FIELD OBSERVATIONS ON FEEDING OF THE LAND SNAIL HELIX ASPERSA MULLER

Feeding of the land snail Helix aspersa (Müller) was observedat monthly intervals. Three natural populations in Galicia (NW-Spain)were studied. At two sites only a few plants 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 observations wereon Mentha suaveolens, Ranunculus repens and Gramineae. The diversity ofthe snails' diet showed seasonal variation with the maximumin the autumn months. Comparison between the availability ofthe different plant species and their contribution to the snails'diet showed that the snails did not eat at random; Urtica dioicawas eaten much more than expected from its occurrence and grasseswere strongly under-represented in the snails' diet. Temporalchanges of availability were significantly correlated with the amountseaten in the case of Urtica, but not for the other food plants.The distribution of the snails in the plot was significantlycorrelated with that of Urtica. Chemical analyses of the foodplants revealed Urtica as the species with the higher protein,ash and calcium contents. The strong preference of Helix aspersafor Urtica dioica could be explained by the value of Urticaas food or by its suitability as habitat for the snails. The largestproportions of green material in the snails' diet occurred inthe spring and juveniles ate more green material than adultsin the three populations. (Received 16 March 1998; accepted 30 November 1998)     

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)     

VARIATION OF WATER CONTENT IN HELIX ASPERSA MULLER IN A NATURAL ENVIRONMENT

1. Daily and seasonal variations occur in the water content ofHelix aspersa Müller.
2. The water content is not constantthroughout 24 hours but fluctuatesby 2 to 10%. There is noapparent daily rhythm.
3. Humidity has an effect on the watercontent. Falls in relativehumidity to 60% or less, triggera decrease of the snails' hydration,over a range of temperatures,as well as a brief pause in activity.
4. Only external climaticconditions produce noticeable fluctuationsof water content,and they more particularly affect juvenileswhich undergo greaterevaporo-transpiration.
5. A seasonal rhythm in water contentexists in Helix aspersa,with maxima during spring and autumnand minima in June andJanuary.
6. The water content of juvenilesis always superior to that ofthe adults.
7. The water contentaffects the duration of the snail's activephase.

(Received 22 September 1988; accepted 5 February 1989)     

SNAIL FARMING: A SURVEY OF BREEDING MANAGEMENT, HYGIENE AND PARASITISM OF THE GARDEN SNAIL, HELIX ASPERSA MULLER

Fifteen farms in 1984 and twenty in 1985 were investigated forbreeding management, hygiene and parasitism. Farm hygiene didnot seem to play an important role on the breeding performancesin our samples. Nematodes were the most frequent parasites:Alloionema appendiculatum were equally prevalent among juvenileand adult snails whereas Angiostoma aspersa and Nemhelix bakeriwere found mostly in reproductive adults. The presence of nematodes,assessed by coproculture, was negatively related to breedingperformances     

VARIABILITY OF EGG CANNIBALISM IN THE LAND SNAIL HELIX ASPERSA IN RELATION TO THE NUMBER OF EGGS AVAILABLE AND THE PRESENCE OF SOIL

Egg cannibalism by hatchlings has been demonstrated in somepulmonate land snails; this behaviour is promoted by a highhatching asynchrony within the egg-batch. Under laboratory conditions,the percentage of new-born snails Helix aspersa having cannibalisedunhatched eggs was not influenced by the soil factor: about70% of them ingested one egg within their first four days oflife whether soil was present or not. The propensity to eggcannibalism in hatchlings of H. aspersa increased with egg density.However, most of the new-born hatchlings consumed a single eggduring the four days following hatching, and only exceptionallytwo. The consumption of one egg increased the snails wet weightby 38.7% within four days. A weak ingestion of soil componentsalso occurred, but it induced a growth that was three-timesless than that due to the consumption of an egg. In addition,the survival of newly hatched snails maintained under non-dehydratingthermohygrometric conditions was high, even when they were submittedto four days food-deprivation. (Received 22 July 1999; accepted 24 November 1999)     

THE LOVE DART OF HELIX ASPERSA MULLER IS NOT A GIFT OF CALCIUM

The phenomenon of dart shooting in several species of land snailshas still not been explained. We were interested in whetherthe dart can function as a nuptial gift of calcium, as previouslyproposed. Donating calcium would increase the fitness of theoffspring and thereby result in a higher reproductive successfor the donor. We confirmed in Helix aspersa that the developingembryo takes up calcium from the egg shell for the formationof its embryonic shell. However, other results from behaviouralobservations and calcium measurements in various reproductivestructures do not support the calcium hypothesis. We found thatthe dart penetrated the skin in 91.7% or the shootings, butit was internalized by the recipient in only 6.3% of the shootings.The amount of calcium in one dart is roughly equal to that ofone egg, and thus it would not contribute significantly to anaverage clutch of 59 eggs. The spermatophore contains virtuallyno calcium, and therefore it is unlikely that the dart signalsa donation of calcium with the sperm. The dart is also unlikelyto influence egg laying since dart shooting does not predicteither the latency or the productivity of egg laying in theshooter or the recipient. We conclude that the love dart ofHelix aspersa is not a gift of calcium. Instead, we suggestthat it is a vehicle to introduce a substance into the partnerto influence the fate of the donated sperm. (Received 8 November 1996; accepted 25 April 1997)     

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)     

GEOGRAPHICAL PATTERNS OF GENETIC DIFFERENTIATION IN THE LANDSNAIL HELIX ASPERSA MuLLER (GASTROPODA: PULMONATA)

In order to investigate the initial processes responsible forthe geographical distribution of the Mediterranean landsnailHelix aspersa, populations from 64 locations in Maghreb andin Europe were studied at 17 enzyme loci by both starch andpolyacrylamide gel electrophoresis. Allele frequencies and genotypedistributions were analysed to provide information on populationstructure and genetic differentiation among populations. Valuesof Nei's unbiased (D) and Rogers (R) genetic distances led tothe formation of two main groups, namely Europe-west Algeriaand east Maghreb (average D = 0.182). Such splitting supportedthe pattern of anatomical variation recently described. Twosamples from Morocco were excluded from these groups becauseof an average D = 0.224. Within this Moroccan entity, geneticdivergences did not distinguish the ‘giant’ snailsH. a.maxima from the brownsnails H. a. aspersa. Such biochemicalsimilarities among morphological heterogeneity throws the subspecificstatus of H. a. maxima into question. Values of fixation indicesindicated that the greatest differentiation between samplesoccurred on the Maghreb territory ( = 0.393). The patterns of population differentiation are discussed inrelation to past geological and historical events. Thus, threescenarios, not mutually exclusive, are considered: the disjunctionof populations by plate tectonics during the Tertiary, the formationof geographically isolated populations during the Pleistoceneglacial/interglacial cycle and the possibility of human introductionas early as the Neolithic period. (Received 9 December 1992; accepted 11 November 1993)     

NATURAL SELECTION REDUCES ENERGY METABOLISM IN THE GARDEN SNAIL, HELIX ASPERSA (CORNU ASPERSUM)

Phenotypic selection is widely recognized as the primary cause of adaptive evolution in natural populations, a fact that has been documented frequently over the last few decades, mainly in morphological and life-history traits. The energetic definition of fitness predicts that natural selection will maximize the residual energy available for growth and reproduction, suggesting that energy metabolism could be a target of selection. To address this problem, we chose the garden snail, Helix aspersa ( Cornu aspersum ). We performed a seminatural experiment for measuring phenotypic selection on standard metabolic rate (SMR), the minimum cost of maintenance in ectotherm organisms. To discount selection on correlated traits, we included two additional whole-organism performance traits (mean speed and maximum force of dislodgement). We found a combination of linear (negative directional selection, β=−0.106 ± 0.06; P = 0.001) and quadratic (stabilizing selection, γ=−0.012 ± 0.033; P = 0.061) selection on SMR. Correlational selection was not significant for any possible pair of traits. This suggests that individuals with average-to-reduced SMRs were promoted by selection. To the best of our knowledge, this is the first study showing significant directional selection on the obligatory cost of maintenance in an animal, providing support for the energetic definition of fitness.     

GENETIC AND CONCHOLOGICAL COMPARISON OF SNAILS (HELIX ASPERSA) DIFFERING IN SHELL DEPOSITION OF LEAD

Populations of snails inhabiting areas with different historiesof Pb contamination differed in their deposition of Pb in shellrelative to soft tissues. Genetic variation, measured usingisozymes, was not related to Pb history nor geographic distancebetween populations. Shell characteristics were significantlydifferent among sites; shell dry weight was strongly relatedto soil calcium levels. Shells of snails from areas with longhistories of Pb contamination were significantly more robust(greater shell width/shell height ratio) than snails from otherlocations. H. asprsa adaptation to Pb contamination may involvesignificant changes in shell characteristics but these do notcorrelate with genetic traits assessed with allozymes (Received 29 December 1994; accepted 15 October 1995)     

FOOD SELECTION IN THE LAND SNAIL LIMICOLARIA FLAMMEA MULLER (PULMONATA: ACHATINIDAE)

Food consumption by juvenile and adult specimens Limicolariaflammea was measured as dry weight of food eaten. The snailwas fed weekly for 15 months on four types of food (lettuce,potato, apple and carrot) and chalk provided, hence an opportunityto select for food. Juvenile snails showed greater selectivitydepending on whether the food was needed for somatic growthor reproduction. On the other hand, when fed exclusively onone each of the same food items and given no opportunity tochoose, the snails grew better on the same food items they selectedin the choice experiment. The absorption efficiencies (AD) werehigh, the utilisation efficiency (ECI) was 1–13% and showeddistinct peaks corresponding to egg production. Values for ECDwere 1.25–15.5%. Variation in growth rates was due mainlyto utilisation efficiencies of the four foods tested. Largeamounts of chalk were eaten during the period of somatic growthand greatly reduced during the reproductive phase of L. flammea.The significance of these findings to the snails ecology isdiscussed. (Received 9 April 1991; accepted 11 June 1991)     

POPULATION DENSITY EFFECTS ON GROWTH IN CULTURE OF THE EDIBLE SNAIL HELIX ASPERSA VAR. MAXIMA

The effects of population density on the growth of H. aspersaMÜller var. maxima under controlled environmental conditionswere examined. Inhibitory effects on snail growth and maturityresulting from increased population density, between 100–800snails m^–2 of floor area, were observed for a range ofcontainer cleaning frequencies. At all population densities,enhanced snail growth was observed when the frequency of containercleaning was increased to a two-day interval. No significantdifferences were recorded, following 19 weeks growth, betweenfinal mean weights of snails from containers cleaned less frequently.The lowest snail mortality was consistently recorded at thelowest population density in the most frequently cleaned containers.At all snail population densities three phases of growth wereobserved: (a) lag (0–5 weeks), (b) rapid (6–15 weeks)and (c) stable (16 weeks and over). During the first three weeksof growth, high population density had a positive effect ondiet consumption, food conversion efficiency and snail growth.Adverse population density effects increased progressively duringphase (b), typically following 9 weeks growth. Juvenile snailstransferred from high to low population densities during phase(b) continued to exhibit slower growth rates associated withhigh population densities. Food conversion efficiency of snailsin all treatments decreased throughout the experimental periodbut with no overall effect of container cleaning frequency apparent.Inherent growth variability of sibling snails was unaffectedby population density or container cleaning frequency. The importanceof the results for intensive snail culture is discussed. (Received 23 June 1994; accepted 1 December 1994)     

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)     

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)     

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)