Cytological evidence of spontaneous androgenesis in the freshwater clam Corbicula leana Prime

 Cytological observations and DNA microfluorometry of the hermaphrodite freshwater triploid clam Corbicula leana revealed unusual androgenetic development as follows: (1) the maternal genome of zygotes was extruded as two polar bodies just after karyokinesis of the first meiosis, (2) only chromosomes derived from one male pronucleus constituted the metaphase of the first cleavage of zygotes, (3) DNA content of 7-day-old veliger larvae was identical to the somatic cells of the parent. This spontaneous androgenetic process in C. leana zygotes is the first case in the phylum Mollusca and may be related to the specialized mode of reproduction; i.e. hermaphroditism and self-fertilization. Received: 22 September 1997/Accepted: 7 December 1997     

Grazing rates of crustacean zooplankton communities on intact phytoplankton communities in Canadian Subarctic lakes and ponds

Quantitative genetic studies in natural populations are of growing interest to speciation research since divergence is often believed to arise through micro-evolutionary change, caused by natural selection on functional morphological traits. The species flock of cichlid fishes in Africa’s oldest lake, Lake Tanganyika, offers a rare opportunity to study this process. Using the cichlid species Tropheus moorii, we assessed the potential for microevolution in a set of morphological traits by estimating their quantitative genetic basis of variation. Two approaches were employed: (1) estimation of trait heritabilities (h ²) in situ from a sample of wild caught fish, and (2) estimation of h ² from first generation offspring produced in a semi-natural breeding experiment. In both cases, microsatellite data were used to infer pedigree structure among the sampled individuals and estimates of h ² were made using an animal model approach. Although power was limited by the pedigree structures estimated (particularly in the wild caught sample), we nonetheless demonstrate the presence of significant additive genetic variance for aspects of morphology that, in the cichlid species Tropheus moorii, are expected to be functionally and ecologically important, and therefore likely targets of natural selection. We hypothesize that traits showing significant additive genetic variance, such as the mouth position have most likely played a key role in the adaptive evolution of the cichlid fish Tropheus moorii.     

Two mitochondrial lineages in Korean freshwater Corbicula (Corbiculidae: bivalvia)

The Korean freshwater Corbicula was surveyed genetically by sequencing 614-bp homologous fragment of mitochondrial cytochrome oxidase I subunit. Among a total of 127 individuals collected from 12 Korean freshwater localities we found only two COI haplotypes and these differed by a total of 9 base substitutions. Although the sequence divergence between the two haplotypes is moderate (p = 1.47%), placing the two mitotypic sequences in the context of Asian mtDNA phylogeny reveals that Korean freshwater Corbicula is comprised of two independent freshwater mitochondrial lineages. These results are in serious disagreement with the long-standing conclusions of earlier conchology-based taxonomic work on Korean Corbicula in which a number of species names (a minimum of 10 nominal species) have been used. This indicates that morphological characteristics alone are poor criteria for species-level identification in this group. In addition, our COI dataset shows that there is an extremely low level of genetic variation in Korean freshwater populations, suggesting that these populations have passed through a severe population bottleneck that greatly reduced their genetic variability. Our data also provide new information on the biogeographic distribution of Korean freshwater Corbicula. When haplotypic frequencies were compared, it was evident that the two Korean freshwater mitochondrial lineages have achieved very different distribution ranges: the predominant lineage (FWKR1) is widely distributed in Korean freshwater systems, whereas the minor lineage (FWKR2) is restricted to a relatively narrow range.     

Relationships between zooplankton community structure and phytoplankton in two lime-treated eutrophic hardwater lakes

1. North Halfmoon Lake and Lofty Lake (Alberta, Canada) were chosen for whole-lake liming experiments as a new restoration technology to enhance calcite precipitation and reduce eutrophication. During a 3-year study (1991–93) the relationships between zooplankton and phytoplankton were assessed, together with the effects of lime additions. 2. Zooplankton communities were numerically dominated by rotifers, while the major contribution to biomass was due to large filter-feeding Daphnia during the first half of the summer season. In Lofty Lake, cladocerans made up to 93% of biomass, whereas in North Halfmoon Lake both cladocerans and calanoids were strongly represented. 3. Total zooplankton and cladoceran biomasses were inversely correlated with chlorophyll a (chl a). The same relationship was found between large Daphnia (≥ 1 mm) and chl a. These relationships suggest that the decline in Daphnia may have been caused by an increase in cyanobacteria biomass during bloom events. 4. There were minor changes in rotifer populations after liming; however, these changes have been caused by natural year-to-year variation rather than liming. In general, cladocerans showed an increase in body size and population biomass when pre and post-treatment data were compared by means of ANCOVA. Statistical analysis showed that there were more cladocerans per unit of chl a after liming; however, further research is needed to relate these patterns unambiguously to the application of lime as a restoration technology.     

Diurnal changes in zooplankton respiration rates and the phytoplankton activity in two Chilean lakes

Phytoplankton spatial distribution patterns in the Abra of Bilbao (a semienclosed coastal body of water) and adjacent shelf waters have been studied during June–July 1983 and May–June 1984. Small naked dinoflagellates, cryptophyceans and an unidentified nanoplankton component, were a common feature in all surveys. In July 1983 a dense bloom of nanoplankton developed inside the Abra which, in contrast to the community in the adhacent waters, contained high densities of small diatoms, naked dinoflagellates, cryptophyceans and the Haptophyta Phaeocystis pouchetii. Microplankton was mainly composed of dinoflagellates in July 1983, and of diatoms in June 1983 and May–June 1984. Microplankton abundance was highest in May–June 1984 and decreased from the shelf to the Abra. A principal component analysis performed separately on each cruise revealed the differences in the structure of the phytoplankton community between the Abra of Bilbao and the adjacent shelf waters.     

Rapid evolution of sessility in an endemic species flock of the freshwater bivalve Corbicula from ancient lakes on Sulawesi, Indonesia

The fauna of ancient lakes frequently contains taxa with highly derived morphologies that resulted from in situ radiation of lacustrine lineages with high antiquity. We employed a molecular mtDNA phylogeny to investigate this claim for corbiculid freshwater bivalves in two ancient lake systems on the Indonesian island Sulawesi. Among the otherwise mobile corbiculid species flock, only one taxon, Posostrea anomioides, in the ancient Lake Poso exhibits a unique habit, i.e. cementing one valve to the substrate. Our data show that Corbicula on Sulawesi is polyphyletic, with the endemic riverine taxa in terminal position, and the lacustrine species flock being paraphyletic. Surprisingly, Posostrea is not confirmed as a genus distinct from Corbicula and genetic distances suggest a rather recent origin from the only other corbiculid species endemic to Lake Poso, the non-cementing Corbicula possoensis. While the cementing anomioides, despite its unique behavioural and morphological characteristics, clusters together with non-sessile Corbicula species, the latter exhibit strong genetic distances in the absence of morphological disparity and fall into several genetically rather distinct clades. These findings suggest that developmental plasticity of animals in ancient lakes rather than the antiquity of lineages might account for the unique morphology of some species.     

Short-term productivity responses of algae and bacteria to zooplankton grazing in two freshwater lakes

SUMMARY. 1. The specific productivities of algae and bacteria were measured in short-term (4 day) experiments consisting of enclosures with natural or reduced zooplankton biomass. Experiments were repeated five times over a season in each of two lakes that differed in the background concentration of dissolved organic carbon (DOC).
2. Algal biomass as estimated by chlorophyll a was suppressed in enclosures with ambient grazer levels in six of ten experiments and enhanced in one experiment. Distribution of chlorophyll among net and nanoplankton was not significantly affected by grazing.
3. Relative to enclosures with reduced zooplankton, normal grazer biomass (97–466μg 1⁻¹ dry weight) enhanced specific algal productivity in only one of five experiments in the low DOC take and had no effect in all five experiments in the high DOC lake. The main effects of grazers on algae was through removal of biomass rather than through indirect changes in turnover rate.
4. Between experiments, bacterial density was either unaffected, or mildly enhanced (4–87%) in enclosures with ambient macrozooplankton compared to those with reduced levels. Bacterial productivity and turnover estimated by incorporation of [³H]thymidine into DNA showed different responses across experiments; increasing, declining or remaining the same with grazer minipulation. This variability was not related to differences in dissolved primary production or to background DOC between lakes or experiments. Comparison of bacterial productivities based on thymidine incorporation rates with changes in cell densities indicated that control of bacterial loss processes by macrozooplankton is more important than control of growth rates.     

Comparative limnology,species diversity and biomass relationship of zooplankton and phytoplankton in five freshwater lakes in Kenya

Comparative studies on the limnology, species diversity and standing stock biomass of phytoplankton and zooplankton in five freshwater lakes, Naivasha and Oloidien, Ruiru, Masinga and Nairobi reservoirs, were undertaken. Phytoplankton chlorophyll a, dissolved oxygen and temperature were also measured. Thermocyclops oblongatus (Copepoda) was dominant in all the lakes. Ceriodaphnia cornuta and Diaphanosoma excisum (Cladocera) dominated in lakes Naivasha and Oloiden, whereas in Ruiru, Masinga and Nairobi reservoirs, Brachionus angularis and Hexarthra mira (Rotifera) were the dominant zooplankters. Phytoplankton biomass as chlorophyll a was lowest in Ruiru dam 5.64 ± 4.0 µg l^-1 and highest in the eutrophic Nairobi dam 71.5 ± 12.02 µg l^-1. The endorheic lakes Naivasha and Oloidien showed medium values of 24.5 ± 4.0 µg l^-1.     

Predation-driven dynamics of zooplankton and phytoplankton communities in a whole-lake experiment

Summary 1. Species compositions of zooplankton and phytoplankton were followed in Tuesday Lake before and after experimental manipulation of its fish populations (addition of piscivorous largemouth bass, removal of planktivorous minnows). Plankton dynamics were compared to those of adjacent, unmanipulated Paul Lake, where piscivorous fish have been dominant historically. 2. Indices of similarity for the zooplankton communities in the two lakes in 1984 prior to the manipulation were low; however, following the manipulation in spring, 1985, similarity of the zooplankton in the two lakes rose considerably and remained high throughout 1986. This was the result of an increase in Tuesday Lake of previously rare large-bodied cladocerans (Daphnia pulex, Holopedium gibberum) which were the dominants in Paul Lake, and the disappearance in Tuesday Lake of the dominant small-bodied copepod Tropocyclops prasinus, a minor component of the Paul Lake zooplankton. These observations are consistent with prior observations of the effects of size-selective predation on zooplankton communities. 3. Phytoplankton communities also responded strongly to the manipulation, with similarity indices for the two lakes rising from low levels in 1984 to high levels of similarity in 1985 and 1986, reflecting the decrease of formerly dominant Tuesday Lake taxa which were unimportant in Paul Lake and the appearance or increase in Tuesday Lake of several taxa characteristic of the Paul Lake phytoplankton assemblage. these results clearly show that food web structure can have pronounced effects on community composition at all levels of the food web, and that, just as zooplankton communities are structured by sizeselective predation, phytoplankton communities are structured by herbivory. These observations may provide some insight into factors governing the complex distributions of phytoplankton species among various lakes.A contribution from the University of Notre Dame Environmental Research Center, funded by NSF grants BSR-83-08918 and BSR-86-06271     

Interaction effects of zooplankton and CO2 on phytoplankton communities and the deep chlorophyll maximum

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The gametogenesis and sexual cycle of the bivalve Corbicula japonica Prime (1864) in the mouth of the Kievka River (Sea of Japan)

Oogenesis, spermatogenesis, and the sexual cycle of the bivalve Corbicula japonica in the estuary of the Kievka River (Primorye, Sea of Japan) were studied. The stages of development of germ cells and the role of auxiliary cells in gonads during vitellogenesis were described. The features of sperm morphology were recorded. It was shown that the reproductive cycle in C. japonica is determined by the water temperature, that gametogenesis starts at the boundary between April and May, that spawning occurs from mid-July to mid-September, and that gonads are in a stage of relative sexual inactivity in the autumn and winter.     

Effects of nutrient recycling by zooplankton and fish on phytoplankton communities

In laboratory experiments we tested the hypothesis that nutrients supplied by fish and zooplankton affect the structure and dynamics of phytoplankton communities. As expected from their body size differences, fish released nutrients at lower mass-specific rates than Daphnia. On average, these consumers released nutrients at similar N:P ratios, although the ratios released by Daphnia were more variable than those released by fish. Nutrient supply by both fish and Daphnia reduced species richness and diversity of phytoplankton communities and increased algal biomass and dominance. However, nutrient recycling by fish supported a more diverse phytoplankton community than nutrient recycling by Daphnia. We conclude that nutrient recycling by zooplankton and fish have different effects on phytoplankton community structure due to differences in the quality of nutrients released. Received: 21 December 1998 / Accepted: 31 May 1999     

Cercariae (Platyhelminthes: Trematoda) as neglected components of zooplankton communities in freshwater habitats

Zooplanktonic animals live in the open water of freshwater habitats, whilst trematodes are ubiquitous parasitic worms of aquatic vertebrates and invertebrates. A defining characteristic of trematode life cycles is the cercariae, a mainly planktonic free-living larval stage that emerges into the aquatic environment from infected molluscan hosts. Cercariae are lecithotrophic (non-feeding) larvae analogous of meroplankton. Although millions of planktonic cercariae enter freshwater habitats on a daily basis and are capable of forming into dense clouds, they are a largely neglected component of the zooplankton community, rarely mentioned in faunal studies. Nevertheless, there is increasing evidence to suggest that cercariae have important secondary roles in aquatic food webs and energy transfer. This article reacquaints freshwater biologists with cercariae, highlighting the key characteristics of their biology and population dynamics, their role in food webs, public health and veterinary importance, and the risk of increased population densities under the influence of climate change. The reasons for their neglect in freshwater biology is evaluated and considered to be associated with zooplankton sampling methodologies being unsuitable for collecting cercariae and the paucity of identification keys in the limnological literature.     

Feeding selectivity of Corbicula fluminea (Bivalvia) on natural phytoplankton

Phytoplankton in water samples from the lower delta of the Paraná river (Argentina) and in the gut contents of a local populaton of the Asiatic clam Corbicula fluminea was identified and counted monthly between April 1992 and March 1993. Relative abundances of algal species were generally similar in the medium and in the stomachs of clams, suggesting the absence of feeding selectivity on the basis of taxonomic group or size-class. Diatoms made up a slightly but consistently larger (yet not significantly different) proportion of the diet of C. fluminea than of the plankton; this pattern is attributed to enhanced preservation of algal siliceous frustules, rather than to feeding selectivity. It is suggested that non-selective feeding by the bivalve is a response to the overall scarcity of food and, probably, to food collection by a combination of deposit and filter feeding.     

The impact of an introduced bivalve (Corbicula fluminea) on the benthos of a sandy stream

1. The effect of an exotic, burrowing bivalve ( Corbicula fluminea ) on the benthic fauna of a sandy-bottomed stream was assessed by field and laboratory experiments. Corbicula differs from other freshwater, non-native bivalves in that it both filter- and pedal-feeds and thus has the potential to influence the streambed community in different ways.
2. In the field, cages were used to vary the abundance of Corbicula in the streambed. Increasing abundance of Corbicula was negatively associated with the abundance of benthic bacteria and flagellates but had no apparent effect on other protists or meiofauna.
3. In the laboratory, we compared the effect of Corbicula on the benthic community when the bivalves were able both to filter- and pedal-feed with that when they were able only to filter-feed. Bivalves restricted to filter-feeding were placed in sediment lacking benthic fauna and organic matter, then the benthos in nearby natural sediment was compared with the community present when Corbicula was able to move freely through the sediment and both to filter- and pedal-feed. Corbicula able to pedal-feed were again associated with a decreased abundance of benthic flagellates and bacteria, as well as diatoms.     

Effects of aquatic macrophytes on water quality and phytoplankton communities in shallow lakes

We investigated aquatic macrophytes, water quality, and phytoplankton biomass and species composition in three shallow lakes with different levels of vegetation cover and nutrient concentration in Kushiro Moor, during August 2000. Trapa japonica can live in a wide range of nutrient levels. This species forms an environment with a steeper extinction of light, higher concentrations of dissolved organic carbon (DOC), lower concentrations of dissolved oxygen (DO) near the bottom, and lower concentrations of nitrate+nitrite and soluble reactive phosphorus (SRP) than other vegetation types. The pH was much higher in a Polygonum amphibium community, and the DO near the bottom did not decrease compared to a T.japonica community in the summer. The relationship between chlorophyll a and the limiting nutrient (total phosphorus (TP) when total nitrogen (TN):TPis 10 and TN/10 when TN:TP is <10) significantly differed between lakes with and without submerged vegetation. The chlorophyll a concentrations at a given nutrient level were significantly lower in water with submerged macrophytes than in water without them. Correspondence analysis showed that the difference in phytoplankton community structure across sites was largely due to the presence or absence of submerged macrophytes, and the ordination of phytoplankton species in the lakes with submerged macrophytes is best explained by environmental gradients of TN, chlorophyll, pH and SRP.     

The annual patterns of photosynthesis in two large, freshwater, ultra-oligotrophic Antarctic lakes

The phytoplankton productivity and biomass of two large, freshwater Antarctic lakes (Vestfold Hills, eastern Antarctica) were investigated over a 12-month period. Crooked Lake was sampled at one site, while Lake Druzhby, a complex lake with two shallow and one deep basin, was subject to a more detailed investigation. Concentrations of chlorophyll a were usually below 1 µg l^-1, indicating ultra-oligotrophic conditions. Despite periodic low nutrient levels, low temperatures (range 0.4-2.8°C) and periodic poor light climate, some degree of photosynthesis was measurable throughout the year, including the dark winter phase. Snow cover had a pronounced impact on the light climate of the water column and inhibited photosynthesis. Mean rates of carbon fixation in the 0- to 15-m water column varied between 0 and 38.47 µg C l^-1 day^-1 in Crooked Lake and 0.24 and 37.68 µg C l^-1 day^-1 in Lake Druzhby. There were significant differences in the seasonal patterns of primary production between the basins of Lake Druzhby. The shallow basins had highest productivity in August, whereas the deep basin had highest rates in summer. Chlorophyll specific rates of photosynthesis or assimilation numbers [µg C (chl. a)^-1 h^-1] varied between 0.05 and 44.9, and photosynthetic efficiency [µg C (chl. a)^-1 h^-1 µmol m^-2 s^-1] between 0.02 and 5.19. The data suggest that the phytoplankton of these lakes is adapted to low irradiance levels, low temperatures and nutrient limitation.     

Distribution of sediment bacterial and archaeal communities in plateau freshwater lakes

Both Bacteria and Archaea might be involved in various biogeochemical processes in lacustrine sediment ecosystems. However, the factors governing the intra-lake distribution of sediment bacterial and archaeal communities in various freshwater lakes remain unclear. The present study investigated the sediment bacterial and archaeal communities in 13 freshwater lakes on the Yunnan Plateau. Quantitative PCR assay showed a large variation in bacterial and archaeal abundances. Illumina MiSeq sequencing illustrated high bacterial and archaeal diversities. Bacterial abundance was regulated by sediment total organic carbon and total nitrogen, and water depth, while nitrate nitrogen was an important determinant of bacterial diversity. Proteobacteria, Acidobacteria, Actinobacteria, Bacteroidetes, Chlorobi, Chloroflexi, Cyanobacteria, Firmicutes, Gemmatimonadetes, Nitrospirae, Planctomycetes, and Verrucomicrobia were the major components of sediment bacterial communities. Proteobacteria was the largest phylum, but its major classes and their proportions varied greatly among different lakes, affected by sediment nitrate nitrogen. In addition, both Euryarchaeota and Crenarchaeota were important members in sediment archaeal communities, while unclassified Archaea usually showed the dominance.     

Effects of biological behaviors of benthic bivalve (Corbicula japonica) on its passive transport

Hydrobiologia - The bivalve Corbicula japonica is a popular food in Japan. The abundance of the clam in a lake changes over time. Clam transport is assumed to be passive even after the clam’s...     

Ecological effects of low-level phosphorus additions on two plant communities in a neotropical freshwater wetland ecosystem

We conducted a low-level phosphorus (P) enrichment study in two oligotrophic freshwater wetland communities (wet prairies [WP] and sawgrass marsh [SAW]) of the neotropical Florida Everglades. The experiment included three P addition levels (0, 3.33, and 33.3 mg P m^–2 month^–1), added over 2 years, and used in situ mesocosms located in northeastern Everglades National Park, Fla., USA. The calcareous periphyton mat in both communities degraded quickly and was replaced by green algae. In the WP community, we observed significant increases in net aboveground primary production (NAPP) and belowground biomass. Aboveground live standing crop (ALSC) did not show a treatment effect, though, because stem turnover rates of Eleocharis spp., the dominant emergent macrophyte in this community, increased significantly. Eleocharis spp. leaf tissue P content decreased with P additions, causing higher C:P and N:P ratios in enriched versus unenriched plots. In the SAW community, NAPP, ALSC, and belowground biomass all increased significantly in response to P additions. Cladium jamaicense leaf turnover rates and tissue nutrient content did not show treatment effects. The two oligotrophic communities responded differentially to P enrichment. Periphyton which was more abundant in the WP community, appeared to act as a P buffer that delayed the response of other ecosystem components until after the periphyton mat had disappeared. Periphyton played a smaller role in controlling ecosystem dynamics and community structure in the SAW community. Our data suggested a reduced reliance on internal stores of P by emergent macrophytes in the WP that were exposed to P enrichment. Eleocharis spp. rapidly recycled P through more rapid aboveground turnover. In contrast, C. jamaicense stored added P by initially investing in belowground biomass, then shifting growth allocation to aboveground tissue without increasing leaf turnover rates. Our results suggest that calcareous wetland systems throughout the Caribbean, and oligotrophic ecosystems in general, respond rapidly to low-level additions of their limiting nutrient.