Production and ecology of eelgrass (Zostera marinal L.) in the Grevelingen estuary,the Netherlands,before and after the closure

The Grevelingen estuary was cut off from the North Sea and from the influences of the river Rhine by a dam in 1971, and became a stagnant salt-water lake. Production and ecology ofZostera marina L. were studied in 1968 and in 1973–1975, both through standing stock estimations, biomass increases in permanent quadrats, and correlation of distribution patterns with ecological factors. After the closure of the estuary the intertidal eelgrass population extended downwards to 5 m below lake level, probably owing to the increased transparency of the water; the area occupied, and the density of the eelgrass beds increased strongly. Eelgrass annual overground production, based on doubled maximum standing crop values in July–August, was estimated at 50 g C/m² in 1968, 121 g C/m² in 1973 and 91 g C/m² in 1975 inZostera beds, and 4 g C/m² in 1968, 18 g C/m² in 1973 and 23 g C/m² in 1975 for the entire Grevelingen area. A minimum estimate of net production inZostera beds at a depth of 0.50–0.75 m, based on short term changes in biomass in 2 permanent quadrats in 1974 and 1975, was 40.5 g C/m²/yr for overground parts and 12.7 g C/m²/yr for underground parts. Horizontal distribution of celgrass is not primarily limited by grainsize distribution, but more by exposure to wave action and currents. On account of irradiance reduction light is a limiting factor in the vertical distribution of the eelgrass population in Lake Grevelingen. Communication no. 146 of the Delta Institute for Hydrobiological Research, Yerseke, The Netherlands.     

The role of macrophytes,especiallyZostera marina,in the carbon cycle of Lake Grevelingen (IV)

Summary Macrophytes in Lake Grevelingen comprise macroalgae (Ulva, Enteromorpha, Chaetomorpha) and eelgrass (Zostera marina). In terms of production of organic matter far out the largest share comes from eelgrass. Eelgrass showed an enormous increase after the closure of the dam in 1971: in 1968 the phanerogam covered an area of 1200 ha, and in 1978 this area was 4400 ha (Fig. 3).     

A simulation model of production,seasonal changes in biomass and distribution of eelgrass (Zostera marina) in Lake Grevelingen

Summary A simulation model has been described, based on data from Lake Grevelingen, The Netherlands, as a predictive tool for lake management. The model has been developed as part of a large-scale aquatic modelling effort in Lake Grevelingen, carried out in close cooperation with the Delft Hydraulics Laboratory and the Delta Department, Environmental Research Division of the Ministry of Transport and Public Works. Available data on growth rates per unit eelgrass biomass, obtained with the leaf-marking technique, and on above- en below ground biomass and shoot density changes per unit area have been used. A space limitation depending on density of the above ground biomass and a growth limitation due to shortage in below ground biomass have been introduced. The seasonal changes in eelgrass production, both above and below ground, have been simulated as functions of the external forcing variables light, water temperature, wind generated water movements and of the internal control variables due to aging of the plant material. The vertical distribution of eelgrass can be partly explained from the modelling results on space, light and below ground biomass limitations. From the shore down to about 1 m waterdepth the above ground eelgrass biomass suffers from space limitation. Between 1 and 2 m production and biomass reach maximum values. Between 2 and 3 m waterdepth the above ground eelgrass growth is limited by the availability of below ground biomass. Between 3 and 5 m waterdepth both below ground biomass and light are the growth-limiting factors. Below 5 m waterdepth light is not sufficient to sustain net growth of eelgrass from rhizomes. Together with additional data — not used in the model — on seed production and growth of eelgrass shoots from seeds the vertical and horizontal distribution of the dominant macrophyte in the lake can be explained.     

Causes of the eelgrass wasting disease: Van der Werff's changing theories

The 1930's wasting disease among the North Atlantic population of eelgrass,Zostera marina, is still an ecological and historical enigma, despite several attractive theories. Van der Werff investigated the die-back of eelgrass in the thirties in the Dutch Wadden Sea, and he considered the micro-organismLabyrinthula as the possible cause of the disease. In 1980, Grevelingen lagoon, harbouring an extensive population ofZostera marina, experienced a major decline of the area covered by the submerged macrophyte. Speculations about the cause of this dramatic decline induced us to think that the wasting disease had struck again. Van der Werff investigated the Grevelingen population and found bothLabyrinthula and a Chaetophoracean endophytic alga to be presumably responsible for the decline. During the quest for the ultimate cause of the wasting disease the question remains whether both micro-organisms are the cause of the disease or simply an effect of decomposition processes triggered by other factors.     

Relationships between bed age,bed size,and genetic structure in Chesapeake Bay (Virginia,USA) eelgrass (<Emphasis Type="Italic">Zostera marina</Emphasis> L.)

Genetic structure and diversity can reveal the demographic and selective forces to which populations have been exposed, elucidate genetic connections among populations, and inform conservation strategies. Beds of the clonal marine angiosperm Zostera marinaL. (eelgrass) in Chesapeake Bay (Virginia, USA) display significant morphological and genetic variation; abundance has fluctuated widely in recent decades, and eelgrass conservation is a major concern, raising questions about how genetic diversity is distributed and structured within this metapopulation. This study examined the influence of bed age (<65years versus<6years) and size (>100ha versus<10ha) on morphological and genetic (allozyme) structure and diversity within Chesapeake Bay eelgrass beds. Although both morphology and genetic diversity varied significantly among individual beds (F _ST=0.198), neither varied consistently with bed age or size. The Chesapeake eelgrass beds studied were significantly inbred (mean F _IS=0.680 over all beds), with inbreeding in old, small beds significantly lower than in other bed types. Genetic and geographic distances within and among beds were uncorrelated, providing no clear evidence of isolation by distance at the scale of 10's of km. These results suggest that local environmental conditions have a greater influence on plant morphology than do bed age or size. They support the hypotheses that eelgrass beds are established by multiple founder genotypes but experience little gene flow thereafter, and that beds are maintained with little loss of genetic diversity for up to 65 years. Since phenotypic and genotypic variation is partitioned among beds of multiple ages and sizes, eelgrass conservation efforts should maximize preservation of diversity by minimizing losses of all beds.     

Decomposition processes of eelgrass,Zostera marina L.

Summary In Lake Grevelingen decomposition of eelgrass was studied in the field with the litter bag method from July 1977 till February 1978. After 6 months only 6% refractory matter remained in the bags. Under aerobic conditions the decomposition of eelgrass is completed within one year. The organic fraction decreased from 80 to 55%. Chlorophyll a was always present in the detritus, but with this parameter no decomposition stages could be distinguished. Fragmentation was mainly physical, and a particle size spectrum showed a maximum towards the small pieces.The POC content was fairly constant, and the N and P content changed during the decomposition. The C/N and C/P ratios increased the first 10 weeks (leaching) and decreased after 10 weeks (microbial colonization). This did not correspond with the three decomposition stages, based on the dry weight loss per day. The C/N ratio does not seem to be a simple index for the decomposition stage in eelgrass.Communication no. 235 of the Delta Institute for Hydrobiological Research, Yerseke, The Netherlands.     

Some reflections about the structure of the pelagic zone of the brackish Lake Grevelingen (SW-Netherlands)

Summary The seasonal succession of the plankton in the marine brackish Lake Grevelingen, a closed sea arm in the S.W.-Netherlands, comprises the initial stagessensu Margalef and is characterized by predominantly small phytoplankton (flagellates, diatoms) and zooplankton (rotifers, tintinnids, copepods), maintaining relatively high levels of production from early spring (February) to late summer (September). The structure of the plankton in the course of seasonal succession is in agreement with the concepts of Margalef.Simplification of the pelagic food web in Lake Grevelingen has occurred as a consequence of the elimination of the tides. Some examples are given in relation to the composition of the phyto- and zooplankton and of its significance. The occurrence of rotifer-dominated zooplankton blooms in early spring is emphasized.Closed sea arms such as Lake Grevelingen, showing the same morphometry as the previous tidal estuary, contain extended shallow areas which influence strongly the pelagic zone. The abundance in the zooplankton of larval stages of several littoral-benthic species demonstrate these influences clearly. The shallows of the lake, occupied by eelgrass beds (Zostera marina) in summer, influence the pelagic zone in several ways: large quantities of detritus are given off after the growing season, sheltered habitats are supplied for small pelagic animals, and eelgrass leaves represent a substrate for epifauna species.Contribution no. 168 of the Delta Institute for Hydrobiological Research.     

Canopy characteristics of the brown alga Sargassum muticum (Fucales,Phaeophyta) in Lake Grevelingen,southwest Netherlands

The present distribution of the invasive brown alga Sargassum muticum in the southwest Netherlands is updated. Populations of the alga were found to remain at their 1985 level in Lake Grevelingen, with a small eastward expansion into the Eastern Scheldt estuary. A new population for the brackish, non-tidal Lake Veere is reported. Within Lake Grevelingen S. muticum forms a persistent, extensive canopy of 100% cover (4,442.5 ± 525.6 g fresh wt m^–2, 640.3 ± 75.8 g dry wt m^–2) that has a marked effect upon the penetration of photosynthetically active radiation (PAR) (reduced by 97% at 0.1 m). Surface sea water temperatures can be elevated by 2.7 °C above water not associated with a Sargassum canopy; furthermore, the dense canopy shades and hence reduces water temperatures below 0.1 m depth. Productivity studies indicate that assimilation occurs in the upper levels of the canopy (57.09 µg C mg dry wt^–1 m^–2 at a mean PAR rate of 106.7 J cm^–2 h^–1). Self-shading and a resultant decrease in the rate of assimilation was evident below the canopy.     

Using light-permeable grating to mitigate impacts of residential floats on eelgrass Zostera marina L. in Puget Sound, Washington

This study evaluated whether light-permeable deck grating could mitigate impacts of residential mooring floats constructed over eelgrass (Zostera marina L.) in Puget Sound, Washington. Eelgrass shoot densities in undisturbed control areas and underneath and adjacent to 11 residential floats (16–50% of each float was grated) were monitored prior to float installation and annually for 3 years following installation. Using linear regression analysis, a decline in eelgrass shoot densities relative to controls was detected underneath three floats (eelgrass was eliminated under only one float) and adjacent to two floats. When control data were used to represent 100% grated, there was a weak relationship between eelgrass bed quality and percent of the deck grated (r = 0.46, p = 0.032), but no relationship when the range of grating was 16–50% (p = 0.90). The percent of a float deck grated did not contribute significantly to a multiple regression model examining change in eelgrass density that included five other dependent variables associated with the design of the floats. We conclude that either there was no effect of grating up to 50% of a float deck or we could not detect an effect. We hypothesize that the large number of site and landscape scale variables associated with a float influenced the effect (and our ability to detect it) of any one variable (such as grating). Consequently, we recommend that managers manipulate as many attributes of a float as possible (including grating) in order to reduce risks to eelgrass.     

Two new and three redescribed species of Viscosia (Nematoda,Oncholaimidae)

Viscosia coomansi sp. nov. and Viscosia heterolaina sp. nov. are described from Lake Grevelingen and Eastern Scheldt (The Netherlands). Viscosia glabra (Bastian, 1865) de Man 1890, Viscosia franzii Boucher 1977, and Viscosia viscosa (Bastian 1865) de Man 1890 are redescribed, taking into account new important characters. Juvenile specimens are depicted for V. viscosa. Viscosia carnleyensis Ditlevsen, 1921 is synonymized with Viscosia glabra (Bastian, 1865). Mononcholaimus viscosus Allgén, 1930 and Mononcholaimus elegans sensu Schuurmans-Stekhoven, 1942, 1950 (nec. Kreis, 1924) are synonymized with Viscosia viscosa (Bastian, 1865).     

The role of hoplonemerteans in the ecology of seagrass communities

Seagrasses of the world harbor a rich and varied fauna, but a review of the literature revealed that little has been done to evaluate the ecological importance of nemerteans in such communities. Monostiliferous hoplonemerteans are common inhabitants of some seagrasses, e.g. eelgrass (Zostera), but generally they are seldom collected or identified or are apparently absent in other species such as schoalgrass (Halodule) or turtlegrass (Thalassia). Nineteen species of hoplonemerteans (four families) have been identified from eelgrass beds around the world; they exist mainly as epifauna, and all except two species are probably suctorial feeders. Some palaeonemerteans (2 species) and heteronemerteans (4 species) are also associated with eelgrass, but mainly as infauna. Suctorial nemerteans (4 species in 3 families) from eelgrass beds located along the mid-Atlantic coast of the United States feed in the laboratory on a variety of amphipod species that inhabit eelgrass. Tubicolous species (e.g. Corophium) seem to be preferred. Zygonemertes virescens feeds on nine species of amphipods belonging to six families, and is the only species to feed on isopods (3 species). Analyses of field studies on the occurrence of hoplonemerteans in eelgrass beds in Virginia and New Jersey, along with available information on the food habits of these worms, were used as a basis for demonstrating their potential importance as predators of peracarids in seagrass systems. More careful methods for collecting and identifying worms, continued studies on food preferences and rates of predation, and emphasis on the population dynamics of worms and prey, are recommended in order to evaluate the role of suctorial hoplonemerteans in the ecology of seagrasses.     

The harbour seal,Phoca vitulina,in the Oosterschelde: decline and possibilities for recovery

Within a timespan of a few decadesm the harbour seal almost completely disappeared from the estuaries in the south-west of the Netherlands. In 1960 a population of around 350 animals still lived in the Oosterschelde and Westerschelde area. About a quarter of this population lived in the Oosterschelde. At present less than 17 animals can be regularly observed in the whole area. Human influences are responsible for the rapid decline of the population. Initially a high hunting pressure and later environmental pollution are the main causes. Loss of habitat and disturbance at the resting places are additional important factors. The Oosterschelde still is a suitable habitat for seals. A short term natural development of a viable population in the area is not to be expected. Only with human help through active management, i.e. reintroduction of rehabilitated seals (preferably originating from that area) and strict conservation of the extant Oosterschelde seal population, accompanied by environmental sanitation of the neighbouring waters, can the current southern Dutch harbour seal population increase.     

Limited recovery following a massive seagrass decline in subarctic eastern Canada

Over the last few decades, there has been an increasing recognition for seagrasses' contribution to the functioning of nearshore ecosystems and climate change mitigation. Nevertheless, seagrass ecosystems have been deteriorating globally at an accelerating rate during recent decades. In 2017, research into the condition of eelgrass (Zostera marina) along the eastern coast of James Bay, Canada, was initiated in response to reports of eelgrass decline by the Cree First Nations of Eeyou Istchee. As part of this research, we compiled and analyzed two decades of eelgrass cover data and three decades of eelgrass monitoring data (biomass and density) to detect changes and assess possible environmental drivers. We detected a major decline in eelgrass condition between 1995 and 1999, which encompassed the entire east coast of James Bay. Surveys conducted in 2019 and 2020 indicated limited changes post-decline, for example, low eelgrass cover (<25%), low aboveground biomass, smaller shoots than before 1995, and marginally low densities persisted at most sites. Overall, the synthesized datasets show a 40% loss of eelgrass meadows with >50% cover in eastern James Bay since 1995, representing the largest scale eelgrass decline documented in eastern Canada since the massive die-off event that occurred in the 1930s along the North Atlantic coast. Using biomass data collected since 1982, but geographically limited to the sector of the coast near the regulated La Grande River, generalized additive modeling revealed eelgrass meadows are affected by local sea surface temperature, early ice breakup, and higher summer freshwater discharge. Our results caution against assuming subarctic seagrass ecosystems have avoided recent global declines or will benefit from ongoing climate warming.     

Influence of Habitat Fragmentation on the Genetic Variability in Leaf Litter Ant Populations in Tropical Rainforests of Sabah, Borneo

Two ant species, Odontomachus rixosus and Pheidole annexus, were studied in the tropical rainforests of Sabah, Malaysia, North Borneo, to analyze the impact of habitat fragmentation on the genetic diversity and population structure of ant populations using RAPD-fingerprinting. Ants were sampled in a contiguous (43,800 ha) and three patches of primary rainforests of varying size (4294, 146 and 20 ha) that were fragmented about 40 years ago. We found a decrease in genetic variability for both species in the fragmented populations compared to the contiguous. Genetic distances between populations resembled the geographical arrangement of populations and can be explained by an effect of isolation by distance. A high degree in population subdivision suggests a lack of meta-population dynamics due to a shortage of gene flow between populations, possibly the result of the high degree of habitat isolation by oil palm plantations. Although the results of this study are limited due to low replication this is the first data on genetic patterns of insect populations in fragmented rainforests and should be seen as starting point for future research. The value of small to medium sized protection areas for conservation needs to be carefully evaluated in the context of this study, as even relatively large areas (4294 ha) may not prevent the critical loss of genetic variability and guarantee long-term survival of organisms.     

The Distribution of Nearshore Fishes in Kelp and Eelgrass Communities in Prince William Sound, Alaska: Associations with Vegetation and Physical Habitat Characteristics

The nearshore (less than 20m depth) demersal fish community in Prince William Sound, Alaska, is dominated by Pacific cod, Gadus macrocephalus, pricklebacks (mostly Arctic shanny Stichaeus punctatus), gunnels (mostly crescent gunnels Pholis laeta), a variety of greenlings (Hexagrammidae) and sculpins (Cottidae). During summer, the spatial distribution of fishes, over scales of 100's of m to 10's of km, varied by habitats characterized by different vegetation types. Juvenile Pacific cod and greenlings were numerically dominant in eelgrass, Zostera marina, beds. Pricklebacks and sculpins were dominant in areas with an understory of the kelps Agarum cribrosum and Laminaria saccharina. Greenlings and sculpins were the most abundant demersal fishes in more exposed sites with a canopy of Nereocystis luetkeana and an understory of L. bongardiana. Measured habitat variables, including vegetation type, slope, vegetation biomass, and substratum type, explained a significant proportion of the variation in the presence or absence of most fishes. The relative importance of different habitat characteristics varied between taxonomic groups of fishes. Vegetation type explained a significant proportion of variation for cod, rockfishes, and ronquils. Juvenile cod were closely associated with eelgrass, while rockfish and ronquils were associated with kelps. Pricklebacks and rockfishes were more frequently observed on steeply sloped shorelines, while ronquils were more often found at sites with higher biomass of vegetation. Within A. cribrosum habitats, more greenlings and sculpins were present at sites where algal biomass was higher. Also, sculpins were more abundant in deeper water and gunnels were more abundant in shallow water within this habitat. These associations may not have been causative. However, evidence suggests that some differences between fish communities in eelgrass and Agarum beds may have been causally related to vegetation characteristics. The possible roles of different vegetation types as refugia from predators or as sources of prey are discussed.     

On the distribution of ‘gonionemus vertens’ a. Agassiz (Hydrozoa,Limnomedusae), a new species in the eelgrass beds of Lake Grevelingen (S.W. Netherlands)

Summary Data about morphology, life history and habits ofGonionemus vertens A. Agassiz (Hydrozoa, Limnomedusae) are given. The distribution of the species is treated in more detail, especially its mechanism in relation to large-scale (global) and small-scale (local) range extensions.The species is endemic in the coastal North Pacific Ocean, possessing a strictly littoral medusa stage (bound to eelgrass beds and algal belts) and a very small sessile solitary polyp phase (attached to stones and shells).The hypotheses trying to explain the spread to Western Europe and the Atlantic coast of North America strongly suggest human influences,i.e. distribution of the polyp via oyster transports (EDWARDS, 1976) and transport of the polyp as a member of the fouling community on ships hulls (TAMBS-LYCHE, 1964). Arguments pro and contra these hypotheses are evaluated. The occurrence ofGonionemus polyps in offshore waters, as mentioned by EDWARDS (1976) but not further discussed by this author, deserves more attention.It is the assumed occurrence of the polyps in shallow as well as in deeper waters with strong tidal movements that explains satisfactorily the previous and recent findings of the medusa in the S.W.-Netherlands. The medusa has been found in theZostera marina beds of the saline Lake Grevelingen. From 1976 onwards the abundance of the medusa increased synchronously with the extension of the eelgrass beds. Medusae were collected during the summer of 1980, fed with isopod crustaceans or mussel meat and kept alive for several months in seawater aquaria in the lab. Polyps have not yet been observed.Communication no. 211 of the Delta Institute for Hydrobiological Research.     

Temperature dependence of sediment-water exchange in Lake Grevelingen,SW Netherlands

Extended abstract Lake Grevelingen is a brackish water lake in the SW Netherlands. The lake has an area of 108 km², a mean depth of 5.3 m (maximum 48 m), a mean chlorinity of 13 to 16%0 Cl⁻, and a hydraulic residence time of about 8 years. Mass budget studies have shown a consistent seasonal pattern in the phosphorus sediment-water exchange in Lake Grevelingen (Kelderman 1980). From May to August a P mobilization from the sediment takes place, estimated atca. 12.5 mg P · m⁻² · day⁻¹. The sediment accumulatesca. 5.5 mg P · m⁻² · day⁻¹ during the rest of the year. Temperature may be an important factor in establishing this pattern. Sediment-water exchange was studied by means of laboratory experiments under specified conditions. Sediment cores (30 cm depth, 11 cm diameter) were taken at four stations in the lake, with sediment types varying from medium- to muddy sand (Fig. 1). The cores with overlying water (ca. 21) were placed in the dark at 5 °C in thermostatically controlled water baths. After a week's incubation time the temperature was slowly raised, such that after three weeks eight cores (four sediment types, duplicates) were at 5 °C, eight were at 10 °C, eight at 15 °C and eight at 20 °C. The same procedure was applied to the four control cores, containing lake water.     

Long-term change in water transparency before and after the loss of eelgrass beds in an estuarine lagoon,Lake Nakaumi,Japan

Long-term changes in Secchi disk transparency in Lake Nakaumi, Japan, from 1932 to the present, which includes the periods before and after the loss of eelgrass (Zostera marina L.) beds, were compiled from previous reports. During the first (July 1932–February 1934) and the second (January 1949–November 1950) periods, the mean transparency was greater than 3 m at all observed stations. Data during the third period (July 1954–March 1956) are only available for the station at the center of the lake. The mean transparency during the third period was significantly lower than that of the second period but higher than the mean during the fourth period. This observation suggests that the decrease of transparency occurred during the third period. Because the decline of eelgrass beds in Lake Nakaumi occurred in the mid-1950s, the decrease of transparency most likely resulted from the shift in primary producers from submerged macrophytes to phytoplankton. Although the maximum transparency sometimes exceeded 3 m, mean transparencies during the fourth period (May 1975–December 2003) were less than 2 m at all stations, significantly lower than those during the first and second periods. The shift in the chief primary producer, from benthic macrophytes to phytoplankton, caused a subsequent shift in secondary producers. The opportunistic filter-feeding bivalve Musculista senhousia, regarded as a biofouling species of local fisheries, increased in Lake Nakaumi. The long-term monitoring data of transparency suggested that restoration of submerged aquatic vegetation may be indispensable for the remediation of the lake environment in this shallow eutrophic lagoon.     

On the habitats,distribution and life-cycles of the Western European species of the genus Helochares Mulsant (Coleoptera: Hydrophilidae)

Adult water beetles were collected from 841 localities in the Netherlands. The distribution maps of the three species of the genus Helochares (Coleoptera; Hydrophilidae) in the Netherlands are given. They are based on specimens from museum collections and this investigation. H. lividus occurs all over the Netherlands, except for the north-east. This northern limit of its European distribution area coincides with the 8.5°C isotherm for the mean year temperature. H. obscurus is limited in its distribution to clay and peat-bog, but in the sandy coastal dune area H. obscurus replaces H. punctatus. H. punctatus is limited in its distribution to soils consisting of peat-moor or sand, but it is absent from the coastal dune area.Co-existence of H. lividus with either H. obscurus or H. punctatus occurs regularly. Co-existence of H. obscurus and H. punctatus has been observed only once, these species exclude each other by different ecological requirements. H. lividus and H. obscurus show two optima in abundance during the year which corresponds with a univoltine life-cycle. H. punctatus shows no clear maximum in abundance. However, on the basis of the presence of females carrying egg-cases and the presence of teneral adults a univoltine life-cycle for all three Helochares spp. can be deduced. A small proportion of the beetles hibernates either as larva or as pupa and emerges in spring.The distribution of Helochares spp. has been related to some environmental variables using the Index of Representation. H. lividus has a clear preference for waters with a high pH, high conductivity and high chlorinity with a luxurious vegetation of floating leaved and submerged plants on mineral soils. Occasionally running waters or waters with a low current velocity are preferred. Preferred habitats are regulated lowland streams, clay- and sand-ditches, and clay- and sand-canals. H. obscurus has a clear preference for stagnant waters with a high pH, high conductivity and high chlorinity (but the optimum is lower than for H. lividus) with a luxurious vegetation of floating leaved and submerged plants. Preferred habitats are clay- and sand-ditches. H. punctatus has a very clear preference for waters with a low pH, low conductivity and low chlorinity. With respect to vegetation and type of current the species shows no significant preferences or aversions. Preferred habitats are moorland pools, peat-canals and peat-cuttings.     

Reproduction and larval development of the gastropodAlaba vladivostokensis in Vostok Bay, the Sea of Japan

The reproduction and development of the prosobranch gastropodAlaba vladivostokensis Bartsch were studied. In Vostok Bay,A. vladivostokensis reproduces in the warmest season at water temperatures of 19–23°C. The females lay egg masses in the shape of flat, coiled bands on eelgrass leaves and on sargosso thalluses. The total period of development from egg laying to larval settling takes 25–27 days. The reproduction and development patterns and larval morphology ofA. vladivostokensis are similar to that ofAustralaba picta (A. Adams), which inhabits the coast of Japan. It is suggested that the population ofA. vladivostokensis in Peter the Great Bay should considered as part of the wide geographical range ofA. picta.