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Abstract

The reproductive cycle of Ruditapes decussatus was studied to assist with understanding the basic biology of this potential candidate for bivalve aquaculture in Egypt. Clams from three sites along the Egyptian coast (near Damietta, Ismailia and Alexandria) were examined over a year. Female:male sex ratio was 1:1.02. There were no significant differences in Mean Gonadic Index (MGI) between males and females or between different locations confirming a simultaneous life cycle in Egyptian waters. Active gametogenesis in females began in January-February and in males in December-January. MGI showed one peak of ripe clams (75-100% ripe) in March for both males and females confirming the beginning of the spawning season, except for Damietta in which the peak of ripe females was a month later (April, 67% ripe). MGI decreased from April till November reflecting partial spawning and gamete release. The onset of spawning for both sexes started in April at a temperature of more than 18°C in the three sites. Spawning peaks were in May in Ismailia males and simultaneously in June for all females in all sites and males of Alexandria and Damietta . Earlier gametogenesis and spawning in Ismailia was attributed to higher food availability. From September to November, most females were classified as either spent or at resting stage while spawning continued till November for males. Size of 50% maturity was 18.03 and 18.82 mm shell length for females and males, respectively. Clam broodstock (≥ 18 mm shell length) should be collected no later than February for conditioning in the hatchery to avoid natural spawning in the field.

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Seasonal variations in the body weight and biochemical composition of the bivalve Ruditapes decussatus were studied over a period of 15 months. Separate analyses were made of foot, mantle edge, siphons, gills, adductor muscle and gonad-visceral mass. Variations in weight, body growth, gonad growth and spawning depended on environmental conditions, especially food availability. The gametogenic cycle comprised two phases: a resting phase (November–December) and gametogenesis, including ripeness and spawning, during the rest of the year. Gametogenesis usually took place during the spring, and spawning in summer (June–August). The highest variation in biochemical composition was largely attributable to a change in the glycogen content (average 14.7%±5.1 S.D. of dry weight). Protein (48.1%±1.8) and lipid contents (5.6%±0.6) remained relatively constant throughout the year. Adductor muscle, foot and siphons contained mainly proteins (67.8%, 51.2% and 65.8% of the dry tissue weight, respectively) and gonad-visceral mass and gills contained the highest amount of lipids (≈7%). The gonad-visceral mass showed the largest variations in all components during the gametogenic cycle. Lipid and glycogen concentrations in the gonad-visceral mass were inversely related: maximum concentrations of glycogen occurred during the resting phase or initial gametogenesis and corresponded with minimum concentrations of lipids, and minimum concentrations of glycogen occurred at maturity when lipids reached maximum concentrations. The gonad-visceral mass contained lower amounts of proteins (
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This study evaluates the influence of different states of energy balance, positive and negative, on the accumulation and utilisation of energy reserves during the period of gonad development in the clam Ruditapes decussatus (L.). Our results show a clear sexual differentiation in the biochemical composition of the clams. The differences were associated with gonad development and were particularly evident in proteins and lipids. These findings imply that separation of the sexes for studying variations in the biochemical composition of sexually developed specimens is essential, and thus questions the results of previous studies in which both sexes were analysed simultaneously. Gonad development occurs under conditions of both positive and negative energy balance, and permits the development of a model of the utilisation of reserves during gonad development, which is closely linked to the clam's energy balance. Under conditions of positive energy balance, the amount of food ingested provides sufficient energy for both metabolic consumption and the accumulation of reserves. The absolute value of all biochemical components increases, in particular for lipids in the case of female clams, with no evidence of consumption of glycogen and other carbohydrates or their conversion into lipid reserves. Conditions of negative energy balance lead to a moderate degree of organic weight loss in the clams. Glycogen and other carbohydrates are rapidly consumed, falling to almost a quarter of initial values, whilst protein and lipid levels remain constant in females. Lipid values decrease slightly in males. Finally, under situations of extreme nutritive stress (when energy reserves are scarce) there is a loss of all biochemical components in a specific order of priority. Most of the glycogen and other carbohydrates are consumed initially (a decrease of 61 and 74%, respectively), followed to a lesser extent by lipids in males (40%) and then proteins (between 20 and 25%). In females, there was no lipid loss until the majority of the carbohydrates and part of the proteins had been consumed, indicating that lipids constitute the reproductive reserve par excellence in this species.
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Through histological analyses, this investigation reveals an unusual high incidence of hermaphroditism in Megapitaria squalida, a gonochoric species. A total of 684 specimens were sampled at monthly intervals from February 2007 to January 2008 at Bahía de La Paz and Bahía Magdalena, Baja California Sur, Mexico. Microscopic examination showed that 21.8% of specimens from Bahía de La Paz and 23.5% from Bahía Magdalena were hermaphroditic; in both areas, hermaphroditism was found throughout the year, although in variable proportions. In general, a significantly higher number of females relative to males (chi-square, P < 0.05) were observed in both areas. Our findings indicate that this unusual incidence of hermaphroditism was likely caused by the low population density resulting from overfishing.
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A study of the reproductive cycle of the Manila clam, Ruditapes philippinarum, was undertaken at two sites in the northwest of Ireland from February 2003 to May 2004. Histological evidence showed that gametogenesis began in February and most gametes were ripe by May. Spawning began in May and continued until September. Gametes, which were ripe in October and November, were resorbed. Minimum and maximum condition indices were attained in February 2003 and June 2003, respectively, and decreased from June to September, signifying the spawning period. Vesicular cells were evident in the later gonadal stages in males. From December to March, most individuals were either spent or in a gonadal resting period. One hermaphrodite was observed.
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Since the introduction of the Manila clam Ruditapes philippinarum (Mollusca: Bivalvia) to aquaculture clam beds, this species has spread to several sites. As a result, an extensive fishery has developed around this species. Its reproductive cycle and that of a native clam species, Ruditapes decussatus (Mollusca: Bivalvia) from two Brittany sites are examined here and the seasonal variations in the condition index, the scale of maturation stages, the histology and the oocyte size-frequencies of both species are compared. The differences observed between the species and between sites were attributed to environmental conditions. Ruditapes philippinarum showed a more extended breeding period and a greater number of spawning events than R. decussatus. In the Morbihan Gulf, the reproductive activity of R. philippinarum appeared particularly great in terms of number of spawning events, and speed of maturation. For both species, synchronism was more apparent during the first maturation, while spawning, and particularly partial spawning, tended to decrease the level of synchronization.
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Aspects of gametogenesis of the carpet-shell clam, Ruditapes decussatus (L.), in two lagoons on the Atlantic coast of north Morocco were studied during December 1986 to February 1988. The clams sampled did not show any sign of hermaphroditism and the percentage of males was 42%. The gonads started developing in mid-winter and were ripe in spring. Successive spawnings and reconstitution of gametes look place simultaneously starting from May up to the end of September. Two major spawning periods were noticed: a partial one in May-June and a complete spawning in August-September. From October to December the gonads were at resting stages. In coastal ecosystems where freshwater input is a very important factor, gametogenesis may be affected if clams are exposed to low salinities (less than 15%) for prolonged periods.
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The reproductive cycle of a natural population of grooved carpet-shell clam, Tapes decussatus, on the coast of northwest Spain was monitored over a year-long period. Gametogenic activity was characterized on the basis of stereological analysis of the gonads. In both sexes, ripe gametes were continually present and continually released between April and August; spawning efficiency over this period was 60% in females and 83% in males. Again in both sexes, major spawning occurred in August/September (spawning efficiency 94% in females, 86% in males). In parallel with monitoring of gametogenic activity, energy storage was also monitored, by staining the visceral–gonadal mass for glycogen α (rapid-access glycogen), glycogen β (protein-bound glycogen), and acid and neutral lipids. The principal storage areas are muscle tissue, vesicular cells and the digestive gland. Muscle tissue stores and utilizes protein-bound glycogen during the resting period and at the start of gametogenesis, while rapid-access glycogen appears in this tissue only during the gamete ripening period. In vesicular cells, protein-bound glycogen bodies (diameter 1–3 μm, up to 6 per cell) accumulate throughout the resting period; as gametogenesis progresses, these are transformed to triacylglycerides (bodies of about 5–9 μm in diameter, up to 5 per cell), which are subsequently transferred to oocytes (bodies of up to 14 μm in diameter). In the digestive gland, rapid-access glycogen is present throughout the year, while triacylglyceride bodies (diameter 2–15 μm) accumulate over the period of gamete ripening. Acidic lipid inclusions (about 1 μm in diameter, probably corresponding to membrane phospholipids) are observed along the borders of muscle fibres, gonadal follicles, vesicular cells and their nuclei, haemocytes and cells of the gastric diverticles. Haemocytes transport both glycogen fractions, and are observed: (a) between muscle fibres, (b) adhering to the external wall of gonadal follicles and within follicles, (c) adhering to the external wall of gastric diverticles and within diverticles, and (d) adhering to ripening oocytes. Stereological analysis likewise allowed evaluation of seasonal variation in the volume fraction (VF) occupied by vesicular cells. The period of peak vesicular-cell VF (October/November; VF 32% in females, 36% in males, 30% in indeterminate individuals) coincided with that of peak intracellular glycogen levels. Minimum vesicular-cell VF were observed during the gamete ripening period (VF 2–8% in females, 0% in males).
Article
Knowledge of the gametogenic cycle and biochemical composition of commercially valuable species is important in aquaculture. Monthly variations in meat yield, gonadal development and biochemical composition of the bivalve Tapes decussatus were studied from January 2002 to December 2002 in the Sufa Lagoon in relation to environmental conditions. Histological evidence showed that gametogenesis began in January and major spawning started in July and appeared to continue into August and September. The majority of individuals were resorbing their gonad, or alternatively in a period of gonadal inactivity in October, November and December. When the gonad stage was ripe in April, the glycogen value was at the maximum level in contrast to protein which was at the minimum value. The glycogen value was at the minimal level during spawning and resting period. During the late gametogenesis the protein value was the highest in March and the following month when the gonad was ripe, protein was at the lowest. Lipid content fluctuated throughout the study. During the gametogenesis, the meat yield was observed to range between 12.7% and 26.8%. The results of this study showed that six stages of gonadal development of T. decussatus were observed in transplanted area. Seawater temperature was the major factor rather than food availability on gametogenesis of carpet shell clam in the study area.
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