The freshwater snail Bulinus tropicus (Planorbidae) in Namibia, characterised according to chromosome number, enzymes and morphology

Freshwater snails collected in central Namibia, south-western Africa, from 15 populations belonging to the Bulinus truncatus/tropicus complex (Planorbidae) are characterised in respect of their chromosome number, morphology, egg proteins and enzymes. The population samples were all consistently diploid and euphallic. The findings are compared with observations on this group of snails in other areas of Africa. It is concluded that the Namibian populations belong to a single species, B. tropicus (Krauss, 1848), of which B. parietalis (Mousson, 1887) is probably a synonym. No evidence was found of any occurrence of the tetraploid species B. truncatus or of snails belonging to the B. africanus group; lack of a potential intermediate host therefore precludes transmission of Schistosoma haematobium in this area.External Scientific Staff of the Medical Research Council of the UK     

Predation on snails by an indigenous fish,Sargochromis codringtonii,in ponds in Zimbabwe

The extent to which Sargochromis codringtonii (a known predator of snails) can control snail numbers was investigated in cementlined ponds in order further to evaluate the fish's potential as an agent for the biological control of fresh water snails, especially Bulinus globosus which is the intermediate host for Schistosoma haematobium in Zimbabwe. Bulinus tropicus of all sizes were vulnerable to predation by fish, but both Melanoides tuberculata and Bulinus globosus with shell heights greater than 10mm seemed to be less so. Surprisingly, the density of B. globosus was lower in ponds without fish than in ponds with fish, while the density of B. tropicus was very high in ponds without fish. It was not possible to conclude whether B. globosus benefited by the presence of fish, or was less palatable to the fish than B. tropicus, or whether the high density of B. tropicus in ponds without fish exerted competitive effects on B. globosus. Consequently, further experiments are required to determine whether S. codringtonii can reduce populations of B. globosus in the absence of B. tropicus.     

Microsatellites in the freshwater snail Bulinus globosus (Gastropoda: Planorbidae) from Zanzibar

Six microsatellite loci were isolated and characterized in Bulinus globosus, a freshwater snail with a wide distribution throughout sub‐Saharan Africa. Bulinus globosus is an intermediate host of Schistosoma haematobium, the causative agent of human urinary schistosomiasis. Microsatellites were tested using 32 snails from four populations collected from Pemba and Unguja islands of Zanzibar. The microsatellite loci displayed relatively low levels of variation, with between two and five alleles per locus. F_ST estimates indicate that gene flow is low, as has previously been suggested for other species of Bulinus.     

Distribution of freshwater snails in the river Niger basin in Mali with special reference to the intermediate hosts of schistosomes

Snail surveys were carried out in various parts of Mali. All areas surveyed are part of the Niger basin being either affluents or irrigation schemes fed by this river. The snail species present varied greatly between areas. The following potential hosts of schistosomes were recorded: Biomphalaria pfeifferi, Bulinus truncatus, B. globosus, B. umbilicatus, B. forskalii and B. senegalensis. In the large irrigation schemes, i.e. ‘Office du Niger’ and Baguinéda, only B. pfeifferi and B. truncatus appear to be intermediate hosts. Snail distribution appeared to some extent to be focal and high snail densities appeared to be associated with human water contact activities, which apparently create favourable biotopes for the snails. This is probably due to an alteration of the vegetation and an increase of the trophic status of the site by contamination with food remnants and other debris. The larger irrigation canals or lakes in these schemes play an important role in the transmission of human schistosomes and transmission appears to be very focal in these habitats. Infected snails are almost exclusively found in well-defined human water contact sites (WCS). Local infection rates with schistosomes were often high (i.e. up to 27% in B. pfeifferi). In urban areas (i.e. Bamako), transmission patterns are more variable. In Bamako schistosome-infected B. truncatus were found in the Niger river. A number of smaller semi-permanent or permanent streams are very important transmission sites, and schistosome infections were recorded from B. pfeifferi, B. truncatus and B. globosus. Schistosome infection rates in B. pfeifferi were often high (up to 30%). In a new lake at Sélingué, B. truncatus was found to be widely distributed only about a year and a half after the dam was constructed, and in some sites schistosome infections were recorded.     

Biomphalaria species in Alexandria water channels

Of the several species of Biomphalaria snails worldwide that serve as the intermediate host for Schistosoma mansoni, Biomphalaria alexandrina is a species that is indigenous to Egypt. Recently, there has been much debate concerning the presence of Biomphalaria glabrata and the hybrid of the species with Biomphalaria alexandrina. Due to this debate, the absence of a clear explanation for the presence of B. glabrata in Egyptian water channels and the probability that they may be reintroduced, we conducted this field study to identify Biomphalaria species present in Alexandria water channels. Laboratory-adapted susceptible snails to Schistosoma mansoni of the following species were used as a reference; Biomphalaria alexandrina, Biomphalaria glabrata and their hybrid. These snails were used to perpetuate the Schistosoma life cycle at the Theodor Bilharz Research Institute (TBRI), Cairo, Egypt. Morphological and molecular studies were conducted on these reference snails as well as on the first generation of Biomphalaria snails from two areas in the Alexandria governorate. The morphological study included both external shell morphology and internal anatomy of the renal ridge. The molecular study used a species-specific PCR technique.The results demonstrated that there was an absence of Biomphalaria glabrata and the hybrid from Alexandria water channels. Moreover, the susceptibility patterns of these reference snails were studied by measuring the different parasitological parameters. It was found that Biomphalaria glabrata and the hybrid were significantly more susceptible than Biomphalaria alexandrina to the Egyptian strain of Schistosoma mansoni. The results demonstrated that if Biomphalaria glabrata was reintroduced and adapted to the local environment in Egypt, it would have important epidemiologic impacts that would have a serious effect on the health of Egyptian people.     

The effect of temperature,darkness, starvation and various food types on growth,survival and reproduction of Helisoma duryi,Biomphalaria alexandrina and Bulinus truncatus (Gastropoda: Planorbidae)

Helisoma duryi has been proposed as a biological control agent in schistosomiasis due to its superiority in laboratory competition experiments with various species of the intermediate host snails. Therefore it was considered important to evaluate the response of this snail species and the intermediate host species, Biomphalaria alexandrina and Bulinus truncatus, to various physical, chemical and biological factors under laboratory conditions in order to obtain information on the similarities in the ecological niches of these species. The factors considered in the present paper are: temperature, darkness, starvation and food. All three species had optimal growth and egg laying at 26–28 °C. Only H. duryi survived for a longer period at 33°C and it was capable of starting egg laying at this temperature although the onset was delayed. However, low temperature (18°C) caused a relatively larger decrease in egg laying of H. duryi than in the other two species. Growth and egg laying was reduced for H. duryi and B. truncatus kept under darkness and B. alexandrina could not tolerate maintenance under darkness. A few days of starvation of juvenile snails had no effect on later growth and egg laying capacity of the survivors, although mortality in B. truncatus was increased. B. alexandrina had a lower tolerance to starvation than the other two species. Egg laying of snails fed only one of the three laboratory food types decreased for all three species in the order: Vov-vov (dog food in dry pellets), Tetramin (fish food) and lettuce. Combinations of lettuce and one or more proteinaceous food types gave optimal growth and egg laying for all three species.     

BULINUS AFRICANUS-GROUP SPECIES IN WEST AFRICA DIFFERENTIATED BY MORPHOMETRIC ANALYSIS (GASTROPODA, PLANORBIDAE)

In the present study 241 shells from snails belonging to theBulinus africanus-group in West Africa, were examined in a morphometricanalysis including ten characters. Principal component analysisand stepwise discriminant function analysis were used for analysisof the data. It was shown that Bulinus umbilicatus, B. jousseaumeiand B. globosus are clearly distinct on morphometric shell characters.Besides a general differentiation between the three species,the characters necessary for differentiation between them twoby two are found. The knowledge of the geographic distributionof B. umbilicatus and B. jousseaumei is significantly improved. (Received 18 March 1988; accepted 4 May 1988)     

INBREEDING DEPRESSION,NEUTRAL POLYMORPHISM,AND COPULATORY BEHAVIOR IN FRESHWATER SNAILS: A SELF-FERTILIZATION SYNDROME

This paper examines the consequences of self-fertilization on life-history traits and neutral genetic polymorphism in natural populations of three species of hermaphrodite freshwater snails: Biomphalaria straminea, Bulinus globosus, and the aphallic species Bulinus truncatus. Life-history traits (fecundity, growth, hatching rate, and survival of offspring) are compared under laboratory conditions between isolated (obligatory selfing) and paired (outcrossing possible) snails in one population of B. straminea and B. globosus and two populations of B. truncatus. The genetic polymorphism of the same four populations is analyzed using electrophoretic markers in B. straminea and B. globosus and microsatellite markers in B. truncatus. In B. truncatus and B. straminea, isolated snails have a higher fecundity than paired snails, whereas the contrary is observed in B. globosus. For all populations, no difference in hatching rate and offspring survival is detected between the two treatments. Genetic analyses using microsatellite markers conducted in B. truncatus on progeny of paired snails reveal a high selfing rate in spite of high copulation rates, highlighting the difficulties of obtaining outcrossing in highly selfing snails. The high survival of selfed offspring in B. truncatus and B. straminea indicates that inbreeding depression is limited. The extent of inbreeding depression in B. globosus is less clear. Overall, fitness decrease in this species is limited to fecundity. The extent of allozyme polymorphism is very limited whereas a much higher variability is observed with microsatellites. Biomphalaria straminea and B. truncatus populations are also characterized by very low observed heterozygosities and large heterozygote deficiencies, whereas the B. globosus population does not exhibit such a deficiency. Overall these results allow the definition of a self-fertilization syndrome in hermaphrodite freshwater snails: selfing populations (such as those of B. straminea and B. truncatus studied here) are characterized by high selfing rates in spite of copulations, limited deleterious effects of selfing, limited neutral genetic polymorphism, and large heterozygote deficiencies.     

New species in <Emphasis Type="Italic">Barleria</Emphasis> sect. <Emphasis Type="Italic">Stellatohirta</Emphasis> (<Emphasis Type="Italic">Acanthaceae</Emphasis>) from Africa

Summary  Three new species are described in Barleria L. sect. Stellatohirta M. Balkwill from tropical Africa: B. aristata from south-central Tanzania, B. aenea from south-western Tanzania and northeast Zambia, and B. purpureotincta from south-western Zambia. Their affinities and conservation status are discussed.     

Differential Parasitism of Native and Introduced Snails: Replacement of a Parasite Fauna

The role of parasites in a marine invasion was assessed by first examining regional patterns of trematode parasitism in the introduced Japanese mud snail, Batillaria cumingi (= B. attramentaria), in nearly all of its introduced range along the Pacific Coast of North America. Only one parasite species, which was itself a non-native species, Cercaria batillariae was recovered. Its prevalence ranged from 3 to 86%. Trematode diversity and prevalence in B. cumingi and a native sympatric mud snail, Cerithidea californica, were also compared in Bolinas Lagoon, California. Prevalence of larval trematodes infecting snails as first intermediate hosts was not significantly different (14% in B. cumingi vs 15% in C. californica). However, while the non-native snail was parasitized only by one introduced trematode species, the native snail was parasitized by 10 native trematode species. Furthermore, only the native, C. californica, was infected as a second intermediate host, by Acanthoparyphium spinulosum(78% prevalence). Given the high host specificity of trematodes for first intermediate hosts, in marshes where B. cumingi is competitively excluding C. californica, 10 or more native trematodes will also become locally extinct.     

Transmission and diversity of Schistosoma haematobium and S. bovis and their freshwater intermediate snail hosts Bulinus globosus and B. nasutus in the Zanzibar Archipelago,United Republic of Tanzania

BackgroundThe Zanzibar Archipelago (Pemba and Unguja islands) is targeted for the elimination of human urogenital schistosomiasis caused by infection with Schistosoma haematobium where the intermediate snail host is Bulinus globosus. Following multiple studies, it has remained unclear if B. nasutus (a snail species that occupies geographically distinct regions on the Archipelago) is involved in S. haematobium transmission on Zanzibar. Additionally, S. haematobium was thought to be the only Schistosoma species present on the Zanzibar Archipelago until the sympatric transmission of S. bovis, a parasite of ruminants, was recently identified. Here we re-assess the epidemiology of schistosomiasis on Pemba and Unguja together with the role and genetic diversity of the Bulinus spp. involved in transmission.Methodology/Principal findingsMalacological and parasitological surveys were conducted between 2016 and 2019. In total, 11,116 Bulinus spp. snails were collected from 65 of 112 freshwater bodies surveyed. Bulinus species identification were determined using mitochondrial cox1 sequences for a representative subset of collected Bulinus (n = 504) and together with archived museum specimens (n = 6), 433 B. globosus and 77 B. nasutus were identified. Phylogenetic analysis of cox1 haplotypes revealed three distinct populations of B. globosus, two with an overlapping distribution on Pemba and one on Unguja. For B. nasutus, only a single clade with matching haplotypes was observed across the islands and included reference sequences from Kenya. Schistosoma haematobium cercariae (n = 158) were identified from 12 infected B. globosus and one B. nasutus collected between 2016 and 2019 in Pemba, and cercariae originating from 69 Bulinus spp. archived in museum collections. Schistosoma bovis cercariae (n = 21) were identified from seven additional B. globosus collected between 2016 and 2019 in Pemba. By analysing a partial mitochondrial cox1 region and the nuclear ITS (1–5.8S-2) rDNA region of Schistosoma cercariae, we identified 18 S. haematobium and three S. bovis haplotypes representing populations associated with mainland Africa and the Indian Ocean Islands (Zanzibar, Madagascar, Mauritius and Mafia).Conclusions/SignificanceThe individual B. nasutus on Pemba infected with S. haematobium demonstrates that B. nasutus could also play a role in the local transmission of S. haematobium. We provide preliminary evidence that intraspecific variability of S. haematobium on Pemba may increase the transmission potential of S. haematobium locally due to the expanded intermediate host range, and that the presence of S. bovis complicates the environmental surveillance of schistosome infections.     

Seasonal influence on the production of Schistosoma haematobium and S. mansoni cercariae in Rhodesia

Batches of Bulinus (Physopsis) globosus and Biomphalaria pfeifferi were exposed to Schistosoma haematobium and S. mansoni respectively each month for a 12-month period. The snails were kept out of doors in Salisbury (highveld) and indoors at Chiredzi (lowveld S. haematobium only) and examined weekly to determine the duration of prepatency, and the number of cercariae produced per infected snail. There was a strong seasonal influence in the highveld experiments which showed sporocyst dormancy in winter and the almost simultaneous maturation of infection in nine batches during early summer. In the lowveld the release of cercariae by infected B. globosus continued throughout the year although numbers fell off in winter. The prepatent period was prolonged in winter, but there was no evidence of sporocyst dormancy.     

Further laboratory studies on the interspecific competition between Helisoma duryi (Wetherby) and the intermediate hosts of schistosoma mansoni sambon: Biomphalaria alexandrina (Ehrenberg) and B. camerunensis (Boettger)

Biological control of schistosomiasis by means of introduction of a competitor, Helisoma duryi, of the intermediate host snails has been proposed. In the present laboratory studies the competitive relationships between H. duryi and two Biomphalaria species were investigated both when direct interference between the two competing species was possible and when not. A reduction of growth and reproduction of Biomphalaria was found when direct interference was involved but the effect of competition was lessened when the two species were separated by a mesh.The data showed that despite continuous presence of food in the aquaria food competition was involved in the direct interference between the two species, while the role of chemical interactions could not be evaluated conclusively from the present experiments.The role of food is discussed.     

Use of indigenous fishes to control schistosome snail vectors in Mala i, Africa

The expansion of water resource development projects in the tropics and subtropics resulted in an increased transmission of human schistosomes. This study was conducted to test the feasibility of using two snail-eating fishes, Trematocranus placodon (Trewavas) and Maravichromis anaphyrmis (Trewavas), as biocontrol agents of schistosomes in fish ponds. The results suggest that T. placodon has a potential for controlling the snail vectors of Schistosoma mansoni (Sambon) and Schistosoma haematobium (Bilharz). The use of M. anaphyrmis, however, to control schistosome-carrying snails was not satisfactory.     

The Effects of Isolation,Grouping and Population Density on Fecundity of Bulinus truncatus

Summary

Bulinus truncatus is the intermediate host of Schistosoma haematobium, a trematode parasite causing bladder bilharziasis in man. In the struggle against this disease, control of snail populations is an important goal and expansion of our knowledge of the reproductive activity of Bulinus is relevant in this regard.

In this study the effects of various factors on the fecundity of Bulinus were investigated in specimens kept under continuous water refreshment conditions. The results are considered to be more indicative of the natural situation than results obtained under the current use of discontinuous water refreshment.

Isolated snails produce considerably higher numbers of egg masses and eggs than grouped snails. The effect of grouping on isolated snails, and the reverse situation, is very rapid, being fully effective within one week. The increase in fecundity of isolated snails is most probably caused by the absence of copulatory activity, since transfer of semen is known to curtail egg laying in freshwater pulmonate snails. Egg mass production was shown to decrease with increasing population density. This may be caused either by the difference in space available or by the possible difference in copulatory activity. Isolated snails showed a faster body growth than grouped snails.

The possible role of hormones involved in establishing the above effects and the consequences of the results for snail control programs are discussed.     

GENETIC AND MORPHOLOGICAL VARIATION OF POPULATIONS BELONGING TO THE BULINUS TRUNCATUS/TROPICUS COMPLEX (GASTROPODA; PLANORBIDAE) IN SOUTH WESTERN ZIMBABWE

Aquatic snails from south western Zimbabwe belonging to theBulinus trunscatus/tropicus complex vary widely in shell formsuggesting that more than one taxon could be present. This possibilitywas investigated by making observations on snail samples from13 populations from the Plumtree area, in respect of allozymevariation (5 polymorphic loci), shell morphology (9 variables),copulatory organ and chromosome number. Comparative data wereobtained from snails from north western Zimbabwe identifieddefinitely as B. tropicus. Analysis of the genetic structurerevealed a high degree of polymorphism (P) ranging from 0.29–0.80among populations from Plumtree and expected heterozygosity(H_e) from 0.02–0.22. No enzymatic diagnostic loci werefound which could differentiate the different morphs or populationsand discriminant function analysis on the morphological datashowed an overlap of morphs among populations. Snails analyzedfor chromosome number were all diploid (2n = 36). Snails exposedto Schistosoma haematobium mira-cidia were all refractory. Thisinformation supports the view of a single species, B. tropicus,which is differentiated due to migration barriers and whereenvironmental variables might be implicated in the morphometricdivergence. (Received 31 July 1995; accepted 15 January 1998)     

SELF-FERTILIZATION VERSUS CROSS-FERTILIZATION IN THE HERMAPHRODITIC FRESHWATER SNAIL BULINUS GLOBOSUS

Self-fertilization depression of fitness in the freshwater hermaphroditic snail Bulinus globosus, an intermediate host of the parasitic trematode Schistosoma, has been studied in a strain originating from Niger. B. globosus is an outcrosser that can self-fertilize when isolated before any copulation has occurred. The self-fertilization depression has been estimated during two successive generations. In the first generation, selfing was compared to outcrossing. Within each mating system group, selfing and outcrossing were compared again in the second generation. A striking difference was shown in favor of cross-fertilization for the number of eggs laid, the survival at birth of young snails and the number of snails reaching sexual maturity. The overall self-fertilization depression is 0.920 after two generations of selfing. We discuss the relative role of selfing and outcrossing in the evolution of freshwater snail populations.     

Dental variation ofProconsulfrom the Tinderet region, Kenya

The morphology of mandibular molars of the twoProconsulspecies,Proconsul majorandProconsul africanus, from the Tinderet region, Kenya was analyzed. While the molar size variability within the TinderetP. majorwas slightly greater than those of local African ape subspecies, the shape variability was comparable. Because the twoProconsulspecies show some differences in cusp areal proportions,P. majoris not just a largerP. africanusin molar morphology. Napak specimens are generally similar to the TinderetP. major.     

Water balance and resistance to desiccation in rock-dwelling snails

We have examined the resistance to desiccation among rock-dwelling land snails of various phylogenetic groups:Cristataria genezarethana (Clausiliidae),Rupestrella rhodia (Chondrinidae) andLevantina caesareana (Helicidae), all from the same location in Israel.L. caesareana was the most resistant andR. rhodia the least resistant to desiccation andC. genezarethana was of intermediate resistance. Differences in the rates of water loss during desiccation were determined mainly by rate of water loss during the first 2 days of desiccation. The high rates of water loss in rock-dwelling species exceed those of other snails in the Mediterranean habitat of Israel. However, snails collected in the field at the end of aestivation were in only a mild state of dehydration, suggesting that the rocky habitat protects its occupants against desiccation. We also suggest that among the rock-dwelling species, the protective role of the rock is more important in the more evolutionarily primitive genera (the chondrinidRupestrella and the clausiliidCristataria) and that physiological capacities are more effective in the more highly evolved helicidLevantina.     

The evolution of phally polymorphism in Bulinus truncatus (Gastropoda,Planorbidae): the cost of male function analysed through life-history traits and sex allocation

In the hermaphrodite freshwater snail Bulinus truncatus, two sexual morphs, euphallic (regular hermaphrodites) and aphallic individuals without a male copulatory organ, co-occur at various ratios in natural populations. Both aphallic and euphallic individuals can reproduce by selfing, but when outcrossing aphallic individuals can only play the female role. A comparison of life-history traits and sex allocation in these two forms provides the opportunity to investigate the evolution and maintenance of sexual polymorphisms. This study was performed to test whether a reallocation of resources from the lost male function to the female function occurs in aphallic snails at the level of both sex organs (sex allocation) and life-history traits. In a first experiment we compared life-history traits over a whole life-cycle under selfing between the two sexual morphs. In a second experiment, the sex organs were weighed to test for a difference in sex allocation between the two morphs. No difference in resource allocation to female function between the two morphs was observed in either experiment. This is in contrast to patterns frequently observed in sexually polymorphic plants, and in a previous study performed on aphally in the same snail species. We discuss the genetic and physiological hypotheses that could explain these results, and their consequences for the evolution and maintenance of phally polymorphism in B. truncatus.