The impairment of mobility and development in freshwater snails (Physa fontinalis and Lymnaea stagnalis) caused by herbicides

1. The pulmonate freshwater snails Physa and Lymnaea and the earthworms Eisenia and Lumbricus can take up and concentrate a carbamate herbicide (chlorpropham). 2. The mobility of freshwater snails was diminished in the presence of the herbicides chlorpropham (carbamate), cycloate (thiocarbamate), pentanochlor (amide) and chloroxurone (urea derivate). 3. The egg-assemblies of Lymnaea stagnalis turned out to be suitable objects for testing the influences of herbicides upon embryonic development. 4. In the presence of chlorpropham, chloroxurone, cycloate, propanil, simazine and terbutryne, all applied in concentrations lower than in practical use, the period of egg-maturing was delayed and the total number of dead embryos increased. 5. Developing snail eggs were very sensitive towards triazine herbicides (simazine, terbutryne) reflecting in an ED50-value lower than 10(-7) M. 6. Similar herbicidal-induced effects suggested that the developing stages of snail embryos may be suitable models for the ecologically more important but experimentally less accessible earthworms.     

Physid snails as sentinels of freshwater nematomorphs.

Freshwater nematomorphs, or gordiids, are parasitic as larvae, but free-living in aquatic environments as adults. Studies based on the collection of adults have reported gordiids to be widespread, but discontinuous in distribution. However, a relatively short adult life span and unknown life history make the detection of adults difficult. An alternative approach to investigate gordiid distribution is to use cysts. Of all paratenic hosts, snails were chosen because they lacked internal defense reactions to the cysts and become easily infected. Here, it is reported that the occurrence of gordiids on the basis of the cyst stage is much more common than previously reported, thus altering the perception of how common these worms are. Using this modified survey procedure, gordiid cysts were found at 70% of sites examined, in an area where extensive sampling over 3 yr yielded adults only at a single site. Of 1,000 snails dissected, 395 were infected with gordiids (intensity range: 1-465). Furthermore, different types of human-modified landscapes did not affect gordiid distribution, suggesting that as urban and suburban areas sprawl, human encounters or pseudoparasitism with nematomorphs may increase. The results of this study indicate that use of organismal-specific sampling techniques can be critical in studies of parasite distribution and biodiversity.     

Larval trematode infections in freshwater snails from the highveld and lowveld areas of Zimbabwe

Between November 1998 and October 2000, freshwater snails were collected monthly from the highveld and lowveld areas of Zimbabwe to determine the occurrence of larval trematodes. A total of 13,789 snails, representing ten species, were collected from 21 sites and 916 (6.6%) harboured patent trematode infections. Eight morphologically distinguishable types of cercariae were identified. Bulinus tropicus had the highest overall prevalence of infection (13.1%). The echinostome was the most common type of cercaria recovered, contributing 38.2% of all infections. Schistosoma cercariae were recovered mainly from the highveld and comprised 8.0% of all infections. Amphistome cercariae contributed 37.6% of all infections and were recorded from both the highveld and lowveld areas with a peak prevalence occurring during the post-rainy period (March-May). The main intermediate host for amphistomes was B. tropicus. Infections in B. globosus, B. forskalii and Biomphalaria pfeifferi with amphistome cercariae are new records for Zimbabwe.     

The radiation of spring snails of the genus Belgrandiella in Austria (Mollusca: Caenogastropoda: Hydrobiidae)

Grain density microautoradiography (MAR) was used to study uptake of dissolved amino acids (DFAA) in microalgal communities from a sandy bay on the west coast of Sweden. A mixture of fifteen ³H-labeled amino acids (final concentration 20–80 nmol l^–1) was added to sediment samples collected from two depths (0.5 and 4 m), on five occasions representing different seasons. On all sampling occasions, the microflora was dominated by diatoms (> 85% of the total biomass). Cyanobacteria occurred in the summer and autumn, but never dominated the biomass. Between 5 and 48% of all counted algal cells showed uptake (1–44% of algal biomass). Uptake was recorded for all majors microalgal groups (diatoms, cyanobacteria and autotrophic flagellates). Uptake was more frequent on the shallow site and the highest proportion of cells showing uptake occurred in May. Although uptake was common among both motile and attached growth forms, on several occasions uptake frequencies were higher for attached cells. As the attached fraction is frequently burried out of the photic zone, and not able to rapidly migrate towards light, this implies that heterotrophic capacity should be valuable. Also at species level, some taxa showed higher uptake frequencies than others, for example populations of motile diatom species that displayed a seasonal behaviour (blooming species), such as Nitzschia cf. dissipata and Cylindrotheca closterium. Other species showing frequent uptake were the coccoid colony-forming cyanobacterium Microcrocis sp., and the filamentous cyanobacterium Phormidium sp.. The results suggest that there is a high potential for DFAA uptake in the microphytobenthic community and that for some growth forms and species this could imply a competitive advantage.     

On the regulatory nature of individual growth: some observations from freshwater snails

Various types of growth models are assessed. Results derived from observations made on growth in the freshwater snail, Planorbis contortus , suggest that in certain instances models based on a mechano-reductionist philosophy are acceptable whereas in others they are untenable. In the latter case a holistic representation is more biologically satisfying. Q10 values provide a useful assessment and summary of an individuals regulatory capacity with respect to growth when temperature disturbances are experienced. Another parameter, k corr is developed to carry out this function when other, particularly food supply, disturbances are involved.     

Interactions between freshwater snails and tadpoles: competition and facilitation

Summary Freshwater snails and anuran tadpoles have been suggested to have their highest population densities in ponds of intermediate size where abiotic disturbance (e.g. desiccation) is low and large predators absent. Both snails and tadpoles feed on periphytic algae and, thus, there should be a large potential for competitive interactions to occur between these two distantly related taxa. In a field experiment we examined the relative strength of competition between two closely related snail species, Lymnaea stagnalis and L. peregra, and between L. stagnalis and tadpoles of the common frog, Rana temporaria. Snail growth and egg production and tadpole size at and time to metamorphosis were determined. Effects on the common food source, periphyton, were monitored with the aid of artificial substrates. Periphyton dry weight was dramatically reduced in the presence of snails and/or tadpoles. There were no competitive effects on growth or egg production of the two snail species when they were coexisting. Mortality of L. peregra was high (95%) after reproduction, but independent of treatment. Growth of L. stagnalis was reduced only at the highest tadpole densities, whereas egg production was reduced both by intraspecific competition and by competition with tadpoles. Differences in egg production were retained after tadpole metamorphosis. Tadpole larval period increased, weight of metamorphosing frogs decreased and growth rate was reduced as a function of increasing tadpole density. However, contrary to expectation, snails had a positive effect on tadpole larval period, weight and growth rate. Further, in experimental containers without snails there was a dense growth of the filamentous green alga Cladophora sp. We suggest that the facilitative effects of snails on tadpoles are due to an indirect mutualistic mechanism, involving competition between food sources of different quality (microalgae and Cladophora sp.) and tadpoles being competitively dominant over snails for the preferred food source (microalgae). In the presence of tadpoles snails will be forced to feed on low-quality Cladophora, increasing nutrient turnover rates, which results in enhanced productivity of microalgae, increasing tadpole food resources. Thus, tadpoles have a negative effect on snails through resource depression, while snails facilitate tadpole growth through an indirect enhancement of food availability.     

Genetic exchange between two freshwater apple snails, Pomacea canaliculata and Pomacea maculata invading East and Southeast Asia

Two species of apple snails, Pomacea canaliculata and Pomacea maculata (formerly Pomacea insularum), have invaded many countries of East and Southeast Asia from their native range in South America. This study investigated the genetic structure of the two species invading these areas. Phylogenetic analysis based on sequences of the nuclear gene elongation factor 1-alpha (EF1α) detected two well-supported clades (Clade C and Clade M). Both P. canaliculata and P. maculata were represented in each clade. Some snails had both Clade C and Clade M EF1α sequences. These results suggest genetic exchange between snails of the two clades. A mating experiment between P. canaliculata with Clade C EF1α sequences and P. maculata with Clade M EF1α sequences resulted in viable F1 progeny under laboratory conditions. The genetic exchange was also inferred in some populations collected from Argentina, suggesting an existence of hybrid in the native range. Simple identification of EF1α types using a restriction enzyme, ApaLI, detected significant geographical structure of the EF1α variants in the invaded area. The divergent geographical structure could have resulted from either the founder effect or the bridgehead effect, although further genetic analysis is needed to clarify this. Average individual egg weight, which is an indicator of egg size, was higher in P. canaliculata than P. maculata in both field and laboratory reared samples, suggesting that some (probably most) P. canaliculata and P. maculata invading East and Southeast Asia still maintain species-specific populations.     

The freshwater snails (Gastropoda) of Iran, with descriptions of two new genera and eight new species

Using published records and original data from recent field work and revision of Iranian material of certain species deposited in the collections of the Natural History Museum Basel, the Zoological Museum Berlin, and Natural History Museum Vienna, a checklist of the freshwater gastropod fauna of Iran was compiled. This checklist contains 73 species from 34 genera and 14 families of freshwater snails; 27 of these species (37%) are endemic to Iran. Two new genera, Kaskakia and Sarkhia, and eight species, i.e., Bithynia forcarti, Bithynia starmuehlneri, Bithynia mazandaranensis, Pseudamnicola georgievi, Kaskakia khorrasanensis, Sarkhia sarabensis, Valvata nowsharensis and Acroloxus pseudolacustris are described as new to science; Ecrobia grimmi (Clessin & Dybowski, 1888), Heleobia dalmatica (Radoman, 1974) and Hippeutis complanatus (Linnaeus, 1758) are reported for the first time from Iran. Additional field work is highly desirable for a more appropriate evaluation of the extant freshwater snail biodiversity in Iran.     

Foodborne uptake and sublethal effects of copper and zinc to freshwater snails

Snail habitat selection was strongly influenced in thisresearch by measured copper (Cu) contribution from a powerplant discharge. Laboratory feeding studies showed that thesnail Leptoxis praerosa significantly bioconcentrated Cuwhen fed aufwuchs containing 564 (±269) µg Cu g^-1but did not have significant cellulolytic enzyme activityimpairment. No foodborne zinc (Zn) bioconcentration was foundfrom aufwuchs containing up to 20 000 (±18 400) µg Zn g^-1. These results suggest that water column Cuconcentrations leading to impairment via feeding mechanismscan be an order of magnitude higher than concentrations causingimpairments via waterborne exposures. Therefore, foodborneuptake might be expected to contribute to toxicity rather thandominate toxicity in most conditions where waterborneconcentrations of metals occur. Changes in food qualityappeared to have a greater overall effect on snail populationsthan foodborne uptake of metals alone. Snail populations took2 years to recover following intensive metal treatment on theplant discharge and subsequent reduction of measured metalconcentrations in both water and aufwuchs.     

Flying shells: historical dispersal of marine snails across Central America

The geological rise of the Central American Isthmus separated the Pacific and the Atlantic oceans about 3 Ma, creating a formidable barrier to dispersal for marine species. However, similar to Simpson's proposal that terrestrial species can 'win sweepstakes routes'-whereby highly improbable dispersal events result in colonization across geographical barriers-marine species may also breach land barriers given enough time. To test this hypothesis, we asked whether intertidal marine snails have crossed Central America to successfully establish in new ocean basins. We used a mitochondrial DNA genetic comparison of sister snails (Cerithideopsis spp.) separated by the rise of the Isthmus. Genetic variation in these snails revealed evidence of at least two successful dispersal events between the Pacific and the Atlantic after the final closure of the Isthmus. A combination of ancestral area analyses and molecular dating techniques indicated that dispersal from the Pacific to the Atlantic occurred about 750 000 years ago and that dispersal in the opposite direction occurred about 72 000 years ago. The geographical distribution of haplotypes and published field evidence further suggest that migratory shorebirds transported the snails across Central America at the Isthmus of Tehuantepec in southern Mexico. Migratory birds could disperse other intertidal invertebrates this way, suggesting the Central American Isthmus may not be as impassable for marine species as previously assumed.     

Inter- and intraspecific trait compensation of defence mechanisms in freshwater snails.

Trait compensation occurs when mechanically independent adaptations are negatively correlated. Here, we report the first study to demonstrate trait compensation in predator-defence adaptations across several species. Freshwater pulmonate snails exposed experimentally to predation chemical cues from fishes and crushed conspecifics showed clear interspecific differences in their behavioural avoidance responses, which were negatively correlated with shell crush resistance. The type of avoidance response varied between species: thin-shelled species (Lymnaea stagnalis and Physa fontinalis) moved to the water-line or out of the water, while those with thick shells moved under cover or showed a mixed response. There were also intraspecific size-linked differences, with an ontogenetic increase in shell strength accompanied by a decrease in behavioural avoidance. Such trait compensation in response to predation has important implications for interspecific interactions and food-web dynamics.     

Mode of origin differentially influences the fitness of parthenogenetic freshwater snails

How parthenogenetic lineages arise from sexual ancestors may strongly influence their persistence over evolutionary time. Hybrid parthenogens often have elevated heterozygosity and ploidy, thus making it difficult to disentangle the influence of reproductive mode, hybridity and ploidy on their relative fitness. By comparing the relative fitness of both hybrid and non-hybrid parthenogens to their sexual ancestors, further insight may be gained into how these three factors influence the maintenance of sexual and parthenogenetic reproduction. In the present study, hybrid and non-hybrid parthenogenetic and sexual snails (Campeloma sp.) were compared for the following characteristics: female size-fecundity curves, offspring size, survivorship, and growth. Compared to nearby sexual populations, triploid hybrid parthenogens from the Florida Gulf coast have similar fecundity and offspring size, five-times higher survivorship, and 60% higher growth. Relative to nearby sexual populations, non-hybrid parthenogenetic C. limum from the Atlantic coast have significantly higher fecundity, smaller offspring size, similar survivorship and slightly lower growth. Given the considerable fitness advantages of parthenogens, especially hybrid parthenogens, it is enigmatic as to why these parthenogens occupy marginal natural habitats.     

Factors influencing the stability of nutrient-enriched freshwater macrophyte communities: the role of sticklebacks Pungitius pungitius and freshwater snails

1. Enclosures, with the following characteristics, were used to investigate the effects of increased nutrient (nitrate and phosphate) loading on freshwater communities: (a) controls with normal densities of invertebrates, and including macrophytes, molluscs and fish (sticklebacks), and communities similar to the controls but with (b) much reduced snail densities and (c) increased fish densities. 2. The addition of nutrients resulted in increases in the biomass of phytoplankton and epiphyton, as well as zooplankton counts, in all the experimental enclosures. 3. A reduction in the snail population density resulted in a significant decrease in phytoplankton density and an increase, compared to controls, in the density of epiphytic algae growing on the leaves of the submerged macrophytes, Potamogeton crispus and Groenlandia densa. It also resulted in a significant increase (3.5 times that in the controls) in chlorophyll-a values of epilithic algae growing on glass slides, an increase in senescent tissue in macrophytes and a decline in the mass of Groenlandia densa by the end of the experiment. 4. An increase in the fish population resulted in a significant decline, compared to the controls, in the densities of zooplankton (including Chydorus sphaericus and copepods), Lymnaea peregra (from May to mid-June), Asellus meridianus, Crangonyx pseudagracilis, Culex (P.) molestus and Physa fontinalis. It also resulted in a change in the composition of the phytoplankton, with an increase in the large colonial Eudorina elegans and a decline in smaller planktonic algae, including Ankistrodesmus and Cryptomonas spp. 5. The possible mechanisms responsible for the effects are evaluated and their relevance to conservation and management of freshwater macrophyte communities are discussed.     

The effects of spatial habitat structure on population variability of freshwater snails

Experiments were conducted to examine the effects of a habitat's spatial structure on population variability in two species of freshwater snails (Physa acuta and Austropeplea ollula). To alter the spatial structure of the habitat, vinyl chloride plates were hung in experimental tanks, providing three types of spatial structure: Complex structure, Simple structure and Control (no structure). In Experiment 1, the average number of individuals in a tank did not differ among the three types of structure 2 months after the introduction of the snails, but the variability of the number of individuals in the Complex structure tanks was lowest, whereas the variability in the Control tanks was highest. In Experiment 2, in addition to the spatial structure of the habitat, three types of species interaction were designed as experimental treatments: only P. acuta was introduced into the tanks (P. acuta tanks), only A. ollula was introduced into the tanks (A. ollulatanks) and both P. acuta and A. ollula were introduced into the tanks (two-species tanks). For the P. acuta tanks, the variability of the number of P. acuta individuals in the Complex structure tanks was lowest, and the variability in the Control tanks was highest when the effect of the number of individuals in a tank was subtracted. For the A. ollula tanks and the two-species tanks, there were no significant differences in the variability of the population size among the different treatments of spatial structure. The spatial distribution of P. acuta was more uniform than the distribution of A. ollula on the plates of complex structure. Our results indicate that the spatial structure of the habitat influences the variability of population size (the variance of the number of individuals in different populations during the earlier period after the introduction of the snails), but the effects depend on the spatial behavior of individuals and the interaction with other species.     

On the defence strategy of Physa fontinalis (L.), a freshwater pulmonate snail

Summary Physa fontinalis (L.) gives a characteristic, chemically mediated escape response when stimulated by the majority of British leeches and flatworms. The snail responds rapidly and consistently to contact with all the molluskivorous leeches but also to three species which may be considered harmless. However, no response was given to Erpobdella octoculata, the most abundant and widespread of the harmless leeches. The flatworms generally evoked less strong reactions. The adaptive significance of the pattern of responsiveness is discussed. A weaker shell-shaking response is elicited in conspecifics and it is shown that this antisocial behaviour leads to a relatively spaced-out dispersion pattern. A possible adaptive advantage is the reduction of risk of detection by shell-crushing fish predators, to which the snails are otherwise extremely vulnerable.     

Effects of starvation on growth and reproductive apparatus of two immature freshwater snails Biomphalaria pfeifferi and Biomphalaria glabrata (Gastropoda: Planorbidae)

Starvation of immature snails of Biomphalaria pfeifferi and B. glabrata results in an arrest of growth and animals remain immature. Spermatogenesis is limited to spermatogonia (B.g.) or spermatocytes 1 (B.p.). The number and the size of oocytes remain inferior to that of controls. Animals show reduced genital tracts.Once feeding is restored, growth is resumed but wet weight and shell diameter do not reach the same level as in controls. Fecundity and gametogenesis do not differ from that in controls. Genital tracts weight is proportional to body weight.