Journal of
The Malacological Society of London
Molluscan Studies
Journal of Molluscan Studies (2015) 81: 502– 511. doi:10.1093/mollus/eyv025
Advance Access publication date: 26 June 2015
The role of the adductor muscle as an energy storage organ in the
pen shell Atrina japonica (Reeve, 1858)
Young-Jae Lee1, Kwang-Sik Choi2, Dae-Sung Lee3, Won Chan Lee4, Hyun Je Park1,
Eun Jung Choy5, Hyung Chul Kim4 and Chang-Keun Kang1
1
School of Environmental Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea;
2
School of Marine Biomedical Science, Jeju National University, 66 Jejudaehakno, Jeju 690-756, Republic of Korea;
3
Natural Products Research Team, National Marine Biodiversity Institute, Chungcheongnam-do 325-902, Republic of Korea;
4
Marine Environment Research Division, National Fisheries Research and Development Institute, Busan 619-705, Republic of Korea; and
5
Korea Polar Research Institute, Korea Ocean Research and Development Institute, Incheon 406-840, Republic of Korea
Correspondence: C.-K. Kang; e-mail: [email protected]
(Received 9 December 2013; accepted 12 May 2015)
ABSTRACT
The biochemical composition and reproductive cycle of the pen shell Atrina japonica were investigated
through separate analyses of the adductor muscle, gonad and the remaining tissues over a 1-year cycle.
Seasonal variations in condition and gonadosomatic indices reflected those of gross weights of biochemical components of whole tissues. During the spring period, growth was initiated in the gonadal tissues
simultaneously with the maximum weight gain in the adductor muscle, in which most of the energy
reserves were stored, indicating that gametogenesis occurs at the expense of immediately ingested food
energy. The increased energy reserves in the gonad during gametogenic development were exhausted
completely during the summer spawning. Protein and carbohydrate reserves in the adductor muscle
were used as catabolic substrates during spawning. The interannual shift in the timing of spawning
appeared to be related to the changes in energy storage and gamete growth during the spring, probably
reflecting changes in nutritional conditions in the ambient environment. Our results suggest that the
adductor muscle of the pen shell plays a critical role as a major organ responsible for energy storage and
that organ-specific biochemical composition can provide information of general relevance to the processes of energy gain and mobilization in bivalves.
INTRODUCTION
In bivalve molluscs, seasonal cycles of energy storage and utilization, as reflected in their biochemical composition, are
related closely to the reproductive cycle (Giese, 1969; Bayne,
1976; Barber & Blake, 1981; Gabbott, 1983; Uddin et al., 2012).
These changes represent the sum of metabolic activities over
time in relation to environmental conditions such as temperature
and food availability (Newell & Bayne, 1980; Gabbott, 1983).
The energy storage and utilization cycle can exhibit interspecific
as well as intraspecific differences. Bayne (1976) distinguished
two different cycles of energy accumulation and gamete production in bivalves. These processes can overlap temporally (e.g. in
Tellina tenuis, Abra alba and Cerastoderma edule) or be clearly separate (e.g. in Macoma balthica, Mytilus edulis and Pecten maximus).
Gametogenesis can occur at the expense of recently ingested
food (in the former group with an opportunistic strategy) or of
energy accumulated during prior feeding (in the latter group
with a conservative strategy). Several studies have reported
interannual or site-dependent variations in the cycles of energy
storage-utilization and reproduction, depending on nutritional
conditions or temperature (Navarro, Iglesias & Larraňaga,
1989; Kang et al., 2000; Ngo et al., 2006; Park et al., 2011).
Accordingly, simultaneous observation of both seasonal cycles in
biochemical composition and in gametogenesis of bivalves is
crucial for assessing their adaptive strategies and population dynamics under varying environmental conditions (see Newell &
Bayne, 1980).
The pen shell, Atrina (Servatrina) japonica (Reeve 1858) is a
large bivalve that is distributed in China, Korea and Japan and
commonly inhabits silty sand or muddy sediments up to a water
depth of 30– 50 m. Its adductor muscle has long been a favourite
type of seafood in East Asian countries and it is a commercially
important species. Since the 1990s, a decreasing trend in annual
production of A. japonica has been observed on the Korean coast
from about 15,300 tons in 1990 to 7,400 tons in 2009 (NFRDI,
2009). Overexploitation, degradation of seashore fishing grounds,
marine pollution and shortage in supply of artificially-produced
seed are recognized as the major causes of the decline in pen
shell production. There have been numerous studies of this
species (in previous literature generally identified as A. pectinata,
but see Schultz & Huber, 2013), for example on age and growth
(Ryu et al., 2001), reproductive cycle (Ceballos-Vázquez et al.,
2000; Baik et al., 2001; Chung, Baik & Ryu, 2006; Chung,
# The Author 2015. Published by Oxford University Press on behalf of The Malacological Society of London, all rights reserved
ENERGY STORAGE IN ADDUCTOR MUSCLE OF ATRINA
Chung & Lee, 2012), aquaculture techniques (NFRDI, 1995),
genetic divergence (Yokogawa, 1996) and transplant experiments on growth and mortality (Wu and Shin, 1998). However,
little is known about the basic biological traits of A. japonica in
relation to the seasonal cycle of energy gain and loss that affect
its production, condition and gametogenesis.
Most of the previous studies on nutrient storage-utilization
cycles of bivalves have assessed their whole-body tissues, because
of difficulty in isolating different organs. In contrast, several
studies on pectinids have shown clearly that the biochemical
composition of different organs such as the gonad, adductor
muscle, digestive gland and mantle can explain more precisely
the storage, extent and mobilization processes involving these
biochemical components in gametogenesis and maintenance
of bivalves (Epp, Bricelj & Malouf, 1988; Pazos et al., 1997;
Lodeiros et al., 2001; Liu et al., 2008; Yan et al., 2010). As with
pectinids, the gonad, adductor muscle and the remaining body
tissues of the pen shell can be isolated easily. In addition, its sex
is clearly distinguishable by gonad colouration and the dissected
gonad can be used to calculate the gonadosomatic index (GSI).
The annual reproductive cycle of A. japonica can be characterized by changes in the GSI.
The adductor muscle accounts for more than half of the
whole-flesh tissue weight in the pen shell. Weight loss of the adductor muscle during the ripe stage of the gametogenic cycle
and spawning indicates that the adductor muscle is the principal
organ of energy storage, as shown in the scallop (Pazos et al.,
1997; Racotta et al., 2003; Uddin et al., 2007). Glycogen content
is much higher in the adductor muscle than in the visceral mass,
including the gonad and digestive diverticula, of A. japonica
(Baik et al., 2001). Stored energy in the form of glycogen in the
adductor muscle may be utilized for the production of gametes,
as commonly indicated in bivalves (Gabbott, 1975; Bayne et al.,
1982). Low protein content in the adductor muscle of A. japonica
in winter may indicate its utilization for metabolic maintenance
during the period of energy imbalance. By contrast, protein and
lipid contents in the gonad of A. japonica peak at the ripe stage of
gametogenesis in spring (Baik et al., 2001). While there have
been attempts to examine biochemical composition and gametogenic cycle of A. japonica, the seasonal dynamics of energy
reserves in each organ have not been systematically investigated.
To address the relationship between energy gain-loss cycles
and the gametogenic pattern in the pen shell, we divided its
flesh tissues into three parts (viz. the gonad, adductor muscle
and remaining tissues) and measured the content of biochemical
components such as protein, carbohydrate, glycogen and lipid
that serve as energy reserves and/or major constituents. The objective of this study was to identify the seasonal variations in
energy storage in each organ of A. japonica and to highlight the
use of these reserves during the gametogenic cycle and in relation to seasonal environmental conditions such as temperature
and food availability.
Figure 1. Map showing the sampling site in Deukryang Bay, Korea.
et al., 1998). The turnover time of the seawater within the bay is
about 1 d. Surface sediment is composed of a deposit of suspended coastal sediment transported east along the southern
coast of Korea (Kong & Lee, 1994). The sediments of the sampling area consist of 60.2% clay, 33.7% silt and 6.1% sand.
Deukryang Bay is relatively free of anthropogenic influence
because of the low level of industrial activity around the bay
compared with other coastal bay systems in Korea. Therefore,
various bivalves and seaweeds have been cultured in the bay.
Pen shell culture was introduced in 1993 and now serves as a
major aquaculture item in the bay. Atrina japonica is cultured in
muddy sediment 10 –50 cm thick at a water depth of 10– 50 m.
Environmental conditions
Temperature and salinity of the seawater were recorded at the
time of sampling using a CTD meter (Sea-Bird Electronics,
Bellevue, WA, USA). Water samples (about 10 l) were collected
with a van Dorn water sampler at 1 m below the water surface
and were prefiltered through a 180-mm Nitex mesh net to
remove zooplankton and large particles. To determine the
chlorophyll a concentration, water samples were filtered through
a Whatman GF/F glass-fibre filter (47 mm in diameter) and the
filters were then extracted with 90% acetone at 4 8C for 24 h.
Chlorophyll a concentration was measured using highperformance liquid chromatography (HPLC). The filters were
ground by a homogenizer (Glas-Col) to aid disruption of the
algal cells. Before analysis, the extracted solution was centrifuged at 2016 g for 10 min to remove particulate materials.
0.25 ml of 1 M ammonium acetate was added to 0.75 ml of
extract. After mixing thoroughly, phytopigment analyses on
1 ml of this solution were performed using reverse-phase HPLC
(Waters System, Milford, MA, USA) as described by Wright
et al. (1991). Canthaxanthin was used as an internal standard.
Photosynthetic pigments were detected using the Waters 2487
absorbance detector (436 nm) and the Waters 474 fluorescence
detector (excitation: 432, emission: 650 nm). Quantification of
the standard pigments was determined by spectrophotometer
using published extinction coefficients (Jeffrey, Mantoura &
Wright, 1997). Water samples for measurement of total suspended particulate matter (SPM) were filtered on preweighed
GF/F glass-fibre filters. The filters were dried at 60 8C for 3 d,
weighed and then ignited at 450 8C for 4 h. The suspended particulate organic matter (POM) was calculated by the ignited
weight loss.
MATERIAL AND METHODS
Study site
The pen shell cultured in Korea has long been identified as
Atrina pectinata (Linnaeus, 1767) (Yang et al., 1995; Yokogawa,
1996; Baik et al., 2001; Chung et al., 2006, 2012; NFRDI, 2009)
or A. pectinata japonica (Reeve, 1858). Recently, Schultz & Huber
(2013) have shown that this species is A. (Servatrina) japonica
(Reeve, 1858). Sampling was conducted in Deukryang Bay, on
the southern coast of Korea (348600 N, 1278100 E), from June
2010 to July 2011 (Fig. 1). The bay is semienclosed, with an
area of about 375 km2 (about 50 km long and 4.6 –11 km wide).
Water depth in the bay averages 7.5 m and the tide is semidiurnal with mean tidal amplitude of approximately 2.4 m (Cho
503
Y.-J. LEE ET AL.
animal. The energy contents of each organ were calculated by
the following conversion factors: 17.5 kJ g21 for carbohydrate,
24.0 kJ g21 for protein and 39.5 kJ g21 for lipid (Gnaiger,
1983). The relationships between different biotic and environmental parameters were examined by Kendall’s rank correlation
coefficient (t). All statistical analyses were performed using
SPSS software (IBMSPSS Statistics v. 12.0; IBM, Armonk, NY,
USA).
Animal collection and biometric measurements
Twenty specimens were collected at random each month from
June 2010 to July 2011 by scuba diving. After immediate transportation to the laboratory under cool conditions in an icebox,
biometric measurements of each individual were conducted.
The shell length, width and height were measured using Vernier
callipers. The specimens were then dissected carefully using a
stainless steel knife and the flesh tissues were divided into three
parts (gonad, adductor muscle and remaining tissues) for the
analysis of biochemical composition. Because of the difficulty in
isolating pure gonadal tissue, specimens of the gonad contained
part of the visceral mass, which accounted for less than 10% of
gonadal mass (Baik et al., 2001; Rodrı́guez-Jaramillo et al.,
2001). The separated tissues were freeze-dried and weighed. The
dried tissues were ground to powder with a mortar and pestle.
The shell valves were dried in an oven at 60 8C for 2 d and the
dry shell weight was measured. The powdered tissue samples
were kept frozen at 270 8C until analysis.
RESULTS
Environmental conditions and biometric measurements
Monthly mean seawater temperature at the sampling site
showed a seasonal pattern typical of the temperate zone, with a
maximum of 25.0 8C (August 2010) and a minimum of 4.6 8C
(January 2011; Fig. 2). Mean salinity ranged from 27.9 psu
(January 2011) to 31.7 psu (September 2011). Chlorophyll a concentration ranged from 1.1 to 5.2 mg l21 and peaked in August
2010 (3.3 mg l21) and April 2011 (5.2 mg l21). SPM concentration fluctuated from 10.3 (September 2010) to 37.0 mg l21 (July
2010) and POM concentration from 1.2 (May 2011) to
6.8 mg l21 (October 2010). Results of monthly biometric measurements showed that individuals of similar size were collected
each month and were compared during the study period
(Table 1). Shell length ranged from 18.7 to 30.0 cm, shell width
from 10.1 to 17.0 cm, shell height from 3.6 to 7.5 cm, total DW
from 8.6 to 50.8 g, adductor muscle DW from 2.8 to 24.9 g,
gonad DW from 0.5 to 17.6 g, the remaining DW from 3.4 to
16.0 g and SW from 63.0 to 258.2 g.
Condition index and GSI
To measure the physiological activity and gonadal development
of the pen shell, the condition index (CI) and GSI were calculated with the following formulae: CI ¼ total dry tissue weight
(total DW)/dry shell weight (SW) (Walne, 1976; Mann &
Glomb, 1978) and GSI ¼ gonad DW/total DW.
Biochemical analysis and standard animal
Protein was extracted with 0.5 N NaOH at 30 8C for 24 h, and
the protein content was measured by the Folin phenol method
(Lowry et al., 1951). Lipid was extracted with a mixture of
chloroform and methanol (Bligh & Dyer, 1959) and the lipid
content was determined spectrophotometrically by the method
of Marsh & Weinstein (1966). To measure the carbohydrate
and glycogen contents, extraction was performed using 15% trichloroacetic acid and glycogen was precipitated by adding 99%
ethanol. Carbohydrate and glycogen contents were then estimated using the colorimetric method of Dubois et al. (1956).
The ash-free DW was measured after ignition at 450 8C for 4 h
in a muffle furnace.
Least-squared linear regression analyses following logarithmic
transformation (base 10) were performed on each sampling
occasion according to the allometric equation: Y ¼ aWb, where
Y ¼ total DW, W ¼ SW, and a and b are fitted constants representing the intercepts and slopes, respectively, of the regression
equations. The whole set of regression equations obtained on
each sampling occasion was analysed by analysis of covariance
(ANCOVA) to test the significance of differences in slope (Sokal
& Rohlf, 1995) and significant differences between estimates of
slopes were tested at a probability of P , 0.05. Then, to evaluate
the physiological state of A. japonica independent of its growth,
the absolute value for total DW was standardized to that of an
individual of 113.6 g SW. This overall mean value of SW of all
specimens analysed in this study was chosen as a standard
animal. Our observed total DWs were adjusted to a standard
animal by the regression slopes of total DW on SW (Packard &
Boardman, 1987). This allowed us to remove variation in total
DW stemming from allometric variation between body size and
total DW. The adjusted total DW values for a standard animal
in each month were compared. The relationships between the
weights of the biochemical components and the total DW were
also analysed by the same procedure; all regressions were statistically significant (P , 0.01). Gross biochemical composition
was then calculated for a given SW by substituting the appropriate values of DW in the regression equations. The results of the
biochemical analysis are expressed in grams per standard
CI and GSI
The CI of A. japonica showed a clear seasonal pattern (Fig. 3).
The CI peaked in June 2010 and May 2011, and then declined
Figure 2. Seasonal variations in water temperature and salinity (A),
chlorophyll a (chl a) concentration (B) and suspended particulate
matter (SPM) and particulate organic matter (POM) (C).
504
ENERGY STORAGE IN ADDUCTOR MUSCLE OF ATRINA
Table 1. Biometric measurements of sampled individuals of the Atrina japonica population.
Month
L
W
H
TDW
ADW
GDW
RDW
SW
June 2010
22.6 – 26.8
11.2 –14.5
4.5– 5.7
15.4 –30.8
6.8 – 12.4
2.7– 10.7
4.4 – 7.7
83.6 –139.0
July
24.0 – 30.0
14.3 –17.0
4.5– 6.6
14.3 –48.7
6.0 – 23.8
1.6– 17.6
5.6 – 12.8
89.4 –215.7
August
23.1 – 26.2
12.1 –14.6
4.9– 6.0
14.3 –48.7
5.0 – 10.8
1.0– 2.0
5.0 – 7.4
83.4 –123.3
September
21.8 – 29.1
11.8 –14.3
4.2– 6.0
5.1 –22.9
1.3 – 12.7
0.5– 2.0
3.4 – 8.9
80.9 –171.0
October
22.3 – 27.2
12.2 –15.7
1.5– 5.9
10.3 –21.9
4.6 – 12.4
1.0– 2.2
4.3 – 9.1
72.4 –127.0
November
23.4 – 26.5
11.4 –14.0
5.1– 6.3
12.4 –25.4
5.2 – 14.2
0.5– 3.1
5.7 – 9.9
90.9 –177.7
December
22.3 – 26.5
12.1 –13.7
4.9– 6.0
12.3 –25.5
4.8 – 14.3
1.3– 3.0
6.0 – 9.1
90.0 –144.2
January 2011
18.7 – 24.8
10.1 –11.8
3.6– 5.6
6.6 –13.9
2.8 – 6.6
0.9– 1.9
2.8 – 5.5
41.4 –91.4
February
22.5 – 26.5
12.5 –14.9
4.0– 6.0
11.2 –21.4
4.5 – 11.4
1.5– 2.4
5.2 – 8.3
76.1 –114.9
March
22.8 – 26.0
12.3 –14.6
4.6– 6.0
11.4 –22.2
3.6 – 11.6
1.1– 2.9
5.3 – 8.1
67.5 –185.4
April
23.1 – 29.2
12.7 –16.5
4.4– 7.5
16.5 –50.8
6.4 – 24.9
2.6– 12.4
5.8 – 16.0
72.4 –258.2
May
22.8 – 27.2
13.0 –15.0
4.7– 6.5
16.5 –50.8
8.5 – 15.2
3.1– 13.3
6.3 – 11.0
86.3 –193.2
June
20.8 – 25.7
11.6 –14.8
4.1– 5.7
11.8 –25.1
4.6 – 12.1
2.0– 6.1
4.0 – 8.0
63.0 –118.3
Abbreviations: L, shell length; W, shell width; H, shell height; TDW, total dry tissue weight; ADW, adductor muscle tissue weight; GDW, gonad tissue weight; RDW,
remaining tissue weight; SW, shell dry weight (g).
Figure 4. Seasonal variations in gonadosomatic index (GSI) of Atrina
japonica.
Figure 3. Seasonal variations in condition index (CI) of Atrina japonica.
rapidly. Seasonal variations in the GSI were also very clear and
displayed a similar pattern to that observed for the CI (Fig. 4).
Both the CI and GSI increased abruptly from March to May
2011, indicating gametogenesis during the spring period. The
GSI peaked in spring (July 2010 and May 2011) and declined
sharply in July to August 2010 and May to June 2011, respectively. This sudden decrease in the CI and GSI in summer indicated that the major spawning occurred during that time.
Standardized gross biochemical composition
Seasonal variations in gross biochemical composition of a standard animal showed different patterns between organs as well as
between years (Fig. 6). Protein was the major biochemical component of the three organs (constituting 45.1– 84.6%). Seasonal
fluctuations in protein weight in the adductor muscle and the
gonad showed a clear seasonal cycle that was consistent with the
gametogenic and spawning cycles. Protein weight in the adductor muscle increased steadily during the spring in both years and
peaked in July 2010, although the peak occurred in May 2011,
which was 2 months earlier than in 2010. The protein weight
decreased suddenly in summer. Protein weight in the gonad also
increased rapidly during early spring in both years and peaked
at the same time as did the protein weight in the adductor
muscle, and then decreased sharply. By contrast, seasonal variation in protein weight in the remaining tissues was less clear,
even during the critical period when gametogenesis and spawning were taking place. The maximum protein weights in the
remaining tissues were recorded in autumn and winter.
Seasonal patterns of carbohydrate and glycogen weight differed between the organs of A. japonica. In addition to protein,
carbohydrate (including glycogen) was a major biochemical
component (11.3 –28.7%) in the adductor muscle and glycogen
accounted for most of the carbohydrate. The seasonal pattern of
carbohydrate weight in the adductor muscle of a standard
animal was similar to that of protein: increasing in spring,
peaking in July 2010 and April–May 2011, and declining sharply
Standardized DW
The ANCOVA test revealed significant differences in the slopes
of the monthly allometric equations relating total DW to SW of
A. japonica (F12, 252 ¼ 6.295, P , 0.001), and the original slopes
and y-axis intercepts in each month were used to calculate the
DW of each biotic component of a standard animal (mean total
DW ¼ 113.6 g; Table 2). The total DW peaked in spring (July
2010 and May 2011) and this peak was followed by a sudden
decline in summer (Fig. 5). Fast growth in total DW was
observed during spring (June –July 2010 and March –May
2011). Seasonal fluctuation in the adductor muscle DW was coincident with that of the whole tissues; both displayed peaks in
spring in both years. Gonadal DW also increased abruptly at the
same time ( peaking in spring) as did the total DW and adductor
DW (July 2010 and May 2011), and remained at a low level
during the rest of the year. Seasonal fluctuations in DW of the
remaining tissues were less clear, as shown by the coefficient of
variation values (10.8 vs 17.4 –75.9 in other organs).
505
Y.-J. LEE ET AL.
Table 2. Allometric equation Y ¼ aWb between dry tissue weight (Y, g) and dry shell weight (W, g) of Atrina japonica.
Month
n
TDW
ADW
GDW
RDW
a
B
r
a
b
r
a
b
r
a
b
r
June 2010
19
1.140
0.604
0.888
0.686
0.533
0.853
0.110
0.821
0.845
0.517
0.515
0.826
July
20
0.482
0.843
0.940
0.116
0.958
0.873
0.339
0.675
0.870
0.132
0.836
0.944
August
18
1.448
0.518
0.869
0.611
0.565
0.853
0.329
0.337
0.817
0.593
0.499
0.777
September
20
0.024
1.321
0.877
0.001
1.882
0.843
0.020
0.874
0.881
0.114
0.823
0.814
October
20
0.114
1.069
0.861
0.029
1.218
0.837
0.015
1.001
0.822
0.096
0.907
0.834
November
18
0.795
0.666
0.901
0.293
0.739
0.860
0.089
0.628
0.837
0.477
0.573
0.851
December
19
0.584
0.726
0.869
0.186
0.813
0.853
0.068
0.700
0.817
0.382
0.631
0.834
January 2011
20
0.230
0.918
0.898
0.045
1.117
0.864
0.109
0.594
0.802
0.156
0.805
0.836
February
19
0.791
0.672
0.860
0.487
0.617
0.779
0.425
0.321
0.829
0.155
0.834
0.838
March
20
1.821
0.491
0.930
0.664
0.559
0.840
1.039
0.148
0.805
0.657
0.504
0.961
April
20
0.879
0.713
0.932
0.224
0.838
0.934
0.542
0.501
0.812
0.331
0.672
0.914
May
20
2.159
0.556
0.905
0.903
0.545
0.861
0.713
0.553
0.800
0.540
0.576
0.783
June
20
0.059
1.251
0.901
0.020
1.319
0.840
0.005
1.438
0.840
0.051
1.050
0.851
Abbreviations: TDW, total dry tissue weight; ADW, adductor muscle tissue weight; GDW, gonad tissue weight; RDW, remaining tissue weight.
the remaining tissues and therefore exhibited no clear seasonal
fluctuations.
Lipid weight of a standard animal stayed at a very low level in
the adductor muscle and the remaining tissues and showed no
clear seasonal patterns. By contrast, lipid weight in the gonad
showed a clear seasonal fluctuation that was consistent with that
of protein. Lipid weight in the gonad increased simultaneously
with the accumulation of protein in the spring of both years and
peaked in July 2010.
Energy content
The adductor muscle had the highest energy content among the
three organs measured; the energy value in the adductor muscle
of a standard animal ranged from 86.9 to 259.9 kJ with two
peaks in July 2010 and May 2011 (Fig. 7). The energy value of
the adductor muscle decreased by 131.5 kJ during the spawning
period in July–September 2010 and increased by 60.5 kJ during
the gametogenic period in March –May 2011. Seasonal fluctuation in the energy value in the gonad was consistent with that
in the adductor muscle: ranging from 21.3 to 176.2 kJ and
peaking in July 2010 and May 2011. In contrast to the adductor
muscle, the energy value in the gonad remained at the lowest
level during autumn and winter. The energy value of the gonad
decreased by 126.2 kJ during the spawning period July–
September 2010 but increased by 138.2 kJ during the gametogenic period March –May 2011. The seasonal pattern of energy
value in the remaining tissues was unclear and ranged from 75.6
to 166.2 kJ even during the critical periods.
Kendall’s rank correlation matrix did not show direct relationships between biometric components and particular environmental parameters (Table 3). By contrast, significant correlations
between CI, GSI and biochemical components in the adductor
muscle and gonad were found. The gross weight of protein in the
remaining tissues and most of the biometric components did not
correlate significantly.
Figure 5. Seasonal variations in dry weight (DW) of whole tissue
(A), adductor muscle (B), gonad (C) and the remaining tissues (D)
normalized to a standard individual of 113.6 g shell dry weight (SW).
The vertical bars represent 95% confidence intervals.
DISCUSSION
Reproductive cycle
in summer. The maximum weights of carbohydrate and glycogen
in the adductor muscle of a standard animal were slightly higher
in April–May 2011 (3.4–3.3 and 2.8–2.3 g, respectively) compared with July 2010 (2.4 and 1.8 g, respectively). By contrast,
levels of carbohydrate and glycogen were low in the gonad and
The seasonal cycle of the GSI in the present study clearly indicated that the reproductive pattern of Atrina japonica is characterized by a year-long unimodal cycle. As indicated by a sudden
decrease in the GSI value, the pen shell spawned once a year, in
506
ENERGY STORAGE IN ADDUCTOR MUSCLE OF ATRINA
Figure 6. Seasonal variations in gross weights of biochemical components (A, adductor muscle; B, gonad; C, the remaining tissues) in a standard
individual of Atrina japonica of 113.6 g dry shell weight. CHO, carbohydrates. The vertical bars represent 95% confidence intervals. The white circles
indicate that the glycogen contents of most individuals analysed were below the detection limit.
July–August 2010 and May–June 2011. The GSI of the pen
shell peaked at 30.6% in July 2010 and 32.6% in May 2011.
These values are likely to point to full ripeness and readiness for
spawning. The peak values of the GSI of the pen shell immediately before spawning were similar to those observed for other
bivalve species. For example, Crassostrea gigas undergoes spawning when the egg mass becomes 40% of the total DW in
Goseoung Bay on the south coast of Korea (Kang et al., 2003).
Crassostrea virginica initiates spawning when the egg mass
accounts for 20% of the body weight in Galveston Bay, Texas
(Choi et al., 1993). Accordingly, our result suggests that the pen
shell may be ready for spawning when its GSI value reaches
around 30%.
It has commonly been observed that the gametogenesis of A.
japonica is initiated in winter when the water temperature is
lowest in the temperate zone (Baik et al., 2001; Chung et al.,
2006). In the present study, an abrupt increase in gonadal mass
was observed in June –July 2010 and March –May 2011. These
periods correspond to the time when mature and ripe gonads are
found (Chung et al., 2001). Although a direct correlation in the
rank correlation matrix between temperature and GSI was undetectable because spawning occurs in the middle of the warm
period, it is obvious that both the gonadal development and the
subsequent increase in gonadal mass were related closely to the
rising temperature during the spring. Such rapid gametogenic
development during the warm temperature of spring has been
well documented in other bivalve populations on the southern
coast of the Korean peninsula (e.g. C. gigas, Kang et al., 2000,
Ngo et al., 2006; Scapharca broughtonii, Park, Kang & Lee, 2001;
S. subcrenata, Park et al., 2011). Several studies have demonstrated
that rising water temperature accelerates gonadal maturation in
bivalves (Mann, 1979a; Ruiz et al., 1992a; Rodrı́guez-Jaramillo
et al., 2001; Chávez-Villalba et al., 2002).
Gametogenesis of bivalves is considered a highly energydemanding process (Ruiz et al., 1992b). In the present study,
Kendall’s rank correlation analysis showed a close relationship
between the GSI and CI (and total DW) of the pen shell. In
particular, synchronous increase in the GSI and gross weight
including the protein and carbohydrate contents of the adductor
muscle was also found in spring (Figs 5 and 6). No energyreserve mobilization from the three analysed organs to the
gonadal tissues was observed during this time (Fig. 5). This
result indicates that the energy from recently ingested food is utilized to fuel both the production of gametes and the growth of
the adductor muscle. This allows us to categorize the pen shell
into a reproductive group using an opportunistic strategy rather
than a conservative one (Bayne, 1976; see Introduction).
Temperature may have a great influence on the gametogenesis of bivalves by controlling the physiological state, as mentioned above. However, although rapid production of gametes
and spawning of the pen shell were concentrated in spring in
Deukryang Bay, spawning took place at different temperatures:
22.8 –25.0 8C in 2010 and 14.9 –18.3 8C in 2011. This result suggests that temperature is not the only determinant of gametogenesis in the pen shell. Similar marked difference in temperature at
spawning has been reported for Mytilus edulis (Hawkins et al.,
507
Y.-J. LEE ET AL.
1985), Cerastoderma edule (Navarro et al., 1989), Ruditapes philippinarum (Park & Choi, 2004) and S. subcrenata (Park et al., 2011).
In addition, temporal discrepancies in the reproductive cycle of
M. edulis and differences in gonadal proliferation rate of C. gigas
have been reported for different populations in similar thermal
conditions (Newell et al., 1982; Kang et al., 2000). These authors
demonstrated that variations in the reproductive cycle of the
bivalves are related to temporal or spatial fluctuations in food
availability. Considering that the pen shell uses an opportunistic
strategy for gametogenesis, it may be necessary for it to assimilate sufficient nutrients to allow synthesis of gametes in spring, as
previously reported for C. edule (Navarro et al., 1989) and Ostrea
edulis (Ruiz et al., 1992b).
The accumulation and utilization of energy reserves in the
bivalve tissues reflect its nutritional status and the consequent
energy dynamics under ambient environmental conditions
(Bayne, 1976; Newell & Bayne, 1980). Accordingly, the reproductive cycle of A. japonica in Deukryang Bay is examined below
according to the temporal patterns of the storage or utilization
of reserve materials in the different organs.
Gross biochemical composition in different tissues
The tissue energy of A. japonica in Deukryang Bay was stored
mainly in the form of protein in all organs. Seasonal variation in
total DW of whole tissues was related directly to seasonal variation of protein reserves in the adductor muscle and the gonad.
Proteins are known to play an important role as an energy
reserve for metabolic maintenance during the energy imbalance
period in other bivalves (Barber & Blake, 1981; Beninger &
Lucas, 1984; Navarro et al., 1989). Seasonal fluctuation of the
protein energy in the adductor muscle accounted for 76.0%
(100.1 kJ) of total energy reduction during the spawning period
of July– September 2010 and 66.4% (40.8 kJ) of total energy increase during the next gametogenic period of March– May
2011, indicating an important role of proteins as an energy
reserve in the pen shell. In contrast, the accumulation of glycogen was negligible in the gonad and the remaining tissues, while
carbohydrate energy fluctuated from 10.6 to 58.1 kJ in the adductor muscle, indicating the importance of this reserve in the
pen shell, as also shown in other bivalves (Pazos et al., 1997;
Soria, Pascual & Cartes, 2002).
Seasonal patterns of the storage or utilization of energy
reserves in bivalves are closely related to food availability
(Navarro et al., 1989; Delgado & Pérez-Camacho, 2007). In the
present study, the tissue energy in the pen shell was replenished
during the short period of autumn after exhaustion by the
summer spawning. This replenishment reflects high food availability in summer in Deukryang Bay, as indicated by the peak in
chlorophyll a concentration of the water column. Phytoplankton
blooms, which occur in response to the heavy rains in the
summer monsoonal period, are a well-known phenomenon on
the southern coast of Korea (Kang et al., 2000; Park et al., 2011).
Figure 7. Seasonal variations in energy storage of each organ (A,
adductor muscle; B, gonad; C, the remaining tissues) in a standard
individual of Atrina japonica of 113.6 g dry shell weight. The numerical
values show the energy variation during the critical period and vertical
bars represent 95% confidence intervals.
Table 3. Kendall’s rank correlation coefficient matrix for temperature (T), salinity (S), chlorophyll a (CHL), condition index (CI), gonadosomatic
index (GSI) and total dry tissue weight (TDW), adductor muscle (A), gonad (G), the remaining tissue (R), gross weights of protein (P), lipid (L),
carbohydrate (C) and glycogen (Gly) of a standard animal of Atrina japonica.
T
T
S
CHL
GSI
CI
S
CHL
GSI
CI
TDW
AP
AC
AGly
GP
GL
RP
0.194
20.152
0.051
0.260
0.039
20.026
0.000
20.128
20.128
20.194
20.026
0.185
20.513*
0.065
0.116
20.090
0.194
0.156
20.013
0.273
0.333
20.452*
0.364
0.303
0.121
0.394
0.443*
0.330
0.415
20.121
0.821**
0.641**
0.410
0.462*
0.658**
0.769**
0.756**
20.026
0.821**
0.487*
0.590*
0.684*
0.897**
0.813**
0.051
0.564**
0.667**
0581**
0.872**
0.785**
0.077
0.538*
0.555**
0.487*
0.414
0.154
0.710**
0.590**
0.613**
0.606**
0.574*
TDW
AP
AC
AG
GP
0.677**
GL
0.103
20.039
0.892**
0.602**
RP
* and ** are significant at 0.01 , P , 0.05 and 0.001 , P , 0.01, respectively.
508
ENERGY STORAGE IN ADDUCTOR MUSCLE OF ATRINA
Our biochemical results also demonstrated that storage of
energy reserve materials differs between the pen shell organs, as
also observed for scallops (Pazos et al., 1997; Uddin et al., 2007).
Indeed, while the energy reserve in the adductor muscle was
stored in the form of protein and carbohydrate (mainly glycogen) during this time, protein alone composed the reserves in the
remaining tissues. By contrast, energy reserves in the gonad
remained at the lowest level and did not increase until the next
spring. One of the most interesting results was the discrepancy in
the variation in energy reserve (and total energy content)
between the adductor muscle and the remaining tissues during
gamete production in spring. While energy reserves (both protein
and carbohydrate) in the adductor muscle increased further
during the spring period of rapid gonadal growth, reserve materials (protein alone) in the remaining tissues decreased progressively during the late winter–early spring when the temperature was
lowest. As mentioned earlier, the pen shell initiates gametogenesis
in winter. The consumption of protein in the remaining tissues
during late winter–spring suggests that these reserves provide the
energy to initiate gametogenesis during the energy imbalance
period. Accordingly, the variation in the total energy content in
the adductor muscle of a standard animal pen shell was quantitatively double that of the remaining tissues.
The increase in the gonadal mass in spring (March –May
2011) was explained by the increase in mainly protein and, in
part, lipid. This result suggests that proteins are the major constituent of pen shell gametes, as observed for other bivalves
(Ruiz et al., 1992a, b; Pazos et al., 1997). Indeed, the reduction
in protein energy during the first spawning period of the pen
shell accounted for 96.1% of the total energy reduction in the
gonad; its accumulation during the next gametogenic period
accounted for 94.0% of the total energy increase. Lipids are also
known to be the principal energy reserve used by bivalve larvae
(Holland, 1978; Utting & Doyou, 1992) and thus constitutes an
important component of bivalve oocytes (Gabbott, 1983; Marin
et al., 2003). Although the levels were low, clear evidence of accumulation of gonadal lipid was found in spring. The water
column was warmer than during the previous winter and the
chlorophyll a concentration peaked during this time, suggesting
that when environmental conditions are favourable (i.e. warm
temperature and high food availability), the pen shell is able to
assimilate sufficient nutrients to allow the synthesis of gametes.
Almost all of the increased protein and lipid contents in the
gonad disappeared during the short spawning period of May–
June 2011 (also July–August 2010). This result indicates that
the increased protein and lipid contents in the gonad during the
spring provide the major constituents of gamete materials
(Holland, 1978; Beninger & Lucas, 1984). Accordingly, the simultaneous increases in the gamete materials ( protein and lipid)
in the gonad and the energy reserves ( protein and carbohydrate) in the adductor muscle of the pen shell confirmed again
that its gamete production and the accumulation of energy
reserves overlap temporally. In addition, although lipid is an important energy reserve in many bivalve species (Holland, 1978;
Ruiz et al., 1992b), the lack of accumulation of lipid in the adductor muscle and the remaining tissues suggests the limited importance of lipid as an energy reserve in the pen shell. By
contrast, rapid consumption of protein and glycogen in the adductor muscle during spawning suggests that these reserves are
used as catabolic substrates during the spawning period, further
indicating the important role of the adductor muscle as an
energy-storage organ for spawning activity in bivalves, as also
observed for scallops (Pazos et al., 1997; Uddin et al., 2007).
2011 (Fig. 4). It has been proposed that a minimum critical
threshold temperature is required for the initiation and completion of gonadal development of bivalves (Mann, 1979b;
Utting & Doyou, 1992; Navarro & Iglesias, 1995). However,
more recent studies have determined that the degrees of
gonadal development of bivalves are the same at different
temperatures and that their gonadal maturation is much
greater at high temperatures when they encounter high food
availability and ingest sufficient food (Navarro & Iglesias,
1995; Navarro et al., 2000; Delgado & Pérez-Camacho, 2007).
The earlier spawning of the pen shell in spring of 2011 in the
present study may have been due to high food availability.
The seasonal variations in total DW and CI reflected that of
GSI. Weight loss by spawning in summer 2010 was compensated partially by the accumulation of reserve materials in the
adductor muscle and the remaining tissues in the autumn.
The recovery of the gross weight of protein during this accumulation period reached its maximum level in the two organs
before spawning. A considerable recovery in carbohydrate
weight was also detected in the adductor muscle. The stored
reserves may be used to overcome a state of energy imbalance
and/or to initiate gametogenesis in winter (Navarro et al.,
1989; Delgado & Pérez-Camacho, 2007).
Subsequent rapid gonadal maturation of the pen shell in
spring may require mobilization of more energy from immediately ingested food. As reported for other bivalves displaying
opportunistic strategies (Ruiz et al., 1992b; Kang et al., 2006;
Park et al., 2011), the accumulation of reserve materials in the
adductor muscle of the pen shell in March – May 2011 indicated that food availability in ambient environments during
the short period is important for rapidly increasing the
gonadal mass. The peak in total energy in the adductor
muscle was much higher in May 2011 than in July 2010, and
the timing of the peak advanced by 2 months in 2011. Rapid
accumulation of reserves and fast growth of gonadal tissues
coincided clearly with the period of maximum chlorophyll a
concentration in 2011.The fast gonadal development of the
pen shell in 2011 probably reflected exposure to a nutritionally more favourable condition during the spring of 2011 compared with 2010 and resulted in the earlier time of spawning.
Similar interannual discrepancies in reproductive cycle
because of variable nutritional conditions have been reported
also for other bivalve species (Newell et al., 1982; Navarro
et al., 1989).
Lastly, another possible explanation for the interannual shift
in the GSI peaks is that the advanced peak in 2011 may reflect
the faster increment in gonadal mass in smaller individuals
(Hofmann et al., 1994). However, although smaller individuals
may have a greater GSI, it is unlikely that differences in body
size can explain the differences in reproductive effort because the
monthly collection of pen shell included individuals with a
similar size range (Table 1).
In conclusion, this study is the first attempt to highlight the
reproductive strategies of A. japonica in relation to its nutrient
storage and consumption in three different organs. This bivalve
may be characterized as an opportunistic species, in which gametogenesis occurs synchronously with the accumulation of energy
reserves during spring. Energy reserves in the adductor muscle
were stored in the form of protein and carbohydrate (mainly
glycogen), which were used as catabolic substrates during the
spawning period. Protein and lipid contents increased in the
gonad during the spring and were the major gamete materials. By
contrast, the accumulation of energy reserves in spring was
unclear in the remaining tissues. Interannual differences in gross
weights of stored reserves and the timing of gonadal development
(and hence spawning) probably reflected different nutritional
conditions in two successive years and the nutritional conditions
during the gametogenic period.
Interannual variation in spawning
The timing of spawning of the pen shell differed between
2010 and 2011 in Deukryang Bay, being 2 months earlier in
509
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ACKNOWLEDGEMENTS
This research was financed by the National Fisheries Research
and Development Institute (grant no. RP-2013-ME-021). The
authors would like to thank associate editor Prof. S. Cragg and
editor Dr David G. Reid for critical and helpful comments on
earlier versions and editing of this manuscript.
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