Seasonal trends in body size of adult male mosquitofish,Gambusia affinis,with evidence for their social control

Synopsis Collections from a natural population of mosquitofish,Gambusia affinis, in southern Indiana showed that males born early in the breeding season mature rapidly at a small body size, while males born later in the breeding season delay maturation and achieve larger body sizes. A field experiment, involving removal of males from pond populations, was conducted to test the hypothesis that delayed maturation by late summer males is, at least in part, under social control. Mean total length (TL) of adult and maturing males in late summer in ponds from which large numbers of males had previously been removed was significantly lower than mean TL of males in control ponds. These data support the hypothesis of social control and support Sohn's competition-predation hypothesis for control of adult male body size inGambusia.     

Muscle esterases of the mosquitofish, Gambusia affinis.

1. Muscle esterases of Gambusia affinis were separable into 14 electrophoretic bands that formed five groups according to differential physical and chemical properties. 2. Nine phenotypic patterns were observed among the populations sampled, based on variation within the EST-3 group of carboxylesterases. 3. A model for the molecular and genetic bases of the EST-3 polymorphism was proposed on the basis of phenotypic patterns and physicochemical characterization. 4. The isozymes of EST-3 are inferred to be dimeric molecules on the basis of electrophoretic patterns observed following incubation of the EST-3 enzymes with neuraminidase. 5. The binding of sialic acid is believed to be an epigenetic mechanism which operates to maintain the structural integrity of these enzymes.     

Effects of 17alpha-ethynylestradiol on sexual development of male western mosquitofish (Gambusia affinis)

Juvenile male western mosquitofish (Gambusia affinis) were exposed to different concentrations of 17alpha-ethynyl estradiol (EE2) in the diet during the period of sexual maturation. A clear inhibiting influence of EE2 on sexual development was apparent. The proportion of males in each treatment group that failed to complete gonopodial development during the 150-day observation period increased significantly with EE2 concentration. There were significant nonlinear trends toward shorter gonopodia in groups exposed to higher EE2 concentrations. Vitellogenin (VTG) was detectable in the blood of all fish exposed to 1.0 or more micro/g food and the concentration increased dramatically with increasing EE2 exposure. A significant negative association was seen between EE2 concentration and spermatophore counts. This study has demonstrated deleterious effects of EE2 exposure on sexual maturation and several indirect measures of reproductive fitness. It supports the biological relevance of vitellogenin in the blood and reduced gonopodium length as biomarkers for estrogen exposure and endocrine disruption in mosquitofish.     

Population development of the mosquitofish,Gambusia affinis,in rice fields

Synopsis Development of mosquitofish,Gambusia affinis, stocks in rice fields following stocking for mosquito control is poorly understood and highly variable. To characterize population development and explain observed variability, size distributions and total numbers of mosquitofish stocks were followed from stocking through the end of the summer rice season in several experimental rice paddies. Instead of the highly variable, logistic growth implied by previous studies in which only adult fish were sampled, we observed a consistent development of size structure between years, with some variability in specific demographic processes. Population development consists of: (1) an initial peak in numbers due to a pulse of recruitment, (2) a period of low recruitment and constant or declining numbers, and (3) a second peak caused by a second pulse of recruitment. Timing of the initial peak appears to depend on rice height. Differences in the rate of decline following the peak are apparently due to different mortality rates. The second peak in reproduction is probably due to a second parturition by stocked females, possibly followed by first reproduction by young born earlier. Since population development in rice fields following stocking in the spring is similar to development of natural populations in temperate regions in which there is no reproduction during the winter, results obtained here are relevant to natural as well as artificially stocked systems.     

Impacts of mosquitofish (Gambusia affinis) predation on plankton communities

An investigation of the effects of mosquitofish (Gambusia affinis) predation was conducted in 12 experimental ponds in southern California over a period of 10 months.Gambusia essentially eliminatedDaphnia pulex andCeriodaphnia sp. populations, reducedDiaptomus pallidus andKeratella quadrata populations, had little impact onCyclops vernalis, and caused large increases inK. cochlearis, Polyarthra sp.,Synchaeta sp., andTrichocerca spp. populations and in total phytoplankton.Gambusia caused a decrease in the PIE (probability of interspecific encounter) of the planktonic crustaceans and an increase in the PIE of the planktonic rotifers. Hemiptera, such as neustonicMicrovelia sp. and nektonicBuenoa sp. andNotonecta sp., andHyla regilla tadpoles were absent from fish ponds but sometimes abundant in control ponds.Gambusia caused higher pH and oxygen levels, presumably via its effect on the phytoplankton. The impact ofGambusia on the pond ecosystems was less in winter, when fish numbers and feeding rates were low, than in summer. Results of other fish-plankton studies are summarized in tabular form. A model is proposed to account for variation in the calanoid/cyclopoid ratio; evidence is summarized suggesting that in general calanoids are more susceptible to predation by predaceous zooplankters while cyclopoids are more susceptible to fish predation. Some parallels are drawn between the effects ofGambusia predation and those of insecticide treatments.     

Size at maturity in male Gambusia heterochir

The size at maturity in many male poeciliid fish is believed to be controlled by 'social control mechanism', and post-maturation growth is insignificant. In the Clear Creek gambusia, Gambusia heterochir . post-maturation growth in male fish is limited. Male G. heterochir matures at a fixed size, and age of maturation is dependent on environmental factors (e.g. temperature). In the presence of an adult male, the maturation size of a young male is largely determined by genetic factors rather than 'social control' factors. A male can switch its status as a 'dominant' or 'subordinate' male, depending on the relative size of rival males in mating competition. Mating success of a large (dominant) male reduces as the number of competing small (subordinate) males increases. These unique reproductive tactics may explain in part why 'social control mechanism' does not play a key role in determining the maturation size of male G. heterochir .     

The effects of induced masculinization on reproductive and aggressive behaviors of the female mosquitofish,Gambusia affinis affinis

Synopsis FemaleGambusia affinis affinis were masculinized with the degraded products of 65% stigmastanol-30% B-sitosterol, a phytosterol. The masculinized females were paired with non-treated males, non-treated females, and other masculinized females. The pairs were analyzed for reproductive and aggressive behaviors exhibited. The behavioral patterns of these pairs were compared statistically to the behavior patterns of the following non-treated pairs: male with male, female with female, and male with female. The masculinized females behaved like males in that they followed, swung, and thrust their gonopodia at non-treated females and larger masculinized females. However, the male-like behavior of the masculinized females was not as intense as that of normal males. Also, the effect of masculinization on the behavior of the treated female was context-dependent as seen when placed with a male or a smaller masculinized female. Under these conditions the masculinized females behaved like typical females and exhibited no masculinized behaviors. The effects of anal spots and size differences are discussed as possible explanations for the variability in behaviors exhibited. The masculinized females displayed no change in aggressive behaviors.     

Geographical and temporal variation of multiple paternity in invasive mosquitofish (Gambusia holbrooki,Gambusia affinis)

Multiple paternity (MP) increases offspring's genetic variability, which could be linked to invasive species' evolvability in novel distribution ranges. Shifts in MP can be adaptive, with greater MP in harsher/colder environments or towards the end of the reproductive season, but climate could also affect MP indirectly via its effect on reproductive life histories. We tested these hypotheses by genotyping N = 2,903 offspring from N = 306 broods of two closely related livebearing fishes, Gambusia holbrooki and Gambusia affinis. We sampled pregnant females across latitudinal gradients in their invasive ranges in Europe and China, and found more sires per brood and a greater reproductive skew towards northern sampling sites. Moreover, examining monthly sampling from two G. affinis populations, we found MP rates to vary across the reproductive season in a northern Chinese, but not in a southern Chinese population. While our results confirm an increase of MP in harsher/more unpredictable environments, path analysis indicated that, in both cases, the effects of climate are likely to be indirect, mediated by altered life histories. In both species, which rank amongst the 100 most invasive species worldwide, higher MP at the northern edge of their distribution probably increases their invasive potential and favours range expansions, especially in light of the predicted temperature increases due to global climate changes.     

Putative chemical inhibition of development by conspecifics in mosquitofish, Gambusia affinis

The objective of this study was to test for a chemical influence of adult Gambusia affinis on the development and maturation of juveniles. Sixty juveniles (<1 day old) were reared (from a total of 157 trials) individually up to 115d in 3liter containers that circulated water bathing either (1) two adult males, (2) two adult females, (3) an adult male and an adult female, or (4) conspecific-free tank water. Randomly chosen juveniles were sampled over time, sacrificed, sectioned, and measurements were taken of standard length and gonadal parameters such as gonad size, number of spermatogenic cysts, or number of oocytes. Measurements were regressed as functions of time, and analyzed for significant differences by ANCOVA. A total of 32 males and 28 females were examined. Males showed no significant differences in any of the parameters, but females showed reduced growth rate and smaller ovary size when reared in water bathing two adult females. Mortality rate in these young fish was high, but Chi-square analysis showed that mortality rate was significantly higher when they were raised in water bathing at least one adult female. Females are known to influence female development in other vertebrates; our findings suggest this may also be true for a fish, and that the interaction may be through chemical communication.     

Environmental regulation of annual reproductive cycle in the mosquitofish, Gambusia affinis

To clarify the environmental factors regulating the annual reproductive cycle of the female mosquitofish, Gambusia affinis, a viviparous teleost, histological changes of the ovary in natural population, and laboratory experiments were examined. The results, extending over two years, suggested that ovarian recrudescence is initiated by the rise in temperature during spring and that ovarian regression is caused by the shorter daylength during late summer. The first rearing experiments using four photoperiod-temperature groups to investigate the factors triggering the onset of reproduction revealed that females with regressing ovaries began reproduction with the rise of temperature regardless of the photoperiod during spring. The results of the second experiment using three different temperature groups indicated that vitellogenesis occurred at over 14 degrees C and pregnancy at over 18 degrees C. The third experiment with four photoperiod-temperature groups was arranged to investigate the factors in the cessation of reproduction. Sexually active females ceased vitellogenesis of the next clutch of oocytes due to the shorter daylength regardless of temperatures during late summer; however, temperature seemed to influence the rate of embryo development. The critical photoperiod is estimated at about 12.5 hr. In nature, it is supposed that vitellogenesis starts when the temperature rises to about 14 degrees C, and final maturation of oocytes occurs when the temperature reaches about 18 degrees C during spring. Then, vitellogenesis of the next clutch of oocytes ceases when the daylength becomes shorter than 12.5 hr during late summer; the last gestation proceeds at a rate dependent on the temperature, and finally reproduction ends by the last parturition. J. Exp. Zool. 286:204-211, 2000.     

Allometric relationship between oxygen consumption and body weight of mosquitofish,Gambusia affinis

Synopsis The allometric relationship between body size and oxygen consumption of Gambusia affinis at 28° C was determined under controlled experimental conditions, using a manometric respirometer. The allometric exponent (b-value) was 0.64 ± 0.02 S.E. (n = 51). Oxygen consumption was not influenced by any time-related factors during the 17 days of measurements. Variance between replicated oxygen consumption trials on individual fish was negligible. Specific oxygen consumption rates of several G. affinis at 28° C that were determined by using a sealed vessel and O₂ electrode respirometry method were similar to the rates measured by the manometric respirometry method in similar-sized G. affinis.     

Sexual selection for small size in male mosquitofish (Gambusia holbrooki)

In many poeciliid fishes, large males which court females coexist with small males which sneak-copulate. It is unclear whether these two tactics represent two evolutionarily stable strategies or if sneaking is a conditional strategy adopted by small, unattractive males. We studied the success of sneaky copulation by looking for sperm in the gonoduct of females after they were kept for 48 h with a male. A logistic regression analysis showed that the probability of a female being inseminated increased with female length and decreased with male length. The length of the male relative to that of the female was the best predictor of success. This result was confirmed using virgin females, thereby excluding any possible confounding effect due to the release of sperm from previous copulations. Sperm counts suggested that large males do not compensate for their reduced copulatory success by releasing larger sperm numbers. Behavioural data indicate that the advantages to small males are twofold: they have a greater chance to approach females from behind without being detected, and manoeuvre better when inserting the gonopodium into the female''s gonoduct. The selective advantage of small size might explain male dwarfism in poeciliids. Our results also suggest that small males adopting the sneaky tactic may be as successful as large males adopting courtship, and that alternative mating strategies may be maintained by negative density-dependent selection.     

Sex differentiation and pubertal development of gonads in the viviparous mosquitofish, Gambusia affinis

The ontogenetic development of gonads from embryo to adult was observed histologically in the viviparous teleost, Gambusia affinis. Primordial germ cells (PGCs) appeared in the subendodermal space of the embryo 14 days before birth, and then transferred to the dorsal mesentery to form paired genital ridges 12 days before birth. The PGCs proliferated in the genital ridge, forming gonadal primordia 10 days before birth. All gonadal primordia differentiated to the ovary containing oocytes 2 days before birth, but then redifferentiated to the ovary and testis just after birth. This indicates that the mosquitofish is a juvenile hermaphroditic species. The characteristics of gonadal sex differentiation just after birth were enlargement of the oocytes in females, and invasion of somatic cells from the hilar region to an inner portion of the gonad in males. The paired ovary fused at the basal area 5 days after birth, then on the ventral and dorsal portions, developing into a single ovary 10 days after birth. During this time a single ovarian cavity was formed on the dorsal portion of the ovary. The paired testes fused only at the basal area and became a single testis having two main lobes 10 days after birth. The oocytes gradually developed and began vitellogenesis 100 days after birth, but did not reach maturation until 110 days after birth. Spermatogenic cells formed cysts at 20 days, began meiosis at 70 days, and matured to form sperm balls 90 days after birth. The male fish sexually matured earlier than the female.     

Domperidone treatment attenuates stress-induced suppression of reproduction in viviparous mosquitofish Gambusia affinis

The aim of this study was to determine the effect of stress on reproduction and the possible involvement of dopaminergic systems in the reproductive stress response in the mosquitofish Gambusia affinis. Exposure of fish to aquaculture stressors (four 10 min episodes of stress, each corresponding to a different stressor such as handling, chasing, frequent netting and low water levels), for a period of 30 days caused reduction in the mean numbers of stage I–IV follicles associated with lower number of pregnant females and embryos in most of the developmental stages compared with experimental controls. Besides, increase in the intensity of labelling and the per cent area of tyrosine hydroxylase (TH; a rate-limiting enzyme in the biosynthetic pathway of catecholamines)- immunoreactive (ir) neurons was observed in the preoptic area (POA) and the nucleus preopticus (NPO) regions of the brain concomitant with reduction in the labelling of gonadotropin releasing hormone–immunoreactive (GnRH-ir) fibres in the proximal pars distalis (PPD) of the pituitary gland in stressed fish compared with experimental controls. Treatment of domperidone (DOM) caused an increase in the number of stage II and V follicles and promoted pregnancy rate concomitant with an increase in the number of embryos at various developmental stages compared with those of experimental controls. Similar treatment to stressed fish caused an increase in the number of stages I–V follicles compared with those in stress alone group. The GnRH fibres showed increased immunolabelling in stress + DOM treated fish compared with stress alone fish. On the other hand, TH-immunoreactivity in the POA and the NPO regions was reduced in stress + DOM treated fish compared with stress-alone group. These results suggest that stress inhibits follicular development and subsequent hatching success through the suppression of GnRH and that the inhibition appears to be mediated through dopamine, for the first time in a viviparous fish.     

Maternal effects on offspring size and number in mosquitofish,Gambusia holbrooki

Given a trade-off between offspring size and number, all mothers are predicted to produce the same optimal-sized offspring in a given environment. In many species, however, larger and/or older mothers produce bigger offspring. There are several hypotheses to explain this but they lack strong empirical support. In organisms with indeterminate growth, there is the additional problem that maternal size and age are positively correlated, so what are their relative roles in determining offspring size? To investigate this, we measured the natural relationship between maternal and offspring size in a wild population of Gambusia holbrooki (eastern mosquitofish), and experimentally disentangled the effects of maternal age and size on offspring size in the laboratory. In combination, our data indicate that the relationship between maternal and offspring size is nonlinear. Small mothers seem to produce larger than average offspring due to integer effects associated with very small broods. For extremely large mothers, which were only sampled in our wild data, these larger than average offspring may result from greater maternal resources or age effects. However, maternal age had no effect on offspring size or number in the laboratory experiment. Our results highlight the importance of sampling the full size–range of mothers when investigating maternal effects on offspring size. They also point to the difficulty of experimentally manipulating maternal size, because any change in size is invariably associated with a change in at least one factor affecting growth (be it temperature, food availability, or density) that might also have an indirect effect on offspring size.     

Gonopodial morphogenesis in female mosquitofish,Gambusia affinis affinis,masculinized by exposure to degradation products from plant sterols

Synopsis Female mosquitofish,Gambusia affinis affinis, were masculinized by exposure to degradation products (presumably steroids) of the plant sterol, stigmastanol. Masculinization was indicated by the induction of a male-like gonopodium in each specimen. The morphogenetic stages of gonopodial development are discussed as are the anatomical specializations produced in the mature gonopodial tip.     

Ejaculate traits in relation to male body size in the eastern mosquitofish Gambusia holbrooki

In the present study, the correlation between sperm number, sperm quality (speed, viability, longevity and length), sperm bundles quality (size and dissolving rate) and male body size has been tested in the eastern mosquitofish Gambusia holbrooki a poecilid species characterized by coercive mating tactics where males do not possess obvious ornaments, and the body size is the key determinant of pre‐copulatory male mating success. The results do not tally with theoretical predictions. Indeed, no correlation between male body size and either sperm or sperm‐bundle traits has been found, evidencing the lack of the theoretically expected trade‐off between the investment in characters involved in mate acquisition and the investment in ejaculate quality. An explanation for the observed pattern comes from the extremely dynamic mating system of G. holbrooki, characterized by variable size‐related male mating success and strong post‐copulatory selective pressure, with all males facing a similar high level of sperm competition. In this situation, a higher investment in growth and maintenance at the expense of ejaculate quality is not expected. These results underscore the necessity to comprehend detailed information on species’ reproductive biology and reproductive environment to understand both the evolution of ejaculate characteristics and possible deviations from theoretical predictions.     

The influence of persistent individual differences and age at maturity on effective population size

Ratios of effective populations size, N(e), to census population size, N, are used as a measure of genetic drift in populations. Several life-history parameters have been shown to affect these ratios, including mating system and age at sexual maturation. Using a stochastic matrix model, we examine how different levels of persistent individual differences in mating success among males may affect N(e)/N, and how this relates to generation time. Individual differences of this type are shown to cause a lower N(e)/N ratio than would be expected when mating is independent among seasons. Examining the way in which age at maturity affects N(e)/N, we find that both the direction and magnitude of the effect depends on the survival rate of juveniles in the population. In particular, when maturation is delayed, lowered juvenile survival causes higher levels of genetic drift. In addition, predicted shifts in N(e)/N with changing age at maturity are shown to be dependent on which of the commonly used definitions of census population size, N, is employed. Our results demonstrate that patterns of mating success, as well as juvenile survival probabilities, have substantial effects on rates of genetic drift.