Number Of Reproductive Individuals Research Articles

Fine-scale genetic structure of Magnolia iltisiana in forest regeneration sites with a history of selective logging

Background: Selective logging is a frequent practice in the Tropical Montane Cloud Forest which can impact forest regeneration and the genetic makeup of successive generations of trees. The spatial clustering of genetically related individuals, fine-scale genetic structure (FSGS), can develop from the reduction of gene dispersal and the decrease in the number of reproductive individuals at the local scale due to selective logging.
 Questions: In regeneration sites with a history of selective logging, does FSGS differ from a site without such a history? Is FSGS stronger in seedlings and saplings relative to juveniles and adults? Is genetic diversity similar among life stages and sites?
 Studied species: Magnolia iltisiana an endemic tree.
 Study site and dates: Sierra de Manantlan Biosphere Reserve, Jalisco, Mexico.
 2020.
 Methods: We evaluated genetic diversity, genetic structure, and FSGS across four life stages (seedlings, saplings, juveniles, and adults) by genotyping 211 individuals with seven nuclear microsatellite loci in two regeneration and one conserved site.
 Results: We found statistically significant FSGS in the two regeneration sites only for seedlings and saplings, while no evidence of FSGS was detected in the conserved site. No differences in genetic diversity estimates and structure were found among life stages.
 Conclusions: Our study does not suggest an effect of selective logging on genetic diversity on the contrasted conditions and an FSGS pattern only in the earlier stages of the regeneration sites in M. iltisiana.

Cascading effects of seed-stem-individual spatial patterns along a grazing gradient.

Studying the seed trait-stem trait-individual spatial pattern system is helpful for understanding the developmental direction of plant dynamics and populations under grazing disturbance as well as the antagonistic relationship between animals and plants, but few systematic analyses of this spatial pattern system have been carried out. Kobresia humilis is the dominant species in alpine grasslands. We studied K. humilis seed traits and their relationship with K. humilis reproductive individuals, the relationship between reproductive and vegetative stems, and the weights and spatial patterns of reproductive and nonreproductive individuals under four grazing treatments: no grazing (control), light grazing, moderate grazing and heavy grazing. We explored the relationship among seed size and seed number with reproductive stems and vegetative stems along the grazing gradient and assessed the spatial pattern changes between reproductive and nonreproductive individuals. The results showed the following: (1) Seed size increased with increasing grazing intensity, and the coefficient of variation for seed size and seed number in the heavy grazing treatment was greater than 0.6. (2) The structural equation model showed that grazing treatment had a positive effect on seed number, seed size and reproductive stem number and a negative effect on reproductive stem weight. (3) Grazing treatment did not affect the resource allocation to reproductive stems and vegetative stems per unit length of reproductive K. humilis individuals. (4) Compared with the number of reproductive individuals in the no grazing treatment, the number in the heavy grazing treatment decreased significantly, and the negative correlation between reproductive individuals and nonreproductive individuals changed from a full-scale negative correlation to a small-scale negative correlation and a large-scale positive correlation. Our study showed that grazing could activate and change the resource allocation pattern of dominant species in a grassland and have significant positive effects on reproductive stem number, reproductive stem weight, seed number and seed size. Along a grazing intensity gradient, with the increase in distance between reproductive and nonreproductive individuals, the transformation of intraspecific relationships from a negative correlation to a positive correlation is an ecological strategy conducive to population survival.

Nepotistic colony fission in dense colony aggregations of an Australian paper wasp

Social insects are highly diverse in their social structures, aside from the consistent presence of reproductive castes. Among social insects, the Australian paper wasp Ropalidia plebeiana constructs extremely dense colony aggregations consisting of hundreds of colonies within a few square meters; however, little is known about the aggregation structures. We genetically analyzed the colony and population structure of R. plebeiana, and concomitant variations in colony sex ratios. In spring, the foundress (candidate queen) group started their colonies on a single old comb from the previous season, subsequently dividing these old combs via relatedness-based comb-cutting. Female philopatry, a prerequisite condition of Local Resource Competition (LRC), was confirmed. The colony sex ratio of reproductive individuals (male and female offspring for the next generation) became slightly male-biased in larger colonies, as predicted under LRC. However, the number of foundresses was positively associated with the number of reproductive individuals, suggesting that Local Resource Enhancement (LRE) also operates. Although the population structure appears to meet the prerequisites of LRC, the sex ratio appears to be modulated by factors other than LRC. Rather, through LRE, the availability of female helpers at the founding stage is likely to mitigate the sex ratios predicted under LRC.

Major Anthropogenic Interactions Determining the Conservation Status of Endemic Mammals of Eastern Africa

Africa, as a continent of diversity, harbors many cosmopolitan and endemic mammals, 17 of the world’s 20 orders of terrestrial mammals. The Horn of Africa alone harbors nearly 220 mammalian species, including many threatened species. Mammals, particularly endemics ones, are threatened by anthropogenic challenges impacting their abundance, the number of reproductive individuals, and geographic ranges. Human population, in Eastern Africa, has been growing fast, and political and civil unrest aggravate human impacts on the environment. In particular, this study focused on identifying factors that are influencing the conservation status of endemic mammals of Eastern Africa using a multinomial logistic regression model. Agricultural expansion and deforestation threatened vulnerable (AOR: 2.650,p<0.05) and critically endangered species (AOR: 4.763,p<0.05) more than any other factors. Habitat loss persists as a major factor when critically endangered species (AOR: 3.520,p<0.05) are compared to near threatened species. Collectively, threatened species are mainly impacted by habitat loss (AOR: 2.678,p<0.05), agricultural expansion, and deforestation (AOR: 2.376,p<0.05). In the next 50 years, threats to biodiversity are likely to grow as human populations increase. There is no a generalized global model to measure the intensity of agricultural expansion, habitat loss, hunting, and human settlement in the protected areas. Attempts should be made to develop conservation strategies that aim to articulate an array of several conservation threats together across space and time.

Diversity of Termite Breeding Systems.

Termites are social insects that live in colonies headed by reproductive castes. The breeding system is defined by the number of reproductive individuals in a colony and the castes to which they belong. There is tremendous variation in the breeding system of termites both within and among species. The current state of our understanding of termite breeding systems is reviewed. Most termite colonies are founded by a primary (alate-derived) king and queen who mate and produce the other colony members. In some species, colonies continue throughout their life span as simple families headed by the original king and queen. In others, the primary king and queen are replaced by numerous neotenic (nymph- or worker-derived) reproductives, or less commonly primary reproductives, that are descendants of the original founding pair leading to inbreeding in the colony. In still others, colonies can have multiple unrelated reproductives due to either founding the colonies as groups or through colony fusion. More recently, parthenogenetic reproduction has shown to be important in some termite species and may be widespread. A major challenge in termite biology is to understand the ecological and evolutionary factors driving the variation in termite breeding systems.

An inhomogeneous controlled branching process∗

We consider a discrete-time branching process in which the offspring distribution is generation-dependent and the number of reproductive individuals is controlled by a random mechanism. This model is a Markov chain, but, in general, the transition probabilities are nonstationary. Under not too restrictive hypotheses, this model presents the classical duality of branching processes: it either becomes extinct or grows to infinity. Sufficient conditions for the almost sure extinction and for a positive probability of indefinite growth are given. Finally, the rates of growth of the process are studied, provided that there is no extinction.

Males and females contribute unequally to offspring genetic diversity in the polygynandrous mating system of wild boar.

The maintenance of genetic diversity across generations depends on both the number of reproducing males and females. Variance in reproductive success, multiple paternity and litter size can all affect the relative contributions of male and female parents to genetic variation of progeny. The mating system of the wild boar (Sus scrofa) has been described as polygynous, although evidence of multiple paternity in litters has been found. Using 14 microsatellite markers, we evaluated the contribution of males and females to genetic variation in the next generation in independent wild boar populations from the Iberian Peninsula and Hungary. Genetic contributions of males and females were obtained by distinguishing the paternal and maternal genetic component inherited by the progeny. We found that the paternally inherited genetic component of progeny was more diverse than the maternally inherited component. Simulations showed that this finding might be due to a sampling bias. However, after controlling for the bias by fitting both the genetic diversity in the adult population and the number of reproductive individuals in the models, paternally inherited genotypes remained more diverse than those inherited maternally. Our results suggest new insights into how promiscuous mating systems can help maintain genetic variation.

Laysan Albatross on Guadalupe Island, México: Current Status and Conservation Actions

Guadalupe Island, off the Baja California peninsula, México, hosts the most important growing Laysan Albatross (Phoebastria immutabilis) breeding colony in the eastern Pacific. Since this seabird's first arrival in 1983, it has been affected by predation from feral cats (Felis catus), present on Guadalupe since the late 19th century. Heavy predation events have been recorded on the island, so we initiated a feral cat control campaign in 2003 and began collecting baseline information for developing an eradication plan. At the same time, we conducted seasonal monitoring of Laysan Albatross reproductive success in order to assess the benefits from control activities. Cat relative abundance on Guadalupe was estimated through spotlight surveys, and control was done at the southernmost end of the island around the 2 locations where Laysan Albatross nest: Colinas Negras and Punta Sur. Laysan Albatross population growth rate was calculated based on the number of reproductive individuals, while breeding success was estimated as the proportion of laid eggs that resulted in fledged chicks. A total of 203 cats were removed from the south end of Guadalupe between 2003 and 2013. During this same period, high reproductive success (0.8) was recorded for Laysan Albatross, suggesting a positive effect of cat control activities. We found significant differences in reproductive success between years with predation and no predation by feral cats. The Laysan Albatross colony on Guadalupe has grown steadily during the past 30 years, increasing from 4 to 143 breeding pairs between 1984 and 2013, respectively, and with a population growth rate of 1.10 between 2004 and 2013.

Growth and reproductive strategies of Thalictrum petaloideum under different stocking rates

Aims Grazing is one of the major measures for grassland management in Nei Mongol steppe. The growth and reproductive strategies of grassland plants are developed under their long-term co-evolution with ungulates in grassland ecosystems. This study was conducted to understand how Thalictrum petaloideum would change its growth and reproductive strategies with increasing stocking rate. Methods By using a grazing experiment in a typical steppe ecosystem initiated by the Nei Mongol Grassland Ecosystem Research Station in 2006, we examined the responses of T. petaloideum to stocking rates at population, individual and organ levels during the 2012 and 2013 growing seasons. Important findings With increasing stocking rate, the number of reproductive individuals decreased and the reproductive process ceased in T. petaloideum at the population level. The threshold for ceasing the reproductive process was at a stocking rate of 7.5 sheep per hm 2 . The plant height, plant biomass and the numbers of reproduc- tive branches, flowers and seeds all decreased with increasing stocking rate at an individual level. Two shifts were detected for the reproductive allometric relationships with increases in stocking rate. With the increase of stocking rates, T. petaloideum reduced the number of flowers per reproductive branch decreased but the individual seed weight increased with increasing stocking rate at the organ level. A trade-off between the number of seed and the weight of seed was found, which might be an important mechanism in T. petaloideum responding to grazing dis- turbance.

Spatial Pattern and Fine-Scale Genetic Structure Indicating Recent Colonization of the Palm Euterpe edulis in a Brazilian Atlantic Forest Fragment

Rainforests worldwide are subject to increasing amounts of external pressure and degradation that dissect continuous species distributions into small and isolated fragments (Whitmore 1997). This spatial isolation and reduction of natural populations negatively affects the reproductive success of many tropical plants (Fuchs et al. 2003; Quesada et al. 2004). A reduction in the number of reproductive individuals in a population represents a decrease in the number of pollen/ovary donors and in the quantity of pollen deposited (Aizen and Feinsinger 1994). This may be accompanied by a decrease in the abundance of pollinators and selection for self-compatibility (Stephenson et al. 2000). Subsequently, such impacts can lead to

Ecologia de duas espécies de Pachyurus (Perciformes, Sciaenidae) do rio Tocantins, na região represada pela UHE Serra da Mesa, Goiás

Pachyurus schomburgkii Günther, 1860 e P. paucirastrus Aguilera, 1983 ocorrem na bacia Tocantins-Araguaia, sendo a segunda endêmica da bacia. Ambas foram encontradas na região do alto Tocantins, onde este foi represado pela usina hidrelétrica Serra da Mesa, tendo sido capturadas somente em dois de 14 pontos amostrados, sendo ambos (FU e PG) a jusante da barragem. Os peixes foram amostrados bimestralmente com redes de espera padronizadas entre dezembro de 1995 e dezembro de 2002. São apresentados dados de distribuição, dieta, condição fisiológica e estado reprodutivo das duas espécies, focando possíveis modificações em decorrência da instalação da barragem. A maioria dos indivíduos capturados eram adultos e o número de reprodutivos diminuiu após o represamento. As duas espécies foram classificadas como invertívoras bentônicas, alimentando-se principalmente de formas imaturas de insetos. A amplitude de nicho foi significativamente maior para P. paucirastrus, e maior na fase anterior ao represamento, para ambas as espécies. A sobreposição interespecífica da dieta foi alta, porém, a partilha de recursos foi maior no ponto imediatamente abaixo da represa (FU) do que naquele mais distante (PG). Os valores obtidos para o fator de condição indicaram que P. schomburgkii deve ter utilizado mais efetivamente os recursos disponíveis após as mudanças ambientais do que P. paucirastrus. Dessa forma, problemas relativos à alimentação podem ter contribuído para o desaparecimento dessa espécie durante a fase de operação da usina. Pachyurus schomburgkii parece ter se ajustado melhor às mudanças e mostrou-se mais persistente na área após o represamento, porém restrita ao ponto mais distal da barragem, tendo sido capturada de forma esporádica até desaparecer, aproximadamente quatro anos após a formação do reservatório. Considerando a série de usinas hidrelétricas projetadas para o rio Tocantins, recomendam-se estudos intensivos sobre P. paucirastrus em outras partes da bacia, de forma que seja avaliado seu real estado de conservação.

Genetic variation and differentiation in populations of a threatened tree, Magnolia stellata: factors influencing the level of within-population genetic variation

Genetic variation and differentiation in Magnolia stellata were studied in 20 populations distributed across most of the species' range using 10 microsatellite markers, and the factors influencing their levels of within-population genetic variation were examined. Generally, populations distributed intermittently from southern Gifu to central Aichi Prefectures showed substantially higher levels of genetic variation (exceptions included populations located at unusually high altitude sites or western and southern edges of the range) than more isolated populations on the Atsumi Peninsula of southern Aichi Prefecture and in northern Mie Prefecture. Significant isolation-by-distance patterns were detected in genetic differentiation among the studied populations, and a neighbor-joining tree based on D(A) distances among the populations reflected well the geographical positions of the populations. The level of within-population genetic variation was significantly influenced not only by the size of the populations (represented by the number of reproductive individuals) but also by their degree of isolation (represented by the number of populations within a radius of 0.5 km around them). Therefore, areas within radii of 0.5 km may encompass M. stellata metapopulations, in which gene flow may usually occur. We suggest that this area may be a suitable standard for constructing conservation units for the species.

The role of schizogenesis in population dynamics of Timarete filigera (Polychaeta: Cirratulidae): 2-years observations in the Port of Bari (South Adriatic Sea)

Different aspects of reproductive biology of the polychaete Timarete filigera (Delle Chiaje) were studied during a period of 2 years (from September 2002 to August 2003 and from September 2004 to August 2005). Timarete filigera specimens were collected monthly in the port of Bari (South Adriatic Sea, Mediterranean Sea). For population analysis, regenerating and entire specimens were separately analyzed, the regenerating individuals were assigned to different categories related to the region of the body from which they derived. Regenerating individuals were present throughout the year. There were three to four times more individuals deriving from the middle part than the anterior and posterior regenerating portions. From the high percentage of regenerating individuals observed in the T. filigera population, it can be supposed that this species reproduces mainly asexually by schizogenesis. However, histological analysis revealed the presence of female gametes in a few individuals. In contrast, males were never collected, probably due to the limited number of reproductive individuals collected. The abundance has shown wide fluctuations during the year, with a decrease from Winter to Spring. The analysis of the size-frequency histograms has shown a growing phase of whole specimens from Spring to Winter. The life cycle of the investigated species is discussed with the hypothesis that strategies could be linked to the colonized environments, agamic reproduction coupled with sexual reproduction assures the presence and expansion of the population after the colonization by a limited number of larvae.

Population structure of Dioon purpusii Rose in Oaxaca, Mexico

Cycads are currently suffering significant habitat loss from deforestation, agricultural expansion, and illegal harvesting. Dioon purpursii Rose has restricted distribution and small populations, which until now are endemic in the state of Oaxaca in Mexico, and it has been included in the category of threatened and vulnerable. Three sites with D. purpusii plants were located in the state of Oaxaca. Structural and dendrometric characteristics of the population and physiognomic, floristic and environmental characteristics of the forest community at each site were determined. Dendrometric variables were evaluated by an analysis of variance (ANOVA), and a similarity analysis between sites was applied to all characteristics, followed by a cluster analysis to express the relationships between sites. The population structure of the three sites is typical of species in which a great frequency of seedlings and juveniles suggests optimal microhabitat conditions for initial establishment. The results of the ANOVA showed that the most developed plants were found at Site 3, followed by Sites 1 and 2. The floristic composition corresponded to the type of tropical deciduous forest vegetation in the region, where D. purpusii formed part of the tree, shrub and herb layers. The phenogram resulting from the similarity and cluster analyses showed that Sites 1 and 2 are more similar (0.97), and Site 3 less similar to the other two (0.84). Although the population structure of D. purpusii at the three sites is similar to that found for other cycad species, the main threats are the small number of reproductive individuals, harvesting of leaves, and the constant threat of habitat alteration, like construction of roads, contributing to extinction.

Sex promotes gamete selection: A quantitative comparative study of features favoring the evolution of sex

AbstractExplaining the maintenance of sexual reproduction remains one of the greatest challenges in biology. The theoretical oddity of sex is based on at least three advantages that asexual organisms have over sexual ones: (1) Asexuals, by not producing males, have a twofold advantage over sexuals, regarding the number of reproductive individuals; (2) sexuals have an evolutionary disadvantage over asexuals, in that a rare alleles that increases fitness of one parent, for example, is not necessarily transmitted to the offspring, due to segregation; and (3) asexuals do not have to find a mate in order to reproduce. Here I present the results of evolutionary tournaments performed using the agent‐based computer simulation Biodynamica. The tournaments explored the relative merits of various features that have been proposed to affect the putative advantages of sexual reproduction, including ploidy, mate selection, large reproductive variance of males, low mutation rates, gamete selection, reduced cost for the production of males, thelytoky vs. parthenogenesis, variable environments, and complex genomes. The model allowed for a quantitative comparison of the effect of these features on the adaptive value of sex. The results showed that the production of large number of gametes (for example “masting” in massive flowering of trees or spermatozoa of most animals) inducing gamete selection, together with nonrandom mate selection, increased variance in male reproductive success and/or low mutation rates, allowed sexual agents to out‐breed asexual ones, despite the three disadvantages for sex cited above. The wok suggests that gamete cycles and alloiogenesis, common to most sexual plants, fungi, and animals, should be viewed as a new level for natural selection to work. Mate and gamete selection are more efficient in the use of biomass, energy, and time than natural selection at the level of organisms, helping to make sexual reproduction an evolutionary success. © 2004 Wiley Periodicals, Inc. Complexity 9: 43–51, 2004

The Effect of the Sterilising Anther-Smut Fungus Microbotryum Violaceum on the Genetic Composition of Silene Dioica Populations

Populations of the outbreeding perennial herb Silene dioica, growing on the islands of an archipelago in the Baltic Sea, are often diseased by the sterilising anther-smut fungus Microbotryum violaceum (synonym Ustilago violaceu). Since diseased host plants are sterilised, island populations with high disease incidences experience large reductions in the number of reproductive individuals. Here, we compared the genetic composition of diseased plants at six allozyme loci with that of the remaining reproductive individuals to determine whether this reduction changes the genetic composition of S. dioica island populations. We found a significant but small difference in genetic composition between diseased and healthy samples using analyses of multiple allelic types. We discuss our results in relation to changes in genetic differentiation among island populations and the impact of disease in the archipelago.

Ecological life history of Polymnia canadensis, a monocarpic species of the North American temperate deciduous forest: demography.

A demographic investigation was conducted to assess variation in life history of Polymnia canadensis (Asteraceae), a geographically-widespread, herbaceous species of deciduous forests in eastern North America. During 1985-1994, 23,063 seedlings of P. canadensis were monitored at five central Kentucky study sites. Numbers at the end were: biennials, 554; triennials, 142; winter annuals, 16; monocarpic perennials, 2; tricarpic perennials (three years), 3; and dicarpic perennials (some skipped years) that matured in the first year of life, 23; in the second year, 60; and in the third year, 9. Weekly cohorts of P. canadensis generally exhibited Deevey Type III survivorship with highest seedling mortality in summer associated with low soil moisture. Wide spatial variation in life history was displayed by the fact that fall germination cohorts at dry sites generally had greater germination and survivorship than at mesic sites during seedling establishment, while the reverse was true for spring cohorts. This led to more reproductive individuals in fall germination cohorts than spring cohorts at dry sites and generally the opposite case at mesic sites. Forest shade in mesic sites caused slower growth and a greater frequency of longer-lived reproductive individuals than in open, dry sites, but it also resulted in higher survivorship than at dry sites during moderate drought. Annual population growth rate averaged across four years was not significantly different between a dry site and mesic site, indicating that despite forest shade, P. canadensis persisted in the mesic site as well as it did in the dry site. Population structure varied among years, seasons, and study sites; at two study plots, a 2-year flowering cycle of mass seeding and senescence persisted for 4 years. At a smaller scale, there was little difference in survivorship between study plots within sites or between quadrats within study plots, while in a few instances there were large differences in the number of reproductive individuals. Weekly cohorts that germinated early within seasonal cohorts had greater number of reproductive individuals than later weekly cohorts, but not consistently greater survivorship. Several droughts induced temporal variation that was as important as spatial variation. During these droughts, the population size of all cohorts that germinated prior to the droughts declined to zero at all sites, and biennials were the longest lived type of reproductive individuals. During drought, Deevey Type I survivorship was prevalent, and lack of seed rain led to dependence on persistent seed banks for recruitment in some seasonal cohorts.

Effective size and polymorphism of linked neutral loci in populations under directional selection.

The general theory of the effective size (Ne) for populations under directional selection is extended to cover linkage. Ne is a function of the association between neutral and selected genes generated by finite sampling. This association is reduced by three factors: the recombination rate, the reduction of genetic variance due to drift, and the reduction of genetic variance of the selected genes due to selection. If the genetic size of the genome (L in Morgans) is not extremely small the equation for Ne is [formula, see text] where N is the number of reproductive individuals, C 2 is the genetic variance for fitness scaled by the squared mean fitness, (1 - Z) = Vm/C2 is the rate of reduction of genetic variation per generation and Vm is the mutational input of genetic variation for fitness. The above predictive equation of Ne is valid for the infinitesimal model and for a model of detrimental mutations. The principles of the theory are also applicable to favorable mutation models if there is a continuous flux of advantageous mutations. The predictions are tested by simulation, and the connection with previous results is found and discussed. The reduction of effective size associated with a neutral mutation is progressive over generations until the asymptotic value (the above expression) is reached after a number of generations. The magnitude of the drift process is, therefore, smaller for recent neutral mutations than for old ones. This produces equilibrium values of average heterozygosity and proportion of segregating sites that cannot be formally predicted from the asymptotic Ne, but both parameters can still be predicted by following the drift along the lineage of genes. The spectrum of gene frequencies in a given generation can also be predicted by considering the overlapping of distributions corresponding to mutations that arose in different generations and with different associated effective sizes.

Evolution of Colony Characteristics in Social Insects. II. Number of Reproductive Individuals

The inclusive-fitness models and single-locus gene frequency models suggest different thresholds for the cost allowing worker reproduction to evolve. The advantage of the inclusive-fitness approach is that it allows combining the invasion and loss of worker reproduction in the same model. According to the results derived here through this model, both polyandry and polygyny can restrict the conditions for the evolution of worker reproduction. There is a conflict between queens and workers concerning both worker reproduction and sex allocation. If a colony becomes either polygynous or polyandrous, the queens and workers can have similar interests concerning reproduction, and the colony could act in harmony to increase the inclusive fitnesses This requires that the workers can recognize the genetic heterogeneity within the colony and adjust their own behavior optimally. If the workers can so behave, polyandry could evolve as the queens' way of reducing the queen-worker conflict and manipulating the workers.

The Population Growth of a Rotifer Brachionus plicatilis and Life History of Amictic Females.

The relation between the individual life history characteristics and the population growth in a rotifer Brachionus plicatilis is discussed, The population growth rates (r) in three different strains of the rotifer calculated theoretically from observed individual life history characteristics, embryonic developmental time, duration of pre-reproductive, reproductive and post-reproductive phase, and number of offspring, is presented. The observed characteristics varied between different strains. Due to these differences, the calculated population growth rate between 15 days and 20 days after the inoculation of one freshly laid egg varied from 0.30 to 0.46. The major factors which in-fluence the population growth rate seem to be spawning interval. Increasing the number of off-spring from one individals and the number of reproductive individuals would not bring about the desired increase in the population growth rate as well as the shortening of developmental time and spawning interval.