Life history traits of the free-living nematode, Plectus acuminatus Bastian, 1865, and responses to cadmium exposure

Free-living nematodes are ubiquitous and play an essential role in ecosystems. However, little is known about their standard life history traits (LHTs), which limits their inclusion in estimations of energy flows and carrying capacities of ecosystems, as well as in modelling population-level responses to toxicants. Thus, we used the hanging-drop method to measure LHTs of Plectus acuminatus with and without exposure to cadmium (2 mg l−1). In controls, the mean lifespan was 68 days and the maximum 114 days. Individuals laid eggs on average 19 days after hatching, while production of offspring peaked at 37 days. Plectus acuminatus individuals were very fertile, producing on average 848 juveniles. Population growth rate of 0.19 was estimated for the control cohort leading to an average population doubling time of 3.65 days. Exposure to cadmium reduced mean lifespan by 62% and affected reproduction as only 22% of individuals produced offspring, leading to a total fertility rate 85% lower than in controls.

Influence of bacterial density and mating on life history traits of Heterorhabditis bacteriophora

Heterorhabditis bacteriophora, associated with Photorhabdus luminescens, is commonly used against insect pests. Dauer juveniles (DJ) develop into self-fertilising hermaphrodites that lay eggs until juveniles hatch inside the uterus and feed on the body content of the mother (endotokia matricida). The life history traits of H. bacteriophora were studied at 2.5 × 109, 5 × 109, 10 × 109 and 20 × 109 cells ml−1 of P. luminescens at 25°C using a hanging drop technique. The number of offspring produced per hermaphrodite increased from 50 at 2.5 × 109 cells ml−1 to 269 at 20 × 109 cells ml−1 of P. luminescens. The bacterial density did not influence the beginning of endotokia matricida, hermaphrodite death, DJ release from the maternal carcass and the percentage of juveniles obtained through endotokia matricida. Mating of automictic females could not increase offspring production and survival. Endotokia matricida is an obligatory developmental step in H. bacteriophora.

Life history traits, liquid culture production and storage temperatures of Steinernema yirgalemense

Using the hanging drop technique with nematode growth gelrite medium, life history traits of Steinernema yirgalemense (strain Sy 157-C) were investigated at a bacterial density of 10 × 109 cells ml−1 of Xenorhabdus indica at 25°C. With the same technique, the exit of dauer juveniles (DJ) from the arrested stage (recovery) was assessed at 5 × 109, 10 × 109 and 20 × 109 cells ml−1 of X. indica. Additionally, S. yirgalemense was incubated in nematode liquid medium at 25, 27 and 30°C. At each culture temperature, DJ recovery, sex ratio at 3 days post DJ inoculation and DJ yield and DJ as a percentage of non-DJ stages at 15 days post DJ inoculation were assessed. DJ survival in Ringer’s solution stored at 4, 15 and 25°C was assessed for 66 days. Steinernema yirgalemense has a total fertility rate and net reproductive rate of 487 and 314 offspring per female, respectively. The intrinsic rate of natural increase was 0.98 day−1, population doubling time PDT = 0.71 days and mean generation time days. The average lifespan of S. yirgalemense females starting from first-stage juveniles was 6.55 days. In liquid culture, DJ recovery ranged from 63-75% at 72 h post DJ inoculation and was not significantly different between the incubation temperatures. Parental male to female ratio was not influenced by incubation temperature and usually was at a ratio of 1:2. The percentage of females that entered into endotokia matricida at 72 h post DJ inoculation was 61% at 25°C, whereas at 27 and 30°C it was 24% and 0.5%, respectively. The highest DJ yield was recorded at 25°C (284 114 DJ ml−1) followed by 27°C (176 932 DJ ml−1) and the lowest at 30°C with 26 298 DJ ml−1. At a storage temperature of 4°C, DJ survival did not exceed 42 days, whereas at 15 and 25°C more than 95% of the DJ survived 66 days. Although S. yirgalemense DJ survived for long periods at both 15 and 25°C in liquid storage, their survival in formulated product and virulence after storage needs further investigation.

Life history trait analysis of the entomopathogenic nematode Steinernema feltiae provides the basis for prediction of dauer juvenile yields in monoxenic liquid culture

Entomopathogenic nematodes (Steinernema spp.) are used in integrated pest management to control insect pests in cryptic environments. The nematodes are mass produced in monoxenic liquid culture with their symbiotic bacteria Xenorhabdus spp. For a better understanding of nematode population dynamics, the life history traits (LHTs) of the entomopathogenic nematode Steinernema feltiae were assessed at 25 °C by observing single pairs of male and female nematodes using a hanging drop technique. To investigate the influence of different food supplies on nematode reproduction, the LHTs were assessed with a daily supply of 5 ×, 10 × and 20 × 10(9) cells ml(-1) of the nematode's bacterial symbiont Xenorhabdus bovienii in semi-solid nematode growth gelrite (NGG) medium. Increasing bacterial density had a significant positive influence on the average number of offspring produced, which ranged from 359 to 813 per female. The intrinsic rate of natural increase r m, which ranges from 1.10 to 1.19 day(-1), was neither influenced by the bacterial density, nor was the mean generation time T (5.12-5.25 days) and population doubling time (PDT) (0.64-0.59 days). The average lifespan of reproductive females, which ranged from 6.7 to 7.3 days, was positively correlated with bacterial density. A positive correlation between female body volume and bacterial density was recorded (R = 0.67) as well as a significant positive correlation between female body size and offspring production (R = 0.89) in hanging drops. Whether these data can be used to predict nematode yields in liquid culture was tested. The total female body volume calculated as the average female body volume × total number of parental females per millilitre 3 days after nematode inoculation was positively correlated (R = 0.72) with nematode yields. The total female body volume on process day 3 is thus a good indicator for the estimation of nematode yield at the end of the process (12-15 days post dauer juvenile (DJ) inoculation) in both Erlenmeyer flasks and bioreactors. With a mean deviation of 9467 DJs ml(-1), the error resembles approximately 5 % of the final DJ yields.

Life-History Traits of the Model Organism Pristionchus pacificus Recorded Using the Hanging Drop Method: Comparison with Caenorhabditis elegans

The nematode Pristionchus pacificus is of growing interest as a model organism in evolutionary biology. However, despite multiple studies of its genetics, developmental cues, and ecology, the basic life-history traits (LHTs) of P. pacificus remain unknown. In this study, we used the hanging drop method to follow P. pacificus at the individual level and thereby quantify its LHTs. This approach allowed direct comparisons with the LHTs of Caenorhabditis elegans recently determined using this method. When provided with 5×10(9) Escherichia coli cells ml(-1) at 20°C, the intrinsic rate of natural increase of P. pacificus was 1.125 (individually, per day); mean net production was 115 juveniles produced during the life-time of each individual, and each nematode laid an average of 270 eggs (both fertile and unfertile). The mean age of P. pacificus individuals at first reproduction was 65 h, and the average life span was 22 days. The life cycle of P. pacificus is therefore slightly longer than that of C. elegans, with a longer average life span and hatching time and the production of fewer progeny.

Life history trait analysis of the entomopathogenic nematode Steinernema riobrave

Life history traits (LHT) of Steinernema riobrave strain Sr 7-12 and Sr HYB19 were assessed at 25°C in monoxenic culture using a hanging drop technique. The LHT were studied with 5 × 109, 10 × 109 and 20 × 109 cells ml−1 of Xenorhabdus cabanillasii in semi-solid Nematode Growth Gelrite. Increased X. cabanillasii densities had a significant positive influence on offspring production on both Sr 7-12 and the hybrid Sr HYB19. At the higher bacterial food density, the total fertility rate (TFR) per female of Sr 7-12 was 2022 offspring and the net reproductive rate () reached 1904 offspring. Similarly, for Sr HYB19, the TFR per female was 2434 and was 1903. The percentage offspring produced via intra-uterine development (endotokia matricida) was relatively higher at 5 × 109 bacterial cells ml−1 than at higher bacterial density, with 64% and 66% of the total offspring produced by Sr 7-12 and Sr HYB19, respectively. A positive correlation () was recorded for offspring production and bacterial food density and for female body volume with bacterial density (). Female nematodes survived longer at higher bacterial food density; however, females of both strains could not survive longer than 7.8 days, beginning from hatching and always ending 1 day after initiation of endotokia matricida. Based on LHT results, there is a potential to maximise yield of dauer juveniles in liquid culture through increasing bacterial food density.

Influence of cell density of Escherichia coli and the dinoflagellate Crypthecodinium cohnii on life history traits of the nematode Panagrolaimus sp. strain NFS 24-5, a potential larval food for marine aquaculture

The nematode Panagrolaimus sp. NFS 24-5 has potential for use as living food for larval shrimps and fish in marine aquaculture. The nematodes are usually produced on bacterial or yeast cells. Nematodes cannot synthesise the long chain fatty acid docosahexaenoic acid (DHA) which is essential for feeding marine aquaculture organisms. The eukaryotic, heterotrophic dinoflagellate Crypthecodinium cohnii consists of approximately 20% DHA. To culture the nematodes and simultaneously enrich them with DHA, single adult male and female individuals were cultured in hanging drops with variable cell density of C. cohnii. Life history traits, such as net reproductive rate (), population doubling time (PDT) and intrinsic rate of natural increase (), were assessed and compared with data obtained from cultures on Escherichia coli. A maximum was recorded at a cell density of 4 × 106 C. cohnii cells ml−1, corresponding to 2478.82 μg dry mass ml−1. The same was achieved with 7× lower biomass of E. coli at a cell density of 3 × 109 cells ml−1, corresponding to 335.63 μg dry mass ml−1. The results exclude the use of the dinoflagellate culture from application in mass production of the nematode for aquaculture food and limit the use to post-harvest enrichment of the nematodes with essential fatty acids. At a density of 3 × 109 E. coli cells ml−1 the PDT was lowest and the was highest, indicating that this cell density might be closest to optimum conditions for nematode reproduction. Exceeding this cell density yielded fewer offspring within a longer time period. Implications for mass production in monoxenic liquid cultures are discussed.