Reproductive cycle of the subcrenated ark shell Scapharca kagoshimensis (Tokunaga, 1906) in Ariake Bay, Japan.
Article Type: Abstract
Subject: Bivalvia (Physiological aspects)
Aquatic ecology (Research)
Authors: Yurimoto, Tatsuya
Mori, Yuichiro
Ito, Shiro
Maeno, Yukio
Pub Date: 12/01/2008
Publication: Name: Journal of Shellfish Research Publisher: National Shellfisheries Association, Inc. Audience: Academic Format: Magazine/Journal Subject: Biological sciences; Zoology and wildlife conservation Copyright: COPYRIGHT 2008 National Shellfisheries Association, Inc. ISSN: 0730-8000
Issue: Date: Dec, 2008 Source Volume: 27 Source Issue: 5
Topic: Event Code: 310 Science & research
Geographic: Geographic Scope: Japan Geographic Code: 9JAPA Japan
Accession Number: 191646295
Full Text: ABSTRACT The reproductive biology and glycogen content of a subcrenated ark shell Scapharca kagoshimensis (Tokunaga, 1906) in Ariake Bay were investigated from April 2004 to May 2005 in relation to environmental factors such as water temperature, salinity, and plankton settled volume. In the male, onset of gamete development was observed in April. The period of mature testis and partial spawning was from July to September, and the spent stage was first found in September. In the female, gamete development was first observed in April, and continued until July. The matured ovary was seen in July, and partial spawning occurred in August, and the spent stage appeared from August. The gonads could not be distinguished in both sexes from December to Match. Spawning in both sexes occurred during the period of highest water temperature (>25[degrees]C). The condition factor index remained high (>28.0) from April to June and rapidly declined to 18.9 during June and July. Glycogen content gradually decreased from 43.1 mg/g to 20.1 mg/g from April to July 2004 and gradually increased from 34.1 mg/g in January to 62.6 mg/g in April 2005. These results suggested that S. kagoshimensis in Ariake Bay was characterized by a spawning period in summer, monocyclic gametogenesis throughout the year, and an inverse relationship between gonad development and glycogen content.

KEY WORDS: subcrenated ark shell, Scapharca kagoshimensis, gametogenesis, reproductive cycle, glycogen content

INTRODUCTION

At the global level, increased harvesting and demand for various Arcidae species has been identified (FAO 2004a, b). Between 1991 and 2000, a 27% overall increase in catch of ark shells was noted, primarily in South America and Asia. Recently, the species have become important to the fishing industry along the east coast of the United States (McGraw et al. 2001, Power & Walker 2002). Additionally, the aquaculture of arks, primarily Scapharca broughtonii (Schrenck, 1867) and Anadara granosa (Linnaeus, 1758), is an important commercial activity in China, Malaysia, Thailand, and Korea (FAO 2004b).

The subcrenated ark shell Scapharca kagoshimensis (Tokunaga, 1906) is an epifaunal bivalve inhabiting the intertidal to subtidal zones along the coasts of central and southern Japan, Korea, and China. In Japan, the major fishing grounds of subcrenated ark shell are Ariake Bay on the island of Kyushu and the Seto Inland Sea. The coastal of northern Ariake Bay has been widely exploited for subcrenated ark shell culture (Nakamura et al. 2003). Annual production of the species in Ariake Bay was 7,300 tons in 2000; this accounted for 75% of Japan's total production (Nakamura et al. 2003). The shell is therefore commercially one of the most important edible bivalves in Ariake Bay.

In the aquaculture of bivalves a critical element is a regular supply of seed. For successful seed collection from the wild, we need knowledge of the species and the reproductive cycle and spawning patterns of its local populations (Gosling 2003). However, little attention has been paid to the biological characteristics of the subcrenated ark shell, especially in terms of the seasonal changes in gonad development and spawning patterns. The objective of this study was to examine gonad development in the subcrenated ark shell--especially in terms of gametogenic processes and maturation--through histological observations. We also examine the relationship between the reproductive cycle and glycogen content.

MATERIALS AND METHODS

Animals

From April 2004 to May 2005, 10-30 individual subcrenated ark shells Scapharca kagoshimensis were collected monthly from natural stock in Ariake Bay by SCUBA diving at depths of 5-10 m (Fig. 1). Water temperature and salinity near the bottom were measured in situ during sample collection. Plankton settled volume as an index of food availability for the bivalve was estimated from the total plankton collected with a plankton net (mouth diameter: 22.5 cm; side length: 80 cm; mesh size: 0.1 mm), from the water column from the bottom to the surface. Shell length, shell width, shell height, and soft tissue weight of each specimen were measured. Each specimen was dissected to obtain the gonad and soft tissues (mantle, gill, adductor muscle, digestive gland, and foot). The gonad was fixed in 10% seawater formalin solution for histology. The soft tissues were frozen, and stored at -40[degrees]C until glycogen content analysis.

Histology

A total of 149 subcrenated ark shells ranging from 17-50 mm in shell length were examined histologically to determine the profile of gametogenesis and reproductive cycle. The fixed gonads were dehydrated in an alcohol series and embedded in paraffin wax. Four micrometer-thick sections in each gonad were routinely stained with Mayer's hematoxylin and eosin (H&E).

Condition Factor (CF) Index

All specimens collected monthly were measured for the shell length, shell height, shell width, and soft tissue weight. The condition factor {CF = [Soft tissue weight (g)/ Shell length (cm) x Shell height (cm) x Shell weight (cm)] x 100} index was calculated in accordance with the method used in a previous study (Toba & Miyama 1991).

[FIGURE 1 OMITTED]

Glycogen Content

Glycogen content of the soft tissue used for CF measurement was determined by the anthrone method (Kamada & Hamada 1985). A piece of the tissue (0.2-0.5 g) was removed and suspended in 1.5 mL of 30% KOH, then saponified by boiling to 100[degrees]C for 20 min. After the boiling, 2 mL of 90% ethanol and 0.25 mL of saturated sodium sulfate was added to the sample solution. Glycogen in the sample was settled by centrifugation (x3,000g, 5 min.) and diluted with deionized water. Each sub sample was added to a one-fifth volume of anthrone-sulfuric acid solution and boiled to 100[degrees]C for 15 min. After cooling, absorbance of the resulting colored complex was measured at a wavelength of 620 nm. Comparisons of the glycogen content in each tissue were analyzed by the one-way ANOVA and Tukey test with [alpha] set at P < 0.01.

RESULTS

Environmental Parameters

Monthly water temperature and salinity values at the time of sample collection are given in Figure 2. A seasonal change in water temperature was observed, peaking in summer (28.4[degrees]C in July 2004) and decreasing gradually until its lowest point winter (7.8[degrees]C in February 2005). The salinity remained relatively stable throughout the year (27.4-30.1 PSU), with the exception of a fluctuation from May to July because of heavy rainfall. Plankton settled volume showed a clear seasonal pattern characterized by three unequally sized peaks (Fig. 3). Peaks were seen in July (32.1 mL/[m.sup.3]) and September (28.8 mL/[m.sup.3]) 2004, with a large one in January to February (31.0-33.2 mL/[m.sup.3]) 2005. In early spring and late autumn, plankton settled volume remained low.

Shell

The gonad of the subcrenated ark shell is not a discrete organ bur is arranged irregularly from foot to digestive gland; it is therefore difficult to determine gonad development from external observation. Macroscopic observations showed only that during the spawning period male gonads were whitish, whereas females had orange to red gonads. Gonad color is therefore an unreliable indicator of sex.

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Gametogenesis

The testis and ovary were composed of several acini surrounded by connective tissue. In males, examination of nuclear characteristics and cell size revealed that the germ cells could be classified into four stages: spermatogonium, spermatocyte, spermatid, and spermatozoa. The spermatogonia were attached largely to the acinar wall. The cells were 6-8 [micro]m in diameter, spherical or oval, with a nuclear diameter of about 3 [micro]m (Fig. 4 A). The cytoplasm was stained lightly with hematoxylin, and the nucleolus was distinguishable within the nucleus. The spermatocytes were almost round and 4-5 [micro]m in diameter (Fig. 4 A). The nucleus was stained with hematoxylin, and was 2 [micro]m in diameter. The spermatids were smaller than the spermatocytes and were located close to the lumen (Fig. 4 B); their nuclei were spherical and about 1.5-2 [micro]m in diameter. The spermatozoa were spherical and 1 [micro]m in diameter. The heads of the spermatozoa were separated from the trabeculae and aligned in rows (Fig. 4 C). The chromatin was small and completely condensed.

In females, early-stage oocytes were attached to the acinar wall and mature oocytes were located in the lumen. Analysis on the basis of cell size and morphology revealed five stages of female germ cell: the oogonium and four stages of oocyte. The oogonium was round and 7-8 [micro]m in diameter, and the cells were attached to the inner side of the acinar wall (Fig. 5 A). The previtellogenic oocyte was 10-20 [micro]m in diameter, and the nucleus was round and 8 [micro]m in diameter (Fig. 5 B). The nuclei contained chromatin, which was dispersed throughout the nucleus, and the cells were attached to the acinar wall. The early vitellogenic oocyte was 25-50 [micro]m in diameter, with a nuclear diameter of 10-25 [micro]m (Fig. 5 B). The cells were still attached to the acinar wall. The late vitellogenic oocyte was larger (more than 50 [micro]m in diameter), with a nuclear diameter of about 30 [micro]m (Fig. 5 C); these cells were round or polygonal. Numerous yolk granules were dispersed throughout the cytoplasm and the nucleolus was clearly visible. The cells were completely detached from the acinar wall. The matured oocyte before spawning had numerous yolk granules and no apparent nucleus: that is, germinal vesicle breakdown was observed (Fig. 5 D). The size of the cell was similar to that of the late vitellogenic oocyte.

Reproductive Cycle

The reproductive cycle of the subcrenated ark shell was examined by histological observation of the gonadal changes. There were five stages of gonad maturation-immature, developing, mature, partial spawning, and spent-in the male and female (Fig. 6).

Immature Stage

This stage was characterized by no traces of gametes; connective tissue filled virtually all the space (Fig. 7). Thus, from December to March the sex of the subcrenated ark shell was unidentifiable.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Male

Developing Stage

The gonad was characterized by expansion of the acinar walls and the appearance of spermatogonia along the walls (Fig. 8 A). Primary spermatocytes proliferated rapidly and moved to the center of the lumen. In the late period of the developing stage, the size of the lumen increased and spermatids were sometimes present in the lumen, but not abundant. The developing stage of the subcrenated ark shell occurred from April to July (Fig. 6).

[FIGURE 6 OMITTED]

Mature Stage

The spermatids and spermatozoa occupied most of the area of the lumen, and arranged themselves in radial rows toward the center of lumen (Fig. 8 B). The mature stage was observed from July to September (Fig. 6).

Partial Spawning Stage

Spermatozoa were released into the water, and the gonad still contained spermatids and spermatozoa in the lumen. The acinar wall was wrinkled and partly collapsed (Fig. 8 C). The partial spawning stage was observed from July to September (Fig. 6).

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Spent Stage

Mature spermatozoa were completely discharged and the acinar wall was almost empty. A few residual spermatozoa were seen around the wall (Fig. 8 D). The wall had become wrinkled or degenerated and the gonad was obviously smaller. The spent stage was first observed in September; all shellfish examined had spent gonads in October, and the gonads continued to be spent until late November (Fig. 6).

Female

Developing Stage

The ovary consisted of a capsule-like structure, which contained oogonia and oocytes attached to the trabeculae (Fig. 9 A). The developing stage was observed from April to July (Fig. 6).

Mature Stage

In the mature ovary, there were abundant late oocytes and matured oocytes. Most oocytes were polygonal (Fig. 9 B). The mature stage in the female occurred from July to August (Fig. 6).

Partial Spawning Stage

The nucleus had sometimes disappeared owing to germinal vesicle breakdown. The follicles of the ovary appeared partly empty, indicating that mature oocytes had been released (Fig. 9 C). The follicles contained mostly late vitellogenic oocytes and matured oocytes. Partial spawning occurred in August (Fig. 6).

Spent Stage

Mature oocytes were completely discharged and the follicles were almost empty (Fig. 9 D). The follicular wall had become folded or degenerated and the gonads had decreased in size. The spent stage of the ovary occurred from August to November (Fig. 6).

Condition Factor (CF) Index

We examined annual changes in CF index in the subcrenated ark shell (Fig. 10). The mean CF stayed relatively high from April to June 2004 (28.0-29.1) and from November 2004 to May 2005 (24.4-30.4). The values rapidly declined to 18.9 during June and July, and then gradually increased from July to November.

Glycogen Content

Analysis of the glycogen contents of the organs (mantle, gill, adductor muscle, digestive gland, and foot) of the subcrenated ark shells in April 2004 is shown in Figure 11. The mean glycogen content (62.6 mg/g) of the foot was significantly higher than those in the other organs examined. The monthly change in the glycogen content of the foot is shown in Figure 12. In 2004, the mean of glycogen content decreased from April (43.1 mg/g), and the minimum value was observed in July (20.1 mg/g). The content then reached a high level in August (36.1-40.9 mg/g) and declined again in September (22.4 mg/g). It then stayed between 34.1 and 40.2 mg/g from October to January 2005, gradually increasing from January onward. The glycogen content peaked at 62.6 mg/g in April 2005.

DISCUSSION

Bivalves undergo an annual reproductive cycle that involves a period of gametogenesis followed by either a single, extended spawning event, or several events. These, in turn, are often followed by a period of gonad reconstitution (Gosling 2003).

In the present study, histological observations of the gonads of subcrenated ark shells in Ariake Bay revealed that gametogenetic development of the male and female commenced in April, when the water temperature had reached 17[degrees]C. The gametes grew until June as the water temperature increased. Mature gonads and partial spawning appeared from July to August at high water temperatures (>25[degrees]C), and in October completely spent gonads of both sexes were found. Our histological observations revealed that gametogenesis of the subcrenated ark shell was characterized by a unimodal cycle in relation to water temperature, and that the spawning period of this species in Ariake Bay is from July to August. The CF stayed high from April to June, when the gonads were developing and matured. The CF then decreased from July to August during the spawning period. Thus, CF was well correlated with gonad development in both sexes.

[FIGURE 9 OMITTED]

In bivalves, changes in temperature, salinity, or food availability have been considered to be physical, exogenous factors influencing the reproductive cycles (Gosling 2003). In previous reports, the spawning period of the subcrenated ark shell has been determined to be June to October in Ariake Bay (Tanaka 1954) and from June to December in Kasaoka Bay in the Seto Inland Sea (Ting et al. 1972). Functional activity of the gonads was not clarified by Tanaka (1954), because the timing of spawning was determined speculatively from the color of the gonads. The spawning period of subcrenated ark shells in our study was shorter than that in Kasaoka Bay (Ting et al. 1972). The water temperature increased slowly from spring to summer, and the duration of high water temperature in Kasaoka Bay is shorter than that in Ariake Bay. The difference in spawning period length at the two sites may reflect the maintenance of gonad activity by exogenous factors, especially the influence of low temperatures in causing late termination of the spawning period.

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Gametogenesis is an energy-demanding process for which the main energy reserve in bivalves is glycogen (Gosling 2003). However, the main storage tissue differs among species. For example, it is the mantle in the mussel Mytilus edulis (Linnaeus, 1758) (Bayne et al. 1982), the digestive gland in the oyster Crassostrea gigas (Thunberg, 1793) (Yamamura & Watanabe 1964, Mori et al. 1965), and the adductor muscle in the scallop Patinopecten yessoensis Jay 1857 (Miyazono & Nakano 2000) and pen shell Atrina pectinata (Linnaeus, 1767) (Yurimoto et al. 2005). Our results revealed that the subcrenated ark shell accumulated glycogen mainly in the foot, which is therefore a suitable tissue for use as an indicator of nutritional composition.

Gametogenesis in marine bivalves occurs at the expense of recently ingested food and/or energy stored in various tissues. In the subcrenated ark shell, the glycogen content remained high from winter to early spring. Gonad development from spring to early summer took place at the expense of the glycogen accumulated previously. This was reflected in the inverse relationship between gonad development and glycogen level and is in agreement with previous data concerning other bivalves, such as Argopecten irradians concentricus (Say, 1822) (Barber & Blake 1981), Argopeeten purpuratus (Lamarck, 1819) (Martinez 1991) and M. edulis (Zandee et al. 1980). These findings also suggested that carbohydrate played an important role as a fuel for gametogenesis in S. kagoshimensis. Interestingly, high glycogen content was maintained during the spawning period, and large numbers of developing oocytes were often present in the subcrenated ark shells after spawning. This suggests that the species spawned more than once during the spawning period, but further work is required to gather information on spawning periodicity in this species. Although there have been many studies of the reproductive cycles of species of Arcidae (Broom 1983, Ito et al. 1998, Park et al. 2001, Power & Walker 2002, Walker & Power 2004, Power et al. 2004, Peharda et al. 2006, Lista et al. 2006), we have had only a poor understanding of the relationship between reproduction and its energy costs. These results indicate that glycogen level and CF are simple and reliable indicators of the reproductive status of S. kagoshimensis in Ariake Bay. Our observations of the reproductive characteristics of S. kagoshimensis should provide a valuable insight into the biology of this species and are crucial for broodstock management and seed production, as well as for the sustainable management of wild stock.

ACKNOWLEDGMENTS

The authors thank Drs M. Minagawa and Y. Kotani of the Seikai National Fisheries Research Institute, Fisheries Research Agency, for their critical comments on the manuscript.

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TATSUYA YURIMOTO, (1) * YUICHIRO MORI, (2) SHIRO ITO (3) AND YUKIO MAENO (1)

(1) Seikai National Fisheries Research Institute, Fisheries Research Agency, Taira, Nagasaki, Nagasaki 8512213, Japan; (2) Saga Prefectural Ariake Fisheries Experimental Station, Ashikari, Saga 8490313, Japan; (3) Saga Prefectural Fisheries Division, Jonai, Saga 8408570, Japan

* Corresponding author. E-mail: yurimoto@fra.affrc.go.jp
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