Spawning Stage Research Articles

Spleen histology in the female Ohrid trout, Salmo letnica (Kar.) (Teleostei, Salmonidae) during the reproductive cycle

The Ohrid trout is an endemic autochthonous species dating from the Tertiary period with an unique ecological and commercial value. We still lack the basic knowledge of normal histology of many visceral organs in this species, including the spleen. To tackle this limitation, the investigation was focused on a histological description of the spleen and the influence of the breeding cycle on some aspects of the spleen (the relative volumes of the red and white pulp). Examination by light microscopy showed the same basic structural features of the salmonid spleen: red and white pulp with randomly distributed melanomacrophage centers, surrounded by a fibrous capsule. White pulp comprises about 35-40% of the volume of the parenchyma, 2-fold less than red pulp. During the breeding cycle, we observed a significant increase of white pulp and decrease of red pulp in the spawning stage compared to earlier stages. After spawning, the relative volumes of white and red pulp exhibit a tendency to decrease and increase, respectively. Based on correlations between the amount of white and red pulp and the ovary somatic ratio, we assumed there could be a connection between the sex steroid status and the pulp content in the spleen.

A numerical study of turbulent mixing parameters at the Nile Delta, north of the Egyptian Coast

A study was conducted in Badagry Creek in Lagos, Nigeria on food and feeding habits and reproduction of Frillfin goby (Bathygobius soporator). Data collected from the study intend to serve as a baseline for carrying out further study on the fishes and fisheries of this water body. B. soporator is one of the numerous and diverse fishes of the family Gobiidae which share certain similarities in their life histories. Gobies are among the most successful fishes. They are resident intertidal species which could be found in pools, rocky pools, lagoons, creeks and estuaries. Specimens were collected from the Badagry Creek via Oto-Awori Fish Jetty between January, 2008 and January, 2009. Foods and feeding habits and reproduction were studied in this creek. Seven groups of food items were encountered in the stomachs of the fish, viz; Crustaceans, Pisces, Insects, Detritus, Bivalves, unidentified food materials and Gastropods. The most eaten of these were the crustaceans contributing 34% by number and 38.30% by frequency of occurrence. The presence of other fish species in the stomachs explained the piscivorous habit while the unidentified food materials demonstrate its herbivorous character. Its predatory and carnivorous tendency was exhibited by the presence of insects, bivalves, crustaceans, and gastropods. Presence of sand grains was an indicator of its benthic nature. The specimens were classified as either male or female. Four hundred and seventy and five specimens representing 98.95 and 1.05% of total catch were classified as males and females, respectively giving 1 (male): 0.01 (female) sex ratio. This is significantly different (P>0.05) from the expected or theoretical one male:one female ratio and not in conformity with sex ratios that were reported for some fishes in the adjacent Ologe, Lagos, Lekki and Epe lagoons which favoured more females. The Gonadosomatic index (GSI) of this species varied from 0.00 to 2.89%, meaning less than 2.89% of the fish’s body mass was converted to gonads for reproduction. The pre-spawning and spawning stages of this species contributed 86.75 and 13.25% of the specimens, respectively. Post spawning stages were absent. Key words: Gonadosomatic index, spawning, Ologe, Badagry, Lekki, Epe.

Metabolic adaptations of oxidative muscle during spawning migration in the Atlantic salmon Salmo salar L.

The adaptability/plasticity of the highly oxidative red muscle in Atlantic salmon was demonstrated during spawning migration. Substrate concentrations and the enzymatic pathways of ATP production were examined in red muscle obtained from Atlantic salmon at different sites along their migratory route in the Exploits River, Newfoundland, Canada. Individuals were chronologically sampled from a seawater site, two sites upstream, and at spawning. The 20% decrease in salmon body weight during the later stages of migration was accompanied by large decreases (mg dry weight(-1)) in both glycogen (P < 0.01) and total muscle lipid (P < 0.01). In contrast, water content and protein concentration (mg dry weight(-1)) of the red muscle increased by 25 and 34%, respectively, at spawning. Enzymes of the glycolytic pathways demonstrated a significant (P < 0.001) decrease in maximal activity as migration proceeded whereas enzymes of the oxidative phosphorylation pathways, specifically the citric acid cycle enzymes, exhibited an increase (P < 0.001) in maximal activity at spawning. The antioxidant enzyme superoxide dismutase also demonstrated an increase (P < 0.001) in maximal activity during the latter stages of migration. These adaptations imply that the red epaxial muscle of Atlantic salmon has a more efficient means of oxidizing lipids, while minimizing free radical damage, during the later stages of migration and spawning, thereby potentially increasing post spawning survival.

Changes in Lipid Content of Oocytes of the PenshellAtrina mauraas a Criterion of Gamete Development and Quality: A Study of Histochemistry and Digital Image Analysis

ABSTRACT Tools of histochemistry and digital image analysis were used to quantify changes in the coverage area of lipid droplets (lipid content) of oocytes of the penshell Atrina maura during oogenesis and to determine its relation to changes in water temperature and seston content. These data led to calculating a lipid index as a criterion of gamete development and quality. Gonads were collected monthly for 18 mo and prepared for histochcmical processing with Sudan Black B for identification of lipids. Finished slides were digitized for determining stages of oogenesis and variations in the size of oocytes. Two periods of greatest reproductive activity occurred during the study, with a lower peak from November through January (∼15°C; 26 mg/g) and a major peak from April through June (∼20°C; 25–40 mg/g). Oocyte area significantly varied during the stages of active development (516–2,743 µm2), ripeness (1,073–2,930 µm2), spawning (145–2,939 µm2), and atresia (331–2,001 µm2). Lipid incorporation into oocyte ...

Development and growth of ontogenetically migrating copepods during the spring phytoplankton bloom in the Oyashio region

We have evaluated development and growth of the ontogenetically migrating copepods in the northwestern subarctic Pacific by frequent sampling and by molting rate experiments during the spring phytoplankton bloom. Since different water masses frequently moved into the surface layers at our sampling station, the copepod abundance and species composition fluctuated greatly. Early copepodite stages of Eucalanus bungii and Neocalanus plumchrus appeared abundantly when a warmer and more saline water mass was present. Despite the fluctuating temporal pattern, the population structure revealed that each copepod species had a different life history strategy during the bloom. Eucalanus bungii were in the stages of gonad maturation, spawning and naupliar development when chlorophyll a concentrations were high. The phytoplankton bloom was utilized for development and lipid accumulation by late copepodites of Neocalanus cristatus and Neocalanus flemingeri and for development of nauplii and early copepodites of N. plumchrus. Molting experiments showed that carbon weight of ‘molters’ in the incubations was greater than that of ‘non-molters’, indicating that primarily animals with heavier body weight are those that molt into the next stage. Mean stage durations estimated by the molting rate method were on the order of 9.7 (C3) to 16.6 days (C4) for N. flemingeri, 13.9 (C3) to 29.1 days (C4) for N. plumchrus, and 12.2 days (C2) for E. bungii. Large fluctuations were observed for stage duration estimates, suggesting different development histories (i.e. ages-within-stage distributions) among the replicate incubations. From these results, we discuss development and growth of the ontogenetically migrating copepods during strongly fluctuating hydrographic conditions in the Oyashio.

Changes in trace metal concentrations in the tissues of the White Sea mussel Mytilus edulis over the reproductive cycle

Seasonal changes in the concentrations and total amounts (contents) of Cd, Zn, and Cu in the mussel Mytilus edulis cultivated in the White Sea were studied over the reproductive cycle (prespawning, spawning, and postspawning stages). The results, when compared with published data on the closely related species M. trossulus from the Sea of Japan, suggest that the seasonal dynamics of trace element contents in mussel tissues are related to specific geochemical conditions, as well as to the dynamics of changes in soft tissue weight over the reproductive cycle. The contents of Cd and Cu in the mussels before, during, and after spawning changed similarly in the mollusks from the White Sea and the Sea of Japan. Changes in the Zn content at different stages of the reproductive cycle of the White Sea mussels were similar to those in mussels from the Sea of Japan but had smaller amplitude. The concentration of Zn in the White Sea mussels was before spawning the highest, but still lower than in mussels from the Sea of Japan. After the spawning, the Zn concentration in the White Sea mussels, in contrast to the Pacific mussels, decreased because of the redistribution of this element during the prespawning period from the somatic tissue into the gonad.

갯장어 Muraenesox cinereus의 난소 발달에 따른 혈중 Estradiol-17β 변동

This study correlated changes in the plasma levels of estradiol-17β (E2) with changes in the gonadosomatic index (GSI) and ovarian development during the annual reproductive cycle of the pike eel Muraenesox cinereus, collected at the Tongyung coast region. Ovarian maturity was classified based on histological observations; the perinucleolus stage (November to February), the oil droplet stage (March to April), the early vitellogenic stage (April to May) and the late vitellogenic stage (June to October). Seasonal changes in the GSI were correlated with water temperature and reflected the degree of ovarian maturity. Plasma E2 levels were correlated with changes in the GSI, which increased from April to a peak in July, and the levels remained comparatively high until October. These data indicated that changes in the GSI and plasma E2 levels are correlated with the annual ovarian activity of the pike eel. In this study, however, female pike eels were not collected during the spawning stage. Therefore, spawning of this species seemed to be closely related to its migration toward the deep sea of offshore.

Post-mortem changes of muscle from fresh water prawn ( Macrobrachium rosenbergii) as influenced by spawning stages

Post-mortem changes of the muscle from pre- and post-spawned fresh water prawn ( Macrobrachium rosenbergii) were comparatively monitored during 7 days of iced storage. During the storage, the muscle of pre-spawned prawn had a greater value of trichloroacetic acid (TCA) soluble peptide, heat soluble collagen and pepsin-soluble collagen (PSC) contents than did post-spawned counterpart. Those components in the muscle of both prawns increased markedly after 3 days of storage ( p < 0.05). Conversely, insoluble collagen (ISC) content, shear force value and texture liking of both prawns decreased ( p < 0.05), indicating the softening of muscle. No changes in protein patterns were observed, except the decreased band intensity of 66 kDa protein in water soluble fraction of both prawns was found after 3 days of storage. T max and enthalpy of PSC from both prawns decreased during the first 4 days of storage ( p < 0.05), suggesting the degradation or denaturation of collagen in the muscle. Light microscopic study showed the lowering of intercellular connection of raw meat and higher gaping in cooked meat when the samples were stored for a longer time. Therefore, post-mortem characteristics of prawn muscle was affected by storage time and spawning stages.

Gonadal Sex Differentiation in Cultured Juvenile Flounder, Paralichthys orbignyanus (Valenciennes, 1839)

Gonadal Sex Differentiation in Cultured Juvenile Flounder, <i>Paralichthys orbignyanus</i> (Valenciennes, 1839)

Oocyte development, fecundity and spawning strategy of large sized jumbo squid Dosidicus gigas (Oegopsida: Ommastrephinae)

Ovaries of 39 large jumbo squid Dosidicus gigas (415–875 mm mantle length (ML)) of different stages of maturity from the Gulf of California and Nicaragua were analysed. Oocyte development is asynchronous during female ontogenesis, with predominance (&gt;85–90%) of small protoplasmic oocytes 0.1–0.2 mm in diameter for all stages of female maturity. Potential fecundity (PF) is determined at late immature stage II with average values around 18–21 million oocytes. The maximum PF estimated, 32 million oocytes, is so far the largest recorded for any cephalopod. PF is closely related to female size. Nicaraguan females, of smaller size (415–720 mm ML), show lower fecundity (5–15 million oocytes). Eggs were analysed in oviducts of mature females (540–875 mm ML), 7 from the Gulf of California and three females from Nicaragua. Eggs measured 0.9 × 1.1 mm in diameter and weighed 0.45 mg on average. The oviducts of the largest female (875 mm ML) held 1.2 million eggs, representing 4.6% of PF and no more than 14% of all vitelline oocyte stock of that squid. Variability in oviduct filling is very high and unrelated to ovary weight. However, a strong correlation was found between oviduct fullness and nidamental glands development. These observations suggest that spawning is extended and intermittent. A female spawns no less than half of the initial PF, and the minimal number of spawning activity events (egg batches) is roughly estimated at 8–12. During this terminal spawning stage females continue actively feeding and grow between egg-mass laying periods.

Spawning pattern and batch fecundity of the European hake (&lt;i&gt;Merluccius merluccius&lt;/i&gt; (Linnaeus, 1758)) in the western Mediterranean

The spawning cycle of the European hake (Merluccius merluccius, (Linnaeus, 1758)) was studied in two western Mediterranean areas, the Catalan Sea and the northern Tyrrhenian Sea, including observation of the monthly seasonal variation of the gonad maturity and the gonadosomatic index (GSI). The estimation of the maturity stages by GSI gave similar values in the two study areas: the spawning stage (IV) was easily distinguished from the other maturity stages and its range of variation showed a low overlap with stage III and no overlap with other stages. Although in both study areas active females were present during all the sampled months, the peak of reproductive activity was concentrated from February to May in the northern Tyrrhenian Sea but from August to December in the Catalan Sea, which was subjected to winter cascading events. Batch fecundity gave similar values in the Catalan and northern Tyrrhenian Seas: 204 and 202 eggs per gonad-free female gram, respectively. An asynchronous oocyte development is suggested for M. merluccius in the western Mediterranean.

Escapement goal analysis and stock reconstruction of sockeye salmon populations (Oncorhynchus nerka) using life-history models

We compare life-history models with the Beverton–Holt approach of escapement goal analysis. We model the life history of a sockeye salmon ( Onchorhynchus nerka ) population from a spawning stage, through juvenile and adult stages, and ending with adults that return to spawn. We fit models to data by statistically comparing predicted and observed numbers of four dominant adult ages. Posterior estimates of parameters from Markov chain Monte Carlo simulations are then used to assess optimal harvest policies. We search for policies that produce the highest average yield. We find that it is possible to detect density dependence with a life-history model where analysis of Beverton–Holt stock–recruitment relationship fails to do so. We find that Beverton–Holt relationships produce policies and long-term yield estimates that are inconsistent with empirical trends. Conversely, we find that optimal spawning stock sizes and maximum sustained yield estimates using the life-history model estimate are consistent with the historical behavior of fisheries examined. Adding smolt data to the analysis does not substantially change predicted optimal spawning stock size, but decreases the variance in estimated posterior parameter distributions and policy variable distributions.

Spatial distribution and geomorphic condition of fish habitat in streams: an analysis using hydraulic modelling and geostatistics

AbstractReach‐scale physical habitat assessment scores are increasingly used to make decisions about management. We characterized the spatial distribution of hydraulic habitat characteristics at the reach and sub‐reach scales for four fish species using detailed two‐dimensional hydraulic models and spatial analysis techniques (semi‐variogram analyses). We next explored whether these hydraulic characteristics were correlated with commonly used reach‐scale geomorphic assessment (RGA) scores, rapid habitat assessment (RHA) scores, or indices of fish biodiversity and abundance. River2D was used to calculate weighted usable areas (WUAs) at median flows, Q50, for six Vermont streams using modelled velocity, depth estimates, channel bed data and habitat suitability curves for blacknose dace (Rhinichthys atratulus), brown trout (Salmo trutta), common shiner (Notropis cornutus) and white sucker (Catostomus commersoni) at both the adult and spawn stages. All stream reaches exhibited different spatial distributions of WUA ranging from uniform distribution of patches of high WUA to irregular distribution of more isolated patches. Streams with discontinuous, distinct patches of high score WUA had lower fish biotic integrity measured with the State of Vermont's Mixed Water Index of Biotic Integrity (MWIBI) than streams with a more uniform distribution of high WUA. In fact, the distribution of usable habitats may be a determining factor for fish communities. A relationship between predicted WUAs averaged at the reach scale and RGA or RHA scores was not found. Future research is needed to identify the appropriate spatial scales to capture the connections between usable patches of stream channel habitat. Copyright © 2008 John Wiley &amp; Sons, Ltd.

The ability of gadoids to take advantage of a short‐term high availability of forage fish: the example of spawning aggregations in Barents Sea capelin

During 11 March to 4 April 2002, the distribution of Barents Sea capelin Mallotus villosus along the coast of Finnmark, northern Norway, was covered four times by combining acoustic survey with trawling, synoptically and simultaneously sampling capelin and its main fish predators; cod Gadus morhua, haddock Melanogrammus aeglefinus and saithe Pollachius virens. The surveys demonstrated how these gadoid predators were able to exploit such a short‐term abundance of forage fish. The predator aggregation as well as the stomach fullness and proportion of capelin in their diet followed the capelin spawning migration, increasing in areas and periods with increasing capelin abundance. Capelin clearly constituted most of the biomass in stomachs of cod (97%), haddock (87%) and saithe (96%). The stomach fullness was highest in cod and lowest in haddock, although in areas with low capelin abundance, saithe had more capelin in their stomachs. The total length (LT) of capelin in predator stomachs increased with predator LT, but the proportion of capelin in the diet was not influenced by predator LT. The capelin in predator stomachs was significantly smaller than capelin in the trawl hauls, also when compared within the same sex, indicating feeding selectivity towards weaker individuals. Female capelin, being significantly smaller than the males, predominated in the diet of haddock, whereas in cod and saithe the sex ratio was more equal. Male capelin predominated in the predator diet during the pre‐spawning period, whereas the females predominated as the spawning commenced. During the overall study period, most of the female capelin in predator stomachs was in a pre‐spawning or a spawning stage, whereas the majority of the males appeared to be spent. Regardless of sex, the percentage of spent, relative to pre‐spawning or spawning capelin in the diet of the predators, followed the capelin spawning dynamics, increasing with time as the spawning progressed.

Spawning and fecundity of Jenyns's sprat, Ramnogaster arcuata, a winter spawner in the temperate waters of the Río de la Plata estuary, Argentina–Uruguay

The reproductive biology of the Jenyns's sprat Ramnogaster arcuata in the Río de la Plata area was studied using macroscopic and histological analysis of ovaries. Samples were collected during two research surveys carried out in June 1998 and June 1999. In addition monthly occurrences of this species from historical data were analysed. Jenyns's sprat specimens were mainly observed in open waters of the estuary during winter months. Most of the females collected during June 1998 and 1999 were in the spawning stage. This pattern differs from fish that reproduce in the estuarine waters of the Río de la Plata, which spawn during spring and summer. These results suggest that conditions of productivity and larval retention in the estuary also remain during winter months. This characteristic might be useful for R. arcuata larvae in avoiding competition with other estuarine spawners. Jenyns's sprat is a multiple spawner with indeterminate annual fecundity. A preliminary estimate of spawning frequency (N = 220) indicated a daily percentage of females with postovulatory follicles of 17.35%, which suggests that this species spawns on average once every six days. Batch fecundity, estimated from counts of hydrated oocytes from 40 females, ranged from 669 to 2026 hydrated oocytes in 1998 and from 570 to 1830 hydrated oocytes in 1999. Batch fecundity was fitted to a linear function of total body weight (without ovary) in 1999. Mean relative fecundity was 234 hydrated oocytes per female gram (ovary free) in 1998 and 155 hydrated oocytes per female gram (ovary free) in 1999, and no significant relationship was observed between this parameter and female size. The dry weight of 100 hydrated oocytes ranged between 3.9 and 6.3 mg, these values being significantly higher than those estimated for other species of the Río de la Plata estuary.

Seasonal prevalence and intensity of follicular atresia in Baltic cod Gadus morhua callarias L.

In the present study, 307 ovaries of eastern Baltic cod Gadus morhua callarias sampled during the prespawning and spawning season 2000 were analysed histologically to estimate the seasonal prevalence and intensity of atresia. The number of atretic oocytes per ovary was estimated using a combination of the physical disector method and volume fraction (Delesse principle). Atretic oocytes were observed in 32% of the ovaries. Prevalence of atresia was independent of female size, but increased significantly with declining female condition from prespawning and through the spawning stages. The relative intensity of atresia, i.e. number of atretic oocytes in relation to normally developed vitellogenic oocytes, was low amounting to 1·4% on average. Similar to prevalence, relative intensity of atresia differed significantly between maturity stages and increased with decreasing female condition. The population egg loss due to atresia amounted to 4·6% indicating that Baltic cod was performing close to maximum productivity, i.e. potential egg production.

Changes in the immunostaining intensities of follicle-stimulating hormone and luteinizing hormone during ovarian maturation in the female Japanese flounder

The role of gonadotropin (GTH) in the reproduction of the Japanese flounder, Paralichthys olivaceus, was studied by assessing the changes in the apparent activity of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in the pituitary gland during gonadal maturation by immunohistochemical analyses. Corresponding changes in plasma levels of testosterone (T), estradiol-17beta (E(2)), and 17alpha,20beta-dihydroxy-4-pregnen-3-one (DHP) were also studied. Reared fish at the early spawning to termination stages were sampled from May to August and wild fish at the previtellogenic to termination stages were caught at 3- to 4-week intervals between April and September offshore from the northern mainland of Japan by gill nets. The gonadosomatic index of the reared fish decreased from the early spawning stage to the termination stage, while that of the wild fish increased significantly from the previtellogenic stage to the early spawning stage and decreased thereafter. In the reared fish, the immunostaining intensities of FSH and LH were high during the spawning period, accompanied by high plasma levels of T, E(2), and DHP. In the wild fish, the immunostaining intensities of FSH and LH were low during the previtellogenic stage but increased during the maturing and spawning stages. These results indicate that both FSH and LH are likely associated with oocyte maturation in the Japanese flounder.

Distribution of Micropogonias furnieri at different maturity stages along an estuarine gradient and in relation to environmental factors

Whitemouth croaker (Micropogonias furnieri) maturity stages distribution and its relation to environmental factors was evaluated from 166 stations sampled within the Río de la Plata estuary (36°S–56°W) during spring 1994–1999 and 2003. A canonical correspondence analysis was used to explain whitemouth croaker distribution and estimate its relation to environmental factors (depth, surface and bottom waters temperature and salinity, bottom salinity and temperature horizontal gradients and vertical stratification). Results indicate that bottom salinity horizontal gradient (BSHG) and depth have a major influence on M. furnieri maturity stages spatial distribution; bottom salinity and temperature play a significant although secondary role. Spent and resting adult stages were associated to low BSHG, high salinity and low temperature while immature, gravid (with hydrated oocytes) and running stages to low salinity and high temperature. Although immature, gravid and running stages were clearly linked to similar bottom salinity they were segregated, with gravid and running stages associated to a strong BSHG. The distinct preferences of the different spawning stages result in a segregated distribution pattern along the main axis of the Río de la Plata estuary. The area with low salinity and high horizontal bottom salinity gradient (habitat for gravid and running stages) was located at the river head. The horizontal retention mechanism associated to BSHG within the spawning habitat favours growth and survival of fish larvae and the subsequent transport to nursery habitats.

피라미에 대한 보 철거 구간에서의 물리서식처 모의(PHABSIM) 적용

하천환경에 대한 사회적 요구에 의해 다양한 하천 복원 및 환경개선사업이 진행되고 있으므로, 이와 같은 사업의 효과를 평가할 방법론이 제공되어야 한다. PHABSIM은 하천 흐름과 관련되어 있는 수생 생물의 수리적 서식처 적합도를 평가하기 위해 제안된 방법이다. 본 연구에서는 보 철거 구간에 PHABSIM을 적용하여, 보 철거에 따른 피라미의 물리서식처 변화와 모형의 적절성을 평가하였다. 그 결과 일반적으로 보를 철거할 경우가 존치할 때보다 물리서식처가 향상된 것으로 나타났다. 그러나 유영력이 약한 산란기의 물리서식처는 보 상류의 유속이 증가되어 감소하는 것으로 나타났으며, 이에 대한 적절한 대책이 필요하다. 본 연구에 적용한 PHABSIM은 보 철거 사업을 포한한 다양한 하천사업의 평가와 필요성을 제시하는 효과적인 방법으로 활용될 수 있을 것이다. River restoration and environmental improvement projects have been peformed by social needs, therefore methodology for evaluating such projects must be provided. The PHysical HABitat SIMulation system (PHABSIM) is proposed as a tool for the assessment of hydraulic habitat suitability for aquatic species related to flow regime in river. This study evaluates the change of physical habitat for Zacco platypus according to small dam removal and the model suitability by applying PHABSIM to the reach where small dam was removed. It is shown that the physical habitat is generally increased and improved where the small dam was removed. However, physical habitat in the spawning stage that has a weak swimming speed is decreased by increased flow velocity in the upstream area of small dam, so proper countermeasure for that condition should be needed. Consequently, PHABSIM can be effectively used to provide methodology for assessment and necessity of various river projects including a removal of out-aged hydraulic structures.

Environmental and physiological controls on shell microgrowth pattern of Ruditapes philippinarum (Bivalvia: Veneridae) from Japan

The reproductive cycle and shell microgrowth patterns of the venerid bivalve Ruditapes philippinarum from Matsukawa-ura, a small inlet facing the Pacific Ocean, northeastern Japan were examined. Histological examination of the gonads revealed that spawning in this species occurred twice in 2005, once between late June and early August and the other between late September and early October. Comparison of the shell microgrowth patterns with the developmental stages of the gonad in each individual revealed that in spawning individuals (mature and spawning stages), the mean lunar-day growth rates were significantly smaller than those in individuals which were not in spawning condition (spent, recovery and growing stages). In non-spawning individuals, the mean lunar-day growth rates were positively correlated with seawater temperature up to 20°C. However, in spawning individuals, no correlation was observed between shell growth and seawater temperatures >16°C. These facts suggest that physiological stress during reproduction has a negative influence on shell growth and results in spawning breaks. The present study indicate that spawning breaks can be used to identify the timing of sexual maturity and to count the number of spawning events occurring per year in extant and fossil bivalves.