Notes on the life history of the clam shrimp,Eulimnadia texana

Several life history measures (growth rate, egg production, molt frequency, age at maturity and lifespan) were measured on several clam shrimp hermaphrodites (Eulimnadia texana Packard) grown in a laboratory setting under optimal growth conditions. Growth rates were high early in life, and then dropped dramatically when egg production began (day 5–6). Early egg production was low, and increased until approximately day 7, after which production leveled off for several days. Reproductive senescence was noted after day 17, with clutch sizes continuously dropping until death. Average molts per day was approximately 1.1, and molting seemed to be more closely associated with egg production than with growth. Growth and egg production were negatively correlated, indicating a possible trade-off between these two traits. No other trade-offs were detected. These shrimp show typical early-colonist life history traits, displaying high initial growth, early reproduction at a high rate, and then early senescence and death. This revised version was published online in August 2006 with corrections to the Cover Date.     

Levels of inbreeding depression over seven generations of selfing in the androdioecious clam shrimp, Eulimnadia texana

Androdioecy (mixtures of males and hermaphrodites) is a rare mating system in both plants and animals. Theory suggests that high levels of inbreeding depression can maintain males in androdioecious populations if hermaphrodites commonly self-fertilize. However, if inbreeding depression (delta) can be 'purged' from selfing populations, maintaining males is more difficult. In the androdioecious clam shrimp, Eulimnadia texana, delta is estimated to be as high as 0.7. Previous work suggests that this high level is maintained in the face of high levels of inbreeding due to an associative overdominance of fitness-related loci with the sex-determining locus. Such associative overdominance would make purging of inbreeding depression difficult to impossible. The current experiment was designed to determine if delta can be purged in these shrimp by tracking fitness across seven generations in selfing and outcrossing treatments. Evidence of purging was found in one of four populations, but the remaining populations demonstrated a consistent pattern of delta across generations. Although the experimental design allowed ample opportunity for purging, the majority of populations were unable to purge their genetic load. Therefore, delta in this species is likely due to associative overdominance caused by deleterious recessive alleles linked to the sex determining locus.     

Maintenance of androdioecy in the freshwater shrimp Eulimnadia texana: sexual encounter rates and outcrossing success

The clam shrimp Eulimnadia texana has a rare mating system knownas androdioecy, in which males and hermaphrodites cooccur butthere are no pure females. In this species, reproduction takesplace by outcrossing between males and hermaphrodites, or byselfing within a hermaphrodite; this system provides a uniqueopportunity to examine the adaptive significance of out-crossingand selfing in animals. Our study examined mating behavior inhermaphrodites and males from two populations to understandthe propensity of these shrimp to mate and to estimate a parameterof a model developed by Otto et al. (American Naturalist 141:329-337),which predicts the conditions for stability of the mixed matingsystem in E. texana. Here we present evidence that mating frequencyis environmentally sensitive, with greater numbers of encountersand matings per male when males are rare and in younger males.However, the effects of shrimp density, relative male frequency,and shrimp age interact in a complex way to determine malemating success. Overall, mating frequency was determined bya combination of encounter rates between the sexes and theproportion of encounters resulting in mating. The mating rateswere then used to estimate one of four parameters of the Ottoet al. model, and these estimates were combined with previousestimates of the other three parameters to examine the fitof the predicted to the observed sex ratios in the two populations.     

Mating behavior and time budget of an androdioecious crustacean,Eulimnadia texana (Crustacea: Conchostraca)

The clam shrimp,Eulimnadia texana (Crustacea, Conchostraca), is found in freshwater ephemeral environments throughtout the United States. Individual clam shrimp of this species are either hermaphroditic or male, a relatively rare mating system for animals known as androdioecy. Comparison of sex ratios between four neighboring populations ofE. texana in Southern New Mexico showed wide variation in the ratio of males to hermaphrodites with males making up as much as 42% of some populations and not occurring at all within others. Since little is known about the behavior of this species, an ethogram and time budget were prepared based on observations of laboratory populations. Males attempt to clasp hermaphrodites prior to mating. Precopulatory mate guarding occurs in this species. Outcrossing generally occurs during mate guarding and after the hermaphrodite molts. Hermaphrodites, however, seem to control the mating process. Successful mating by males never occured if the hermaphrodite struggled with him; hermaphrodite will self in the presence of males.     

Relative fitness of two hermaphroditic mating types in the androdioecious clam shrimp,Eulimnadia texana

Androdioecy (populations of males and hermaphrodites) is a rare reproductive form, being described from only a handful of plants and animals. One of these is the shrimp Eulimnadia texana, which has populations comprised of three mating types: two hermaphroditic types (monogenics and amphigenics) and males. In a recent study, the amphigenic hermaphrodites were found to be in greater abundance than that predicted from a model of this mating system. Herein, we compare the relative fitness of offspring from amphigenic and monogenic siblings, attempting to understand the greater relative abundance of the former. Populations started with offspring from selfed monogenic hermaphrodites had a net reproductive rate (R) 87% that of offspring from their amphigenic siblings. Additionally, within populations of amphigenic offspring (which included males, monogenics and amphigenics), amphigenics survived longer than monogenics. These differences help to explain the increased relative abundance of amphigenics in natural populations, but amphigenics continue to be more abundant than expected.     

Positive effects of an arbuscular mycorrhizal fungus on aphid life history traits

Two generations of two aphid species (Myzus ascalonicus and M. persicae) were reared on Plantago lanceolata plants, with and without root colonization by the arbuscular mycorrhizal fungus, Glomus intraradices. Life history traits of the aphids measured were nymphal development time, teneral adult weight, growth rate, total fecundity, adult longevity and duration of post-reproductive life. For both aphids in both generations, mycorrhizal colonization increased aphid weight and fecundity, while other traits were unaffected. The increases were consistent between generations. In a second experiment, M. persicae was reared on plants with and without the fungus, under varying N and P regimes. The results of N addition were inconclusive because there was high aphid mortality. However, under P supplementation, positive effects of the mycorrhiza on aphid growth were seen at low and medium P levels, while at high P levels these effects disappeared. The positive effects of mycorrhizal colonization reported here are contrary to the majority of previous studies with chewing insects, which have reported negative effects. A number of possible mechanisms for this apparent discrepancy are discussed. Received: 1 February 1999 / Accepted: 22 March 1999     

Dietary effects on life history traits of riverine Bosmina

1. We compared growth, reproduction and life history characteristics of Bosmina raised on Ohio River seston versus a unialgal culture (Scenedesmus acutus), to assess potential nutritional constraints experienced by riverine populations. 2. Bosmina grew well in both treatments during their juvenile stage. Analysis of variance showed that Bosmina growth prior to the start of reproduction did not differ significantly between the treatments. After the onset of reproduction Bosmina fed on Scenedesmus grew faster and exhibited higher fecundity than their counterparts fed river seston. 3. Significant quantities of triacylglycerol (visible lipid droplets) were gradually accumulated in Bosmina fed on Scenedesmus. Visual lipid ovary indices were positively correlated with adult growth rate and fecundity and negatively correlated with longevity. 4. Biochemical analysis showed that Scenedesmus had significantly greater total ω3, ω6 and polyunsaturated fatty acids (PUFA) whereas river seston had more eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Thus, in contrast to Daphnia, EPA and DHA do not appear to influence Bosmina growth or reproductive performance, but linolenic acid or total PUFA may be important. 5. Comparatively weak performance by Bosmina feeding on river seston suggest that nutritional constraints may be important despite high particulate organic carbon and low C : N and C : P of river seston.     

The Maturity Index,the evolution of nematode life history traits,adaptive radiation and cp-scaling

Nematodes are increasingly being used in environmental studies. One of the potential parameters to measure the impact of disturbances and to monitor changes in structure and functioning of the below-ground ecosystem is the nematode Maturity Index; an index based on the proportion of colonizers (r-strategists s.l.) and persisters (K-strategists s.l.) in samples. In this paper the original allocation of nematode taxa on the colonizer-persister scale, and the tolerance and sensitivity of colonizers and persisters are discussed from an evolutionary viewpoint. The phenomenon that neither relative egg size nor body length is an unequivocal character to scale nematodes suggests that the main selection for life history traits occurred independently in the major evolutionary branches. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of dietary restriction on lifespan,growth, and reproduction of the clam shrimp Eulimnadia texana

Hydrobiologia - Diet has been linked to lifespan in a broad range of animals. In particular, pronounced caloric restriction has been associated with increased longevity. Herein, the relationship of...     

The effect of domestication on some life history traits of sea trout and Atlantic salmon

During the period 1968–1991, certain morphological traits of Atlantic salmon Salmo salar and sea trout S. trutta have been recorded regularly at the hatchery at Älvkarleby, central Sweden. Total body length, weight (for females both before and after stripping), number of eggs, egg size and date of ovulation. A smaller data set for fish marked and released as smolts, providing information about total body length of released smolts, time spent at sea and body size of the recovered adults, was also available for analysis. According to theory and empirical data, the process of artificial breeding results in an evolutionary divergence of the cultured strain from the wild phenotypic norm. The reason for such a divergence is that both natural and sexual selection pressures are altered or relaxed during the process of artificial breeding, as well as random genetic processes, such as founder effects and in- and outbreeding. A path analysis of both species and sexes revealed that the size of the released smolts had increased during the study period. Time spent in sea has decreased for both female and male sea trout, but not for Atlantic salmon. Adult body size for female and male trout have increased as well as female trout condition factor. The increase found in egg size of both species was greater for the salmon than for the trout, indicating that female salmon invest more in egg size with increasing body size. These results support the view that domestication probably has a significant and selective impact on the life history traits of the two salmonid species studied.     

Heritability estimates of life history traits in small white butterflyPieris rapae crucivora

Heritabilities and genetic correlations of life history characters (pupal weight, age-specific fecundities, and egg weight) of small white butterfly Pieris rapae crucivora are estimated by a quantitative genetic method (sib analysis). The results indicate moderate or high heritabilities and a largely negative genetic correaltion in age-specific fecundities.     

Manipulation of habitat quality: effects on chironomid life history traits

1. To understand how habitat heterogeneity affects organisms, the present authors measured the response of chironomid life-histories to single patch types in a controlled laboratory experiment. The objective was to determine whether the size and type of leaf particles in the substratum affect development rate and survival of larvae, and the fecundity of the resulting adults.
2. Chironomus riparius larvae were raised in sand supplemented with dogwood, Cornus florida, (a rapid decomposer) or American sycamore, Platanus occidentalis, (a refractory decomposer) leaves fully crossed in a factorial design with four leaf particle sizes (0.2, 0.5, 1 and 5 cm²), or in sand only.
3. Development time was shorter in dogwood habitats than sycamore habitats, and shorter in sycamore habitats than in sand habitats. Survival was higher and larval head capsule lengths of instars III and IV were larger in dogwood than in sycamore or sand habitats. Development time was longer and head capsule lengths of instars III and IV were smaller in habitats with 5-cm² leaf particles than in 0.2-, 0.5- and 1-cm² particle size treatments. Female thorax length (a measure of fecundity) did not differ among treatments.
4. The present authors conclude that, for C. riparius confined to a single patch type, fitness is significantly enhanced when the patch contains small particles of dogwood (labile) leaves.     

Food effects on life history traits and seasonal dynamics of Ceriodaphnia pulchella

1. This paper describes the effects of differences in food quantity and quality on selected life history traits of Ceriodaphnia pulchella . Animals were fed with four concentrations of two green algae, Scenedesmus obliquus and Chlamydomonas globosa , given separately as well as in a 1 : 1 mixture.
2. In most cases growth, reproduction and condition of the animals and consequently the rate of population increase, r , increased with food concentration. Life history parameters of animals raised on S. obliquus were lowest, whereas no significant differences were found between animals fed with C. globosa and with the mixture of the two algal species.
3. In contrast to the large effects of food quantity and quality found in the laboratory, an analysis of the population dynamics of C. pulchella in Lake Tjeukemeer showed that the effect of changes in food conditions during the growing season of this species was limited, although overall food levels seemed to be poor throughout the year.
4. Changes in the birth rate of the population are mainly governed by changes in temperature, and possibly by changes in the densities of the invertebrate predator Leptodora kindtii .     

Food effects on life history traits and seasonal dynamics of Ceriodaphnia pulchella

1. This paper describes the effects of differences in food quantity and quality on selected life history traits of Ceriodaphnia pulchella . Animals were fed with four concentrations of two green algae, Scenedesmus obliquus and Chlamydomonas globosa , given separately as well as in a 1 : 1 mixture.
2. In most cases growth, reproduction and condition of the animals and consequently the rate of population increase, r , increased with food concentration. Life history parameters of animals raised on S. obliquus were lowest, whereas no significant differences were found between animals fed with C. globosa and with the mixture of the two algal species.
3. In contrast to the large effects of food quantity and quality found in the laboratory, an analysis of the population dynamics of C. pulchella in Lake Tjeukemeer showed that the effect of changes in food conditions during the growing season of this species was limited, although overall food levels seemed to be poor throughout the year.
4. Changes in the birth rate of the population are mainly governed by changes in temperature, and possibly by changes in the densities of the invertebrate predator Leptodora kindtii .     

Variation in life history and genetic traits of Hawaiian mosquitofish populations

Mosquitofish (Gambusia affinis) were collected from 17 reservoirs on three islands in Hawaii, USA. Genetic and life history traits for adult females from these populations were used to evaluate hypotheses concerning short-term evolutionary divergence of populations recently established from a common ancestral source. The effects of founder events and drift on genetic variability and population differentiation were also examined. Significant differences in life history characteristics, allele frequencies, and multi-locus heterozygosities (H) were found among fish populations collected from different reservoirs and between reservoirs classified as stable or fluctuating on the basis of temporal fluctuation in water level. Females from stable reservoirs exhibited greater standard length (35.1 vs 32.8 mm), lower fecundity (11.9 vs 15.2 embryos), lower reproductive allocation (18.2% vs 22.8%), but larger mean embryo size (1.95 vs 1.67 mg) than females from fluctuating reservoirs. Consistency in means among replicates of each reservoir class and concordance in direction and magnitude of differences reported here and results of sampling conducted from these same locations 10 years previously (Stearns, 1983a) suggest that ecological factors intrinsic to these two environments are important in determing population life history traits. Females from stable reservoirs exhibited lower heterozygosity than females from fluctuating reservoirs (0.134 vs 0.158, respectively). Levels and direction od differences in heterozygosity, the high proportion of polymorphic loci and lack of fixation of alternative alleles argue against a purely stochastic explanation for genetic and life history variation among reservoir populations. Levels of genetic variability and interpopulation differentiation were similar to those observed in mainland populations of this species. A high proportion of the genetic diversity was apportioned between populations and within populations due to differences between juveniles and adults. Significant genotypic differences between adult and juvenile age classes suggest that the genetic divergence of local populations may occur over short periods of time.     

Context-dependent effects of ranaviral infection on northern leopard frog life history traits

Pathogens have important effects on host life-history traits, but the magnitude of these effects is often strongly context-dependent. The outcome of an interaction between a host and an infectious agent is often associated with the level of stress experienced by the host. Ranavirus causes disease and mortality in amphibian populations in various locations around the world, but most known cases of ranaviral infection have occurred in North America and the United Kingdom. While Ranavirus virulence has been investigated, the outcome of Ranavirus infection has seldom been related to the host environment. In a factorial experiment, we exposed Northern leopard frog (Lithobates pipiens, formerly Rana pipiens) tadpoles to different concentrations of Ranavirus and investigated the effect of host density on certain life-history traits, namely survival, growth rate, developmental stage and number of days from virus exposure to death. Our results suggest a prominent role of density in driving the direction of the interaction between L. pipiens tadpoles and Ranavirus. We showed that increasing animal holding density is detrimental for host fitness as mortality rate is higher, day of death earlier, development longer and growth rate significantly lower in high-density tanks. We observed a linear increase of detrimental effects when Ranavirus doses increased in low-density conditions, with control tadpoles having a significantly higher overall relative fitness. However, this pattern was no longer observed in high-density conditions, where the effects of increasing Ranavirus dose were limited. Infected and control animals fitness were consequently similar. We speculate that the host may eventually diverts the energy required for a metabolic/immune response triggered by the infection (i.e., direct costs of the infection) to better cope with the increase in environmental "stress" associated with high density (i.e., indirect benefits of the infection). Our results illustrate how the net fitness of organisms may be shaped by ecological context and emphasize the necessity of examining the direct/indirect costs and benefits balance to fully understand host-pathogen interactions.     

Comparative biology of mammalian telomeres: hypotheses on ancestral states and the roles of telomeres in longevity determination

Progressive telomere shortening from cell division (replicative aging) provides a barrier for human tumor progression. This program is not conserved in laboratory mice, which have longer telomeres and constitutive telomerase. Wild species that do/do not use replicative aging have been reported, but the evolution of different phenotypes and a conceptual framework for understanding their uses of telomeres is lacking. We examined telomeres/telomerase in cultured cells from > 60 mammalian species to place different uses of telomeres in a broad mammalian context. Phylogeny‐based statistical analysis reconstructed ancestral states. Our analysis suggested that the ancestral mammalian phenotype included short telomeres (< 20 kb, as we now see in humans) and repressed telomerase. We argue that the repressed telomerase was a response to a higher mutation load brought on by the evolution of homeothermy. With telomerase repressed, we then see the evolution of replicative aging. Telomere length inversely correlated with lifespan, while telomerase expression co‐evolved with body size. Multiple independent times smaller, shorter‐lived species changed to having longer telomeres and expressing telomerase. Trade‐offs involving reducing the energetic/cellular costs of specific oxidative protection mechanisms (needed to protect < 20 kb telomeres in the absence of telomerase) could explain this abandonment of replicative aging. These observations provide a conceptual framework for understanding different uses of telomeres in mammals, support a role for human‐like telomeres in allowing longer lifespans to evolve, demonstrate the need to include telomere length in the analysis of comparative studies of oxidative protection in the biology of aging, and identify which mammals can be used as appropriate model organisms for the study of the role of telomeres in human cancer and aging.     

Food quality effects on life history traits and fitness in the generalist herbivore Daphnia

Summary Life table experiments were conducted on the generalist suspension feeder Daphnia galeata, using as food the two green algae (Chlorophyta) Scenedesmus acutus and Oocystis lacustris. Oocystis was hypothesized to be a lower quality food because it is convered with a thick sheath, believed to reduce digestibility. Results showed that Oocystis is a lower quality food for Daphnia, but only at relatively low food concentrations (0.15 mg C/L) and not at higher concentrations (1.0 mg C/L). At 0.15 mg C/L, Daphnia intrinsic rate of increase (r) when grown on Oocystis was only half that when grown on Scenedesmus. Daphnia r was similar at 0.15 mg C/L Oocystis and 0.075 mg C/L Scenedesmus, indicating that Daphnia requires twice as much Oocystis as Scenedesmus to achieve the same fitness. Intrinsic rate of increase was lower on Oocystis mainly because age at first reproduction was greatly delayed compared to that on Scenedesmus (13.6 vs 7.3 d). In addition, juvenile growth and survivorship were reduced on Oocystis compared with Scenedesmus. Clutch sizes were similar on the two foods, indicating that once individuals reached adulthood, the two foods were similar in quality. In contrast, at high food concentrations (1.0 mg C/L), the two algae were similar in quality for both juveniles and adults, and r was not significantly different on the two foods. Ingestion and assimilation rate experiments whowed that Daphnia consumes the two algae at identical rates, and that adults assimilate the two algae at similar rates. However, juveniles assimilate Oocystis at much lower rates than Scenedesmus, possibly accounting for reduced juvenile growth and delay in age at maturity at low concentrations. Thus, Daphnia exhibits an ontogenetic shift in its ability to utilize Oocystis, and this can result in juvenile bottlenecks in which survival and growth of young age classes are of critical importance in determining population dynamics. Because food quality effects were manifested primarily in juveniles and at low concentrations, food quality effects in nature will depend on phytoplankton abundance and age-structure of Daphnia populations.     

Parental environmental effects on life history traits in Arabidopsis thaliana (Brassicaceae)

Environmentally induced maternal effects on offspring phenotype are well known in plants. When genotypes or maternal lineages are replicated and raised in different environmental conditions, the phenotype of their offspring often depends on the environment in which the parents developed. However, the degree to which such maternal effects are maintained over subsequent generations has not been documented in many taxa. Here we report the results of a study designed to assess the effects of parental environment on vegetative and reproductive traits, using glasshouse-raised maternal lines sampled from natural populations of Arabidopsis thaliana . Replicates of five highly selfed lines from each of four wild populations were cultivated in two abiotic environments in the glasshouse, and the quality and performance of seeds derived from these two environments were examined over two generations. We found that offspring phenotype was strongly influenced by parental environment, but because the parental environments differed with respect to the time of seed harvest, it was not possible to distinguish clearly between parental environmental effects and the possible (but unlikely) effects of seed age on offspring phenotype. We observed a rapid decline in the expression of ancestral environmental effects, and no main environmental effects on progeny phenotype persisted in the second generation. The mechanism of transmission of environmental effects did not appear to be associated with the quantity or quality of reserves in the seeds, suggesting that environmental effects may be transmitted across subsequent generations via some mechanism that generates environment-specific gene expression.