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;     

HOW IS FOOD AROUSAL MANIFESTED IN THE POND SNAIL LYMNAEA STAGNALIS?: AN OVERVIEW

Latency to bite and spontaneous rasping activity in the absenceof food were measured. On presentation of a food stimulus (sucrosecrystals) to Lymnaea, latency to bite was reduced for c. 15min (increased food arousal). There was a progressive decreasein latency to bite and subsequent interbite intervals duringa meal. However, while hungry (2-day starved) snails showedno difference in latency to bite compared with well-fed controls,the number of spontaneous rasps made in the absence of foodprior to the food stimulus increased. Spontaneous rasps wereorganised in bouts. Log survivorship curves showed these tobe controlled by 2 processes generating within- and between-outintervals. The probability of within-bout intervals occurringincreased in hungry snails. These data suggest that it is importantto consider ‘level of activation’ as well as responsivenesscriteria (latency to bite) when assessing food arousal.     

INBREEDING AND FITNESS IN THE FRESHWATER SNAIL LYMNAEA PEREGRA: AN EVALUATION OVER TWO GENERATIONS OF SELF-FERTILIZATION

Inbreeding depression was estimated from an outbreeding population of the freshwater snail Lymnaea peregra, on the basis of two successive generations of enforced selling and outcrossing, and 70 maternal lineages. Outcrossing was analyzed under two treatments, groups of two and five individuals. The fitness parameters measured included fecundity, growth, and survival. In the first generation, we contrasted three treatments (selfers vs. paired outcrossers and group outcrossers). Very similar results were obtained between the two outcrossing treatments. A strong self-fertilization depression (which includes parental fecundity and progeny fitness) was detected in the selling treatment (about 90%). In the second generation, there was again marked evidence for self-fertilization depression, with the highest contributions coming from parental fecundity and progeny hatching rate. Our results suggest that the decreased parental fecundity is a consequence of the mating system in the previous generation, although the role of partial self-incompatibility and the copulation behavior could not be ruled out. Hatching rate and early survival data are suggestive of purging of lethal mutations. Significant variation in fitness among selfing lineages was found for most fitness traits. Our experimental design also allowed to test for interactions among fitness loci. Only one trait of the nine studied behaved as expected under synergistic interactions. However we cannot rule out some purging during the experiment, which could have biased results towards linearity. Inbreeding depression was also inferred from the change of inbreeding level across generations in the same population. We obtained a value similar to the experimental estimate.     

ALLOZYME VARIATION IN SOME POPULATIONS OF THE FRESHWATER SNAILS LYMNAEA PEREGRA, L. AURICULARIA AND L. STAGNALIS (GASTROPODA: PULMONATA)

Three species of the genus Lymnaea (Gastropoda, Pulmonata, Basommatophora)were examined for genetic variation at 11 enzymatic loci insamples collected in Brittany (North-Western France). Variabilitywas greatest in L. percgra, but the limited area of samplingdid not allow specific inferences. The UPGMA clustering of unbiasedgenetic distances (Nei, 1978) isolated first L. auriculariasamples, and secondly the single L. stagnalis sample from theL. peregra set. This scheme of relationships was opposed tothe identification of alleles shared by the three species atthe studied loci Analysis of L. peregra population structure by F-statistics(Weir & Cockerham, 1984) suggested high inbreeding and raisedthe possible role of self-fertilization. The contradiction betweenhigh variability and high homozygosity is discussed. Furthermore,population differentiation was rather weak (although statisticallysignificant), despite some remarkable differences among loci.Nm estimates using Wright's (1951) and Slatkin's (1985) methodsrevealed a gene flow unable to prevent the effect of geneticdrift among populations. The relative homogeneity of populationswith weak genetic exchange is discussed (Received 24 May 1993; accepted 21 March 1994)     

ENVIRONMENTAL STRESS AND THE MAINTENANCE OF SEX IN A FRESHWATER SNAIL

Synergism among mutations can lead to an advantage to sexual reproduction, provided mutation rates are high enough (the mutational deterministic hypothesis). Here we tested the idea that competition for food can increase the advantage to sexual reproduction, perhaps by increasing the synergism among mutations in asexual individuals. We compared the survivorship of sexual and asexual snails (Potamopyrgus antipodarum) under two treatments: starved and fed. We predicted higher mortality for asexual snails when starved, but found that sexual and asexual individuals survived at the same rate, independent of treatment. These results suggest that the distribution of sex in this snail may not be explained by variation in competition among populations.     

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.     

SELF-FERTILIZATION VERSUS CROSS-FERTILIZATION IN THE HERMAPHRODITIC FRESHWATER SNAIL BULINUS GLOBOSUS

Self-fertilization depression of fitness in the freshwater hermaphroditic snail Bulinus globosus, an intermediate host of the parasitic trematode Schistosoma, has been studied in a strain originating from Niger. B. globosus is an outcrosser that can self-fertilize when isolated before any copulation has occurred. The self-fertilization depression has been estimated during two successive generations. In the first generation, selfing was compared to outcrossing. Within each mating system group, selfing and outcrossing were compared again in the second generation. A striking difference was shown in favor of cross-fertilization for the number of eggs laid, the survival at birth of young snails and the number of snails reaching sexual maturity. The overall self-fertilization depression is 0.920 after two generations of selfing. We discuss the relative role of selfing and outcrossing in the evolution of freshwater snail populations.     

POPULATION DYNAMICS OF THE FRESHWATER SNAIL BIOMPHALARIA PFEIFFERI IN EASTERN ZAIRE

The population dynamics of the freshwater snail Biomphalariapfeifferi, an intermediate host of the parasitic worm Schistosomamansoni, was studied in the Virunga stream in Eastern Zaïre. Methods are developed to estimate age-specific survivorshipfrom regular quadrat-sampling data without assuming a constantmortality rate, and to approximate age-specific fecundity whenthe fecundity function is known in the laboratory. The population dynamics in the field was found to be very differentfrom that in the laboratory: it is basically discontinuous,with one main generation per year; a massive mortality occursafter hatching; fecundity is reduced considerably; and thereare marked seasonal variations in both survivorship and fecundity.Time-dependent demographic parameters were estimated by monthinstead of by cohort. They show that the environmental conditionsare favourable to an increase of the population only duringa short period of the year. The intrinsic rate of natural increase,r, was found to be well correlated with the changes in egg density. The environmental factors determining the population dynamicsin the Virunga are discussed; current speed seems critical. (Received 4 February 1987;     

鄱阳湖沿岸带螺类的物种组成及其生境

2011年5月和6月在鄱阳湖沿岸带的62个样点采集了淡水螺类样本并调查了生境类型,分析了淡水螺类的物种组成及其生境分布特征。结果表明:共采集到淡水螺类19种,分属田螺科（8种）、觿螺科（8种）、椎实螺科（2种）和黑螺科（1种）。与前人的研究相比,本次调查的螺类物种数有较大的下降,但田螺科和觿螺科的物种仍是整个螺类群落的主要组成部分。淤泥生境的螺类物种组成与其他生境的差异较大,淤泥生境的螺类物种丰富度显著高于沙土、砂石和水草3种生境（P0.05）,表明螺类对生境具有一定的栖息偏好性。最后详细讨论了淡水螺类种数变化、物种组成及其生境分布的影响因素。       

THE RELATIONSHIP OF BODY SIZE AND GROWTH TO EGG PRODUCTION IN THE HERMAPHRODITIC FRESHWATER SNAIL, HELISOMA TRIVOLVIS

Among several other factors, body size has been found to influenceegg production in several species of hermaphroditic snail. Wetested whether this relationship between body size and egg productionexists in Helisoma trivolvis, a freshwater hermaphroditic species.We isolated 50 H. trivolvis from a laboratory population, measuredshell diameter, and monitored egg production for seven weeks.We found a positive relationship between body size and totalnumber of eggs produced, as well as body size and number ofeggs per egg mass. When body size and egg production are linked,it should be adaptive for larger individuals to act as femalesand smaller individuals as males. Since body size is relatedto female fecundity in this species, the relative size of snailsshould determine, at least in part, which individual acts asmale and which as female during copulation. However, the relationshipbetween body size and egg production is not nearly as strongas it is in other snail genera. Other factors such as age, genotypeand previous experience may be important in determining egg-layingcapacity and therefore gender choice in this species. In addition,we found a negative relationship between growth during thisperiod and egg production. This relationship has been foundin other pulmonates, and is evidence of resource allocationtradeoffs. (Received 3 August 2004; accepted 2 August 2005)     

THE EFFECTS OF PREDATION RISK ON MATING SYSTEM EXPRESSION IN A FRESHWATER SNAIL

Environmental effects on mating system expression are central to understanding mating system evolution in nature. Here, I report the results from a quantitative‐genetic experiment aimed at understanding the role of predation risk in the expression and evolution of life‐history and mating‐system traits in a hermaphroditic freshwater snail (Physa acuta). I reared 30 full‐sib families in four environments that factorially contrast predation risk and mate availability and measured age/size at first reproduction, growth rate, a morphological defense, and the early survival of outcrossed/selfed eggs that were laid under predator/no‐predator conditions. I evaluated the genetic basis of trade‐offs among traits and the stability of the G matrix across environments. Mating reduced growth while predation risk increased growth, but the effects of mating were weaker for predator‐induced snails and the effects of predation risk were weaker for snails without mates. Predation risk reduced the amount of time that individuals waited before self‐fertilizing and reduced inbreeding depression in the offspring. There was a positive among‐family relationship between the amount of time that individuals delayed selfing under predation risk and the magnitude of inbreeding depression. These results highlight several potential roles of enemies in mating‐system expression and evolution.     

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;     

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)     

GENETIC STRUCTURE OF COEXISTING SEXUAL AND CLONAL SUBPOPULATIONS IN A FRESHWATER SNAIL (POTAMOPYRGUS ANTIPODARUM)

We examined clonal diversity and the distribution of both clonal and sexual genotypes in a single population of freshwater snails (Potamopyrgus antipodarum) in which diploid sexual individuals and triploid parthenogens coexist. A genetic analysis of individuals from three habitat zones in Lake Alexandrina, New Zealand revealed extremely high clonal diversity: 165 genotypes among 605 clonal individuals. The frequency of triploid clonal individuals increased with increasing depth in the lake, and most of the individual clones were habitat specific, suggesting that differences among habitats are important in structuring the clonal subpopulation. There were also high levels of clonal diversity within habitats, suggesting frequent origins of habitat-specific clones. In contrast, diploid sexual individuals were proportionately more common in the shallow regions of the lake (where infection by trematode larvae is highest), and there was no significant spatial structure in the sexual subpopulation. We suggest that habitat specialization by clones, as well as parasite-mediated selection against common clones, are important factors affecting the structure of this mixed population of sexual and clonal snails.     

大沼螺的生物学研究

大沼螺在自然条件下的雌雄性比为1:0.69。交配期为10月底至次年6月。2、3月开始产卵,8月结束。5月初卵开始孵化。性腺一般当年成熟,次年螺可繁衍后代。幼螺经过4—5个月的生长,到10月份可达13—15毫米高。随后水温下降,螺停止生长,次年5、6月水温上升再继续长高。最大个体为18毫米左右。3个采集点的螺在繁殖、生长方面无明显差异。壳高与壳宽呈直线相关,而壳高、壳宽与湿重呈指数相关。本文首次报道了大沼螺是吸虫的中间宿主。       

PARTHENOGENESIS IN A FRESHWATER SNAIL: REPRODUCTIVE ASSURANCE VERSUS PARASITIC RELEASE

Two alternative (but not mutually exclusive) hypotheses were contrasted for their abilities to explain the distribution of parthenogenesis in the freshwater snail Potamopyrgus antipodarum: the reproductive assurance hypothesis, which predicts that parthenogenesis will be favored in sparse populations where mates are difficult to find, and the Red Queen hypothesis, which predicts that parthenogenesis will be favored in populations that have a low risk of parasitism. The results were inconsistent with the prediction of the reproductive assurance hypothesis; male frequency was not significantly or positively correlated with snail density. Thus, there was no support for any of the hypotheses for the maintenance of sex that rely on selection for reproductive assurance to explain the distribution of parthenogenesis (e.g., recombinational repair). The results, however, were consistent with the Red Queen hypothesis; male frequency was positively and significantly correlated with the frequency of individuals infected by trematodes. This correlation suggests that parthenogenetic females have replaced sexual females in populations where parasites are rare, and that sexual females have persisted in populations where parasites are common.