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Recent characterization of nuclear ribosomal small subunit (SSU) genes has shown that variant nucleotides within this region could be useful for species and species group identi®cation within the genus Lymnaea (Gastropoda: Lymnaeidae). This study aimed to characterize a range of populations of Lymnaea natalensis Krauss, 1848 on Madagascar, and addressed two related questions. First, is there any evidence of intraspeci®c variation of the SSU and, if so, what might be its signi®cance? Secondly, might this variation jeopardize the use of SSU for lymnaeid taxonomy and phylogeny? Lymnaea natalensis (n = 212) was collected from 17 sampling localities, spanning the northern and southern ends of the island. Variation within a selected region of the SSU known to vary between species, the V1 and V2, was assayed by polymerase chain reaction (PCR) linked restriction fragment length polymorphism (RFLP) and denaturing gradient gel electrophoresis (DGGE) analysis. The PCR-RFLP pro®les indicated a striking dimorphism across populations at two restriction site loci (CfoI & MspI) within the E10-1 helix of the V2 region. The observed RFLP variation was con®rmed by direct sequencing and by genomic digestion with subsequent hybridization. Putative heterozygotes were also encountered and in these individuals the SSU arrays composed of two distinct types approximately 1% divergent. A severe departure from Hardy±Weinberg equilibrium with a highly statistically signi®cant ( P < 10 -5 ) heterozygote de®ciency was found and genetic variation among populations was highly structured (F st = 0.53). The geographic distribution of the variants was mapped, revealing that one variant was restricted to higher, predominately colder environments and was thought to be an adaptation. The molecular basis of the SSU variation was caused by single nucleotide polymorphisms (SNPs). To test for the possibility of cryptic taxa, an analysis of individuals representative of the SSU variant types with isoenzyme analysis (ISA), randomly ampli®ed polymorphic DNA (RAPDs) and PCR-RFLP analysis of the ribosomal Internal Transcribed Spacer (ITS) was performed. Little variation was revealed and none that correlated to the groups suggested by SSU, con®rming that the SSU variation was intraspeci®c. The levels of intraspeci®c divergence of the V1 and V2 within Lymnaea were not appreciably different (1%) from interspeci®c and would therefore question the validity of these data for lymnaeid taxonomy and phylogeny.

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A survey of freshwater snails on Ma®a Island has been conducted that mapped the distribution and, by using a timed searching strategy, documented the relative levels of abundance of species. Twenty-®ve freshwater habitats around the island were surveyed; snails were found in 15 sites (60%). From the total number of snails collected (n = 240), prosobranchs (59%) outnumbered pulmonates (41%). Potential intermediate hosts of urinary schistosomiasis were encountered and these Bulinus species were further characterized by analysis of six enzyme systems (GPI, PGM, MDH, HBDH, GDH & AcP), by inspection of DNA sequence variation within mitochondrial cytochrome oxidase sub-unit I (COI) and nuclear ribosomal internal transcribed spacer (ITS). Three species of Bulinus were found: a single B. africanus group taxon, B. nasutus, and two B. forskalii group taxa, B. forskalii and a species presently identi®ed as Bulinus sp. in this paper. The genera Pila, Lanistes and Bulinus were widespread across the island and dominated the fauna. The remaining taxa were found in lower numbers and often only at single sites. Three prosobranchs: Lanistes purpureus, Pila ovata and Melanoides tuberculata were found. In addition to Bulinus, the pulmonates included a small discoidal planorbid (?)Ceratophallus and an ancylid (a freshwater limpet). The factors that shape the dispersal and distribution of molluscs on Ma®a were considered. Water chemistry could be an important variable as both pH (mean 6.37 0.99 sd) and conductivity (mean mS = 149 135 sd) were signi®cantly different from similar habitats on neighbouring Zanzibar Island (P < 0.001). The Ma®an fauna is clearly associated with that of mainland Africa and Madagascar.

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