THE EFFECTS OF POPULATION DENSITY ON GROWTH RATE IN LIMICOLARIA FLAMMEA MULLER (PULMONATA: ACHATINIDAE)

Density effects on growth and sexual maturity in laboratory-keptLimicolaria flammea MÜller are described; high densityresults in low growth, higher mortality and a delay in sexualmaturity. (Received 9 April 1991; accepted 11 June 1991)     

A POPULATION STUDY OF AN EQUATORIAL LAND SNAIL, LIMICOLARIA MARTENSIANA (ACHATINIDAE)

Near the equator in Uganda, the herbivorous land snail, Limicolaria martensiana , occurs in well-defined populations. These populations may be very large and the snails may in places occur at exceedingly high densities. The snails breed all the year round, but with two peaks that correspond with the two annual dry seasons. They enter dispause whenever the weather is very dry. The size of adult snails seems environmentally determined and it varies in different populations : the largest snails occur on soil rich in calcium. The shells of the snails are highly variable and polymorphic in colour and pattern. In particular, there is a streaked form that occurs in all populations, and four distinct pallid dilute forms. The relative frequency of the forms varies markedly in different populations. Polymorphism is greatest at high population densities and the snails are all streaked where they are rare. Polymorphism can be distinguished in Pleistocene fossil populations.     

THE INFLUENCE OF INGESTIVE CONDITIONING ON FOOD CHOICES IN THE LAND SNAIL HELIX ASPERSAMULLER (GASTROPODA: PULMONATA: STYLOMMATOPHORA)

An experiment of food choice in one-day-old naïve landsnail Helix aspersa Müller was carried out, involving threephases. First, a training period on a monophagous diet of leafdiscs of Taraxacum officinale or Urtica dioica was given for1, 5, 10, 15 or 30 days. The area of leaf discs consumed byeach snail during this stage was assessed. In the intermediatephase, the animals were not fed for 24hours. Finally, food choicewas tested over 24 hours, when animals could eat leaf discsof each species of plant usedfor training. A coefficient offood preference for Urtica dioica was calculated (UPC = areaof Vrtica leaf disc consumed/area of Urtica and Taraxacum discsof leaves consumed for 24 hours). The length of the training phase did not reduce the percentageof individuals preferring the plant previously consumed. Onthe contrary, after 15 days of monophagous diet, animals formerlyfed on Urtica showed a coefficient of food preference for Urticasignificantly higher than Taraxacum fed animals, and after 30days of training this phenomenon was more pronounced. Thus,during the choice test, snails ate more of the previously eatenplant leaf. Our experiment highlighted the feeding behaviourplasticity of H. aspersa. The significant correlation betweenthe area of Urtica eaten during the training phase and the coefficientof food preference for Urtica is discussed. (Received 10 April 1994; accepted 10 January 1995)     

GENETIC STUDIES OF ALLOZYME VARIATION IN LEUCINE AMINOPEPTIDASE IN THE LAND SNAIL HELIX ASPERSA (MULLER)

In the land snail H. aspersathe enzyme LAP has two loci, LAP-1and LAP-2, both of which arc monomeric enzymes under the controlof multiple alleles, the alleles being codominant. None of theobserved ratios in the pheno types in the experimental progenywere significantly different from Mendelian expectation. ^* Present Address: Bournside School, Cheltenham, Glos. (Received 1 September 1981;     

NATURAL HISTORY AND POPULATION DYNAMICS OF THE LAND SNAIL HELIX TEXTA IN ISRAEL (PULMONATA: HELICIDAE)

Helix texta is endemic to the Mediterranean regions of Israel.It has a seasonal activity pattern which starts in the autumn,with the first rains, and dwindles towards the spring, whenthe snails dig into the ground for a six months long aestivation.A cold spell of 0°C will, however, terminate the activeseason of the adult snail, even in the middle of the rainy season.Survival of the young is very low and most of them (90%) donot survive their first year, because of the winter cold andthe summer drought. Massive predation of adult snails by wild boar was observedin December 1986: within a few days, about 50% of the adultsin the study in the area were eaten. A very rapid growth ofyoung and subadults was observed immediately after this predation.These observations suggest that the extent of recruitment ofnew adults to the population may be partly controlled by existingadults, through a growth-inhibiting pheromone in the mucus.After predation this inhibiting factor disappears, enablinga subsequent rapid growth of the young. In this manner, massive,irregular predation of the adults by a large predator, and changesin juvenile survival, result in sharp fluctuations in the agestructure of the population. The resulting pattern of unstablepopulation dynamics is different from that described for theEuropean species of Helix. (Received 16 January 1989; accepted 17 April 1989)     

FIELD OBSERVATIONS ON FEEDING OF THE LAND SNAIL HELIX ASPERSA MULLER

Feeding of the land snail Helix aspersa (Müller) was observedat monthlyintervals. Three natural populations in Galicia (NW-Spain)were studied. At two sites only a fewplants constituted thebulk of the diet and in spring the snails' diet had the highestdiversity(H'). In the third population feeding and distributionof Helix aspersa(Müller) were observed in a small plotwith permanent patches of Urtica dioica.Nearly one half offeeding snails fed upon Urtica dioica. Most of the other observationswereon Mentha suaveolens, Ranunculus repens and Gramineae. The diversityofthe snails' diet showed seasonal variation with the maximumin the autumn months.Comparison between the availability ofthe different plant species and their contribution to thesnails'diet showed that the snails did not eat at random; Urtica dioicawas eatenmuch more than expected from its occurrence and grasseswere strongly under-represented in thesnails' diet. Temporalchanges of availability were significantly correlated with theamountseaten in the case of Urtica, but not for the other food plants.The distribution ofthe snails in the plot was significantlycorrelated with that of Urtica. Chemical analysesof the foodplants revealed Urtica as the species with the higher protein,ash and calciumcontents. The strong preference of Helix aspersafor Urtica dioica could beexplained by the value of Urticaas food or by its suitability as habitat for the snails. Thelargestproportions of green material in the snails' diet occurred inthe spring and juvenilesate more green material than adultsin the three populations. (Received 16 March 1998; accepted 30 November 1998)     

POPULATION STRUCTURE AND LEVELS OF GENE FLOW IN THE MEDITERRANEAN LAND SNAIL ALBINARIA CORRUGATA (PULMONATA: CLAUSILIIDAE)

The amount of gene flow among local populations partly determines the relative importance of genetic drift and natural selection in the differentiation of such populations. Land snails, because of their limited powers for dispersal, may be particularly likely to show such differentiation. In this study, we directly estimate gene flow in Albinaria corrugata, a sedentary, rock-dwelling gastropod from Crete, by mark-recapture studies. In the same area, 23 samples were taken and studied electrophoretically for six polymorphic enzyme loci. The field studies indicate that the population structure corresponds closely to the stepping-stone model: demes are present on limestone boulders that are a few meters apart, and dispersal takes place mainly between adjacent demes. Average deme size (N) is estimated at 29 breeding individuals and the proportion of migrants per generation at 0.195 (Nm = 5.7). We find no reason to assume long-distance dispersal, apart from dispersal along occasional stretches of suitable habitat. Genetic subdivision of the population, as derived from F_ST values, corresponds to the direct estimate only at the lowest spatial level (distance between sample sites < 10 m), where values for Nm of 5.4 and 17.6 were obtained. In contrast, at the larger spatial scales, F_ST values give gene-flow estimates that are incompatible with the expected amount of gene flow at these scales. We explain these discrepancies by arguing that gene flow is in fact extremely limited, making correct estimates of Nm from F_ST impossible at the larger spatial scales. In view of these low levels of gene flow, it is concluded that both genetic drift and natural selection may play important roles in the genetic differentiation of this species, even at the lowest spatial scales.     

CONSUMPTION OF AMPHIPODS BY THE NEW ZEALAND LAND SNAIL WAINUIA URNULA (PULMONATA: RHYTIDIDAE)

An unusual carnivorous diet was documented for the rhytididsnailWainuia urnula urnula (Pfeiffer) from the southern NorthIsland,New Zealand. Eighty-two percent of 315 samples of faeces orgutcontents contained remains of landhoppers (Crustacea:Amphipoda).Earthworms (Oligochaeta) were the second most common foodtypeidentified, but occurred in only 4 percent of samples. In thelaboratory,W. urnula urnula captured landhoppers by rapidlyeverting theTVU-section odontophore beneath the prey and immediatelydrawingit into the mouth in a single action. Diet samples from theallopatriccongeners W. edwardi (Suter) (n 5 51) andW. clarki Powell (n5 7) contained no remains of landhoppers,and these snails didnot eat landhoppers in the laboratory. The dietof these specieswas dominated by earthworms. The subspeciesW. u. nasuta ateboth earthworms and amphipods with similarfrequency (n 5 40).Individual teeth on the radula of W. urnulaurnula had a simpleblade-like cusp like other Wainuia species,but there were fewerteeth per row and fewer rows on the whole radulathan in otherspecies. W. u. nasuta had a similar number ofrows of teethto W. clarki and W. edwardi, but thenumber of teeth per rowwas intermediate between these species andW. u. urnula. Recordsof pulmonate species preying onarthropods are rare, yet inthe case of W. urnula thisspecialization has been accompaniedby minimal morphological change. (Received 22 December 1998; accepted 11 March 1999)     

ANNUAL ACTIVITY CYCLES OF THE LAND SNAIL HELIX ASPERSA MULLER IN NATURAL POPULATIONS IN NORTH-WESTERN SPAIN

Activity and inactivity (dormancy) periods of the land snailHelix aspena MÜller were monitored at monthly intervalsduring two years in two natural populations in Galicia (NW-Spain),one m an area of Mediterranean climatic influence and anotherin an Atlantic -climate area The annual activity cycles of Haspena in Galicia are consistent with latitudinal adjustmentsof the life cycle of this species which shows a very wide distributionrange, but also showed some special features related to theparticular microclimate of each locality (Received 4 December 1995; accepted 6 May 1996)     

HETEROGENIC COPULATORY BEHAVIOUR PRODUCES NON-RANDOM MATING IN LABORATORY TRIALS IN THE LAND SNAIL HELIX ASPERSA MULLER

This study reports the results of mate-choice experiments betweenresident and non-resident individuals of the simultaneous hermaphonteland snail Helix aspersa. Snails from different sites differedin their mating proclivity, which resulted in non-random matingin laboratory mate-choice tests. Those snails with the highermating propensity therefore tended to mate with their own typeand tended to be chosen as partner because they were also activelyseeking mates. Keeping snails under crowded conditions did noteffect a lasting influence on mating behaviour in laboratorytests, despite the decline in copulatory behaviour under crowdedfield conditions. (Received 7 November 1994; accepted 31 August 1995)     

THE LIFE CYCLE, POPULATION DYNAMICS, GROWTH AND SECONDARY PRODUCTION OF THE SNAIL BRADYBAENA FRUTICUM (MULLER, 1774) (GASTROPODA PULMONATA) IN NORTHERN GREECE

The life cycle, population dynamics, growth and secondary productionof the pulmonate land snail Bradybaena fruticum were studiedin northern Greece. The demographic analysis of the populationsof B. fruticum revealed that a) two cohorts exist in the fieldthroughout the year b) the reproductive period started in thebeginning of summer and c) growth was increased during springand autumn. According to von Bertallanffy' method B. fruticum needs 5 yearsto attain its maximum size in the field of 25.40 mm. Mortalityrate increases and life expectancy decreases with increasingage. Net reproductive rate (Ro) was equal to 3. 15 and per capitarate of increase (r^c) was equal to 1. Annual secondary production, calculated by Hynes' size frequencymethod, revealed a mean annual density of 5.9 individuals/m²,a mean standing crop (B) of 8.09 g/m²/year and an annual production(P) of 1.92 ± 0.11g/m²/year. Annual turnover ratio (P/B)was equal to 2.37. (Received 25 May 1989; accepted 17 March 1989)     

THE REPRODUCTIVE BIOLOGY OF THE AMPULLARIID SNAIL POMACEA URCEUS (MULLER)

Investigations of the reproductive biology, life cycle and populationdynamics have been undertaken to assess the potential of Pomaceaurceus as a culture species in Trinidad. The species is dioeciousand evidence from microscopic gonad analyses, together withmaturity indices and size frequency distributions in the population,all indicate that the reproductive cycle is annual. Adults spawnat the end of the rainy season months. Hatching and early developmentof the young occur in the dry season (January to May) whileadult females aestivate. The total developmental period variedbetween 22 to 30 days. Mean fecundity is 54 eggs/female witha range of 21 to 93 for egg masses deposited in the field andthose produced under laboratory conditions. ^*Present address: Zoology Dept., University of Aberdeen, TillydroneAve., Aberdeen AB9 2TN. Scotland. (Received 13 January 1988; accepted 11 April 1988)     

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)     

GENETIC AND MORPHOLOGICAL VARIATION IN THE LAND WINKLE POMATIAS ELEGANS (MULLER) (CAENOGASTROPODA: POMATIASIDAE)

Population genetics and shell morphology have been studiedin 11 populations of the poorly colonizing land caenogastropodPomatias elegans. The total area of suitable habitats in northwestEurope is shrinking and many isolated populations are becomingvulnerable to extinction. In this study we tested whether theconcept of management units (i.e. groups of population withsignificantly different allele frequencies due to demographicindependence) is applicable to the conservation of P. elegans.F_st values indicated strong genetic differentiation and thuslittle genetic exchange between populations. Allozyme differentiationcould be explained with an isolation by distance model, whereasmorphological differentiation could not. A morphological differenceexists between sexes but not sufficient to discriminate malesand females. A Mantel test showed no significant relationshipbetween morphological distance (size corrected or not) and geneticdistance. Since allele frequencies differed even among populationsin areas where P. elegans is not threathened, we conclude thatin the case of P. elegans, defining management units with allozymesmay not be an appropriate way to select the most suitable populationsfor conservation. (Received 3 April 2000; accepted 25 August 2000)     

DISTRIBUTION OF THE SCHISTOSOME VECTOR SNAIL, BIOMPHALARIA STRAMINEA (PULMONATA: PLANORBIDAE) IN HONG KONG

Biomphalaria straminea (Dunker) is a snail intermediate hostof Schistosoma mansoni Sambon which has invaded Hong Kong inrecent years from its home range in Brazil. This study examinesits distribution pattern in terms of habitat type and hydrologicalfactors. The data revealed very strong colonizing power, bothin terms of number of sites occupied as well as numerical dominanceat these sites. An inability to colonize lotic habitats mayhave been related to low levels of dissolved minerals and nutrientsbut the effect of current speed on colonization has yet to beresolved. Principal component analysis revealed that hydrological factorsimportant for colonization included high levels of dissolvedcalcium, magnesium, iron, chloride, total nitrogen, sulphateand phosphate. pH had only a secondary effect. The significanceof these findings is discussed with special reference to thelikelihood of introduction of B. straminea elsewhere, and possiblemanagement strategies. (Received 16 December 1988; accepted 12 February 1989)     

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)     

GROWTH, SURVIVORSHIP, AND POPULATION SIZE IN THE LAND SNAIL RHAGADA CONVICTA COX, 1870 (PULMONATA: CAMAENIDAE) FROM A SEMIARID ENVIRONMENT IN WESTERN AUSTRALIA

Basic characteristics of a population of the camaenid land snailRhagada convicta were studied in a semiarid environment nearNorth West Cape, in Western Australia. Consistent evidence frommark-recapture studies and analysis of size-frequency distributionindicates an average annual increase in shell diameter of 3.6mm, with an average period of 5 years from hatching to reachadult size. These rates are low, compared with most other landsnails studied, including cama-enids from wetter environments.Estimates of mortality rate and the rate of recruitment intothe adult population indicate that the snails live an averageof approximately 5 years after reaching adult size, which meansthat the period of turnover for this population is approximately10 years. The population density was estimated to be 0.8 adultsper m², with a total population size (± s.e.) of 875± 164. The population is very localised, with an estimateddiameter of the neighbourhood area of only 38 m. (Received 28 August 1990; accepted 12 February 1991)     

METABOLIC ADAPTATION IN THE POLYMORPHIC LAND SNAIL ARIANTA ARBUSTORUM

The oxygen consumption of brown and yellow morphs of autumnand winter Arianta arbustorum was determined after adaptationto several laboratory microclimatic conditions. The data onmetabolism of autumn and winter collected snails were treatedstatistically by using factorial analysis of variance for eachseason. There are highly significant differences in the metabolicrate of autumn and winter animals resulting from the influencesof microclimatic elements like adaptation temperature, relativehumidity, environmental temperature as well as the effect ofphenotypic shell colour. There are also significant differencesin the interactions of these factors which relate the animalsto natural ecological influences. The energy utilization averagedabout 25 percent less by the brown morph than the yellow oneat 20–30°C. Comparisons were made between oxygen consumptionof the animals at these two seasons. The winter snails are characterizedby a depressed respiration rate, and significant lowering ofrespiratory Q₁₀. The results are discussed in the light of physiologicalselection by different responses of metabolic rate to microclimaticfactors on the two morphs. This is important in the maintenanceof genetic diversity and polymorphism in A. arbustorum. ^*Current address: Department of Zoology, University of El-Azhar,Egypt. (Received 23 March 1983;