Freshwater snails of Oman,South Eastern Arabia

A systematic account is given of the extant freshwater snail fauna of Oman, based on recent collections made in Dhofar and in the northern mountainous areas. Also included are certain species found in brackish coastal localities. A total of 8 freshwater species is regarded as belonging to the fauna of normal freshwater; 7 have been found alive (Thiara scabra, Melanoides tuberculata, Lymnaea natalensis, Gyraulus piscinarum, Biomphalaria arabica, Bulinus wrighti and Indoplanorbis exustus) and one is known only as shells (Paludestrina glaucovirens). This report of G. piscinarum is the first for Oman and may be the first reliable identification for Arabia. Three taxa from brackish water, Gangetia miliacea, Iravadia quadrasi and Melanoides sp., are new for the fauna of Oman and of the whole Arabian peninsula. The finding of G. miliacea provides the first instance of any member of the Stenothyridae living west of the Gulf of Oman and identified from whole snails rather than empty shells. I. quadrasi is the first member of its genus to be found alive west of the Gulf of Oman.     

Environmental change over two decades since dredging and excavation of the lower Boro River, Okavango Delta, Botswana

The Okavango Delta, southern Africa's largest wetland, is situated on the fringe of the semi-arid Kalahari Desert. It is a large alluvial fan, occupying a graben structure which is an extension of the East African Rift system. Of the 16 km³ of water which enters the Delta each year, 96% is lost to the atmosphere by evapotranspiration, 2% to groundwater and only 2% leaves as surface flow. In order to increase surface outflow to meet human needs, the distal Boro channel–floodplain system was dredged, excavated and bunded between 1971 and 1974. The immediate impact of these measures was the destruction of in-channel flora. After 20 years, the aquatic flora has recovered in the excavated channel. However, little recovery has occurred along the channel reach which was dredged. Moreover, there has been significant encroachment of terrestrial species onto the floodplain in the region of the dredged channel. In addition, dredging created a nick point which has been migrating upstream by headward erosion since dredging ceased. The average rate of advance of the nick point has been about 500 m per year. Incision associated with nick point migration has produced a channel which is indistinguishable in form from the dredged channel and, like the dredged channel, is almost completely devoid of in-channel aquatic flora. The adverse environmental impact of dredging has therefore continued to propagate in an upstream direction, although the height of the nick point has decreased, suggesting a natural attenuation process. This is likely to result in eventual elimination of the nick point.     

湘江中游江段螺类的种类组成及区系分析

报道了湘江中游江段螺类的调查结果,计有螺类31种(其中5种为湖南省新记录),分别隶属于腹足纲6科11属,其中主要是田螺科的种类.对它们的种类组成、区系、种群大小、分布与栖息环境的关系进行了分析,并讨论了城镇污水未经处理直接排放及修河拦坝对螺类生存环境的影响.     

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     

Vertical migrations in two hygrophilous species of micro‐snails in relation to time of the year and habitat type

The aim of this study was to investigate vertical migration behaviors in two species of hygrophilous micro‐snails, Vertigo moulinsiana and Vertigo angustior, in relation to the time of the year (spring and summer) at two sites that differ in ground water level (periodically inundated site and non‐inundated site). The study shows different patterns of vertical migrations in the studied species. Vertigo angustior demonstrated a strong affinity to the litter layer (a weak tendency for vertical movements), independent of the time of year and site studied. By contrast, V. moulinsiana showed a clear tendency for vertical migrations, which differed depending on the time of year and site. These differences may be related to the spatial segregation of microhabitats used by these two species at the sites studied and to differences in the ability to resist inundation. Vertigo angustior is associated with microhabitats which are not subjected to prolonged inundation and tolerates a brief submersion. The periodic character of vertical migrations may suggest the effect of endogenous factors related to reproduction in V. moulinsiana. Some plasticity of this behavior in relation to habitat conditions demonstrated by this species may be an adaptation to live in unpredictably flooded environments.     

A nomenclator of extant and fossil taxa of the Valvatidae (Gastropoda,Ectobranchia)

A compilation of all supra- and (infra-) specific taxa of extant and fossil Valvatidae, a group of freshwater operculate snails, is provided, including taxa initially described in this family and subsequently classified in other families, as well as names containing errors or misspellings. The extensive reference list is directly linked to the available electronic source (digital view or pdf-download) of the respective papers.     

Aquatic snails of the Bulinus africanus group in Zambia identified according to morphometry and enzymes

The Bulinus africanus species group (Planorbidae) of freshwater snails has been reported to be represented in Zambia by two species, B. africanus (Krauss) and B. globosus (Morelet), both named as intermediate hosts for Schistosoma haematobium. Uncertainty in identification of these snails from morphology led to the present investigation, combining morphometry (shell and copulatory organ) with enzyme analysis. Observations of both kinds were made usually on the same individual snails, from collecting sites mostly in the Lusaka area or at Lake Kariba. Particular attention was given to the proportional relationship between the penis sheath and the preputium of the copulatory organ, a character used previously to distinguish B. africanus from B. globosus in south-eastern Africa. The enzyme profile MDH-1, AcP-2, PGD-1 and PGM-2 was common to all snails examined from 25 populations; GPI and HBDH were polymorphic. The enzyme data indicate that the samples represent a single species. Shell characters varied continuously. The copulatory organ was generally of the form known for B. globosus. Although the copulatory organ of a few individuals had proportions overlapping the range reported for B. africanus, the present variation was continuous and was not bimodal. It is concluded that all these specimens are conspecific and may be identified as B. globosus. Previous identifications of B. africanus from Zambia appear to need substantiation and it seems that if this species is present at all in the sampled areas, it must be uncommon. It is relevant in regard to possible strain differences within S. haematobium in Zambia, that our observations indicate that only a single species of intermediate host is involved in transmission.     

Aquatic snails of the Jonglei region,southern Sudan,and transmission of trematode parasites

An account is given of the aquatic gastropod molluscs obtained in a survey made to assess future ecological effects of the Jonglei Canal now under construction in the Sudd region of southern Sudan. A total of about 23 species of aquatic snail lives in this region, of which a number are utilised as intermediate hosts by trematode parasites infecting man, livestock and wild herbivores, causing the diseases schistosomiasis, fascioliasis and amphistomiasis. Public health and veterinary authorities should be alert to the possibility that the Jonglei Canal will increase snail populations and raise the prevalence of parasitic diseases.Jonglei Range Ecology Survey.     

High diversity and regional endemism in land snails of eastern Tanzania

In February/March 1995 we collected land snails (including slugs) at 12 stations in eastern Tanzania. A total of 571 person-hours yielded 9174 snails assigned to 159 morpho-species. The richest two sites each (<4ha of uniform forest) had 50 species (Amboni Cave) and 48 species (near Amani, Usambaras), nearly as great as the most species-rich sites known in the world; sieving of litter and soil would probably yield more species. In lowland (coastal) forests, both diversity and endemism seemed to decrease from north to south. Most snail species were found within only one of four coastal or one montane geographic regions, indicating substantial regional endemism. Only one species (Achatina fulica) appeared in all five regions, and 84% of all other species were found in only one (61%) or two regions (23%). The predatory streptaxids comprised about half the species and a third of the individuals at the Usambara site, an extremely high ratio of carnivores. Small snails (< 5mm greatest adult shell dimension) – many of which are probably undescribed species – comprise a substantial proportion of Tanzanian molluscan diversity; more surveys are needed, especially because of human pressures on the few forest patches remaining.     

Nutrient Budget in the Seasonal Wetland of the Okavango Delta, Botswana

The nutrient (P and N species) and chloride budgets were investigated in a representative floodplain in the seasonal wetlands of the Okavango Delta, Botswana. A variety of sources of nutrients in the surface water were considered, namely ion species coming with the floodwater, those generated from dry floodplain soils and those from water-soluble dust deposition (both local and long-range sources). Concentrations of total-nitrogen and chloride in surface water were below 1 mg l⁻¹. Total-phosphorus concentrations were 0.05 mg l⁻¹, reflecting the oligotrophic character of the system. Dust deposition rates were highest for chloride at 2.44 g m⁻² year⁻¹ followed by 0.79 g m⁻² year⁻¹ for total-N, 0.40 g m⁻² year⁻¹ for ammonia and only 0.02 g m⁻² year⁻¹ for total-P, respectively. Chloride was derived primarily from long-range transport, while N and P species were of more local origin. Dissolution rates for these ions combined were calculated to be 3.9 g m⁻² for the flooded area in the 1999 season and thus all dry deposits must be re-dissolved. The accumulation of dust deposits on dry surfaces and their subsequent dissolution causes 2–5 times higher concentrations of nitrogen, phosphorus and chloride with the onset of the flood, thus boosting the nutrient stock in the crucial phase of the onset of flooding. Chloride dissolved from dry soil surfaces and dust contributed approximately 40% to the overall floodplain budget. Although contributions from the soil surface and dust to the nitrogen and phosphorus pools of the floodplain are less prominent (with 10% of total), they nonetheless represent a significant source of nutrients in the entire system. Extrapolation to annually flooded swamps (10,000 km²) indicates a maximum contribution of 40% for total-nitrogen and 60% for total-phosphorus from dust deposition on wet or dry surfaces to the nutrient pool of the water body.     

Some observations on the geomorphological impact of hippopotamus (Hippopotamus amphibius L.) in the Okavango Delta, Botswana

The Okavango Delta is a large wetland system situated in northern Botswana. The hippopotamus forms an integral part of this dynamic ecosystem, as it exerts a catalytic effect on geomorphological change. In the distal reaches of the wetland, regular movement of hippos to feeding grounds results in the development of incised channels, which are kept clear of vegetation and act as nodes for swamp expansion. Hippos maintain pathways in backswamp areas, which lead to the development of new channel systems during channel avulsion. They create breaches in the vegetation levees which flank channels in the permanent swamps, causing diversion of water and sediment to backswamp areas. Their paths often lead to lakes in the permanent swamps. During channel avulsion, diversion of a channel into lakes may occur via these paths, which can result in lake closure.     

Ecology of freshwater snails in south-western Nigeria. I: Distribution and habitat preferences

Dry and rainy season investigations of diverse freshwater habitats in south-western Nigeria revealed fourteen species of snail comprised of nine pulmonates: Biomphalaria pfeifferi Krauss, Bulinus globosus Morelet, Bulinus rohlfsi Clessin, Lymnaea natalensis Krauss, Physa ( Aplexa) waterloti Germain, Bulinus forskali Ehrenberg, Gyraulus costulatus Krauss, Ferrissia sp, Segmentorbis sp. and five prosobranchs namely, Lanistes libycus Morelet, Lanistes ovum Peters, Pila wernei Philippi, Potadoma moerchi Reeve and Melanoides tuberculata Müller.The influence of diverse chemical and physical properties of water on their occurrence as well as interspecific and snail-plant relationships are discussed and maps showing the distribution of the species encountered are presented. Among the intermediate hosts of Schistosoma, B. globosus was widely distributed and common, B. pfeieri was also widely distributed but infrequent and B. rohlfsi was rare.     

Taxonomic revision of the Pyrgulopsis gilae (Caenogastropoda,Hydrobiidae) species complex,with descriptions of two new species from the Gila River basin,New Mexico

We describe two new species of springsnails (genus Pyrgulopsis) for populations from the middle Fork and upper East Fork of the Gila River Basin (New Mexico) that had been previously identified as P. gilae. We also restrict P. gilae to its originally circumscribed geographic range which consists of a short reach of the East Fork Gila River and a single spring along the Gila River (below the East Fork confluence). These three species form genetically distinct lineages that differ from each other by 3.9–6.3% for mtCOI and 3.7–8.7% for mtNDI (the latter data were newly obtained for this study), and are diagnosable by shell and penial characters. Collectively the three species form a strongly supported clade that is distinguished from other congeners by the unique presence of two glandular strips on the dorsal surface of the penial filament. These findings suggest that the conservation status of P. gilae, which was recently removed from the list of candidates for listing as endangered or threatened by the United States Fish and Wildlife Service, should be revisited and that the two new species may also merit protective measures given their narrow geographic ranges.     

Low genetic diversity in a snail intermediate host (Biomphalaria pfeifferi Krass, 1848) and schistosomiasis transmission in the Senegal River Basin

Population genetic perturbations of intermediate hosts, often a consequence of human pressure on environmental resources, can precipitate unexpectedly severe disease outbreaks. Such disturbances are set to become increasingly common following range changes concomitant with climate shifts, dwindling natural resources and major infrastructure changes such as hydroprojects. Construction of the Diama dam in the Senegal River Basin (SRB) reduced river salinity, enabling the freshwater snail intermediate host Biomphalaria pfeifferi to rapidly expand its distribution. A serious public health problem ensued, with an epidemic of intestinal schistosomiasis occurring in the previously schistosome‐free Richard‐Toll region within 2 years. The current study aimed to assess the population variability of B. pfeifferi in the SRB, and speculate upon its subsequent impact on host‐parasite interactions following such engineered ecological change. Genetic variation at nine polymorphic microsatellite loci revealed little population differentiation in SRB snails compared with those from natural habitats in Zimbabwe, where Schistosoma mansoni transmission is much lower. ‘Open’ SRB habitats are associated with greater water contact, smaller population sizes and less genetic diversity, with sites downstream of Richard‐Toll showing greater inter‐ and intrapopulation variation, concomitant with less frequent human contact. These observations may be explained by rapid expansion into pristine habitat selecting for high fecundity genotypes at the expense of schistosome resistance, presenting S. mansoni with genetically homogenous highly fecund susceptible populations around the focal point, promoting development of a highly compatible host‐parasite relationship. Longitudinal study of such systems may prove important in predicting public health risks engendered by future environmental engineering projects.     

Biology of Latia neritoides Gray 1850 (Gastropoda,Pulmonata, Basommatophora): the Only Light-producing Freshwater Snail in the World

This report on the freshwater pulmonate basommatophoran Latia neritoides of the North Island of New Zealand is based on field and laboratory studies. It deals with aspects of the habitat, describes biological adaptations of organs and organ systems in Latia, reviews the chemical basis of bioluminescence in this species, discusses the possible roles of light production in Latia, and addresses zoogeographic and phylogenetic questions.