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)     

PLASTICITY OF THE LIFE CYCLE OF XEROPICTA DERBENTINA (KRYNICKI, 1836), A RECENTLY INTRODUCED SNAIL IN MEDITERRANEAN FRANCE

Xeropicta derbentina (Krynicki, 1836), a native of Eastern MediterraneanEurope, was introduced to southeastern France during the 1940sand is now widely spread across Provence. In summer it aggregateson plants, making its populations clearly visible. However,its life cycle within the Mediterranean basin is poorly documented.While X. derbentina in its native area exhibits an annual lifecycle, this species has been found in Provence to have a bienniallife cycle. Moreover, in southeastern France, field studieswithin a restricted area show variations in demographic structure.In consequence, the life cycle of X. derbentina and the demographicpatterns observed require clarification. Five populations withvarious demographic structures were studied over 1 year in thesame location, i.e. under the same climatic conditions. Thefield study was complemented by laboratory observations on mating,egg-laying and hatching. Xeropicta derbentina appears to bea semelparous species, with an annual life cycle being foundon four plots. The reproductive period begins at the end ofsummer and lasts until the beginning of winter. First egg-layingoccurs within 1 week after mating and lasts up to 30 days. Hatchingtakes place 15–20 days after egg-laying. Xeropicta derbentinapossesses multiple mating and egg-laying sessions, involvingsuccessive hatching. Populations are mainly characterized bytwo growth stages, the first in spring when newly-hatched snailsevolve into juveniles, and the second in late summer when theyreach maturity. However, on the highest density plot, a bienniallife cycle is observed for some newly-hatched snails that showan interrupted growth during summer and evolve into juvenilesonly in the second autumn. Moreover, this life cycle not onlyvaries among plots but also at a 1-year interval within plots.Hence, the life span of X. derbentina is between 12 and 20 months,but can be extended up to 30 months according to whether hatchingoccurs early or late and whether they survive the first andsecond winters. Xeropicta derbentina is thus able to have variousgrowth speeds and life spans, and appears to switch from anannual life cycle to a biennial cycle in response to populationdensity or climatic conditions. (Received 8 October 2004; accepted 15 December 2004)     

LIFE CYCLE AND HABITAT SHIFT OF THE TROCHID SNAIL DILOMA SUAVIS (PHILIPPI) WITHIN INTERTIDAL MUSSEL ZONES

The life cycle of the trochid snail Diloma suavis (Philippi, 1849),was studied on an intertidal rocky shore at Shirahama, Wakayama Prefecture,where two mytilid bivalves, Septifer virgatus (Wiegmann) andHormomya mutabilis (Gould), formed vertically contiguous musselbeds in the upper-middle and lower zones, respectively. At lowtide in April, the snail density increased with decreasing shoreheight and was greatest at the middle level of the H. mutabilisbed. Then, the density decreased towards the lower littoralfringe. Newly settled juveniles smaller than 2 mm in shell heightappeared abundantly in late summer and autumn within algal turfon the lower shore. As snails grew larger than 2 mm, they appearedwithin the gaps of the H. mutabilis bed and the S. virgatusbed. They increased in size monotonically towards the next summer,but rate of growth in shell height tended to be great in autumnand small in winter. Seasonal change in the density of snailsfound within the gaps of the mussel beds was remarkable during athree year period, increasing from autumn to winter and then decreasingtowards next summer. Reproduction occurred in summer, and adultsnails disappeared by September. It is thus suggested that this specieshas a one year lifespan and shows a habitat shift from algal turfto the gaps of the mussel beds with growth. (Received 12 October 1998; accepted 2 March 1999)     

Field and laboratory studies on the life-cycle, growth and feeding preference in the hairy snail Trochulus hispidus (L., 1758) (Gastropoda: Pulmonata: Hygromiidae)

For the first time the life cycle of the common land snail Trochulus hispidus was completely described in Central Europe (Poland). This is a semelparous species predominantly with an annual life cycle and the reproductive period lasting from April till October. The first young snails hatch in spring, grow rapidly in summer and reach ca. 4 whorls until winter. In spring of the next year they mature and reproduce. After that they die. There is hardly any growth from late autumn till early spring. The average proportional growth rate is ca. 0.3 whorl/month in the wild. The fastest growth is present in the youngest snails and then gradually decreases over the course of their age. Laboratory and field observations allowed for establishing the following life cycle parameters: eggs calcified, almost spherical, ca. 1.5 mm, laid in spring and summer in batches of between 1 and 47. Time to hatching is 6–24 days, hatching is asynchronous; newly-hatched snails have approximately 1.5 whorls. Analysis of food preferences revealed, that T. hispidus tends to restrict its diet during the life. Generally the youngest snails equally consumed leaves of all four tree species offered (Fraxinus excelsior, Acer pseudoplatanus, Tilia cordata and A. platanoides) whereas adults preferred F. excelsior over A. pseudoplatanus and A. platanoides.     

REPRODUCTIVE CYCLE OF THE CUTTLEFISH, SEPIA OFFICINALIS (L.) IN THE NORTHERN PART OF THE BAY OF BISCAY

The sexual cycle of the cuttlefish, Sepia officinalis, fromthe northern part of the Bay of Biscay was followed over severalyears (1988 to 1990 and 1992 to 1993). Successive maturity stagesare reached at the same time regardless of site in the northernpart of the Bay. In this area, the majority of cuttlefish reproduceduring their second year of life (group II) whereas the remainderreproduce in their first year (group I). The first visible signsof sexual development concern the testis in males and the genitaltract in females. Males mature earlier than females: the firstspermatophores appear in July (group II) and October (groupI) while mature eggs appear from December (group II) and March(group I). The breeding season lasts from about mid-March tolate June (3.5 months). (Received 2 February 1996; accepted 30 May 1996)     

LIFE HISTORY AND POPULATION DYNAMICS OF COCHLICELLA ACUTA (MuLLER) (GASTROPODA: HELICIDAE) IN A PASTURECEREAL ROTATION

The conical snail Cochlicella acuta was sampled over a 3-yearperiod in a pasture-cereal rotation and a nearby roadside wastelandat Hardwicke Bay, South Australia. The life cycle of C. acuta was primarily biennial in the agriculturalfields, with offspring being produced in large numbers in thepasture phase but not the cereal phase of the rotation. Thebreeding season lasted from autumn to spring. Snails were mostabundant in spring and summer, especially near the edges offields. During summer, snails aggregated on robust weeds suchas Reseda lutea. Many snails were killed by burning pasturesin autumn, prior to sowing crops. Snails were more abundantand smaller in size in the roadside wasteland than in the agriculturalfields. Strategies for the control of C. acuta in pasture-cereal rotationsare discussed. (Received 9 April 1990; accepted 10 August 1990)     

COPULATION AS FEMALES AND USE OF ALLOSPERM IN THE FRESHWATER SNAIL GENUS BULINUS (GASTROPODA:PLANORBIDAE)

Each of 8 snails in 2 groups of Bulinus (Physopsis) globosus,1 group raised in isolation and 1 group raised in community,were paired for 14 consecutive days with a male-acting partnersnail. In each group, the experimental snails, which were notallowed to act as males, were able to copulate as females onapproximately 94% of the days paired. Two copulations as female,with the same male partner, occurred on 50% of the days thatthe snails were paired, in the 2 groups combined. Non-receptivefemale behaviour by the experimental snails occurred frequently,and copulation was prevented by such behaviour during 6 pairings,3 in each group. Young B. (P.) africanus first copulated as females when theywere 31–33 days old. The accessory sex glands of the femalereproductive tracts of these young female-acting snails containedmoderate to large amounts of secretion. B. (P.) africanus, which were raised in pairs, laidcross-fertilizedeggs in isolation for an average of 76 days, and 1559 eggs/snailwere deposited before cross-fertilization ceased. Cross-fertilizedeggs were produced for as long as 120 days. After 1 copulation as female, virgin B. (P.) africanus laidcross-fertilized eggs for an average of 78 days and deposited3654 eggs/snail before crossfertilization ceased. Cross-fertilizedeggs were produced for as long as 113 days. After 2 copulationsas female, 1 copulation on each of 2 consecutive days, virginB. (P.) africanus laid cross-fertilized eggs for an averageof 102 days and produced 4397 eggs/snail before cross-fertilizationceased. Cross-fertilized eggs were produced for as long as 123days. Snails which were homozygous for an allele governing mantlepigment pattern were raised with a partner which was homozygousfor a different pigment pattern. Young produced in a 4-day periodafter the snails were isolated were 100% heterozygous. The snailswere then rearranged into pairs with a partner of the same genotypefor 4 days, during which time 26% of the young produced werehomozygous. The snails were again isolated for 4 days, and 49%of the young produced during this 4-day period were homozygous.The results of this experiment strongly suggest that multipleoutcrossing occurred. In B. (P.) africanus, stored allosperm were used to fertilizeeggs after 1, 4 and 7 weeks of starvation; after 1 and 4 weeksof 15°C low temperature and 4 weeks of 15°C + 4 weeksof 10°C low temperature; and after 1 and 4 weeks of desiccation.After 8 weeks of desiccation, 2 of 3 surviving snails reproducedby self-fertilization and 1 snail did not reproduce. Too fewsnails survived 8 weeks of desiccation for a conclusion to bereached on the ability of allosperm to survive. (Received 1 June 1984;     

SOME ASPECTS OF THE BREEDING BIOLOGY OF PLANAXIS SULCATUS (BORN) (GASTROPODA: PLANAXIDAE)

Sex ratio and breeding habits of Planaxis sulcatus were studiedfor Japanese and other Indo-Pacific populations. On the coastof the Kii Peninsula, Japan, the gonad of P. sulcatus is developedfrom June through August, when pairing of the snails is observed.The females contain embryos in their brood pouches and releaseveligers. New recruits appear in autumn, grow during the nextspring and summer, and merge with the older-year cohort in sizein early autumn. The position on the shore of the first-yearand the older-year snails is lower in spring and early summerthan in other seasons. Sex ratio was significantly differentbetween populations, seemingly due to different percentagesof large snails which were mostly females. Inspection of thesamples from a wider geographic range, from Japan to India,detected various sex ratios among the localities; females weredominant in the samples composed exclusively of large individuals.The most developed embryos in the female brood pouches wereveligers. Parthenogenetic development and rearing of embryosup to crawling juveniles, which have been reported for the populationsin the inner Arabian Sea, does not seem to hold for these otherIndo-Pacific populations. (Received 20 February 1996; accepted 8 July 1996)     

ASPECTS OF LIFE CYCLE, POPULATION DYNAMICS, GROWTH AND SECONDARY PRODUCTION OF THE PULMONATE SNAIL CEPAEA VINDOBONENSIS (FERUSSAC, 1821) IN NORTHERN GREECE

The life cycle, population dynamics, growth and secondary productionof the land snail C. vindobonensis were studied in northernGreece. Demographic analysis of the populations of C. vindobonensisrevealed that a) three cohorts were present in the field throughoutthe year, b) the reproductive period started in late April-Mayand the newly hatched snails appeared in the beginning of June,and c) increased growth rates were observed during spring andearly summer, but also during autumn for the newly hatched snails. According to von Bertalanffy's method C. vindobonensis needs7 years to attain its maximum size measured in the field. Mortalityrate is very high during the first year of life, while lifeexpectancy is higher during the second year of life and decreasesafterwards. Net reproductive rate (R_o) was equal to 3.1 andthe finite capacity for increase (antilog_er_c) was equal to 1. Estimated annual secondary production with Hynes' frequencymethod revealed a mean standing crop (B) of 0.99 g/m²/year anda production (P) of 1.3 ± 0.11 g/m²/year. Annual turnoverratio (P/B) was equal to 1.31. (Received 7 April 1997; accepted 2 October 1997)     

A DEMOGRAPHIC ANALYSIS OF y-NEOTRICULA APERTA (GASTROPODA: POMATIOPSIDAE) POPULATIONS IN THAILAND AND SOUTHERN LAOS, IN RELATION TO THE TRANSMISSION OF SCHISTOSOMIASIS

The results of field work undertaken along the tropical reachesof the Mekong river in Thailand and Laos are presented. Demographicdata concerning the schistosome-transmitting snail y-Neotriculaaperta are examined and related to the life-cycle of this species.On the basis of these data, and a review of the literature,the only available, empirically substantiated, model for thelife-cycle of y-N. aperta (see Upatham et al., 1980) is shownto be equivocal. In this model snails survive the annual spatein the Mekong river as eggs, which remain dormant below theflood waters, whilst the adults die soon after the waters beginto rise. Here, the above described model of ‘egg survival’is shown to fail to balance the demographic equation for y-N.aperta. An alternative model of delayed oviposition with post-spatesurvival of mature snails is proposed and shown to be a betterfit to the available data. The implications of this alternativemodel for the evolutionary biology and ecology of N. apertaare discussed. The possible effects of the Pak-Mul dam project(Northeast Thailand) on local populations of schistosome-transmittingsnails are assessed and the hydrodynamic conditions conduciveto vector snail proliferation are considered. Attenuation ofthe normal flood-cycle of the Mekong river anc alteration ofthe river bed topography will probably lead to increased numbersof N. aperta at Pak-Mul. These changes, together with the influxof casual labour from endemic areas in Laos, may result in anepidemic of human schistosomiasis in Northeast Thailand. (Received 18 April 1994; accepted 23 June 1994)     

DEVELOPMENT OF ACCESSORY SEXUAL ORGANS IN BIOMPHALARIA GLABRATA (PLANORBIDAE) IN RELATION TO TIMING OF INFECTION BY SCHISTOSOMA MANSONI: CONSEQUENCES FOR ENERGY UTILIZATION PATTERNS BY THE PARASITE

The growth rates of ovotestis and individual accessory sexualorgans (ASO) of Biomphalaria glabrata snails were studied forcontrols and for immature and mature snails infected with Schistosomamansoni. The infection of immature B. glabrata strongly delaysgrowth of the ovotestis and inhibits the development of theaccessory sexual organs. There is no significant differenceup to 2 weeks post infection in the volume of the ovotestisand the ASO between mature infected B. glabrata and controlsnails. From 3 to 4 weeks post infection there was a reductionin the volume of the ovotestis and the ASO of infected matureB. glabrata; then growth of the ovotestis, albumen gland andfemale organs was stopped, but the effect of infection was lessconsistent for the male organs. For a parasite, immature andmature snails have to be considered as two different resourceenvironments, each having at infection time a particular patternof resource allocation, towards growth for juvenile and towardsreproduction for adult snails, changing the possible energyutilization patterns which can be used by the trematode. (Received 29 January 1993; accepted 22 April 1993)     

EVOLUTION, DURING DIGESTION, OF THE BACTERIAL FLORA IN THE ALIMENTARY SYSTEM OF HELIX ASPERSA (GASTROPODA: PULMONATA): A SCANNING ELECTRON MICROSCOPE STUDY

The use of scanning electron microscopy (SEM) allowed a studyof the distribution of bacteria in the various digestive organsof the snail Helix aspersa Müller. The bacteria are enclosedby mucous secretions (mucous film or mucous grains) and sometimesattached on the cilia of some of the digestive walls. Accordingto the food that was given to the snails, different morphologicaltypes appeared, two of which dominated. Adult snails were fasted for 4 days, given a dehydrated artificialfood and then sacrificed at different times during digestion.The presence of bacteria may be related to the time of digestion.In fact, bacteria seem to accompany the food mass; they developmostly in the stomach and in the intestine where they may helpdigest the food. Fasting or hibernating snails do not possess bacteria in thealimentary lumen or on the digestive walls. However, the residualfaeces localized in the distal, intestinal lumen, lodge greatquantities of bacteria. From these results, the endogenous or/and exogenous existenceof the bacterial flora in alimentary system of Helix aspersais discussed. (Received 26 June 1989; accepted 16 October 1989)     

THE EARLY LIFE-CYCLE STAGES OF HYDROBIA VENTROSA AND HYDROBIA NEGLECTA WITH OBSERVATIONS ON POTAMOPYRGUS JENKINSI

The egg mass of Hydrobia neglecta contains a single egg, whilethat of H. ventrosa contains up to three eggs. At hatching H.neglecta has a significantly greater shell length than H. ventrosaand the late embryos and young snails of the two species canbe separated according to the pigmentation of the head region.The young snails can also be separated on the basis of surfaceornamentation of the shell and comparisons with the closelyrelated H. ulvae and Potamopyrgus jenkinsi show how the basicpattern of shell sculpturing differs in the four species. Thesedifferences are discussed in relation to the mode of development. (Received 14 February 1980;     

FOOD PREFERENCES AND DIETARY OVERLAP BY TERRESTRIAL SNAILS IN LOGOS AREA (EDESSA, MACEDONIA, NORTHERN GREECE)

Among the five coexisting species of snail in Edessa (N. Greece),Bradybaena fruticum and Helix lucorum dominated in density andbiomass in comparison with Xeropicta arenosa, Monacha cartusianaand Cepaea vindobonensis. Resting places of each species inrelation to plant cover and diet preferences are examined seasonallyand throughout a year. Discriminant analysis for the annualdata showed that 83% of the snail group species could be differentiatedby the plants on which they were attached. Each snail specieswas usually found sitting on one of the most abundant plants.Cepaea preferred to sit on and eat senescent material even inspring time and Bradybaena green material. In spring, the snailsdid not consume plant material in proportion to the occurrenceof those plants but they usually did so in summer and in autumn.Hedera helix was not consumed even when it was abundant. Inspring and autumn the snails had consumed plants to which theywere found attached, but in summer they did not. (Received 9 August 1993; accepted 14 February 1994)     

COPULATORY ACTIVITY AND SPERM PRODUCTION IN BULINUS (PHYSOPSIS) GLOBOSUS (GASTROPODA: PLANORBIDAE)

Young Bulinus (Physopsis) globosus performed male copulatoryactivity and cross-fertilized other snails before their femalereproductive tracts were mature. The two most immature snailsshowed preputial eversion when secretion was present only inthe muciparous gland and at the carrefour region of the oviduct.Sixteen snails showed preputial eversion and four snails cross-fertilizedother snails when their oothecal glands and/or major portionof the oviducts contained either no secretion or only scantyamounts. When paired with a partner snail for 12 or 20 consecutive days,adult snails copulated as males on approximately 60% of thedays paired and up to 8 consecutive days. Virgin snails raisedin isolation copulated as male at the same rate as non-virgin,community-raised snails. Ability to copulate as male was notdependent upon previous experience as male or female. Aftera single copulation as male after 7 days isolation, the hermaphroditicducts of maleacting snails contained 87 000 sperm. Sperm productionoccurred at approximately 50 000 sperm.d-¹, until at 10 dayspost-copulation, snails contained 639 000 sperm. (Received 25 May 1982;     

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)     

The development of behavioral defenses: a mechanistic analysis of vulnerability in red-eyed tree frog hatchlings

I examined the development and effectiveness of behavioral defensesof red-eyed tree frog hatchlings, Agalychnis callidryas, against predatoryshrimp Macrobrachium americanum. Arboreal eggs of A. callidryashatch early if attacked by egg predators and later if undisturbed,producing tadpoles that enter the water at different developmentalstages. Older hatchlings survive better than young hatchlings withaquatic predators, including shrimp. Hatchlings respond to shrimpby both increasing activity and avoiding the bottom microhabitat.Older hatchlings are more active and, in the presence of shrimp,avoid the bottom more than young hatchlings. These ontogeneticchanges in behavior improve survival. Specifically, the likelihoodof fleeing from shrimp increases with hatchling age, and fleeingis an effective defense. In contrast to results from experimentswith odonates, immobility does not reduce risk of shrimp attack, thusthere is no trade-off between fleeing and motionless crypsis.Shrimp spend most of their time on the bottom, where attacksare more often successful. Avoidance of the bottom microhabitatby tadpoles therefore improves survival. Evasive maneuvers alsofunction in defense, but evasiveness does not change with age.Morphology may limit microhabitat use by younger hatchlings.Failure to flee may reflect unresponsiveness to disturbance,which would reduce unnecessarily early hatching and limit exposureof young A. callidryas to aquatic predators.     

DISTRIBUTION AND PRODUCTION OF THREE HYDROBIA SPECIES (GASTROPODA: HYDROBIIDAE) IN A SHALLOW COASTAL LAGOON IN THE BAY OF CADIZ, SPAIN

The abundance, life span, growth and production of the mud snailsHydrobia minoricensis, H. ulvae and H. ventrosa in a semi-naturallagoon system were studied by taking monthly samples at threesites during 1991 and 1992. The most abundant species, H. minoricensisoccurred at mean densities of 12834 to 26264 snails m^–2(10.7 to 25.8g dry weigh m^–2), depending on the site.The least abundant species, H. ulvae, occurred at mean densitiesof 185 to 353 snails m^–2 (3.2 to 2.2g dry weight m^–2).The numerical abundance and biomass of the three Hydrobia specieswere positively related to the biomass of benthic macroalgae(P<0.01). Although H. ulvae egg capsules were recorded throughoutthe year, newly hatched snailsof this species were not observed,in contrast to the other two species. The early spring and summercohorts of H. minoricensis and H. ventrosa seemed to be themost numerous. The average life spans of these two species wereestimated to be about 18 and 13 months respectively. Annualproduction estimates for the whole lagoon system were 29.0 (6.3),5.5 (0.8) and 5.2 (1.0)g dry weight (ash-free dry weight) m^–2yr^–1 for H. minoricensis, H. ulvae and H. ventrosa respectively.The annual P/B ratio was about 2 for H. minoricensis and H.ventrosa. (Received 5 July 1994; accepted 5 October 1994)     

孵化水热环境对渔异色蛇孵化卵和孵出幼体的影响

渔异色蛇卵孵化时能从环境中吸收水分导致质量增加,卵质量的增加与初始卵质量和孵化基质湿度有关。较大幅度的孵化基质湿度变化对孵化期、孵化成功率、胚胎动用孵内物质和能量、孵出幼体的性比、大小和质量无显著影响。孵化期随温度升高而缩短,并显示极强的窝间差异。温度对孵出幼体的性别无影响,但显著影响孵化成功率、胚胎对卵内物质和能量的动用、幼体的大小和质量、躯干和剩余卵黄的质量。孵出幼体总长的两性差异不显著,但雌体体长大于雄体而尾长小于雄体。３２℃不适于孵化渔异色蛇卵,该温度下孵出的幼体躯干发育不良,剩余孵黄较多,尾部均呈畸形,孵化过程中能量转化率较低。２４℃和２６℃中孵出的幼体躯干发育良好,孵化过程中能量转化率较高,各项被测定的幼体特征指标均极相似。     

THE EFFECT OF WATER QUALITY ON OVIPOSITION IN BIOMPHALARIA GLABRATA (SAY, 1818) (PLANORBIDAE), AND A DESCRIPTION OF THE STAGES OF THE EGG-LAYING PROCESS

With the overall goal of developing a method to reliably induce ovipositionin the freshwater pulmonate Biomphalaria glabrata, the effectsof water quality on female reproductive physiology were examined.Groups of snails were subjected to controlled experimental conditionsconsisting of a daily regimen of feeding and water change. Aftera period of acclimatiz-ation, egg mass (EM) output under theseconditions was relatively stable, and snails laid a majority(82.5%) of their EM during the initial 4 h following daily waterchange. When this regimen was perturbed by halting water changefor 24 h (dirty-water treatment), EM output was significantly inhibited.When water change was resumed, EM output returned to previouslevels within 4 h post-water change (PWC). This dirty-water treatmentfollowed by water change also resulted in a significant increasein mean EM size during the 4 h PWC when compared to controls.To better describe the events preceding egg-laying in B. glabrata,we then used these experimental manipulations to induce ovipositionin groups of snails, and dissected them during the 4 h followingwater change. Observations of the reproductive tracts of stimulatedsnails allowed us to divide the egg-laying process, from ovulationto oviposition, into discrete stages, after de Jong-Brink, Koop,Roos & Bergamin-Sassen (1982). Stage I was characterized bythe presence of ova in the hermaphroditic duct and carrefour,and fertilized, packaged eggs in the oviduct and muciparousgland. Stage II was characterized by the presence of packagedeggs in the othecal gland embedded in a mucous layer, constitutingthe egg mass to be laid on the substratum. No packaging eventswere occurring in the carrefour/albumen gland region duringthis stage. When snails were dissected immediately after oviposition(Stage III), unpackaged ova were observed in the hermaphroditicduct, carrefour, and oviduct. The mean time it took for snailsto reach Stage III was 120 6 49 min (SD), and this value wasstatistically different from the mean time to Stages I and II,showing that our induction protocol results in a temporal progressionthrough the egg- laying process. Gonadal oocyte density (oocytes/mm²of ovotestis) was quantified as a function of these stages ofthe reproductive cycle, and was found to be significantly lowerduring Stage II (fully formed egg mass in othecal gland) thanall other stages examined. Taken together, these results showthat female reproductive activity can be experimentally controlledthrough the manipulation of water quality, and that such a protocolis a valuable tool for addressing specific questions regardingthe reproductive physiology of B. glabrata. The implicationsof these results as they pertain to the regulation of femalereproductive activity in B. glabrata are discussed. (Received 15 January 1999; accepted 17 May 1999)