INTRASPECIFIC LIFE HISTORY VARIATION IN A POND SNAIL: THE ROLES OF POPULATION DIVERGENCE AND PHENOTYPIC PLASTICITY

Two approaches were used to determine the degree of divergence in life histories among populations of the pond snail, Lymnaea elodes. Juvenile snails were reciprocally transferred between ponds differing in permanence and productivity, and the resulting variation in life history traits was recorded. In a second experiment, parents and their offspring from both a vernal and a permanent pond population were reared in the same pond. Proximal factors had by far the greatest effects on life history traits in the transfer experiment, with snails reared in a more productive pond showing earlier reproduction at a larger size, higher fecundity, and longer life cycle length. Snails from the more uncertain pond in terms of drying date did reproduce at an earlier age and smaller size and grew less in each pond. However, these population differences, for the most part, disappeared when snails were reared for two generations in the same environment. Much of the intraspecific variation in life histories seen in this species must therefore be considered the result of phenotypic plasticity. I argue that the plasticity in life histories itself may be adaptive to this inhabitant of unpredictable, vernal ponds.     

REPRODUCTIVE CYCLE OF THE FEMALE MEDITERRANEAN MONK SEAL IN THE WESTERN SAHARA

Mediterranean monk seals at Cap Blanc, western Sahara, pup annually throughout the year, although births are more frequent in autumn (October). Consequently, subsequent events of the cycle are not seasonal. This lack of synchronicity in reproduction can be attributed to the subtropical location of the colony and the presence of a semipermanent upwelling in the area. This contrasts with the consistency in the timing of parturition of individual females, which results in an interbirth period of one year, which varies by a maximum of only 15 d. The duration of lactation, which can last up to five months, correlates with that of the interbirth period. The interbirth and "molt-to-parturition" periods are relatively constant, and the only part of the cycle that appears to compensate for variation in lactation length is the "weaning-to-molt" period, which correlates negatively with the duration of lactation. As a result of the long lactation period, esrrus probably does not occur at the end of weaning, as it does in most other phocids.     

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)     

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)     

ASPECTS OF THE LIFE CYCLE, POPULATION DYNAMICS,GROWTH AND SECONDARY PRODUCTION OF THE EDIBLE SNAIL HELIX LUCORUM LINNAEUS, 1758 (GASTROPODA, PULMONATA) IN GREECE

This paper examines the biology and ecology of Helix lucorumL. which lives in mainland Greece, as well as its growth andsecondary production. A demographic study revealed that (a)3 cohorts exist at any time during the year (when adults ofall generations belong to the same cohort) (b) egg-laying andhatching occur during the months of July and August respectively,(c) the most rapid growth takes place during spring. Study ofH. lucorum genitalia in relation to age showed that the snailsaxe sexually mature 3 years after hatching, when the largestdiameter of their shell (D) is equal to or greater than 35 mm Von Bertallanffy's method suggests that Helix lucorum may liveup to 14 years or more in order to reach its possible maximumsize (48.80 mm) The study of relative growth of D in relation to P_s (peristomesurface) of Helix lucorum shows that D grows faster than P_swhen D12.50 mm; juveniles change their growth rate when theirD arrives at 22.05 mm, and growth becomes slower when adultsarrive at 36.27 mm Annual secondary production calculated by the size frequencymethod gave a mean annual density of 3.39 individuals per m²,a mean annual crop (biomass) of 4.04 g-m^–2 and an annualproduction (P) of 5.02 g · m^–2. The annual turnoverratio (P//b) is equal to 1.24 (Received 23 June 1987;     

粘孢子虫生活史的研究进展

粘抱子虫除少数种类寄生在蠕虫、两栖类、爬行类外,绝大部分寄生于鱼类,可以说是鱼类特有的一类寄生原生动物。自ouge首次发现粘抱子虫以来,至今国内外报道的有近12OO种,其中在我国淡水鱼类中寄生的就有600余种,它几乎能在鱼体的所有器官中寄生,并可在鱼的体表或内脏形成抱囊,使商品鱼失去价值,严重时可导致鱼类大量死亡,给鱼类养殖业带来巨大的经济损失。       

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)     

LIFE HISTORY TRAITS OF THREE MORPHS OF MELANOIDES TUBERCULATA (GASTROPODA: THIARIDAE), AN INVADING SNAIL IN THE FRENCH WEST INDIES

The life history traits of three morphs of the parth-enogeneticsnail Melanoides tuberculata were studied under laboratory conditions.Differences in growth size, mortality and reproduction rateswere observed between morphs, indicating adaptation to differentdemographic strategies. These results are discussed in relationto the invasion of the species which occurred in Guadeloupeand Martinique. (Received 24 February 1992; accepted 30 April 1992)     

范尼道佛吸虫的生活史研究

范尼道佛吸虫的生活史包括毛蚴、胞蚴、尾蚴、囊蚴和成虫5个阶段。它的第一中间宿主为淡水壳菜,第二中间宿主为多种小型鲤形目和鲇形目鱼类,终末宿主是鳜。文中对这种吸虫的胞蚴、尾蚴、囊蚴和成虫的形态特征以及尾蚴的行为特征进行了观察和描述。在用人工方法成功地将尾蚴感染青后,对不同发育阶段的囊蚴在鱼体内的形态变化和器官发育进行了观察。       

THE LIFE CYCLE AND DEVELOPMENT OF DICTYOMORPHA GEN. NOV. (FORMERLY PRINGSHEIMIELLA), A GENUS OF THE AQUATIC FUNGI

Mullins , J. Thomas . (U. Florida, Gainesville.) The life cycle and development of Dictyomorpha gen. nov. (formerly Pringsheimiella), a genus of the aquatic fungi. Amer. Jour. Bot. 48(5): 377–387. Illus. 1961.—The chytrid, Pringsheimiella, originally described by Couch (1939), is renamed Dictyomorpha, because of the previous use of the name Pringsheimiella for an alga. The development and life cycle of the fungus are described. It has been found that the resting spores are initiated by the fusion of isoplanogametes. Couch's observations on heterothallism are confirmed. The validity and relationships of Dictyomorpha are discussed.     

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.     

MACROCYSTS IN THE LIFE CYCLE OF THE DICTYOSTELIACEAE. II. GERMINATION OF THE MACROCYSTS

Macrocyst germination was demonstrated in the five species of the Dictyosteliaceae known to produce these structures. The morphological changes that occurred during germination appeared to be identical in all of the strains examined, showing the following stages: (1) swelling of the dark, contracted content of the dormant cysts, (2) gradual loss of color and reappearance of cells within what previously appeared as a homogeneous protoplasmic mass, and (3) rupture of the heavy cellulosic cyst wall to liberate the myxamoebae. The age of the macrocyst appeared to be the most critical factor in determining whether or not germination would occur, since the cysts in many of the strains needed to age for several weeks or months before germination could be demonstrated. In Dictyostelium mucoroides strain DM-7, upon which the current study was centered, light was necessary to stimulate germination of young macrocysts—a requirement that gradually diminished as the cysts aged. The rate of germination and the temperature permitting germination were also age dependent: older macrocysts germinated more rapidly and at considerably higher temperatures than did young cysts. Although light was not essential for germination in every strain, the results obtained with strain DM-7 seem to be generally applicable to the germination process.     

黑斑蛙双睾吸虫的生活史研究

本文首次研究了黑斑蛙双睾吸虫的生活史。实验证明这种单殖吸虫的生活史具有两种不同的途径,即直接外生周期和直接内生周期。前一种发育途径为一般多盘吸虫所共有,后一种发育途径仅在少数多盘吸虫中存在。这两种发育途径的交替存在与其两栖动物宿主生态学有关,并且可能反映了这类多盘吸虫的一种种群调节机制。     

MACROCYSTS IN THE LIFE CYCLE OF THE DICTYOSTELIACEAE. I. FORMATION OF THE MACROCYSTS

Macrocysts, a morphogenetic phase that is alternative to sorocarp formation in the life cycle of some cellular slime molds, are known for two genera and five species of the Dictyosteliaceae. In all of these macrocyst formation was found to be strongly affected by four factors: light, temperature, moisture, and the composition of the medium. In general, macrocyst formation was suppressed and sorocarp formation was enhanced by exposure to light, by incubation at temperatures lower than 20 C, by buffering nutrient media with phosphates, and by reducing atmospheric moisture through the use of clay covers on Petri dishes. The extent to which these environmental factors, singly or in combination, inhibited the production of macrocysts varied among the different strains and species.     

THE LIFE CYCLE OF PEDIASTRUM SIMPLEX1

A sexual cycle and a newly discovered mode of asexual reproduction by azygotes arc described for Pediastrum simplex. In the sexual cycle isogametes from colony cells fuse in pairs, develop into zygotes, which grow and divide, producing motile cells which become angular polyeders. Polyeders may divide internally to form 4-celled colonies, or they enlarge, cleave, and produce zoospores or nonmotile cells. Zoospores are released in vesicles, swarm, adhere to each other, and form, flat colonies I cell thick. The nonmotile cells adhere to each other, enlarge, break out of the polyeder, and become spheroidal colonies. In the asexual cycle, nonfusing gametes, and motile cells derived from colony cells, become 4-spined azygotes which enlarge, divide internally, and release motile cells developing into polyeders. These polyeders develop further like those from zygotes.     

LIFE HISTORIES, DENSITY AND BIOMASS IN A WOODLAND SNAIL COMMUNITY

Data, based on 8 monthly sets of random samples, are presentedon the life histories, densities and biomass of snail populationsin a mixed deciduous wood with moderately rich soil. Discus rotundatus, Oxychilus helveticus, O. cellarius and Trichiahispida have approximately annual life cycles, with rather littlepost-reproductive survival. Aegopinella nitiduta and Cochlodinalaminata take two years to reach maturity, and adults of thelatter may survive for several years. Breeding seasons are prolonged,but annual species peak in the autumn. Density does not vary much with season. Mean annual densityof all species combined (37.m^–2) is much lower than thoserecorded for similar woods elsewhere, but the estimate of meanannual standing crop (175 mg ashfree dry weight m^–2) doesnot show the same discrepancy. Comparisons reveal that the maincause of low density is the absence or rarity of several verysmall litter-dwelling species which have little effect on overallbiomass, and reasons for this are discussed. (Received 27 January 1981;