
3a at locality 3. Profiles of the relative thickness of the composite prismatic structure in the outer layer are the same as those shown in Fig. 7. Arrow indicates growth break. In the
graph of relative thickness, dark gray, light gray, and white areas indicate composite prismatic structure, crossed lamellar structure, and lost section of the outer layer or thin part
of the layer, respectively. Shaded areas in a graph of shell isotopic data indicate the range of observed temperature (shown in Fig. 4). The years (gray bars) in graphs were estimated by the δ18O record.
4. Discussion
In specimens 2a and 3a, the fluctuations in the relative thickness
of the two microstructures were synchronized with seasonal
changes in water temperatures (Figs. 8 and 9). At cooler temperatures, the outer layer was multilayered, and the thickness proportion
of the composite prismatic structure was increased. From the calculated water temperature from δ 18O, it appears that both the
specimens can form their shells at temperatures higher than approximately 12 °C without recording lower winter temperatures. Because the winter peaks showed cuspate peaks near the positions of
growth breaks, shell growth likely stoped below 12 °C.
Temperature records in specimens 2a and 3a were absent in the
fall, around October and November, after a high temperature peak
in September. Sasaki (1997) reported that the spawning season of
S. broughtonii in Miyagi Prefecture was from June to September,
and its peak was from mid-August to early September. In addition,
Minobe (2007) reported that the spawning season in Aomori Prefecture was from July to September with a peak in approximately
mid-August. As documented by Kanno (1968), S. broughtonii shows
an ovipositional response at approximately 18–20 °C that corresponds to water temperature in the spawning season as previously
mentioned. Hence, saw-toothed peaks of δ 18O values and the proportions of shell structures in summer likely indicate a decrease in
the shell growth as a result of spawning. Growth breaks after the
peaks of high water temperature may indicate spawning breaks
after the peak season of spawning, with a 1–2 month cessation of
shell growth. In specimen 2a, the relative thickness of the composite
prismatic structure was decreased, especially in winter, as the specimen aged (Figs. 8 and 9). Such a change may be affected by aging
and marked by a decline in shell secretion.
The difference in the cultivation methods may affect the relative
thickness of the two structures to some extent although the prime
controlling factor is seasonal change in water temperature. The relative thickness of the two shell structures was partially irregular in
specimen 1 that was cultured in a hanging net in a water column
away from the sea bottom. On the contrary, specimen 3a was cultured in a cage in the bottom sediment. Yurimoto et al. (2007)
reported that cultivation methods affect the shell growth of
Scapharca kagoshimensis. They compared the shell growth of specimens hung in a set and cultivated in cages at the sea bottom, and
reported that the monthly growth rate of the former specimen was
lower than that of the latter specimen because of unstable positioning by waves. Thus, specimen 3a experienced less growth stress than
specimen 1.
Ontogenetic changes in bivalve shell microstructures within a
single shell layer have been observed in various taxa of Bivalvia, as
listed in Table 1. These changes may also appear in other families
that have not been investigated. The inner layer of Geukensia demissa
collected at various times of the year showed a seasonal change in
shell microstructures (Lutz and Clark, 1984). In summer, the inner
layer shows step-like patterns of regular hexagonal nacreous tablets,
while in winter, the layer is formed of smaller, irregularly stacked
nacreous tablets or fine granules (Lutz and Clark, 1984). In their
study, northern populations exhibited three types of aragonitic
microstructures including granular, simple prismatic, and nacreous
structures, and the percentage of granular structure increased
at higher water temperatures. In contrast, granular structure was
absent in shells from southern populations. Nishida et al. (2011)
reported the microstructural variations in a cold seep-associated bivalve, Conchocele bisecta, living in the deep sea. Although environmental conditions are more stable in deep waters than those at
shallow depths, this clam seasonally changed its microstructures
within a single shell layer. Yamaguchi et al. (2006) demonstrated
that in the brackish clam, Corbicula japonica, the outer layers with a
complex crossed lamellar structure vary from opaque to translucent
from early summer to winter, depending on the amount of available
organic materials. From the these studies, it can be generally presumed that molluscan shells preserve records of environmental
and physiological changes in their microstructures in various environments, including brackish water and the intertidal zone to the
deep sea.
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Fig. 10. Growth curves of specimens 1, 2a and 3a. The curves were estimated by
summer growth breaks and the positive peaks of the fluctuations in the two
microstructures.
example, shell microstructural observations in fossil specimens are
useful for the reconstruction of paleoclimates. The methods used in
this study can be applied to determine the summer or winter breaks
without growth line analysis. The degree of sample preservation is a
significant issue in geochemistry. Our method, which focuses on
shell microstructure, can estimate seasonal fluctuations in molluscan
fossils even when shell carbonate is not available for isotopic
analysis.
Acknowledgments
We thank Yuji Kuyama, Makoto Fukui (Kudamatsu Institute of
Mariculture, Yamaguchi Prefecture, Japan) and other members of
this institute, Shizuka Murakami (Kudamatsu city, Yamaguchi Prefecture, Japan), Hiroyuki Izumo (Miyagi Federation of Fisheries Cooperative Associations, Yuriage branch), and other members of this
association for donated specimens. We also thank the Yamaguchi Institute of Fisheries Science, Miyagi Prefecture Fisheries Technology
Institute, Japanese meteorological Agency, and Japan Oceanographic
Data Center for environmental data.
Table 1
Known examples of bivalves showing cyclical shell microstructural changes in a single
shell layer.
Fig. 9. Covariance of the calculated temperature from δ18O data and relative thickness
of composite prismatic (CP) structures in the outer layer. (A) Specimen 2a. (B) Specimen 3a. (C) Specimens 2a and 3a. Linear regression (straight line) and its equation
are shown in Fig. 3C. Calculated p-values of each data group: p b 0.001.
5. Conclusions
In S. broughtonii, the proportions of the thicknesses of composite
and crossed lamellar structures in the outer layer change cyclically
with ontogeny. The changes in the thickness of shell microstructures
were synchronized with seasonal changes in water temperature. The
crossed lamellar structure thickens at higher temperature in summer. Shell δ 18O records indicate that the shells are formed at temperatures higher than approximately 12 °C. The results of this study can
contribute to paleoecological and paleoenvironmental studies. For
Family
Genus and/or species name
Shell
layer
Arcidae
Anadara ninohensis, genus
Scapharca
Outer
Mytilidae
Mytilidae
Idasola argentea
Mytilus californianus, Gaukensia
demissa, Modiolus modiolus
Ostreidae
Pholadidae
References
Kobayashi and Kamiya
(1968); Kobayashi
(1976a, b)
Middle Carter (1980c)
Inner
Dodd (1964); Taylor et
al. (1969); Lutz and
Clark (1984)
Middle Taylor et al. (1969)
Outer
Taylor et al. (1973)
genus Ostrea
Barnea candida, Pholas chiloensis,
Pholas dactylus
Veneridae
Callista brevisiphonata, Saxidomus Outer
purpuratus, Mercenaria mercenaria,
Callithaca adamsi, Protothaca
(Novathaca) euglypta, Protothaca
(Novathaca) jedoensis
Inner
Veneridae
Ruditapes philippinarum,
Ruditapes variegata, Protothaca
(Novathaca) euglypta
Thyasiridae Conchocele bisecta
Outer,
inner
Chamidae
genus Chama, genus Arcinella
Inner
Arcticidae
Arctica islandica
Inner
Shimamoto (1988,
1991); Kobayashi
(1979)
Shimamoto (1988,
1991)
Nishida et al. (2011)
Kennedy et al. (1970)
Kennish (1980)
108
K. Nishida et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 363–364 (2012) 99–108
We also thank Fumiko Yoshitani (The University Museum, The
University of Tokyo) for assistance in scanning electron microscopy,
Kei Sato (The University of Tokyo) for revising our manuscript,
Kazuyoshi Tanabe, Kazuyoshi Endo, and Rie Sakai (The University
of Tokyo) for suggestions on research methods and comments.
This work was supported by grants-in-aid from the Mikimoto Fund
for Marine Ecology and grant-in-aid for Challenging Exploratory
Research (no. 24654167) from the Japan Society for the Promotion
of Science. This research was supported by a grant for the Global
COE Program, “From the Earth to “Earths””, MEXT, Japan. The authors would like to thank Enago (www.enago.jp) for the English
language review.
Natori city (Miyagi Prefecture, Japan) is one of the famous fishery
grounds of S. broughtonii. We used these specimens in cooperation
with the Miyagi Federation of Fisheries Cooperative Associations,
Yuriage branch. The city was severely damaged by the March 11,
2011, earthquake and tsunami. We express our deepest condolences
to the victims in the associations.
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