THE ROLE OF LYMNAEA AURICULARIA (LINNAEUS) AND LYMNAEA PEREGRA (MULLER) (GASTROPODA: PULMONATA) IN THE TRANSMISSION OF LARVAL DIGENEANS IN THE LOWER THAMES VALLEY

Eleven species of larval digeneans are reported for the firsttime in the UK from the freshwater snail Lymnaea (Radix) auricularia.The latter replaced its congener L. peregra in a gravel pitnear Wraysbury, Berkshire in the Lower Thames Valley and hostedan assemblage of larval digeneans similar to that previouslytransmitted by L peregra. The larval digeneans of L. peregrain small bodies of water in the area were also similar to thosefound in L. auriculania in the gravel pit. This replacementin the role of the host is explained on the basis of morphological,physiological and ecological similarities of the two lymnaeidspecies. (Received 19 February 1992; accepted 27 April 1992)     

POPULATION GENETICS OF LYMNAEA PEREGRA (MULLER) (GASTROPODA: PULMONATA) IN LAKE GENEVA

Using gel electrophoresis, a study was made of genetic exchangeamongst populations of Lymnaea peregra in Lake Geneva. Geneticvariability is high. A within-population heterozygote deficiencyis shown and the role of null alleles as an explanation of thisdeficiency is discussed. High values of gene flow show thatthe inter-population structure is weak. Exchange between populationspropably occurs by both active and passive means. (Received 9 January 1989; accepted 15 August 1989)     

ALLOZYME VARIATION AMONG POPULATIONS OF BIOMPHALARIA CAMERUNENSIS (BOETTGER, 1941) (GASTROPODA: PLANORBIDAE) IN CAMEROON

Allozyme electrophoresis was used to study genetic diversityamong populations of the freshwater, hermaphroditic snail Biomphalariacamerunensis in Cameroon. Five of 19 loci studied in 15 enzymesystems were polymorphic. Intrapopulation variation was evidentin 8 of 12 populations sampled and heterozygotes were presentin 6 of these. Two populations with large sampled sets werepanmictic. Although results for the remaining 4 populationswith heterozygotes present were inconclusive, our findings supportthe hypothesis that Biomphalaria are out-breeders. The allelesfor two loci, asparate aminotransferase-1 (AAT-1) and 6-phosphogluconatedehydrogenase (6-PGD), showed distribution patterns that suggestthey may be limited by climate or habitat. The slow migratingAAT-1 was the only allele found in the mountainous regions ofthe southwest. This region falls under the Cameroon-type climateand is the wettest region in the country. The faster allelewas restricted to the lowland rain forest of the south. For6-PGD the fast allele was found throughout the range of B. camerunensis,but the slower allele was found only in the mountainous regionswithin the Cameroon-type climate. (Received 27 April 1989; accepted 9 September 1989)     

ALLOZYME VARIATION AMONG POPULATIONS OF BIOMPHALARIA PFEIFFERI (KRAUSS, 1848) (GASTROPODA: PLANORBIDAE) IN CAMEROON

Allozyme electrophoresis was used to study genetic diversityamong the freshwater, hermaphroditic snail Biomphalaria pfeifferiin Cameroon. Four of 19 loci studied were polymorphic. The allozymesfor two loci, aspartate aminotransferase-1 (AAT-1) and isocitratedehydrogenase (IDH), showed distribution patterns related toclimatic zones. AAT-1¹⁰⁰ and IDH¹⁰⁰ occurred predominantly inthe Tropical Climatic Zone, located in the northern half ofthe country. AAT-1¹⁴⁰ and IDH⁹⁰ were common in the EquatorialZone. Intrapopulation variations occurred in 5 of 19 populationsampled. Only one population, polymorphic for AAT-1 locus, waspanmictic. The other 4 populations were not in Hardy-Weinbergequilibrium. The deficiency of heterozygotes in those populationsis unusual for a member of the genus Biomphalaria and may bedue to a predominance of self-fertilization. Other explanationsinclude ecological factors or the presence of segregated sub-populationsthat do not outcross. This latter hypothesis is supported bythe total absence of heterozygotes for PGM alleles. (Received 24 August 1989; accepted 14 September 1989)     

ALLOZYME VARIATION AMONG POPULATIONS OF BULINUS FORSKALII (EHRENBERG, 1831) (GASTROPODA: PLANORBIDAE) IN CAMEROON

Allozyme electrophoresis was used to study genetic diversityamong populations of the freshwater, hermaphroditic snail Bulinusforskalii in Cameroon. Three of 15 loci studied in 13 enzymesystems were polymorphic. Intrapopulation variation occurredin 8 of 32 populations sampled and heterozygotes were presentin 2 of these. Neither of these populations were in Hardy-Weinbergequilibrium. These findings are in agreement with reports thathave demonstrated a low genetic diversity in this very widelydistributed, eurytopic species and that have indicated thisspecies reproduces principally by self-fertilization. The restricteddistributions of rarer alleles in three loci may be due to anarrower habitat range for those phenotypes. For isocitratedehydrogenase and phosphoglucoisomerase, the faster alleleswere found throughout the range of B. forskalii extending fromthe Sahelian regions of the north to rain forest of the south.The slower, rarer alleles for these loci were restricted tothe equatorial rain forest regions. For hydroxybutyrate dehydrogenase,the opposite was true for the rarer allele, which was restrictedto a small region in the arid, tropical climate. (Received 27 April 1989; accepted 5 September 1989)     

SURFACING AND WATER LEAVING BEHAVIOUR OF THE FRESHWATER PULMONATE SNAILS LYMNAEA PEREGRA (MULLER), BIOMPHALARIA GLABRATA (SAY) AND BULINUS JOUSSEAUMEI (DAUTZENBERG)

Surfacing and water-leaving behaviour (quitting) of aquaticpulmonate snails was investigated in the laboratory. Regressioncoefficients of surfacing rate per hour for 16 snails in 200cm³ of water were 1.28 for L. peregra, 1.35 for B. glabrata(albino form), 0.47 for B. glabrata (pigmented form) and 0.17for B. jousseaumei. Disturbance, snail size and conditioningof the water were relatively unimportant factors; water volumeadjusted by depth was also unimportant up to 50 mm depth. Importantfactors controlling surfacing appeared to be population density,time of year, temperature, water volume adjusted by perimeterand dissolved oxygen concentration. The response to the latterwas not clear and both B. glabrata and L. peregra orientatedalmost as well to a nitrogen/water interface as to an air/waterinterface. Water-quitting regressed on volume, which had beenadjusted by perimeter variation, revealed a curvilinear relationshipfor B. glabrata, with maximum quitting at 1 snail per 7cm³ (1snail per 14cm of climbable perimeter). L. peregra showed anegative exponential curve, with maximum quitting at 1 snailper 3 cm³ of water (1 snail per 8 cm of climbable perimeter).These data could be relevant to design and management of irrigationchannels where bilharzia might occur. (Received 28 November 1990; accepted 22 October 1991)     

STARVATION, DESICCATION AND USE OF ALLOSPERM IN THE HERMAPHRODITE FRESHWATER SNAIL BIOMPHALARIA GLABRATA (GASTROPODA: PULMONATA)

Pairs of albino and pigmented snails were used to test the hypothesisthat the hermaphrodite freshwater gastropod Biomphalaria glabratawas able to conserve and use allosperm, despite periods of desiccationand starvation. In laboratory experiments, lots of twenty snailswere subject to 0, 5, 10, 15 AND 20 days starvation; similarlots of 20 snails were subjected to 0, 2, 4, 6, 8, 10 weeksof desiccation at R.H. 100%. After all periods of starvationand desiccation, albino parents were still producing significantnumbers of pigmented offspring, suggesting preferential cross-fertilizationusing stored allosperm. (Received 30 September 1993; accepted 23 December 1993)     

THE INFLUENCE OF HUMIDITY, PHOTOPERIOD, AND TEMPERATURE ON THE DORMANCY AND ACTIVITY OF HELIX LUCORUM L. (GASTROPODA, PULMONATA)

The dormancy of Helix lucorum from N. Greece is controlled primarilyby low humidity. Photoperiod and temperature have no directeffect, except in the anomalous combination of long cool daysor short hot days. Adults hibernate more readily than juveniles.The duration of hibernation (before rejection of the calcareousepiphragm) is only slightly shorter at high temperatures. (Received 15 December 1985;     

GROWTH AND DEMOGRAPHY OF POPULATIONS OF BIOMPHALARIA PFEIFFERI (GASTROPODA, PLANORBIDAE) IN THE LABORATORY

The growth and demography of the freshwater snail Biomphalariapfeifferi, an intermediate host of the parasitic worm Schistosomamansoni in Eastern Zaire, were investigated in the laboratoryunder two conditions of crowding. Both individual and populationgrowth were depressed by crowding. Crowding reduced growth rateand all the variables and parameters related to reproduction(fecundity, instantaneous birth rate, net reproduction rate),but did not affect maximum size, survivorship, instantaneousdeath rate and life expectancy. The mechanism involved in theseeffects is likely to be exploitation competition for especiallyprofitable food items such as diatoms. (Received 10 November 1986;     

THE FINE STRUCTURE OF DIFFERENTIATING SPERMATOZOA AND SERTOLI CELLS IN THE GONAD OF THE POND SNAIL, LYMNAEA STAGNALIS

The fine structure has been examined, of spermatogonia, spermatocytes,early spermatids, late spermatids and early spermatozoa nestlingagainst Sertoli cells in the gonad of Lymnaea stagnalis. Changes in the Sertoli cells are linked with the phases of spermdifferentiation. Details on differentiation particularly ofthe head of the sperm, are presented. (Received 14 March 1981;     

AN EVALUATION OF RANDOM AMPLIFIED POLYMORPHIC DNA (RAPD) FOR THE IDENTIFICATION AND PHYLOGENY OF FRESHWATER SNAILS OF THE GENUS BULINUS (GASTROPODA: PLANORBIDAE)

The Random Amplified Polymorphic DNA (RAPD) assay was used tostudy genetic variation within and between 9 species of thegenus Bulinus and to determine whether RAPD profiles could beused as markers for identification purposes. RAPDs were generatedwith 8 primers of two different sizes (l0mers & 15mers)and were visualised using both polyacrylamide gel electrophoresis(PAGE) with silver staining and agarose gel electrophoresiswith ethidium bromide staining. The species groups of Bulinushad few similarities in their RAPD profiles and there was interspecificvariation within groups. Intrapopulation variation was observed,with all primers, for B globosus collected from a single sitein Zimbabwe PAGE/silver staining methods visualised a greaternumber of RAPDs in comparison with agarose/ethidium bromidemethods. Phenetic analysis indicated that distance estimatesbetween taxa were sometimes non-additive and the phylo-geneticanalysis of such non-metnc data is discussed. The resultantphenograms, constructed using a least squares method, were constrainedalmost into a polytomy with topologies often differing betweendata sets. It was concluded that this phenomenon was most likelyattributable to large nucleotide divergences between the speciesgroups which go beyond the phylogenetic scope of RAPD analysis.RAPD profiles, when used in conjunction with other taxonomicmethods, may contribute to the identification of species ofBulnus on a regional basis, but the observed variability ina natural population suggests that a diagnostic RAPD profilefor each species throughout its geographic range is unlikely. (Received 19 April 1995; accepted 1 September 1995)     

新疆托木尔峰黄土地层中蜗牛化石组合及其意义(腹足纲:肺螺亚纲:柄眼目)

本文记述了采自新疆托木尔峰黄土地层中的蜗牛化石,经鉴定有１２种和１待定种,隶属于６科７属。依据现生蜗牛的生活习性,栖息环境及地理分布相比较,划分蜗牛化石２个组合,即琥珀螺Ｓｕｃｉｎｅａ属组合和华蜗牛Ｃａｔｈａｉｃａ属组合及其所反映的生态环境,讨论了新疆托木尔峰黄土地层形成时期的生物气候环境。     

GENETIC VARIATION IN SYMPATRIC AND ALLOPATRIC POPULATIONS OF HYBRIDIZING FRESHWATER SNAIL SPECIES(VIVIPARUS ATER AND V. CONTECTUS)

To estimate the geographical extent of introgression, we studiedthe genetic structure of sympatric and allopatric populationsof hybridizing freshwater snail species Viviparus ater and V.contectus in central Europe. Six allozyme loci which were variablein Lake Garda, Italy in a previous study (five nearly diagnosticloci between the two species and one highly polymorphic locusin V. contectus) were analyzed from ten sympatric locationsand four allopatric populations each for the two species. Presumablyintrogressed genes (low allele frequencies) were found fromat least one locus in seven out of the ten sympatric sites.These seven sites covered most of northern Italy. The data indicatethat introgression has occurred from Viviparus contectus toV. ater and vice versa. Therefore, there is a possibility ofwidespread introgression or mosaic zones in nature. However,we cannot rule out that the observed patterns are due to theshared ancestry. V. ater possessed low genetic variation (thejackknifed mean of Wright's F_ST±S.E. over four loci was0.041±0.004). On the other hand, V. contectus showedhigh genetic differentiation (the jackknifed mean of F_ST±S.E. over six loci was 0.546±0.166). Although introgressionmay have caused evolutionary changes in V. ater and V. contectus,it was not strong enough to level out the genetic differencesbetween the two species, which may have originated from isolationamong populations in V. contectus and a past bottleneck eventin V. ater. (Received 21 June 1996; accepted 31 January 1997)     

RELATIVE IMPORTANCE OF AIRBORNE ODOURS AND TRAILS IN THE GROUP HOMING OF LIMACUS FLAVUS (LINNAEUS) (GASTROPODA, PULMONATA)

The relative importance of airborne odours and con-specifictrails in the group homing of Limacus flavus has been studiedin the laboratory using an Y-olfac-tometer and trail followingtests. The slugs showed positive anemotaxis along air flowspassing through shelters inhabited or recently inhabited byconspecifics. In the choice between single and group shelters,the animals headed preferentially towards the latter ones. L.flavus is able to discriminate between conspecific and heterospecific(Limax maximus) shelters. Trail following tests failed to showsignificant trail superimposition, but did show a significantdeflection of the test slug path towards the marker after crossing.In direct comparison, distant chemoreception prevailed overtrail following in the orientation towards conspecifics (Received 10 August 1987;     

INITIATION AND MODIFICATION OF RHYTHMIC BUCCAL MOTOR OUTPUT IN THE ISOLATED CENTRAL NERVOUS SYSTEM OF LYMNAEA STAGNALIS

The neural circuitry underlying generation of rhythmic feedingmovements in Lymnaea stagnalis has been described in detail.Three types of higher order inter-neurone modulate the outputof the feeding rhythm generator. When stimulated, the Slow Oscillatorand Cerebral Ventral 1 interneurones initiate and maintain patternedmotor output. The serotonergic Cerebral Giant Cells (CGCs) canalso initiate the rhythm, but may suppress or abolish an ongoingrhythm. Application of serotonin to the central nervous systemmimicks the effects of stimulating the CGCs. Another monoamine,dopamine, reliably activates the feeding rhythm generator. Otherneuroactive substances, acetylcholine and FMRFamide, inhibitrhythmic motor output. The variety of routes by which feeding motor output may be controlledexperimentally suggests that the system is highly flexible.This would allow for adaptation to a range of sensory environments.     

THE STRUCTURE OF THE SECONDARY GILLS OF SIPHONARIA CAPENSIS (GASTROPODA: PULMONATA)

The secondary gill of the pulmonate limpet Siphonaria capensisis located in the dorsal portion of the mantle cavity. Eachlamella of the gill is triangular in shape and bears ciliarytufts which have a density of about 400/mm². The free ends ofsome of the cilia are enlarged biconcave discs. Each gill lamellais covered by a single layer of cuboidal (4x4 µm) epitheliumon each side, separated by a central haemocoelic space. Ciliatedand mucus secreting cells are interspersed amongst the epithelialcells. The haemocoelic space is spanned at intervals by trabeculaecontaining longitudinal and transverse muscle fibres. (Received 29 August 1986;