Response of <Emphasis Type="Italic">Chrysaora quinquecirrha</Emphasis> medusae to low temperature

Because of their high abundance in Chesapeake Bay, Chrysaora quinquecirrha medusae may be an important reservoir of organic matter. The timing and location of the decomposition of biomass from medusae may have implications for carbon cycling in the bay. Our objective was to identify the cause of C. quinquecirrha medusa disappearance to better understand when and where decomposition occurs. A time series of visual surface counts and vertical net hauls in the Choptank River, a tributary of Chesapeake Bay, showed that as temperatures approached 15°C, C. quinquecirrha medusae disappeared from the surface, but persisted in net hauls until temperatures reached 10°C. In order to test whether medusae sink upon cooling, we exposed C. quinquecirrha medusae to low temperatures in large static tanks and measured their depth and pulsation rates twice daily for at least 6 days. This procedure was repeated three times through the 2008 jellyfish season. On average, individuals exposed to temperatures below 15°C were found deeper and pulsed slower than those in the warmer control tank. This suggests that low temperatures cause the medusae to sink before cooling to the limit of their physiological tolerance and may have implications for the deposition of organic matter associated with the seasonal disappearance of medusae from Chesapeake Bay.     

Large medusae in surface waters of the Northern California Current: variability in relation to environmental conditions

Blooms of jellyfish around the world have been correlated with climatic variables related to environmental causes. Sizeable populations of large medusae, primarily Chrysaora fuscescens and Aequorea sp., appear annually in shelf waters of the Northeast Pacific Ocean. Previous research has shown that C. fuscescens is abundant seasonally in the inner shelf and exhibits high feeding rates on zooplankton. We examined medusae caught in surface trawls over an 8-year period (2000–2007) using (1) mesoscale surveys sampling 8–10 transects in May, June, and September, and (2) biweekly surveys along two transects from April to August, relating abundance to environmental parameters. C. fuscescens abundances generally peaked in late summer, whereas Aequorea sp. peaked in May or June. General additive models of the mesoscale data indicated that station catches for both species correlated with latitude, temperature, salinity, and distance from shore (and chlorophyll a for Aequorea sp.). Analysis of interannual variability revealed that highest catches of medusae correlated with cool spring–summer conditions, or negative anomalies of the Pacific Decadal Oscillation, and low winter–summer runoff from the Columbia River. Results confirmed our hypothesis of connections between jellyfish populations and regional climate conditions in a region known for strong physical forcing of ecosystem processes.     

Growth, development and temporal variation in the onset of six Chironex fleckeri medusae seasons: a contribution to understanding jellyfish ecology

Despite the worldwide distribution, toxicity and commercial, industrial and medical impacts jellyfish present, many aspects of their ecology remain poorly understood. Quantified here are important ecological parameters of Chironex fleckeri medusae, contributing not only to the understanding of an understudied taxon, the cubozoa, but also to the broader understanding of jellyfish ecology. C. fleckeri medusae were collected across seven seasons (1999, 2000, 2003, 2005-07 and 2010), with growth rates, temporal variation in the medusae season onset and differences in population structure between estuarine and coastal habitats quantified. With a mean of 2 September ± 2 d (mean ± 95% confidence limits), the earliest date of metamorphosis was temporally constrained between seasons, varying by only 7 d (30 August to 5 September). Juvenile medusae appeared to be added over an extended period, suggesting polyp metamorphosis was an ongoing process once it commenced. At a maximum of 3 ± 0.2 mm d(-1) IPD, medusae growth to an asymptotic size of ~190 mm IPD was rapid, yet, with the oldest medusae estimated to be ~78 d in age, medusae did not appear to accumulate along the coastline. Furthermore, a greater proportion of juveniles were observed along the coastline, with estuarine populations typified by larger medusae. With key aspects of C. fleckeri's ecology now quantified, medusae season management protocols can be further developed.     

Effects of pH on asexual reproduction and statolith formation of the scyphozoan, <Emphasis Type="Italic">Aurelia labiata</Emphasis>

Although anthropogenic influences such as global warming, overfishing, and eutrophication may contribute to jellyfish blooms, little is known about the effects of ocean acidification on jellyfish. Most medusae form statoliths of calcium sulfate hemihydrate that are components of their balance organs (statocysts). This study was designed to test the effects of pH (7.9, within the average current range, 7.5, expected by 2100, and 7.2, expected by 2300) combined with two temperatures (9 and 15°C) on asexual reproduction and statolith formation of the moon jellyfish, Aurelia labiata. Polyp survival was 100% after 122 d in seawater in all six temperature and pH combinations. Because few polyps at 9°C strobilated, and temperature effects on budding were consistent with published results, we did not analyze data from those three treatments further. At 15°C, there were no significant effects of pH on the numbers of ephyrae or buds produced per polyp or on the numbers of statoliths per statocyst; however, statolith size was significantly smaller in ephyrae released from polyps reared at low pH. Our results indicate that A. labiata polyps are quite tolerant of low pH, surviving and reproducing asexually even at the lowest tested pH; however, the effects of small statoliths on ephyra fitness are unknown. Future research on the behavior of ephyrae with small statoliths would further our understanding of how ocean acidification may affect jellyfish survival in nature.     

Acoustic survey of a jellyfish-dominated ecosystem (Mljet Island,Croatia)

Acoustic techniques have been proposed as a new tool to assess jellyfish populations. However, the presence of mixed echoes from jellyfish and other organisms that share their distribution often prevent accurate estimates of their abundance and distribution being obtained. The isolated population of Aurelia inhabiting the Veliko Jezero (Big Lake-BL) of Mljet Island, in the South Adriatic Sea, offered a good opportunity to employ acoustic techniques to assess an entire jellyfish population. During October 2–5, 2006, combined video and acoustic methods were used in BL to determine the vertical distribution of medusae. Two synoptic acoustic surveys were performed during the day and night. In the daylight echograms, medusae were clearly discernible from the acoustic data, and their presence verified by video camera images, as forming a layer of varying density at and below the thermocline (15–30 m). The depth of the jellyfish layer also coincided with the depth of maximum dissolved oxygen concentration. The echointegration of these daylight data enabled quantification the Aurelia population, at a frequency of 120 kHz. In the night echograms, the acoustic signals of Aurelia were at least partially masked by pelagic and demersal fish, which disaggregated from schools and formed a layer associated with a strong thermocline at 15 m. An average target strength (TS) of −76.4 dB was obtained in situ corresponding to a mean length of 10.8 cm and a mean wet weight of 134 g measured from sampled medusae. These results were combined with echo-integration values to provide an estimate of 4,238,602 individuals and a biomass of 568 tons of Aurelia in BL. This study provided a synoptic view of Mljet Lake and illustrated the potential of acoustic surveys of jellyfish populations to contribute to ecosystems studies. Guest editors: K. A. Pitt & J. E. Purcell Jellyfish Blooms: Causes, Consequences, and Recent Advances     

Environmental influences on seasonal distribution of coastal and estuarine fish assemblages at Wemindji,eastern James Bay

Synopsis The coastal fish assemblages of Wemindji, eastern James Bay, were studied in 1987 and 1988 to describe seasonal utilization of the Maquatua River estuary and the adjacent coastal waters by marine and anadromous fishes. Fish diversity was low (11 sp.) and experimental gill net catches were highly variable between sites in the estuary and coastal waters, and also seasonally at a given site. During summer, the estuarine fishes were numerically dominated by two marine species, the fourhorn sculpin,Myoxocephalus quadricornis, and the slender eelblenny,Lumpenus fabricii, and also by juvenile cisco,Coregonus artedii, and juvenile lake whitefish,C. clupeaformis. In coastal waters, three marine species were abundant: the shorthorn sculpin,M. scorpius, the arctic sculpin,M. scorpioides and the Greenland cod.Gadus ogac. In contrast with the estuary, large (> 270 mm) cisco and lake whitefish were abundant in coastal waters indicating extensive movements of these species in James Bay during the summer. Distribution patterns were influenced by a combination of physical conditions (salinity and temperature) and biological characteristics (habitat choice, migration and reproduction) depending on the season.     

Influence of jellyfish blooms on carbon,nitrogen and phosphorus cycling and plankton production

Due to their boom and bust population dynamics and the enormous biomasses they can attain, jellyfish and ctenophores can have a large influence on the cycling of carbon (C), nitrogen (N) and phosphorus (P). This review initially summarises the biochemical composition of jellyfish, and compares and contrasts the mechanisms by which non-zooxanthellate and zooxanthellate jellyfish acquire and recycle C, N and P. The potential influence of elemental cycling by populations of jellyfish on phytoplankton and bacterioplankton production is then assessed. Non-zooxanthellate jellyfish acquire C, N and P predominantly through predation on zooplankton with smaller contributions from the uptake of dissolved organic matter. C, N and P are regenerated via excretion of inorganic (predominantly ammonium (NH₄ ⁺) and phosphate (PO₄ ³⁻)) and dissolved organic forms (e.g. dissolved free amino acids and dissolved primary amines). Inorganic nutrients excreted by jellyfish populations provide a small but significant proportion of the N and P required for primary production by phytoplankton. Excretion of dissolved organic matter may also support bacterioplankton production but few data are available. In contrast, zooxanthellate medusae derive most of their C from the translocation of photosynthetic products, exhibit no or minimal net release of N and P, and may actively compete with phytoplankton for dissolved inorganic nutrients. Decomposition of jellyfish blooms could result in a large release of inorganic and organic nutrients and the oxygen demand required to decompose their tissues could lead to localised hypoxic or anoxic conditions. Guest editors: K. A. Pitt & J. E. Purcell Jellyfish Blooms: Causes, Consequences, and Recent Advances     

Numerical increases and distributional shifts of Chrysaora quinquecirrha (Desor) and Aurelia aurita (Linné) (Cnidaria: Scyphozoa) in the northern Gulf of Mexico

Fisheries resource trawl survey data from the National Marine Fisheries Service from a 11–13-year period to 1997 were examined to quantify numerical and distributional changes of two species of northern Gulf of Mexico scyphomedusae: the Atlantic sea nettle, Chrysaora quinquecirrha (Desor), and the moon jelly, Aurelia aurita (Linné). Trawl surveys were grouped into 10 statistical regions from Mobile Bay, Alabama to the southern extent of Texas, and extended seaward to the shelf break. Records of summertime C. quinquecirrha medusa populations show both an overall numerical increase and a distributional expansion away from shore in the down-stream productivity field of two major river system outflows: Mobile Bay and the Mississippi-Atchafalaya Rivers. In addition, there is a significant overlap between summer C. quinquecirrha and lower water column hypoxia on the Louisiana shelf. In trawl surveys from the fall, A. aurita medusae showed significant trends of numerical increase in over half of the regions analyzed. For both species, there were statistical regions of no significant change, but there were no regions that showed significant decrease in number or distribution. The relationships between natural and human-induced (e.g. coastal eutrophication, fishing activity and hard substrate supplementation) ecosystem modifications are very complex in the Gulf of Mexico, and the potential impact of increased jellyfish populations in one of North America's most valuable fishing grounds is a most critical issue. Several hypotheses are developed and discussed to guide future research efforts in the Gulf of Mexico.     

Biomass of scyphozoan jellyfish, and its spatial association with 0-group fish in the Barents Sea

An 0-group fish survey is conducted annually in the Barents Sea in order to estimate fish population abundance. Data on jellyfish by-catch have been recorded since 1980, although this dataset has never been analysed. In recent years, however, the ecological importance of jellyfish medusae has become widely recognized. In this paper the biomass of jellyfish (medusae) in 0–60 m depths is calculated for the period 1980–2010. During this period the climate changed from cold to warm, and changes in zooplankton and fish distribution and abundance were observed. This paper discusses the less well known ecosystem component; jellyfish medusae within the Phylum Cnidaria, and their spatial and temporal variation. The long term average was ca. 9×10⁸ kg, with some years showing biomasses in excess of 5×10⁹ kg. The biomasses were low during 1980s, increased during 1990s, and were highest in early 2000s with a subsequent decline. The bulk of the jellyfish were observed in the central parts of the Barents Sea, which is a core area for most 0-group fishes. Jellyfish were associated with haddock in the western area, with haddock and herring in the central and coastal area, and with capelin in the northern area of the Barents Sea. The jellyfish were present in the temperature interval 1°C<T<10°C, with peak densities at ca. 5.5°C, and the greatest proportion of the jellyfish occurring between 4.0–7.0°C. It seems that the ongoing warming trend may be favourable for Barents Sea jellyfish medusae; however their biomass has showed a recent moderate decline during years with record high temperatures in the Barents Sea. Jellyfish are undoubtedly an important component of the Barents Sea ecosystem, and the data presented here represent the best summary of jellyfish biomass and distribution yet published for the region.     

Spread of Exotic Cordgrasses and Hybrids (Spartina sp.) in the Tidal Marshes of San Francisco Bay, California, USA

Four species of exotic cordgrass (Spartina sp.) occur in the San Francisco estuary in addition to the California native Spartina foliosa. Our goal was to map the location and extent of all non-native Spartina in the estuary. Hybrids of S. alterniflora and S. foliosa are by far the most numerous exotic and are spreading rapidly. Radiating from sites of deliberate introduction, S. alterniflora and hybrids now cover ca. 190 ha, mainly in the South and Central Bay. Estimates of rate of aerial increase range from a constant value to an accelerating rate of increase. This could be due to the proliferation of hybrid clones capable of rapid expansion and having superior seed set and siring abilities. The total coverage of 195 ha by hybrids and other exotic cordgrass species is slightly less than 1% of the Bay's tidal mudflats and marshes. Spartina anglica has not spread beyond its original 1970s introduction site. Spartina densiflora has spread to cover over 5 ha at 3 sites in the Central Bay. Spartina patens has expanded from 2 plants in 1970 to 42 plants at one site in Suisun Bay. Spartina seed floats on the tide, giving it the potential to export this invasion throughout the San Francisco estuary, and to estuaries outside of the Golden Gate. We found isolated plants of S. alterniflora and S. densiflora in outer coast estuaries north of the Bay suggesting the likelihood for the San Francisco Bay populations to found others on the Pacific coast.     

Telomerase activity is not related to life history stage in the jellyfish Cassiopea sp.

The polyp (scyphistoma) of the jellyfish Cassiopea sp. can be maintained in culture for a long time, as polyps repeatedly reproduce asexually via formation of vegetative buds or propagules. The medusa, which is the sexually reproducing stage, typically has a relatively short life span. As a first step to understand the difference in life spans of the polyp and medusa stages of Cassiopea sp., we measured telomerase activity in different life cycle stages. We found telomerase activity in tissues of aposymbiotic polyps and propagules and symbiotic ephyrae (newly budded medusae) and adult medusae. No significant difference in telomerase activity was found between polyps and the bell region of the medusae. The cloned elongation products of the stretch PCR contained the TTAGGG repeats suggesting that the jellyfish has the ‘vertebrate’ telomere motif (TTAGGG)_n. This is the first study to show that somatic tissues of both polyp and medusa stages of a cnidarian had telomerase activity. Telomerase activity in somatic tissues may be related to the presence of multipotent interstitial cells and high regenerative capacity of cnidarians.     

GROWTH OF THE GREEN ALGA CODIUM FRAGILE IN A CONNECTICUT ESTUARY1

An in situ comparison of environmental and physiological factors was undertaken in 1971–72 (15 months) in the Niantic River estuary to elucidate some of the important aspects, of the growth and development of the seaweed Codium fragile. In general, Codium in the estuary has a growing season of from 6 to 9 months. Growth increments during this period are relatively constant. Temperature and salinity are the main limiting factors to growth, although low summer concentrations of inorganic nitrogen may also be involved. Reproduction by means of swarmers occurs only in late summer or autumn. Codium appears highly adapted to the role of a colonizing species. It not only possesses the capability to occupy the harshest of environments, but also a system of reproductive alternatives which facilitates rapid colonization. The availability of substrate for attachment is the chief factor limiting its spread in this estuarine system.     

Phylogeography and post-glacial colonization patterns of the rainbow darter, Etheostoma caeruleum (Teleostei: Percidae)

Aim To examine the effects of historical climate change and drainage isolation on the distribution of mitochondrial DNA cytochrome b genetic variation within the rainbow darter, Etheostoma caeruleum (Percidae: Etheostomatinae). Location Eastern North American streams including tributaries to the Mississippi River, Great Lakes, Potomac River and Hudson Bay drainages. Methods Parsimony analyses, Bayesian analyses and haplotype networks of mitochondrial DNA sequences. Results Four major clades were recovered from sampled populations of E. caeruleum. Three of four clades are distributed in the western portion of the species’ range (primarily west of the Mississippi River). Samples from this region do not form a monophyletic group, and sequences often vary greatly between samples from adjacent stream systems (up to 7.2% divergence). A basal clade includes samples from the White River system in the Ozark Highlands. The northern Ozarks–upper Midwest clade includes samples from Missouri River tributaries and the upper Midwest (Hudson Bay, upper Mississippi River, and western Lake Michigan drainage). The eastern clade is composed of individuals from the Ohio River, Great Lakes and Potomac River. The Mississippi River corridor clade includes samples from middle and lower Mississippi River tributaries. Main conclusions The four major clades of E. caeruleum are deep allopatric lineages with well‐defined boundaries and have additional phylogeographical structure within each clade. The Ozark Highlands have the greatest levels of diversity relative to distributional area, with marked cytochrome b subdivisions between adjacent stream systems. Samples from previously glaciated areas do not have a subset of the cytochrome b diversity found in unglaciated areas, but four separate source areas are identified based on phylogenetic analyses. Dispersal into previously glaciated areas followed several known glacial outlets and, based on sequence divergence between populations, may have occurred during different glacial or interglacial stages. The disjunct distribution and cytochrome b pattern of E. caeruleum in the Mississippi River corridor clade is consistent with late Pleistocene and Recent changes in the course and characteristics of the middle and lower Mississippi River. Phylogeographical boundaries between clades of E. caeruleum correspond to independent sources of biogeographical information and provide insight into historical stream drainage relationships, post‐glacial colonization and drainage isolation patterns.     

GENE FLOW VERSUS LOCAL ADAPTATION IN THE NORTHERN ACORN BARNACLE, SEMIBALANUS BALANOIDES: INSIGHTS FROM MITOCHONDRIAL DNA VARIATION

Abstract In reciprocal transplant experiments, Bertness and Gaines (1993) found that Semibalanus balanoides juveniles that had settled in an upper Narragansett Bay estuary survived better in that estuary that did juveniles from coastal localities. The observed pattern of survivorship led to the claim that local adaptation may result from a combination of limited gene flow between and strong selection within these habitats. Here we test the hypothesis that limited gene flow has led to habitat‐specific population differentiation using sequence and restriction fragment length polymorphism analyses of the mitochondrial DNA D‐loop region of S. balanoides. Samples were analyzed from replicated coastal and estuary localities in both Narragansett Bay, Rhode Island, and Damariscotta River, Maine. The patterns of F_ST indicate that gene flow between coast and estuary is extensive (Nm > 100) and is not lower in the estuary with lower flushing rates (Narragansett Bay). Given the high estimate of genetic exchange, adaptations for unpredictable environments seem more likely than local adaptation in this species because loci that respond to selection in one generation are essentially homogenized by the next seasons' settlement. Nevertheless, these estimates of neutral gene flow can help identify the strength of selection necessary for local adaptation to accumulate in Semibalanus.     

Spatio‐temporal variability in reproductive ecology of sand flathead,Platycephalus bassensis,in three Tasmanian inshore habitats: potential implications for management

Temporal and spatial variability in gonad development, duration of spawning period, and size/age at maturity were investigated in sand flathead, Platycephalus bassensis. A 3‐year study (2001–2003) revealed that variation in gonad weight with somatic weight was a function of an interaction between season and study location (Coles Bay, Georges Bay, and Tamar River estuary). Highest gonad weight was recorded in Coles Bay in early summer, which in comparison to Georges Bay, was approximately 50% higher and occurred 3 months later. Tamar River mature individuals were reproductively inactive during the spawning season. The proportion of mature individuals at different stages of maturity differed significantly among the three locations across all times. Coles Bay individuals were reproductively active from October to March, while in Georges Bay females with hydrated oocytes were seen in September but were absent from the population by November. Interannual variation in initiation and duration of spawning activity was evident in each location. Apart from male size at maturity, for both sexes the size and age at maturity (L₅₀) was higher in Georges Bay compared to Coles Bay. The results emphasize the necessity of temporal and spatial management based on population differences in reproductive ecology.     

Prey selection and diel feeding of the freshwater jellyfish, Craspedacusta sowerbyi

1. The diet of the invasive freshwater jellyfish, Craspedacusta sowerbyi (Lankester), was assessed by analysing its stomach contents. 2. The medusae ingested various zooplankton and benthic prey in the 0.1–3.0 mm size range. The selectivity indices for prey showed that larger zooplankton (0.4–1.4 mm) and active prey such as copepods were preferred; small potential prey, and the loricate rotifer Keratella cochlearis in particular, were almost never consumed. Measurements suggest that spacing between the tentacle branches could determine prey selection. Another possibility is that some prey are too small or slow to activate nematocysts after collision with the predator. Prey > 1.4 mm can probably escape or are otherwise too large to be handled. 3. The medusae of Craspedacusta medusae stayed in deep water during daytime and migrated upwards at night. The greatest number of prey in freshwater jellyfish stomachs was detected at night. 4. Calculation of consumption rates, based on mean stomach contents and digestion times of 4–5 h, showed that, on average, one medusa takes 190 zooplankton prey day^??1 at a mean prey density of about 100 L^??1. At the observed medusae density of 1 m^??3, the cropping rate was about 0.2 zooplankton prey L^??1 day^??1.     

Use of Washington Estuaries by Subadult and Adult Green Sturgeon

Green sturgeon, Acipenser medirostris, are the most marine-oriented of North American sturgeons. However, their estuarine/marine distribution and the seasonality of estuarine use are largely unknown. We used acoustic telemetry to document the timing of green sturgeon use of Washington estuaries. In the summers of 2003 and 2004, uniquely coded acoustic transmitters were surgically implanted in green sturgeon captured using commercial gillnets. All sturgeon tagged were greater than 1.2 m total length. They were caught, tagged, and released in both Willapa Bay (n = 49) and Columbia River (n = 11) estuaries. We deployed an array of four fixed- site acoustic receivers in Willapa Bay to detect the estuarine entry and exit of these and any of over 100 additional green sturgeon tagged in other systems during 2003 and 2004. Green sturgeon occurred in Willapa Bay in summer when estuarine water temperatures exceeded coastal water temperatures by at least 2°C. They exhibited rapid and extensive intra- and inter- estuary movements and green sturgeon from all known spawning populations were detected in Willapa Bay. We hypothesize that green sturgeon optimize their growth potential in summer by foraging in the relatively warm, saline waters of Willapa Bay and we caution that altering the quality of estuarine habitats could negatively affect this species throughout its range.     

Habitat use by harbour seals (Phoca vitulina) in a seasonally ice-covered region, the western Hudson Bay

Over the last few decades, the period of ice cover in Hudson Bay has decreased, owing to climate warming, with breakup occurring approximately 3 weeks earlier than it did 30 years ago. The trend towards lengthening of the open water season has led to speculation that ringed seal numbers would decline, but then harbour seals might become numerous enough to replace ringed seals in the diet of polar bears. The movement patterns of 18 harbour seals equipped with satellite-linked transmitters in the Churchill River estuary (western Hudson Bay) were examined, as well as the dive behaviour of 11 of these seals. During the ice-free period, seals followed a general central place-foraging strategy, making repeated trips between their haul-out site in the Churchill River estuary and nearshore areas (<20 km) near the river mouth and estuary. Seal behaviour changed significantly as ice started to form along the coast of western Hudson Bay: animals remained significantly farther from the Churchill River haul-out site and from the coast and performed longer and deeper dives. However, throughout the entire tracking period, whether ice was present or not, all animals restricted their movements to a narrow band of shallow coastal waters (<50 m depth) along a 600-km stretch of the western Hudson Bay coastline, centred on the Churchill River estuary haul-out site. This natural self-limitation to nearshore shallow waters could restrict the potential for the population to increase in size and replace ringed seals as a primary energy resource for polar bears.     

Comparison of the life cycles of two sympatric estuarine polychaetes, Hediste diadroma and H. atoka (Polychaeta: Nereididae), in the Nanakita River estuary, northeastern Japan

The life cycles of two species of estuarine polychaetes, Hediste diadroma and H. atoka, were investigated using field surveys and colonization experiments in the Nanakita River estuary, Japan. Lactate dehydrogenase (LDH) allozyme banding patterns in starch gel electrophoresis were used to distinguish between the species. We found differences in the life cycle, reproductive ecology, and distribution of the two polychaetes. H. diadroma exhibited conspicuous reproductive swarming behavior during the spring tide in April to early May. After hatching, the trochophore larvae may be transported out to sea and return to the river estuary at the =3-setiger stage. By late July, juveniles settled into brackish water, where they completed the 1-year life span. In contrast, H. atoka never exhibited this conspicuous reproductive swarming behavior. The reproductive period of this species spanned most of the year, except winter. H. atoka juveniles hatched mainly as 3-setiger nectochaetes, which immediately became benthic. The juveniles actively dispersed during the 4-to 7-setiger stage, although they could also do so at any later stage. The life span of H. atoka was about 6 months. In small river estuaries, river flooding may flush sediment and benthic animals out to sea and create vacant habitats for colonization by polychaetes. In the Nanakita River estuary, H. atoka maintained a large and stable population in Gamo Lagoon, located 250m upstream from the river mouth, whereas H. diadroma was rarely found there. Swimming excursions of juvenile H. atoka at any stage in the life cycle may ensure its distribution to new vacant habitats, and its life-cycle characteristics may allow large populations to build rapidly. As a result, H. atoka was able to maintain dominance in both the river and the lagoon in the Nanakita River estuary.     

Salinity Adaptations of the Gastropods Littorina mandshuricaand L. squalidafrom a Marine Bay and an Estuary

The authors examine the effect of decreased salinity on adult and larval periwinkles Littorina mandshuricaand L. squalidafrom two different habitats, a marine bay and an estuary in Vostok Bay, Sea of Japan. Considerable differences were found in salinity tolerance of adult periwinkles, larval growth and survival, and morphology of the egg capsules. Using the stepwise acclimation method, no genotypic differences were found in salinity tolerance limits of gastropod mollusks from different populations. It is assumed that despite marked physiological differences these populations appear not to be different physiological races.