Life history characteristics of three types of females in Brachionus calyciflorus Pallas (Rotifera) fed different algae

This study describes the life history characteristics of amictic, unfertilized mictic and fertilized mictic females of the rotifer Brachionus calyciflorus cultured individually on two different algae at 0.1 mg ml^–1 food concentration and 27 °C. The duration of the juvenile period of amictic females was significantly shorter on Chlorella pyrenoidosa Chick than on Scenedesmus obliquus Kütz or both algae together. The duration of the juvenile period of unfertilized mictic females was significantly longer, and the number of eggs produced by amictic females was significantly larger on Chlorella pyrenoidosa than on S. obliquus. When fed the same type of alga, the duration of the juvenile period of the fertilized mictic females was the longest among the three types of females, and the durations of the reproductive period of the amictic females and the post-reproductive period of the fertilized mictic females were longer than, or equal to those of the other two types of females, respectively. The number of eggs produced by an unfertilized mictic female was the largest among the three types of females, and that of amictic females was larger than or equal to that of fertilized mictic females, depending on the type of diet.     

孤雌生殖累积世代数和雌体年龄对萼花臂尾轮虫繁殖的影响

采用单个体培养方法,研究了孤雌生殖的累积世代数和雌体年龄对萼花臂尾轮虫混交雌体形成和产卵量的影响,结果表明,随着轮虫孤雌生殖累积世代数和增加,各代中的总混交雌体百分率呈减小的趋势,年轮的雌体可产生较多的混交雌体,非混交雌体所产后代中的总混交雌体百分率具有随其祖母年龄的增大而增大的趋势,孤雌生殖的累积世代数对轮对轮虫非混交雌体的平均产卵量无显著的影响,非混交雌体的年龄对其报代的平均产卵量亦无显著的影响。     

Feeding preference and population growth of Asplanchna brightwelli (Rotifera) offered two non-evasive prey rotifers

Population growth rates of the predatory rotifer Asplanchna brightwelli were determined at 25 °C using a large (Brachionus calyciflorus) and a small (Anuraeopsis fissa) rotifer prey species in three concentrations (0.5, 0.1 and 2.0 g dry weight ml^-1) and in five combinations. The prey ingestion time by the predator was also measured. For B. calyciflorus the ingestion time (22.97–8.95 s) was more than six times that of A. fissa (3.68 ± 0.93). Regardless of prey type, the population growth of Asplanchna increased with increasing food density. There was a direct correlation between densities of amictic and mictic fernales. The maximum rate of population growth (1.01 ± 0.10 d^-1) was higher at high density of A. fissa prey than that at the same density of B. calyciflorus. Progressive increase of A. fissa density in the offered food combination resulted in a corresponding increase of the predator's number. Gut content analysis of A. brightwelli revealed that the number of prey ingested increased with increasing prey densities.     

Effect of food concentration on the production and viability of resting eggs of the rotifer Brachionus: implications for the timing of sexual reproduction

1. Life‐table experiments with Brachionus calyciflorus test several hypotheses related to the idea that sexual reproduction in monogonont rotifers should occur when food resources are favourable. 2. The food concentration necessary for a fertilised mictic female to produce one phenotypically normal resting egg was higher than that for an amictic female to produce one daughter. At the lowest concentration of Cryptomonas erosa (1.25 × 10³ cells mL^?1), the lifetime fecundity of these two types of females was 0.9 and 1.4, respectively. 3. The lifetime fecundity of both fertilised mictic females and amictic females increased with food concentration to 3.4 resting eggs and 15.2 daughters female^?1, respectively. The approach to maximal fecundity with increasing food concentration was more rapid for fertilised mictic females, such that their lifetime fecundity relative to that of amictic females gradually decreased from 0.64 (at 1.25 × 10³ C. erosa mL^?1) to 0.22 (at 2.5 × 10⁴ C. erosa mL^?1). 4. The probability of a fertilised mictic female producing one or more abnormal resting eggs during her lifetime was high (approximately 75%). The mean proportion of abnormal eggs produced per female varied among the different food‐concentration treatments (26–38%) but was not higher at the low food concentrations. 5. The proportion of normal resting eggs that hatched was high (51–71%); those produced at low food concentrations were no less likely to hatch than those produced at high food concentrations. No abnormal resting eggs hatched. 6. The probability of a fertilised mictic female producing an abnormal resting egg increased rapidly with her age at all food concentrations. The probability of a normal resting egg hatching declined with maternal age at the low food concentration in one of two experiments. 7. The results support the idea that induction of mictic females should occur when food resources are good. Coincidence of sexual reproduction with low food availability risks low production of resting eggs for several reasons. Population size may be small, with a low probability of encounters between young mictic females and males, and fertilised mictic females may be unable to mature and produce resting eggs.     

High density culture of the freshwater rotifer,Brachionus calyciflorus

The freshwater rotifer, Brachionus calyciflorus is one of the live food organisms used for the mass production of larval fish. In this study possibility of obtaining high density cultures of the freshwater rotifer B. calyciflorus were investigated. The two culture systems used differed in their air and dissolved oxygen supplies using three temperatures in each case: 24, 28 and 32 °C. Rotifers were batch-cultured using 5 l-vessels and fed with the freshwater Chlorella. The growth rate of rotifers significantly increased with an increase in temperature. The maximum density of the rotifers with air-supply at 24 °C, 6500 ind. ml^–1, was significantly lower than those cultured at 28 and 32 °C, i.e. 8600 and 8100 ind. ml^–1, respectively. Dissolved oxygen levels decreased with time and ranged from 0.8 to 1.4 mg l^–1 when the density of freshwater rotifer was the highest at each temperature. The highest density (19200 ind. ml^–1) of freshwater rotifer was obtained in cultures with a supply of oxygen at 28 °C. Densities of 13500 and 17200 ind. ml^–1 were found at 24 and 32 °C, respectively. Levels of NH₃-N increased with time and a dramatic increase of NH₃-N was observed at high temperatures. Levels of NH₃-N at 24, 28 and 32 °C were 13.2, 18.5 and 24.5 mg l^–1, respectively. These levels coincided with the highest rotifer density at each of the three temperatures. When rotifers were cultured with an oxygen-supply and pH was adjusted to 7, the maximum density of rotifer reached 33500 ind. ml^–1 at 32 °C . These results suggested that high density culture of freshwater rotifer, B. calyciflorus could be achieved under optimal conditions with DO value of exceeding 5 mg l^–1 and NH₃-N values of lower than 12.0 mg l^–1.     

Incomplete induction of mixis in <Emphasis Type="Italic">Brachionus calyciflorus</Emphasis>: patterns of reproduction at the individual level

In the rotifer Brachionus calyciflorus mictic-female production is density-dependent and appears to be induced by a chemical—a quorum sensing molecule—produced by the females themselves. Even at the highest densities, however, populations never become entirely mictic: i.e., some amictic females continue to be produced. Surprisingly, the phenomenon also occurs in clonal laboratory populations with genetically identical individuals. Here, we study how this ecologically adaptive phenomenon is generated at the level of individual reproducing females. In a life-history experiment we subjected 123 amictic females of a clone of B. calyciflorus separately to a daily renewed stimulus of culture medium conditioned at a density of 30 females ml⁻¹. For each of these mothers we isolated the lifetime offspring individually and recorded whether these females were amictic or mictic. Mothers produced on average 16 offspring but none of the mothers produced 100% mictic offspring; the average proportion of mictic females was 30%, despite the extremely strong stimulus. The distribution of amictic vs. mictic offspring was not uniform over the mothers’ lifetime. Early and late offspring had a low probability of being mictic whereas mid-aged mothers produced the highest proportion of mictic daughters (up to 56%). We conclude that not all oocytes of B. calyciflorus can be turned into mictic females, even when the mictic-female-inducing stimulus is extremely high. Propensity to become a mictic female also depends on the rank of an egg within a female’s offspring production. Despite these regularities, we observed considerable stochastic variability with respect to individual mothers’ life histories. Guest editors: S. S. S. Sarma, R. D. Gulati, R. L. Wallace, S. Nandini, H. J. Dumont & R. Rico-Martínez. Advances in Rotifer Research.     

Induction of sexual reproduction and resting egg production in Brachionus Plicatilis reared in sea water

Brachionus plicatilis raised in our laboratory in sea water reproduces asexually even under high crowding conditions (at least 40 individuals per ml). Amictic females were induced to produce mictic females, males and resting eggs by reducing the concentration of the sea water culture medium. Mictic females and males appeared predominantly among the progeny produced by the amictic females during 4 days following their transfer into 25% sea water. Resting eggs appeared first 5–12 days after the onset of the experiment. Following the disappearance of males, the culture consisted of amictic females.Resting eggs produced by the method described above may be preserved for at least three months at –14°C or by desiccation at room temperature. Under the appropriate experimental conditions, resting eggs hatch into amictic females. Since B. plicatilis is one of the most commonly used food sources of fish larvae in aquaculture, the methods reported here may offer an easy and versatile way of preserving rotifer culture stock to be used on demand.     

Endogenous regulation of environmentally induced sexuality in a rotifer: a multigenerational parental effect induced by fertilisation

SUMMARY 1. Sexual reproduction in the heterogonic life cycle of many rotifers occurs when amictic females, which produce diploid eggs developing parthenogenetically into females, are environmentally induced to produce mictic females. Mictic females produce haploid eggs which develop parthenogenetically into males or, if fertilised, into resting eggs – encysted embryos which develop into amictic females after an obligatory diapause. 2. A Florida strain of Brachionus calyciflorus was used to test the prediction that amictic females hatching from resting eggs (Generation 1), and those from the next few parthenogenetic generations, have a lower propensity to produce mictic daughters in response to crowding than those from later parthenogenetic generations. In 10 replicate clones, populations initiated by amictic females from generations 1, 5, 8, 12 and 18 were exposed to a standardised crowding stimulus, and the proportion of mictic females in the populations was determined. These proportions varied significantly across generations and clones. They were very low in the early generations and gradually increased to a mean of about 0.5 at Generation 12. 3. The mechanism for the transgenerational plasticity in response to crowding is not known. One possibility is that resting eggs contain an agent from their fertilised mictic mother's yolk gland that prevents development into mictic females and is transmitted in increasingly low concentrations through successive parthenogenetic generations of amictic females. 4. This parental effect may contribute to clonal fitness by ensuring that a clone developing from a resting egg will attain a higher population size through female parthenogenesis before maximising its commitment to sexual reproduction, even in the presence of a crowding stimulus from a high population density of other clones. Therefore, the number of resting eggs to which a clone contributes its genes should be maximised. 5. The clonal variation in propensity to produce mictic females in this strain indicates genetic variation in the trade‐off between maximising population growth via female parthenogenesis and increasing the probability of producing at least some resting eggs before local extinction from the plankton.     

Influence of the age of algae fed to rotifers (Brachionus plicatilis O.F. Müller) on the expression of mixis in their progenies

Summary The sequence of the appearance of mixis in the rotifer Brachionus plicatilis was followed among the descendents of amictic rotifers transferred from a high salinity media (40 S) to a low one (9 S). All the neonates that hatched from the amictic eggs, after being transferred to a low salinity, were amictic. Each one of these neonates was cultured individually and its offspring removed periodically every 8–10 h. It was observed that throughout their reproductive phase, these parental females retained their potential to produce either mictic or amictic offspring. All the first produced neonates developed into amictic females, but among those produced later, three patterns were prevalent. The prevalent pattern (type A) was one in which the probability of a neonate being mictic increased towards the middle of the parents' reproductive phase and was followed by a slow decline. In the second pattern (type B), the probability of a daughter being mictic was constant throughout the parents' reproductive phase. It is suspected that the quality of food supplied to the rotifers determines the appearance of patterns, A, B or C. It is postulated that the innate capacity of rotifers to undergo mixis is genetically controlled, while its expression is modulated by environmental conditions.     

Effect of some vertebrate and invertebrate hormones on the population growth, mictic female production, and body size of the marine rotifer Brachionus plicatilis Müller

Eight vertebrate and invertebrate hormones were screened for theireffect on population growth, mictic female production, and body size of themarine rotifer Brachionus plicatilis. Growth hormone (GH) or human chorionicgonadotropin (HCG) at 0.0025–25 I.U. ml^–1 andestradiol-17 (E₂), triiodothyronine (T_{_3}),20-hydroxyecdysone (20-HE), 5-hydroxytryptamine (5-HT), gamma-aminobutyricacid (GABA) or juvenile hormone (JH) at 0.05–50 mg l^–1were added to 5-ml of Nannochloropsis oculata suspension (7×10⁶ cells ml^–1). From an initial densityof 1 individual ml^–1, rotifers were cultured with hormones for48 hours in 22 ppt seawater at 25 °C, in darkness.Rotifers were counted and classified into female types and transferred to anew algal food suspension without hormone every other day until day 8 whenbody size was measured. Population growth was significantly higher intreatments exposed to GABA (50 mg l^–1), GH (0.0025 and 0.025I.U. ml^–1), HCG (0.25 and 2.5 I.U. ml^–1), and5-HT (5 mg l^–1). E₂ caused a decrease inpopulation growth, whereas JH, 20HE, and T₃ had no effect.Mictic female production was significantly higher at 0.05 and 0.5 mgl^–1 JH and 0.05 and 5 mg l^–1 5HT. GH (0.0025 and0.025 I.U. ml^–1), E₂ (50 mg l^–1),GABA (0.5, 5 and 50 mg l^–1), and 20-HE (0.05 mgl^–1) treatments had significantly higher mictic femaleproduction only on day 8, 6, 4, and 6, respectively. T₃ andhCG had no effect on mictic female production. Lorica length increased by9.6% and 4.4% in rotifers treated with JH (0.05 mgl^–1) and GABA (5 mg l^–1), respectively. Correspondingly, lorica width increased by 8.9% and 2.6% inthese treatments. In comparison, 20-HE-, T₃-, and HCG-treatedrotifers were smaller (3.9–8.2%) and GH, 5-HT andE₂ had no effect on rotifer body size.     

Studies on functional response and prey selection using zooplankton in the anostracan Chirocephalus diaphanus Prevost, 1803

Freshwater cladocerans and rotifers were used as prey to study functional response and prey selection by adult females of Chirocephalus diaphanus under laboratory conditions. For functional response studies, we offered three rotifer species (Brachionus calyciflorus, B. patulus and Euchlanis dilatata) and three cladoceran species (Alona rectangula, Ceriodaphnia dubia and Moina macrocopa) at various densities ranging from 0.5 to 16 ind. ml^–1. We found increased zooplankton consumption with increasing prey density but beyond 4 ind ml^–1 cladocerans and 8 ind. ml^–1 rotifers, the number of animals eaten plateaued. In general, C. diaphanus consumed fewer large prey (cladocerans) and many more smaller zooplankton (rotifers). For prey selection experiments, we used B. calyciflrous, B. patulus, C. dubia and M. macrocopa, offered at the ratio of two rotifers: one cladoceran and at three prey densities (total zooplankton numbers: 3, 6 and 12 ind. ml^–1). Prey selectivity patterns followed the functional response trends. In general, regardless of prey types, with an increase in the available zooplankton, there was an increase in the number of prey consumed. At any given prey density, C. diaphanus consumed higher numbers of rotifers than cladocerans. Among the prey offered, B. patulus and M. macrocopa were positively selected. Results are discussed in light of possible control of zooplankton by anostracans in temporary ponds.     

Induction of mictic females in the rotifer Brachionus: oocytes of amictic females respond individually to population-density signal only during oogenesis shortly before oviposition

1. One at a time during the reproductive period of amictic females, oocytes fill with yolk and undergo a mitotic maturation division (oogenesis), are oviposited as single cells, and then develop parthenogenetically into females. Sexual reproduction in Brachionus and several other genera is initiated when amictic females are crowded and oviposit some eggs induced to differentiate into mictic females. Mictic females produce haploid eggs that can develop parthenogentically into males or be fertilised and develop into diapausing embryos called resting eggs. 2. This study examines the time when oocytes in amictic females respond to maternal population density. Is the fate of all oocytes in the germarium irreversibly determined during the early postnatal life of the mother, or is each oocyte labile until just before oviposition? In the former case, the probability of an amictic female producing a mictic daughter at any time throughout her reproductive period would reflect the population density she experienced while young and not that at the time she oviposited an egg. 3. Amictic females of two clones of a Florida strain of B. calyciflorus were cultured singly from birth at a low or high density (in a large or small volume) until about halfway through their reproductive period and then switched (experimental treatment), or not (control treatment), to the other density condition. The results indicate that the female fate of an oocyte is determined by maternal population density during oogenesis. Eggs oviposited soon after transfer from low to high density had the same, or a higher, probability of becoming mictic females compared with those produced by control females kept at the high density; eggs oviposited after transfer from the high to the low density had the same low probability of becoming mictic females as those produced by control females kept at the low density. 4. Control females kept at the high density were less likely to produce mictic daughters as they aged. This decline is not because of a decreased propensity of older females to respond to crowding, as older females responded maximally when transferred from a low to a high population density. 5. As oocytes in amictic females respond to maternal population density only during oogenesis, there is a negligible lag between the population‐density signal in the environment and the commitment to sexual reproduction. This minimises the obligatory two‐generation lag between this signal and production of resting eggs, and thus reduces the possibility that crowding will lead to food limitation before production of these eggs.     

Optimal rates of bisexual reproduction in cyclical parthenogens with density-dependent growth

This work explores theoretical patterns of reproduction that maximize the production of resting eggs and the long-term fitness of genotypes in cyclical parthenogens. Our focus is on density-dependent reproduction as it influences the consequences of a trade-off between producing amictic daughters – which reproduce parthenogenetically and subitaneously – and producing mictic daughters – which undergo meiosis and bisexual reproduction. Amictic females increase competitive ability and allow the population to achieve a larger size; mictic females directly contribute to population survival through harsh periods by producing resting eggs. Although morphologically indistinguishable, the two types of females differ greatly in their ecological and reproductive roles. What factors underlie the differential allocation of resources to produce amictic and mictic females? Using a demographic model based on readily accessible parameters we demonstrate the existence of a frequency of mictic females that will maximize the population's long-term fitness. This frequency, termed the optimal mictic ratio, m_o, is 1 ? (q/b)^1/2, where q is the mortality rate and b is the maximum birth rate. Using computer simulation we compared the fitness of a population with this constant mictic ratio with populations having multiple switches from complete parthenogenetic growth to complete allocation in mixis (mictic ratio either 0 or 1). Two important conclusions for optimal mixis in density-dependent growth conditions are: (1) intermediate mictic ratios are optimal, and (2) optimal mictic ratios are higher when habitat conditions are better. Physiological cues responding to differences in birth and death rates are common so that it is possible that populations may adjust their relative rates of mictic and amictic female production in response to environmentally induced changes to the optimum mictic ratio. Our analysis demonstrates that different patterns of mixis are expected in different type of habitats. Since the optimal mictic ratio is sensitive to the effects of a variety of environmental challenges, our model makes possible a new means to evaluate life history evolution in cyclical parthenogens.     

Effect of Aeolosoma sp. (Aphanoneura: Aeolosomatidae) on the Population Dynamics of Selected Cladoceran Species

Although oligochaete worms naturally coexist with cladocerans in many shallow freshwater ponds and lakes, their influence on the latter is not well established. In this work we studied the effect of Aeolosoma sp. on the population growth of Alona rectangula, Ceriodaphnia dubia, Daphnia pulex, Macrothrix triserialis and Moina macrocopa. Population growth studies were conducted at one algal food density (1 × 10⁶cells ml^–1 of Chlorella vulgaris). The experimental design was similar for all five cladoceran species, where we used 100 ml capacity transparent jars containing 50 ml of EPA medium with the desired algal density and three replicates for each treatment. The test medium was changed daily and fresh algal food was added. The initial density of each of the cladoceran species in the population growth studies was 0.4 ind ml^–1 while that of the worms 1.0 ind ml^–1. Following inoculation, we estimated daily the number of cladocerans and the worms for duration of 21 days. Regardless of the presence of worms, Moina macrocopa and Macrothrix triserialis showed rapid population growth while A. rectangula took more than 2 weeks to reach peak abundances. With the exception of M. triserialis, all the other our cladoceran species declined in the presence of Aeolosoma sp. The lowest peak population density (about 1 ind ml^–1) was observed for M. triserialisin controls. The remaining species had peak densities of about 3–5 ind ml^–1. The rates of population increase per day varied from 0.03 to 0.19 depending on the cladoceran taxa and the treatment. In general we found that pelagic taxa were more adversely affected by the presence of the worms than were the littoral cladocerans.     

Environmental and endogenous control of sexuality in a rotifer life cycle: developmental and population biology

Induction of mictic females, and hence initiation of sexuality, in the life cycle of some Brachionus requires an environmental stimulus associated with crowding. The inducing stimulus appears to be a taxonomically specific chemical released into the environment by the rotifers. Oocytes are induced to develop into mictic females before they are oviposited by their amictic mothers and begin cleavage divisions. Thus, the inducer affects the oocyte in the maternal body cavity either directly or indirectly by altering the physiology of its mother. The level of sexual reproduction expressed in populations of a Florida strain of B. calyciflorus is controlled by two types of endogenous factors and by the degree of crowding. First, some fraction of genetically identical oocytes in a clonal population fails to respond to even extreme crowding conditions, thus ensuring some potential for continued population growth by female parthenogenesis. Second, the propensity of amictic females to produce mictic daughters is extremely low when they hatch from fertilized resting eggs and then gradually increases to an asymptote after about 12 parthenogenetic generations. This multigenerational parental effect likely is due to a cytoplasmic factor in fertilized eggs that inhibits expression of the mictic-female phenotype and that is gradually diluted in successive parthenogenetic generations. The effect may increase a clone's genetic contribution to the resting-egg bank by increasing its population size through parthenogenetic generations before mictic females are induced.     

Impact of cyprinids on zooplankton and algae in ten drainable ponds

To study the impact of cyprinids on algae, zooplankton and physical and chemical water quality, ten drainable ponds of 0.1 ha (depth 1.3 m) were each divided into two equal parts. One half of each pond was stocked with 0 + cyprinids (bream, carp and roach of 10–15 mm), the other was free of fish. The average biomass of the 0 + fish at draining of the ponds was 466 kg ha^–1, to which carp contributed about 80%.The fish and non-fish compartments showed significant differences. In the non-fish compartments the density of Daphnia hyalina was 10–30 ind. l^–1 and that of Daphnia magna 2–4 ind. l-^–1, whereas in the fish compartments densities were c. 1 ind. l^–1. Cyclopoid copepods and Bosmina longirostris, however, showed higher densities in the fish compartments. The composition of algae in the two compartments differed only slightly, but the densities were lower in the non-fish compartments. The significant difference in turbidity was probably caused by resuspension of sediment by carp. No significant difference in nutrient concentration between the compartments was found.     

Induction of diapausing amictic eggs in Synchaeta pectinata

Amictic females of a clone of S. pectinata from Star Lake (Norwich, Vermont) may produce diapausing as well as non-diapausing (subitaneous) eggs. The proportion of diapausing eggs produced in cultures was unaffected by temperature (12 vs 19 °C) or rotifer population density (minima of 0.33 vs 3 ind. ml^–1) at 19 °C. However, at 19 °C this proportion was higher in cultures maintained at a low food level suppressing reproduction (5 × 10³ cells ml^–1 Cryptomonas erosa) than in those maintained at a high food level (2 × 10⁴ cells ml^–1); the treatment effect was marginally significant (p=0.067). Consistent with the effect of low food availability, a period of starvation was very effective in inducing the development of diapausing eggs. None of 19 females cultured individually from hatching at 19 °C on C. erosa (2 × 10⁴ cells ml^–1) in 1-ml volumes produced any diapausing eggs in 4 days (0 out of 349 eggs), while 13 out of 16 females subjected to a 15-hour starvation period 6 hours after birth produced one or more diapausing eggs during that time (34% of the 158 eggs produced by the 16 females were diapausing). Diapausing eggs produced and left at 19 °C hatched after 4 to 13 days. Those produced in cultures with a low food level took significantly longer to hatch (9.7 days) than those produced in cultures with a high food level (8.1 days) (p=0.022). In natural communities, S. pectinata should be able to respond directly and rapidly to poor food conditions by producing eggs that undergo an obligatory dormant period before resuming development.     

Control of sexuality in Asplanchna brightwelli: threshold levels of dietary tocopherol and modification of tocopherol response by exogenous and endogenous factors

Ingestion of one prey rotifer containing about 0.02 pg tocopherol was sufficient to cause young amictic females to produce a high proportion of mictic daughters. Varying the concentration of emulsified tocopherol ol and the population density of amictic females suggested that mictic-female induction approached an all-or-nothing response at relatively high population densities and increased with population density only when population densities were very low. Amictic females hatching from resting eggs were less likely to produce mictic daughters than those hatching from parthenogenetic eggs.     

Low crowding threshold for induction of sexual reproduction and diapause in a Patagonian rotifer

1. This study investigates the basis and ecological significance of the extremely high propensity for mixis (sexual reproduction) observed in laboratory populations of Brachionus calyciflorus from a temporary pond in Patagonia. 2. Experiments with stem females hatched from resting eggs showed that these females were exclusively amictic but produced mictic daughters even at very low population densities. In six experiments, newly hatched stem females were cultured individually in different volumes (1.5, 12, 40 and 150 mL). The percentage of mictic daughters (mixis ratio) was high in the smaller volumes (e.g. 44–83% in 1.5 mL) and lower in the largest volume (6–21% in 150 mL). A regression analysis combining the data from these experiments showed a highly significant decrease in mixis ratio with volume and indicated that the lowest population density at which mixis still occurs (mixis threshold) would be 3.4 females L^?1 (95% CL 2.9–4.0 females L^?1). This value is considerably lower than mixis thresholds for other rotifers (25–250 females L^?1 for many species and 9000–477 000 females L^?1 for some Spanish Brachionus plicatilis). 3. In three additional experiments, stem females and their amictic daughters were cultured individually in 150 mL. The percentage of mictic daughters produced by these two generations of females was not significantly different, showing that the mixis response to crowding is not inhibited in the stem‐female generation. 4. Laboratory experiments showed that two common predators of the Patagonian B. calyciflorus (the calanoid copepod Parabroteas sarsi and the backswimmer Notonecta vereertbruggheni) each ate 60–70 B. calyciflorus predator^?1 per day and cleared all rotifers from c. 250 mL per day. Thus, a very low mixis threshold and high maximal mixis ratio may ensure production of some resting eggs soon after colonisation of the pond and before complete removal from the plankton. 5. Two laboratory experiments showed that resting eggs of the Patagonian B. calyciflorus hatched at variable rates (28 and 81%) after a brief diapause when kept in the conditions under which they were produced and oviposited (20–21 °C; L: D 16: 8). Early hatching of resting eggs from pond sediment may allow multiple periods of colonisation and resting egg production in a season. This may offset the fitness cost of limited population growth through female parthenogenesis in the face of unpredictable and abrupt risk of extinction because of predators.     

Sexual reproduction and diapause of Hexarthra sp. (Rotifera) in short-lived ponds in the Chihuahuan Desert

1. In the life cycle of monogonont rotifers it is generally assumed that diapausing eggs invariably hatch into amictic stem females which produce female offspring parthenogenetically. Diapausing eggs are only produced by later generations after sexual reproduction has been induced by environmental cues. 2. We show that populations of an undescribed Hexarthra species inhabiting small temporary ponds in the Chihuahuan Desert deviate from this life cycle pattern. These ponds may dry within days and up to 85% of females were mictic. Females producing male offspring and diapausing eggs were observed 1 or 2 days, respectively, after ponds had filled with water. 3. Under laboratory conditions, 7–46% of females hatching from re‐hydrated sediments were sexual. Male offspring of these females can fertilise other mictic stem females leading to diapausing egg formation. In laboratory experiments, females produced fully developed diapausing eggs within 1.9 days at 20 °C and 1.2 days at 30 °C. 4. In addition, embryonic development time (1.1–0.3 days at temperatures between 12 and 30 °C) and juvenile period (2.1–0.5 days for the same temperature range) are shorter than those of other rotifer species. In short‐lived habitats, the potential for rapid population development and production of new diapausing eggs may be crucial in the long‐term survival of populations.