Semilunar Cycle Research Articles

Mating behavior of the Iranian fiddler crab Austruca iranica Pretzmann, 1971 (Decapoda: Brachyura: Ocypodidae), with an emphasis on the mate choice of burrow-mating females

Abstract Fiddler crabs exhibit two modes of mating: burrow and surface mating. In burrow mating, males wave and attract mate-searching females for mating inside their burrows, whereas in surface mating males search for and mate with females on the surface. We studied the reproductive behavior of Austruca iranica (Pretzmann, 1971), an Indo-West Pacific species, specifically female-mate choice with regard to the size of both male individuals and their burrows. We also compared the size of male and female burrows in the population. Austruca iranica performed both surface and burrow mating despite the different size of the burrow of the two sexes, within the semi-lunar tide cycles at the study site. There was strong size-assortative mating in burrow mating. Burrow-mating females generally chose larger males and wider burrows and even though chosen and rejected males were not different in burrow length, females sampled males with longer than average burrows. Surface-mating females also possessed burrows that were longer than those of the females in the population and this may have enabled them to mate on the surface and breed alone. Regardless of its similarities to some species that only perform burrow mating; the ecology of A. iranica has made it possible for this species to mate on the surface as well as in male-defended burrows. The choice of large males by burrow mating females may provide benefits to females from both the large male itself (e.g., protection) and his wider burrow (e.g., better incubation temperature).

Semilunar postlarval settlement periodicity ensuing from semilunar larval release cycle in the Nihonotrypaea harmandi (Crustacea, Decapoda, Axiidea, Callianassidae) population on an intertidal sandflat in an open marine coastal embayment

AbstractMany decapod crustaceans in marine intertidal habitats release larvae toward coastal oceans, from which postlarvae (decapodids: settling‐stage larvae) return home. Decapodid settlement processes are poorly understood. Previous studies showed that in Kyushu, Japan, the callianassid shrimp population on an intertidal sandflat of an open bay joining the coastal ocean near a large estuary released eight batches of larvae basically in a semilunar cycle from June through October and that decapodids performed diel vertical migration, occurring in the water column nocturnally. We conducted (a) frequent sampling for population density and size‐composition on the sandflat through one reproductive season, (b) planktonic and benthic sampling for decapodids around the bay mouth, and (c) current meter deployment at three points across the bay mouth for tidal harmonic analysis. On the sandflat, six batches of newly‐settled decapodids (settlers) occurred in a semilunar periodicity until October, with peaks occurring 0–3 days before syzygy dates except for the first one. For larval Batches 1–4, buoyancy‐driven shoreward subsurface currents during July to mid‐October would transport some pre‐decapodid‐stage larvae (zoeae) toward the bay. The absence of expected settler Batches 7–8 would be due to the converse subsurface currents caused by water‐column mixing and seasonal winds after mid‐October, carrying zoeae offshore. Once in the bay, phasing of night and nighttime‐averaged shoreward tidal current explained the settlement pattern for Batches 1–4. For Batches 5–6 occurring in mid‐September to mid‐October, water currents generated by seasonal wind and tidal forcings may have caused peak settlement after the time expected from tidally‐driven decapodid transport.

Plankton community and copepod production in a temperate coastal lagoon: What is changing in a short temporal scale?

Coastal lagoons are often exposed to intense short-term environmental changes and strong anthropogenic pressures influencing zooplanktonic communities and production. However, most works focus on long-term temporal scales using monthly or seasonal sampling strategies. The present study analysed the structure of the mesozooplanktonic assemblages, the production (egg production rates) and physiological condition (RNA:DNA ratio) of the copepod Acartia clausi in a temperate coastal lagoon (Ria Formosa) during the summer, using an intensive sampling approach. Salinity was the main factor affecting the short-term variability of mesozooplankton composition, followed by tidal phase (ebb tides) and semilunar cycle (spring tides). There was a positive relationship between the abundance of Appendicularia and the cladoceran Penilia avirostris with the toxic dinoflagellate Gymnodinium catenatum, suggesting no deleterious effects. The egg production rate of A. clausi was influenced by salinity and ammonia concentration, with a positive correlation between the egg productivity and the macronutrient, showing a possible adaptation of this calanoid species. The RNA:DNA index was positively related to egg production rate, suggesting that it is a good proxy for the reproductive output of copepods, even in short-term periods. This study shows that different timescales need to be included in regular monitoring of planktonic assemblages in coastal lagoons in order to understand the influence of environmental and anthropogenic variables on marine organisms.

Facies and the Architecture of Estuarine Tidal Bar in the Lower Cretaceous Mcmurray Formation, Central Athabasca Oil Sands, Alberta, Canada

In this study, data obtained from the Lower Cretaceous McMurray Formation in the central Athabasca Oil Sands, northeastern Alberta, Canada, are examined and used to establish the architecture of stacked fluvial and estuarine tidal bar deposits. A total of 13 distinguishable facies (F1–F7, F8a–F8b, and F9–F13) corresponding to stacked fluvial and estuarine deposits are recognized. These facies are then reassembled into four facies associations: fluvial deposits, tidal flat, tidal bar complex, and tidal bar cap. Of these, the lower fluvial deposits show a highly eroded channel lag and tidal influences in the cross-stratified sand and wavy interbeds. The fluvial deposits pass upwards into upper tidal-dominated tidal flats and a massive homogeneous tidal sand bar complex. Very thick tidal-influenced facies (F8a–F8b, up to 22 m) caused by semi-diurnal and semi-lunar cycles are also observed in tidal flats. Based on studies of the facies and facies associations, a three-dimensional (3-D) architecture model is finally established and used to analyze the internal distribution of the stacked fluvial and estuarine deposits. This is the first time that a 3-D model of the paleo-estuary tidal bar has been constructed. The results of this study will assist future research analyzing the architecture of stacked fluvial and estuarine deposits.

Assessment of the impact of seasonal patterns climatic conditions on spawning events of the white bream Blicca bjoerkna (Linnaeus, 1758) in astronomical and biological time

This paper describes spawning events of the white bream Blicca bjoerkna (Linnaeus, 1758 )in astronomical and biological time depending on seasonal patterns of climatic conditions. The research was conducted in the Dnipro-Orilskiy Nature Reserve (Ukraine) during the years 1997–2015 in four habitat types. The timing of the spawning event (start, end) was evaluated using astronomical time (number of days from January 1 each year), with correction for the lunar and the semilunar cycle, and using biological time (number of days from the end of spawning in the previous year to the beginning and end of spawning in the current year). The spawning phenology of the white bream was found to be dependent on the dynamics of weather conditions over time. Patterns of varying climatic regimes which are expressed by means of multivariate principal component are the most informative predictors of the spawning events. Evaluation of occurrence of spawning events in astronomic or biological time gives somewhat different models of the impact of climate regimes. The impact of the principal components indicating variability of rainfall during the year in which spawning occurs is statistically significant for almost all regression models. Models using the solar calendar are also more sensitive to the course of air temperatures within the year when spawning occurred. Correction of time based on the lunar cycle allows us to assess events which are sensitive also to the temperature and rainfall variation during the second half of the previous year. Biological time was shown to be sensitive to environmental influences over time from the end of spawning in the previous year to spawning in any given year.

Lunar cycles of reproduction in the clown anemonefish Amphiprion percula: individual-level strategies and population-level patterns

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 594:193-201 (2018) - DOI: https://doi.org/10.3354/meps12540 Lunar cycles of reproduction in the clown anemonefish Amphiprion percula: individual-level strategies and population-level patterns J. R. Seymour, T. A. Barbasch, P. M. Buston* Department of Biology and Marine Program, Boston University, Boston, MA 02215, USA *Corresponding author: buston@bu.edu ABSTRACT: Lunar or semi-lunar cycles of reproduction are a widespread phenomenon in marine organisms. It is common practice to infer the adaptive value of these reproductive cycles based on the population-level pattern. This practice may be flawed if there are multiple types of individuals within the population employing different reproductive cycles. Such phenotypic diversity can be thought of in terms of alternative, mixed or conditional strategies. Here, we capitalized on a long-term field study and a carefully controlled laboratory experiment of individually identifiable clown anemonefish Amphiprion percula to investigate the individual-level strategies underlying population-level patterns of reproduction. The field study revealed that A. percula exhibit a lunar cycle of reproduction at the population level, and that there is naturally occurring variation among and within individuals in the number of times they reproduce per month. The laboratory experiment revealed that the number of times individuals reproduce per month is dependent on their food availability. Individuals employed a conditional strategy with 3 tactics: reproduce 1, 2 or 3 times mo-1, depending on food availability. Breaking down the population-level pattern by reproductive tactic, we showed that each reproductive tactic has its own non-random lunar, semi-lunar (½ lunar) or trient-lunar (⅓ lunar) cycle of reproduction. We conclude that the adaptive value of A. percula reproductive cycles, and likely the reproductive cycles of many other marine organisms, should not be inferred from the population-level pattern. Instead, the adaptive value of lunar, semi-lunar and trient-lunar cycles should be investigated for the individuals that express them. KEY WORDS: Reproductive cycles · Spawning patterns · Phenotypic plasticity · Animal behavior · Adaptive value · Clownfish · Coral reef fish · Marine ecology Full text in pdf format PreviousNextCite this article as: Seymour JR, Barbasch TA, Buston PM (2018) Lunar cycles of reproduction in the clown anemonefish Amphiprion percula: individual-level strategies and population-level patterns. Mar Ecol Prog Ser 594:193-201. https://doi.org/10.3354/meps12540 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 594. Online publication date: April 26, 2018 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2018 Inter-Research.

Light- and circadian-controlled genes respond to a broad light spectrum in Puffer Fish-derived Fugu eye cells

Some cell lines retain intrinsic phototransduction pathways to control the expression of light-regulated genes such as the circadian clock gene. Here we investigated the photosensitivity of a Fugu eye, a cell line established from the eye of Takifugu rubripes, to examine whether such a photosensitive nature is present. Microarray analysis identified 15 genes that showed blue light-dependent change at the transcript level. We investigated temporal profiles of the light-induced genes, as well as Cry and Per, under light-dark, constant light (LL), and constant dark (DD) conditions by quantitative RT-PCR. Transcript levels of Per1a and Per3 genes showed circadian rhythmic changes under both LL and DD conditions, while those of Cry genes were controlled by light. All genes examined, including DNA-damage response genes and photolyase genes, were upregulated not only by blue light but also green and red light, implying the contribution of multiple photopigments. The present study is the first to identify a photosensitive clock cell line originating from a marine fish. These findings may help to characterize the molecular mechanisms underlying photic synchronization of the physiological states of fishes to not only daily light-dark cycles but also to various marine environmental cycles such as the lunar or semi-lunar cycle.

Temporal windows of reproductive opportunity reinforce species barriers in a marine broadcast spawning assemblage.

Prezygotic isolating mechanisms act to limit hybridization and maintain the genetic identity of closely-related species. While synchronous intraspecific spawning is a common phenomenon amongst marine organisms and plays an important role in reproductive success, asynchronous spawning between potentially hybridizing lineages may also be important in maintaining species boundaries. We tested this hypothesis by comparing reproductive synchrony over daily to hourly timescales in a sympatric assemblage of intertidal fucoid algae containing selfing hermaphroditic (Fucus spiralis and Fucus guiryi) and dioecious (Fucus vesiculosus and Fucus serratus) species. Our results confirm that gametes are released on semi-lunar cycles in all species. However, sister species with different mating systems showed asynchronous spawning at finer circadian timescales, thus providing evidence for a partial reproductive barrier between hermaphroditic and dioecious species. Finally, our data also emphasize the ecological, developmental, and/or physiological constraints that operate to restrict reproduction to narrow temporal windows of opportunity in the intertidal zone and more generally the role of ecological factors in marine speciation.

TIDAL PERIODICITY IN MICROGROWTH BANDS OF BLOOD CLAM Anadara indica (Bivalvia: Arcidae): A POSSIBLE ENVIRONMENTAL SCLEROCHRONOMETER IN THE TROPICS

This study examining the microgrowth periodicity of wild type Anadara indica (Gmelin, 1791) being submerged for 29 d in the cage and field plot. Individual clams showed daily increment growth that conforms to semilunar tidal cycle of 14.8 d. The shell therefore provided record about the absolute growth history of the organism, presumably including the environmental conditions under which shell deposition took place. Shell accretion decreased in rhythm to the gonad development and spawning measured as Condition Index. The significantly departed sex ration to 0.25:1 in favour of > 35 mm shell length females indicating the occurrence of male to female sex reversal, likewise its con-specific A. granosa (Linne, 1758) and A. antiquata (Linnaeus, 1758). It is thought that habitat overlap between A. granosa and A. indica, combined with irresponsible fisheries of the local people which commercially caught small size male clams caused the shift in population dominance among these two species, i.e., from A. granosa to A. indica. However, this study showed that A. indica might as well thrive in areas close to intertidal region likewise A. granosa, where tidal periodicity role as a forcing function to the environment.

LPXRFamide peptide stimulates growth hormone and prolactin gene expression during the spawning period in the grass puffer, a semi-lunar synchronized spawner

Gonadotropin-inhibitory hormone (GnIH) plays as a multifunctional neurohormone that controls reproduction in birds and mammals. LPXRFamide (LPXRFa) peptide, the fish ortholog of GnIH, has been shown to regulate the secretion of not only gonadotropin (GTH) but also growth hormone (GH) and prolactin (PRL), which are potentially important for gonadal function. To investigate the role of LPXRFa peptide on reproduction of the grass puffer, which spawns in semilunar cycles, we examined changes in the levels of gh and prl expression over the several months during the reproductive cycle, and the effects of goldfish LPXRFa peptide-1 (gfLPXRFa-1) on their expression were examined using primary pituitary cultures. The expression levels of both gh and prl showed significant changes during the reproductive cycle in both sexes with one peak in the spawning and pre-spawning periods for gh and prl, respectively. Particularly, gh showed substantial increase in expression in the spawning and post-spawning periods, indicative of its essentiality in the advanced stage of reproduction. gfLPXRFa-1 stimulated the expression of both gh and prl but there was a marked difference in response between them: gfLPXRFa-1 stimulated gh expression at a relatively low dose but little effect was observed on prl. Combined with the previous results of daily and circadian oscillations of lpxrfa expression, the present results suggest that LPXRFa peptide is important in the control of the cyclic reproduction by serving as a multifunctional hypophysiotropic factor that regulates the expression of gh and prl as well as GTH subunit genes.

Temporal and spatial variation in female mating preferences in a fiddler crab

Female mating preferences can vary temporally, with females choosing different males at different times; and spatially, with females in different populations preferring different males. This level of complexity is now well established, but we know of no evidence for a mosaic of female preferences within a single population. Here we show that, in the banana fiddler crab, Uca mjoebergi, female preferences vary both temporally and spatially. Females living in the high inter-tidal zone changed their mating preference for male size over the duration of the 9-day mating period every semi-lunar cycle: early mating females selected larger males with cooler burrows, slowing embryonic development; those mating later, selected smaller males with warmer burrows, accelerating development. Females living lower in the inter-tidal zone, however, did not show this temporal variation: they select the same sized males throughout the mating period. It is only in the high inter-tidal zone, at the start of the fortnightly mating period, that large size confers a mating advantage to males.

Evidence from data storage tags for the presence of lunar and semi-lunar behavioral cycles in spawning Atlantic cod

Understanding the environmental processes determining the timing and success of reproduction is of critical importance to developing effective management strategies of marine fishes. Unfortunately it has proven difficult to comprehensively study the reproductive behavior of broadcast-spawning fishes. The use of electronic data storage tags (DSTs) has the potential to provide insights into the behavior of fishes. These tags allow for data collection over relatively large spatial and temporal scales that can be correlated to predicted environmental conditions and ultimately be used to refine predictions of year class strength. In this paper we present data retrieved from DSTs demonstrating that events putatively identified as Atlantic cod spawning behavior is tied to a lunar cycle with a pronounced semi-lunar cycle within it. Peak activity occurs around the full and new moon with no evidence of relationship with day/night cycles.

Prezygotic barriers to hybridization in marine broadcast spawners: reproductive timing and mating system variation.

Sympatric assemblages of congeners with incomplete reproductive barriers offer the opportunity to study the roles that ecological and non-ecological factors play in reproductive isolation. While interspecific asynchrony in gamete release and gametic incompatibility are known prezygotic barriers to hybridization, the role of mating system variation has been emphasized in plants. Reproductive isolation between the sibling brown algal species Fucus spiralis, Fucus guiryi (selfing hermaphrodite) and Fucus vesiculosus (dioecious) was studied because they form hybrids in parapatry in the rocky intertidal zone, maintain species integrity over a broad geographic range, and have contrasting mating systems. We compared reproductive synchrony (spawning overlap) between the three species at several temporal scales (yearly/seasonal, semilunar/tidal, and hourly during single tides). Interspecific patterns of egg release were coincident at seasonal (single peak in spring to early summer) to semilunar timescales. Synthesis of available data indicated that spawning is controlled by semidiurnal tidal and daily light-dark cues, and not directly by semilunar cycles. Importantly, interspecific shifts in timing detected at the hourly scale during single tides were consistent with a partial ecological prezygotic hybridization barrier. The species displayed patterns of gamete release consistent with a power law distribution, indicating a high degree of reproductive synchrony, while the hypothesis of weaker selective constraints for synchrony in selfing versus outcrossing species was supported by observed spawning in hermaphrodites over a broader range of tidal phase than in outcrossers. Synchronous gamete release is critical to the success of external fertilization, while high-energy intertidal environments may offer only limited windows of reproductive opportunity. Within these windows, however, subtle variations in reproductive timing have evolved with the potential to form ecological barriers to hybridization.

Spatio-temporal dynamics of a fish spawning aggregation and its fishery in the Gulf of California

We engaged in cooperative research with fishers and stakeholders to characterize the fine-scale, spatio-temporal characteristics of spawning behavior in an aggregating marine fish (Cynoscion othonopterus: Sciaenidae) and coincident activities of its commercial fishery in the Upper Gulf of California. Approximately 1.5–1.8 million fish are harvested annually from spawning aggregations of C. othonopterus during 21–25 days of fishing and within an area of 1,149 km2 of a biosphere reserve. Spawning and fishing are synchronized on a semi-lunar cycle, with peaks in both occurring 5 to 2 days before the new and full moon, and fishing intensity and catch are highest at the spawning grounds within a no-take reserve. Results of this study demonstrate the benefits of combining GPS data loggers, fisheries data, biological surveys, and cooperative research with fishers to produce spatio-temporally explicit information relevant to the science and management of fish spawning aggregations and the spatial planning of marine reserves.

Hood construction as an indication of the breeding period of the fiddler crab Uca (Leptuca) leptodactyla Rathbun, 1898 (Decapoda, Ocypodidae) from Guaratuba Bay, southern Brazil

Studies were carried out on the annual breeding period, lunar reproductive rhythm and daily activities rhythm of the western Atlantic fiddler crab Uca (Leptuca) leptodactyla Rathbun, 1898 from Guaratuba Bay, southern Brazil. This species reproduces from September to March, when ovigerous females are present and males construct their hoods. The monthly average air temperature fluctuates from 19.2 to 24.8°C in this period. Hoods were constructed in accordance with the semilunar cycle of the breeding period; it lasted eight days, with the maximum frequency exactly on syzygy days (full or new moon). On a sunny full moon day in midsummer, males began to construct hoods 1.5 h after the substrate became exposed to the air, as the tide went out; the maximum density of hoods occurred after 3.5 h. The breeding patterns of most fiddler crabs of the world including U. leptodactyla are not determined by species-specific rules but by environmental conditions of the habitat, while other species respond in different manner to the same climatic conditions. Construction of hoods by males is a secure indication of the reproductive period of the fiddler crab U. leptodactyla from southern Brazil. The timing of hood construction within a day is similar to that in its consubgeneric species in spite of differences in local climatic conditions.

Subsurface and outcrop characterization of large tidally influenced point bars of the Cretaceous McMurray Formation (Alberta, Canada)

Outcrop and subsurface investigations highlight that the main reservoirs within the Cretaceous McMurray Formation, Alberta, Canada, were located in an ancient estuary and that the major economic targets are represented by large point bar deposits. The underlined morphological characteristics of the associated McMurray-valley drainage show morphometric similarity to modern river systems like the Mississippi River. Hydrological parameters estimated by some geomorphologic data suggest that a paleodischarge of about 15,000m3/s (including the tidal prism) and a paleomeander migration rate of between approximately 30m/year and 65m/year characterized the McMurray.However, tidal influence and marginal-marine affinity are evidenced by abundant erosion and reactivation surfaces, mud clast breccia deposits, cleaner channel sands and bioturbated heterolithic sands interfingered with cleaner channel sands. Thus, internal structure observed on the Steepbank River outcrops appears to be more complex than the usual bi-partite system composed of cross-stratified sands at the base and inclined heterolithic stratified sands upward. The highlighted internal structures of large tidally influenced point bars of the McMurray Formation are quite similar to compound dune deposits in which inclined heterolithic sands (IHS) are interfingered with clean sands. These deposits are also associated with ichnological associations characterizing a brackish environment for the McMurray-valley drainage.A modern analog of the tidally influenced point bar deposits of the McMurray Formation is the meandering tidally influenced channels of the Garonne River (located in the Aquitaine basin, SW France). Facies described in cores acquired in both areas are very similar, which allows a possible comparison of these point bars, in spite of the size difference between rivers. The Garonne river point bar is located 95km from estuary mouth near the city of Bordeaux. In this area, tidal currents are the dominant dynamic agent and can reach velocities up to 2m/s. In the point bar core, the facies of the IHS exhibit 3 typical sedimentary structures: mud clast breccias more abundant at the base of the channel, heterolithic stratified sands recording semi-lunar cycles (up to 28 tides) and thick clay layers at the top of the channel. In the overbank deposits, classic tidal bedding recording the semi-diurnal and semi-lunar cycles is observed. Levees and crevasse splays are absent.This study aims to provide a depositional model and geometric framework for tidally influenced point bar reservoirs.

Coupling between microphytobenthic biomass and fiddler crab feeding

Among the organic matter ingested by fiddler crabs, microphytobenthos is of fundamental importance because it is their main N source. Microphytobenthos abundance generally develop semilunar changes as the dynamics of tidal exposures and day-night cycle are not held constant across days, modifying the balance between growth and mortality. In this study we explored the coupling between temporal dynamics in microphytobenthos abundance and crab feeding activity. We measured the Chlorophyll a content in the 2mm surficial sediment surrounding the burrows and the crab feeding activity over two semilunar cycles. Chlorophyll a and crab feeding activity showed biweekly cyclic dynamics. Crabs did not concentrate feeding activity around days with maximum abundance of microhytobenthos. This phase difference between both dynamics could be the result of the crab feeding impact, but a crab experimental exclusion showed that the temporal dynamics of Chlorophyll a content stayed unchanged when feeding activity was removed. Comparisons between fed and unfed sediment suggest that the feeding efficiency changes with tidal dynamic. Crabs achieved more than 50% of Chlorophyll a extraction during days of highest feeding activity, and less than 30% during days of low feeding activity or low microhytobenthos abundance. Furthermore, comparisons of fed sediment between consecutive days indicated that Chlorophyll a was completely replenished during days with high flooding tides, but partially replenished during days near neap tides. Environmental conditions affecting feeding efficiency may select crabs to concentrate feeding activity before days with the highest microhytobenthos abundance. The low feeding impact on microphytobenthos dynamics suggests that fiddler crabs would not control microhytobenthos abundance and thus unable to absorb the increasing eutrophication of studied estuarine areas.

Tower construction by the manicure crab Cleistostoma dilatatum during dry periods on an intertidal mudflat

Animals living on upper intertidal mudflats experience habitat desiccation during neap tides when water does not flood the habitat. Individuals of the manicure crab Cleistostoma dilatatum construct cone-shaped towers at the entrance of their burrows, in which they remain during neap tides. These towers are the tallest known structures compared to body size built by crabs living on intertidal flats. The frequency of tower construction followed semilunar tidal cycles with most building done prior to neap tides when few crabs were active on the mudflat surface. Bigger crabs tended to make taller and wider towers with a wider pinhole on the top. These towers may regulate the microclimate in burrows.

Reproductive cycle and sexual maturity of Sphoeroides annulatus (Jenyns, 1842) (Tetraodontiformes, Tetraodontidae) from the coast of Mazatlan, Sinaloa, Mexico

Summary The reproduction of any fish species may be influenced by environmental factors, knowledge of which is required for an adequate control of the reproductive process to improve culture practices. Thus, the reproduction of a wild population of bullseye puffer, Sphoeroides annulatus (Jenyns, 1842), and the influence of temperature, photoperiod, lunar cycle and tide level were analyzed. Ovarian ripeness is asynchronous, and the ovary may ripen again at least once following spawning. Testes also display an asynchronous ripeness, but once sexual maturity is attained, spermatozoa are continually produced and released. The reproduction is highly seasonal, with an intense spawning period during the spring-summer, when the sea surface temperature is 22.5–30.9°C and a 11–14 h photoperiod. The observations suggest that the timing of spawning is synchronized by a semi-lunar cycle together with the rise of the average tide level. Size at first maturation was similar for females (28.2 cm TL) and males (28.6 cm TL). However, some specimens may start their gonad maturation when are as small as 19 cm TL.

SALT WATER INTRUSION AND ITS RELATIONSHIP WITH ESTUARINE ECOLOGY IN THE CHIKUGOGAWA ESTUARY

The spatiotemporal distribution of seawater intrusion in the Chikugogawa estuary was analyzed, and its relationship with the temporal variation of phytoplankton and engraulid fish, Coilia nasus, was discussed. The Chikugogawa estuary is vertically well mixed for most of the year, and a salt wedge is observed only when the tidal range decreases to 2 m or less. We found that the ratio of chlorophyll-a to pheophytin-a during the semilunar cycle varied according to the change in the mixing conditions in the estuary. Further, the number of sampled estuarine fish was related to the salinity and tidal range within the estuary; the fish was caught when the salinity was low and the tidal range was large. It is necessary to regulate the fresh water discharge in the estuary in order to maintain a low-salinity region that is suitable for the migration of fish and their spawning areas to the downstream of the river mouth barrage.