Freshwater Snail Species Research Articles

Genetic subdivision and morphological variation in a freshwater snail species complex formerly referred to as Viviparus georgianus (Lea)

Genetic and morphological variation was studied in a brooding (ovoviviparous) and morphologically variable freshwater snail [Viviparus georgianus (Lea)] species complex in southern Georgia and Florida. Eleven populations were clustered into three genetically isolated, allopatric groups characterized by 7 to 14 diagnostic loci out of the 38 loci examined. These allopatric groups were identified as V. georgianus (in eastern and southern Florida), V. limi (in the Ochlockonee River) and V. goodrichi (in the Florida panhandle). Nei's standard genetic distances between the species were large (0.23-0.52) compared to intraspecific distances (0.00-0.06). Hierarchical F-statistics for differentiation among sites within drainage systems (FSD) for V. goodrichi and V. georgianus were large (0.519 and 0.387, respectively). The FDT values (differentiation among drainage systems within the total area sampled) were negative or close to zero, so most of the intraspecific genetic differentiation was due to differences among populations within drianage systems, rather than to differences among systems. Canonical discriminant analysis of nine shell morphological measurements separated all three species with little overlap. Principal components analysis of the shell morphology revealed intraspecific variation among sites possibly due to environmental effects. This study demonstrates the marked genetic and morphological variation among and within drainage systems in freshwater prosobranchs.

Population genetic structure of the schistosome-vector snail Bulinus globosus: examining the role of genetic drift, migration and human activities.

Bulinus globosus, one of the intermediate hosts of the genus Schistosoma, is an hermaphrodite freshwater snail species. This species occupies different kinds of environments in Africa, which are generally subjected to large variations in water availability. The mating system of this species is outcrossing, although selfing has been suggested for one population. Here we investigate the genetic structure of populations of B. globosus in natural and human-modified environments from Niger and Ivory Coast using protein electrophoresis. Our goal is to analyse the respective role of the environment (genetic drift and gene flow), the mating system and human activities. Our results indicate a low intrapopulation polymorphism. A large differentiation among populations occurs on the scale of the whole study area whereas neighbouring populations can show little differentiation. These results point to the role of genetic drift and gene flow as the main factors shaping the genetic structure, with human activities playing a role occasionally.

A PROBLEM WITH THE ESTIMATE OF SELF-FERTILIZATION DEPRESSION IN THE HERMAPHRODITE FRESHWATER SNAIL BULINUS TRUNCATUS: THE EFFECT OF GROUPING.

The evolution of self-fertilization has been the subject of both numerous theoretical and empirical studies (reviewed by Jain 1976; Charlesworth and Charlesworth 1987; Uyenoyama et al. 1993; Jarne and Charlesworth 1993). These studies have shown that inbreeding depression, that is, the lowered fitness of inbred as compared with outbred individuals, is an important factor determining this evolution. Although most research has focused on plants, some studies have been performed on hermaphrodite animals (see Jarne and Charlesworth 1993) and particularly freshwater snails (see Jarne et al. 1993). Differences in survival rates between progeny of selfing and outcrossing individuals have been shown and interpreted as resulting from inbreeding depression. Selfing snails also have a lower fecundity than outcrossing snails. The whole effect (fecundity of parents and inbreeding depression of their progeny) has been referred to as self-fertilization depression (Jarne et al. 1991). However, specific problems may arise when estimating self-fertilization depression in animals. In experiments on hermaphroditic freshwater snails, selfed progenies have been produced usually by isolated virgin adults, and outcrossed progenies have been produced by grouped adults (review in Jarne et al. 1993). The effect of the mating system (outcrossing) and the effect of grouping are therefore associated when analyzing the results of outcrossing. It has been shown for various species of freshwater snails that both population density and size influence growth, survival, and fecundity (Chernin and Michelson 1956a,b; Thomas et al. 1974; Thomas and Benjamin 1974; Ahmed et al. 1986; Chaudhry and Morgan 1987). Increased density and population size is generally associated with a reduced fecundity and could lead to an underestimate of self-fertilization depression. In this paper, we investigate whether previous estimates of self-fertilization depression may have been biased by a grouping effect. By using aphallic individuals of the hermaphrodite freshwater snail Bulinus truncatus (see below), we can estimate the effect of grouping selffertilized for isolated and grouped snails. This is done by comparing the fitness of isolated snails with that of individuals at various group size and densities. Bulinus truncatus is a self-fertile hermaphrodite with two sexual morphs, euphallic and aphallic individuals, occurring in natural populations. Euphallic snails can self-fertilize as well as outcross as males or as females. Aphallic snails are characterized by the absence of the male copulatory organ (Larambergue 1939). They can selffertilize and receive sperm from an euphallic partner. However, they cannot transfer their own sperm to a partner (Larambergue 1939). No mutual exchange of sperm occurs during copulation in hermaphrodite freshwater snails: one individual acts as the female when its partner acts as the male. The determinism of aphally is still unclear; some results indicate a genetic determinism (Larambergue 1939), whereas others point strongly to the role of temperature (Schrag and Read 1992). A consequence of aphally is that purely aphallic populations can only self-fertilize. By using aphallic snails, the consequences of grouping can therefore be estimated without interference of the mating system. The aim of the present work is to test (1) whether a grouping effect occurs in the conditions previously used to analyze the self-fertilization depression (about 150 mL of water per snail with group size of 5 to 10 snails) (Jarne and Delay 1990; Jarne et al.

Fatty acid composition of Goniobasis virginica, Physa sp. and Viviparus malleatus (Mollusca: Gastropoda) from Lake Musconetcong, New Jersey

Gas—liquid chromatographic studies were done to determine the fatty acid composition of Goniobasis virginica Physa sp., and Viviparus malleatus (Mollusca: Gastropoda), from Lake Musconetcong, NJ, U.S.A. Palmitic acid was the predominant saturated fatty acid, followed by stearic acid, in all three molluscan species. The chief monoenes were 16:n−7, 18:1n−7; 20:1n−11+9, and 22:1n−11+13, which together accounted for all monoene fatty acids and one-quarter of the total fatty acids. Considerable amounts of linolenic acid (2.7−4.1%) and arachidonic acid (7.8−12%) were found in all three species. The percentage composition of docosahexaenoic acid (22:6n−3) was low compared to that of eicosapentaenoic acid (22:5n−3). Non-methylene interrupted dienes (20:2 NMID), characteristic of marine molluscs, ranged from 2 to 3% in the three species of freshwater snails.

Do self-fertilization and genetic drift promote a very low genetic variability in the allotetraploidBulinus truncatus(Gastropoda: Planorbidae) populations?

SummaryBulinus truncatus, one of the intermediate hosts of the genusSchistosomais an hermaphrodite freshwater snail species occupying a variety of environments over almost all Africa. These environments are subjected to large variations in water availability.B. truncatusis allotetraploid and its populations exhibit various frequencies of aphallic individuals (unable to reproduce as male). Both traits probably favour a reproduction by self-fertilization. Here we investigate the genetic structure of populations ofB. truncatusof Niger and Ivory Coast using protein electrophoresis to analyse the influence of the environment and of both the last traits. To obtain an estimate of the true heterozygosity in this allotetraploid species, we analyse independently the two diploid loci at each tetraploid locus. Our study indicates (i) an extremely low intrapopulation polymorphism with most alleles fixed and the total absence of heterozygotes and (ii) low differentiation between populations. These results indicate high gene flow between populations. However, the existence of private alleles sometimes at high frequency, the low polymorphism and the lack of heterozygotes point to the role of both genetic drift and self-fertilization, the second amplifying the genetic consequences of the first.

Dynamics of Upstream Migration in Two Species of Tropical Freshwater Snails

Much of the fauna of the Río Claro on the Osa Peninsula of Costa Rica, including snails, fishes, and freshwater shrimp, migrates upstream, sometimes in large aggregations. We investigated the dynamics of the migratory behavior of the snails Neritina latissima and Cochliopina tryoniana, which both migrate upstream in huge, mixed aggregations (>500,000 individuals) during the dry season. Yet these two species have very different life histories. Neritina latissima has pelagic larvae which are swept downstream to the ocean. Metamorphosed individuals migrate upstream. Cochliopina tryoniana has direct development, with no marine stage. We analyzed the size distribution of the migrating, stationary, and drifting portions of the populations of these two species. In the drift, we collected only larval N. latissima. Analysis of the size distribution of the migrating and stationary snails suggested that the migration takes >1 yr for N. latissima and perhaps <1 yr for C. tryoniana. In both species, the migration behavior was size and age dependent. Juveniles and small adults composed the bulk of the migrating populations, whereas large adults were almost never observed to migrate. The mean size of individuals in migrating aggregations increased over time and distance upstream. Sphoeroides annulatus, a marine molluscivorous fish, was found only in the lower reaches of the stream. Where S. annulatus was common, N. latissima was less abundant and located on the underside of rocks. The distance migrated may be a balance between size-dependent energetic constraints on upstream movement and increased predation pressure downstream.

The growth of juvenile snails in water conditioned by snails of a different species

Two species of freshwater snails, Physa acuta and Lymnaea sp. aff. columella, were collected from Asabata marsh, Shizuoka Japan. Individuals of both species inhabit the same plants. Individuals of P. acuta are more abundant than those of L. sp. aff. columella. Experiments were conducted to examine the effect of water conditioned by snails on the growth of 10-day-old juvenile P. acuta snails. Juvenile snails in water conditioned by L. sp. aff. columella grew faster than those in water conditioned by P. acuta or only lettuce. The effects of water conditioned by P. acuta differed among the litters. The results suggest that juvenile P. acuta snails experience accelated growth in the presence of L. sp. aff. columella. The freshwater snails interacted through resource competition as well as through substances disolved in the water.

Food selection by freshwater snails in the Gezira irrigation canals, Sudan

Stomach content analysis was carried out on samples of the freshwater snail species Biomphalaria pfeifferi, Bulinus truncatus, Bulinus forskalii (Pulmonata, Planorbidae), Lymnaea natalensis (Pulmonata, Lymnaeidae), Melanoides tuberculata, Cleopatra bulimoides (Prosobranchia, Thiaridae) and Lanistes carinatus (Prosobranchia, Ampullariidae) from different irrigation canals in Sudan. In order to evaluate overlap in diet selection among these species, sites with two or more of the above-mentioned species present were selected. For some species food choice was examined in relation to size groupings. In addition, samples of Marisa cornuarietis (Prosobranchia, Ampullariidae) from small ponds in Sudan, samples of Biomphalaria pfeifferi and Helisoma duryi (Pulmonata, Planorbidae) from drainage canals in an irrigation scheme in northern Tanzania, and samples of H. duryi from fish ponds in the coastal area of Kenya were also analysed.

Toxicity evaluation of ammonium sulphate and urea to three developmental stages of freshwater snails

Studies were performed to evaluate the toxic effects of ammonium sulphate and urea (chemical fertilizers currently applied in ricelands of Cameroon) against eggs, juveniles, and adults of two species of freshwater snails (Helisoma trivolvis and Biomphalaria havanensis). Results obtained from ammonium sulphate tests indicated 24-h LC50 values of 558 mg/L and 669 mg/L for eggs; 393 mg/L and 526 mg/L for juveniles, and 701 mg/L and 657 mg/L for adults of H. trivolvis and B. havanensis, respectively. Similar analysis with urea revealed LC50 values of 14,241 mg/L and 13,532 mg/L for eggs; 18,255 mg/L and 24,504 mg/L for juveniles and 30,060 mg/L and 26,024 mg/L for adults of H. trivolvis and B. havanensis, respectively. Following 48 h exposure, the concentrations of ammonium sulphate killing 100% of snails were 1,250 mg/L and 1,000 mg/L for the adults of H. trivolvis and of B. havanensis, respectively. Those of urea were computed to be 25,000 mg/L for H. trivolvis and 35,000 mg/L for B. havanensis. In rice culture in Cameroon, these fertilizers are applied at doses of 100 kg/ha (ammonium sulphate) and of 150 kg/ha (urea); hence, the above found concentrations lethal to snails appeared to be 10 to 13 times (ammonium sulphate) and to be 165 to 235 times (urea) higher assuming an average water depth of 10 cm in these ricefields. Therefore, the use of ammonium sulphate and urea as chemical fertilizers in ricelands of the Republic of Cameroon might adversely affect the survival of freshwater snails only in the case of spills or of stressful environmental conditions. Under normal laboratory conditions, both chemicals show a low molluscicidal activity with urea being about 25 to 35 times less potent than ammonium sulphate.

Isolation and identification of a slug-specific molluscicide from quack grass (Agropyron repens, L. Beauv.)

A quackgrass extract fraction containing phenolic glycosides showed both dermal and gastrointestinal toxicity toward the two slug species Deroceras reticulatum and D. laeve. These same phenolic glycosides were tested, with no apparent effect, against three freshwater snail species and a garden soil worm. The active compound in the phenolic glycoside fraction, 6-hydroxy-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid (6-HTβC-3-COOH), was identified by use of TLC, CIMS, UV and IR spectrophotometry, and comparison of the isolate to authentic compounds(.)

Avoidance responses of some indigenous and exotic freshwater pulmonate snails to leech predation in South Africa

The avoidance behaviour elicited from four species of freshwater pulmonate snails, Physa acuta and Aplexa marmorata (Physidae), Bulinus tropicus (Planorbidae) and Lymnaea natalensis (Lymnaeidae), following contact with a molluscivorous leech Hebbdella conifera (Glossiphoniidae), was examined experimentally. Although H. conifera showed no species preference for any of those species it attacked, the physids, both exotics, were killed less often than would be expected by chance. The indigenous species, B. tropicus and L natalensis, were more susceptible to leech attack than the introduced species. Size-preference trials using P. acuta showed decreased susceptibility to leech attack with an increase in snail size. These results are interpreted with reference to the mode of leech attack and to differences in snail morphology.

Freshwater molluscs of Yemen Arab Republic

A list is provided of the extant freshwater molluscan fauna of Yemen Arab Republic, based on collections made in wadis, brooks, and wells. A total of 12 freshwater snail species is recorded, some of which serve as intermediate hosts for Trematodes of the genera Schistosoma and Fasciola.

Inventaire des mollusques d'eau douce à Mayotte, archipel des Comores

Aucune trématodose n'est actuellement connue à Mayotte malgré les liens historiques de celle île avec sa voisine Madagascar où l'incidence de ces affections est importante. Aucun hôte intermédiaire n'y avait été décrit dans les études précédentes. Un inventaire des mollusques d'eau douce a été réalisé de 1985 à 1988 dans 53 stations durant les saisons humide et chaude : 3940 gastéropodes, appartenant à 8 espèces différentes, ont été récoltés. Seule la présence de Lymnaea natalensis constitue une nouveauté à Mayotte, portant à 14 le nombre d'espèces. Son rôle dans le cycle de transmission de Fasciola gigantica est connu, en particulier à Madagascar. L'intérêt du contrôle vétérinaire des importations de bovins peut être ainsi souligné.

Effects of buffer composition, pH and temperature on oxygen binding by planorbid snail haemoglobins

The oxygen-binding properties of the haemoglobins of four species of freshwater planorbid snails Bulinus africanus, Biomphalaria glabrata, Bulinus tropicus and Heliosoma trivolvis were examined. Samples chromatographed on Sephadex G75 in CO-saturated buffer (0,1 mol dm−3 MgCl2, 0,05 mol dm−3 Tris-HCl, 0,5 mmol dm−3 phenylmethyl sulfonyl fluoride (PMSF), 1,0 mmol dm−3 Dithioerytritol, ionic strength 0,3) gave the best stability against methaemoglobin formation. The highest haemoglobin oxygen affinity (P50 = 0,35 mmHg) was measured with MgCl2-buffer with a Hill coefficient, nmBX, of 1,16. An nmax value of 2,2 was obtained with snail buffer (4 mmol dm−3 KCl, 50 mmol dm−3 NaCl, 1 mmol dm−3 MgCl2, 6 mmol dm−3 CaCl2, 50 mmol dm−3 Tris-HCI, 0,5 mmol dm−3 PMSF, ionic strength 0,075, pH 8,0) that gives a P50 value of 1,58 mmHg. A Bohr effect of between −0,44 and −0,11 was measured in the physiological range of pH 8,0–7,1 in snail buffer for the three species, B. glabrata, B. africanus and B. tropicus. For these pl...

SUITABILITY OF TROPICAL FISH FOODS FOR LABORATORY CULTURE OF FOUR SPECIES OF FRESHWATER SNAILS ACTING AS INTERMEDIATE HOSTS FOR ECONOMICALLY IMPORTANT HELMINTH PARASITES IN SOUTH AFRICA

SUMMARY Four different kinds of tropical fish food from Tetra Werke, West Germany, as well as a mixture of two of these, were evaluated as possible nutrients for the laboratory culture and maintenance of four freshwater snail species of economic importance in South Africa. These included the intermediate hosts of both the local human schistosome species, i.e. Biomphalaria pfeifferi (Krauss) (intestinal bilharzia) and Bulinus africanus (Krauss) (urinary bilharzia). The other two species, Bulinus tropicus (Krauss) and Lymnaea natalensis Krauss are both of veterinary importance acting as inter mediate hosts of the trematode parasites causing paramphistomiasis and fascioliasis respectively. Age-specific life-tables were compiled for five cohorts of each snail species, each fed on one of the five nutrients and the values of the relevant population parameters compared to each other. The results showed that Tetra Min Baby Food ‘E’ for egg-layers was superior to all the other nutrients tested in respect of juveni...

Cercarial shedding of Echinostoma cinetorchis and experimental infection of the cercariae to several kinds of snails

The development of Echinostoma cinetorchis in several snail species reared in laboratory aquaria was observed. The eggs from adult flukes collected from the intestine of rats were cultivated to miracidia, and exposed to Hippeutis sp. snails. Observations were made for cercarial shedding from the exposed snails. The cercariae shed from the snails were again exposed to several species of fresh water snails in order to observe metacercarial formation in the snails and their infectivity to final hosts. The results obtained in this study were as follows: 1. Twenty miracidia were exposed to each snail of Hippeutis sp. About 58.3% of the above snails (7 out of 12) were dead before shedding the cercariae, and the remainder shed the cercariae for a period of 7 to 9 days before death. 2. Cercarial shedding from the infected snails started from the 25th day after the exposure to miracidia, and the total number of cercariae shed per snail was 684 in average (range; 482-904). 3. The size of rediae developed in the infected Hippeutis sp. snails was 1,242 x 214 microns in average, and the number of rediae per snail was 350 in average (range; 120-510). 4. About 40 to 50 cercariae shed from the Hippeutis sp. snails were each exposed to several species of snails reared in the laboratory. The metacercarial formation was confirmed by dissecting the infected snails, 12 to 16 days after the infection.(ABSTRACT TRUNCATED AT 250 WORDS)

Relative vulnerability of six freshwater gastropods to the leech Nephelopsis obscura (Verrill)

SUMMARY. 1. The vulnerability of six species of freshwater snails to the leech Nephelopsis obscura was determined in laboratory predation experiments.2. Nephelopsis was unable to prey on prosobranch snails with an operculum, but did consume certain pulmonate species though predation rates were low, ranging from one to two snails per leech per night at 20°C. Apparently, Nephelopsis does not actively select prey, but merely consumes those species most easily handled.3. Among pulmonates, leeches captured species in the following order: Physa gyrina (Say), Helisoma anceps (Menke). Lymnaea emarginata (Say) and Helisoma trivolvis (Say). Susceptibihty of size classes varied among snail species.4. Neither environmental structure, such as cobble or macrophytes, nor gastropod escape behaviors, such as clamping to substrates or vertical migration away from bottom‐feeding leeches, lowered capture rates by Nephelopsis.5. In 2 years sampling, only one of seven lakes, however, had appreciable abundances of Nephelopsis. It is therefore suggested that the rarity, low feeding rates, and lack of strong gastropod preference limit the effect of Nephelopsis on field gastropod distributions.

Organic acids in the excretion products of five freshwater snail species

1. 1. A comparative study of the organic acids excreted by five freshwater snail species ( Bulinus globosus, Bulinus tropicus, Biomphalaria glabrata, Lymnaea natatensis and Helisoma duryi ) was carried out at 4°C and 25°C. 2. 2. It appeared that a limited variation occurred regarding the type of organic acid excreted by the various snail species. 3. 3. The concentrations of these acids varied not only from species to species, but also in the same species at the different experimental temperatures. 4. 4. It was found that the concentrations of most of the acids excreted were higher at 4°C than at 25°C for all the species studied, except L. natalensis. 5. 5. It is regarded unlikely that organic acids in excreta will play a role in the species-specific snail-miracidium interactions.

The amino acid composition of the excretion products collected at 4°c and 25°c from five freshwater snail species: Bulinus globosus, Bulinus tropicus, Biomphalaria glabrata, Lymnaea natalensis and Helisoma duryi

1. 1. The amino acid composition of the excretion products collected at 4 and 25°C from 5 freshwater snail species ( Bulinus globosus, Bulinus tropicus, Biomphalaria glabrata, Lymnaea natalensis and Helisoma duryi) was studied. 2. 2. The amino acid composition differed not only from snail species to species, but also in the same species at the 2 exposure temperatures. 3. 3. In the light of the finding that the concentration of the excreted amino acids varied greatly, the possible effect of temperature on the concentration of these acids could not be determined.

The organic acid content of the haemolymph and excretion products of five freshwater snail species

1. 1. The organic acids present in the haemolymph and excretion products of five freshwater species were identified. The species were Bulinus globosus, Bulinus tropicus, Biomphalaria glabrata, Lymnaea natalensis and Helisoma duryi. 2. 2. In general, there was little variation in the occurrence of the organic acids in the species investigated, except for the organic acid composition in the case of L. natalensis. 3. 3. The organic acids in the excretion products were largely representative of haemolymph organic acid composition, indicating that these acids were excreted from the blood via the kidney.