Poeciliid Fish Research Articles

The effects of competitor density on aggressive behaviour and resource defence in a Poeciliid fish

Summary To test the hypothesis that competitor density and time spent at a food resource influences aggressive behaviour in male swordtail, Xiphophorus sp., fish were kept at two densities (low: 18 fish m−3; high: 54 fish m−3) and aggressive behaviour was recorded with the time at the food resource used as a covariate. Competitor density is likely to affect the cost:benefit ratio of food defence. When density is low, there should be sufficient food for all individuals and intra-specific interactions are expected to be rare, while at high densities, increased intra-specific encounter rates mean that individuals may spend more time defending a resource than utilising it. Food resource defence should occur at intermediate densities. The frequency of aggression, i.e. bites, chases, and display behaviour was significantly positively influenced by time spent at the food resource (Bites: F1,31 = 8.186, P = 0.007; Chases: F1,31 = 6.439, P = 0.016; Displays: F1,31 = 4.435, P = 0.043) suggesting that food resource defence occurred. Competitor density had no effect on food resource defence and minimal effect on the frequency of aggression with only one type of aggressive behaviour, male–male displays, showing a difference between densities (F1,31 = 6.975, P = 0.013). This finding is suggested to be a result of the formation of dominance hierarchies in this species. Aggression from the dominant individual may be directed at only subordinates of the next dominance rank and subordinate behaviour may be restricted by status rather than immediate threat. In such a situation, aggression may be independent of competitor density.

Relation between Morphological Antisymmetry and Behavioral Laterality in a Poeciliid Fish

Many animals show lateral bias in various behaviors. In fish, behavioral laterality has been studied from the perspective of its relation with brain lateralization. On the other hands, in some fishes, head dimorphism (righty or lefty) corresponds to behavioral laterality, such as foraging behavior. To examine the correlation between morphological asymmetry and behavioral laterality associated with brain lateralization, we conducted two behavioral tests (the detour test and fast-start test) using a poeciliid fish, Girardinus metallicus. In both behavioral tests, some individuals tended to move rightward, while others tended to move leftward, in a manner associated with head incline. In the detour-test, righty individuals primarily detoured leftward, whereas lefty individuals primarily detoured rightward. In the fast-start tests, the reverse tendency was seen; righty individuals tended to escape rightward, whereas lefty individuals tended to escape leftward. Such results indicate that brain lateralization may be also associated with morphological asymmetry.

Non‐Visual Mate Choice Ability in a Cavefish (Poecilia mexicana) is not Mechanosensory

AbstractOne of the most important decisions any female has to make is selecting a mate. In many animals, this involves a strong visual component, but in cave‐dwelling fishes, mate preferences have to be exercised in darkness. In a cave‐dwelling Poeciliid fish, the cave molly (Poecilia mexicana), visual mate choice is still possible because they have functional, albeit reduced eyes. In a previous laboratory study, it was documented that females prefer larger males in light and in darkness. In the present study, we investigated the role of the mechanosensory lateral line in this mate preference. We temporarily knocked out the lateral line using antibiotics. We found that even in the functional absence of the lateral line, females showed a clear preference for larger males, indicating that the lateral line is not crucial for mate choice. This points toward a strong role of chemical communication in cave molly mate choice.

Can mixed-species groups reduce individual parasite load? A field test with two closely related poeciliid fishes (Poecilia reticulata and Poecilia picta).

Predation and parasitism are two of the most important sources of mortality in nature. By forming groups, individuals can gain protection against predators but may increase their risk of being infected with contagious parasites. Animals might resolve this conflict by forming mixed-species groups thereby reducing the costs associated with parasites through a relative decrease in available hosts. We tested this hypothesis in a system with two closely related poeciliid fishes (Poecilia reticulata and Poecilia picta) and their host-specific monogenean ectoparasites (Gyrodactylus spp.) in Trinidad. Fish from three different rivers were sampled from single and mixed-species groups, measured and scanned for Gyrodactylus. The presence and abundance of Gyrodactylus were lower when fish of both species were part of mixed-species groups relative to single-species groups. This is consistent with the hypothesis that mixed-species groups provide a level of protection against contagious parasites. We discuss the importance of potentially confounding factors such as salinity and individual fish size.

Incongruence between the sexes in preferences for body and dorsal fin size in Xiphophorus variatus

Female preference for male fin enhancements in poeciliid fishes may be driven by a preexisting perceptual bias for increased male lateral projection area (LPA). This hypothesis suggests that a male with enlarged body and/or fin size projects a larger image onto the female's retina at a given viewing distance, eliciting a greater sensory and thus behavioral response out of the female than a smaller male. Given the shared sensory/neural systems of opposite sex conspecifics, we might expect the LPA bias to also be present in males of at least some poeciliid species. However, we need not expect congruence between the sexes in the state of the bias over evolutionary time. To examine whether the sexes share a bias for sailfin-like dorsal fins, a trait not present in their evolutionary history, the bias favoring increased dorsal fin size and LPA observed in female Xiphophorus variatus, among other poeciliids, was investigated by testing male preference for dummy females varying in dorsal fin size, body size, and dorsal fin:body size ratio. In three sets of simultaneous choice experiments, males preferred females of larger body size when fin size was held constant and when total LPA was held constant, but showed no preference for larger fins when body size was held constant. The LPA bias is therefore less permissive in males than females with selection favoring a male's ability to discriminate between female body size – an indicator of fertility/fecundity – and fin size, which offers no known fitness benefits.

Ecology and Evolution of Poeciliid Fishes

Ecology and Evolution of Poeciliid Fishes

Physiological adaptation along environmental gradients and replicated hybrid zone structure in swordtails (Teleostei:Xiphophorus)

Local adaptation is often invoked to explain hybrid zone structure, but empirical evidence of this is generally rare. Hybrid zones between two poeciliid fishes, Xiphophorus birchmanni and X.malinche, occur in multiple tributaries with independent replication of upstream-to-downstream gradients in morphology and allele frequencies. Ecological niche modelling revealed that temperature is a central predictive factor in the spatial distribution of pure parental species and their hybrids and explains spatial and temporal variation in the frequency of neutral genetic markers in hybrid populations. Among populations of parentals and hybrids, both thermal tolerance and heat-shock protein expression vary strongly, indicating that spatial and temporal structure is likely driven by adaptation to local thermal environments. Therefore, hybrid zone structure is strongly influenced by interspecific differences in physiological mechanisms for coping with the thermal environment.

Mechanisms of Speciation

Mechanisms of Speciation

Ecology and Evolution of Poeciliid Fishes

Ecology and Evolution of Poeciliid Fishes

Sex cells in changing environments: can organisms adjust the physiological function of gametes to different temperatures?

AbstractOver the past decade, hundreds of studies have examined the abilities of whole organisms to modify their physiology and behaviour in response to environmental temperature changes; despite this, virtually nothing is known about the ability of sex cells to adjust to different temperature conditions. In fact, a recent meta‐analysis based on studies of 309 species and 112 physiological and ecological traits found no studies examining the influence of temperature on gamete function. Because sex cells play a critical role in the adaptation and persistence of species, this represents a severe oversight in physiological studies of thermal adaptation. Our study examines whether sex cells can respond phenotypically to variation in the thermal environment that is experienced by the whole‐organism. Specifically, we studied the thermal dependence of sperm swimming and the critical thermal limits of sperm cells in males of the poeciliid fish, Gambusia holbrooki. This species is well known for its ability to modify physiological function and maintains burst and sustained swimming performance and mating ability across a wide range of thermal conditions. In contrast, we found that sperm cells from male G. holbrooki did not adjust their physiological function as predicted by adaptive models. After acclimation of adult males to cool or warm temperatures, we found that the critical thermal limits of sperm function remained unchanged, as did the effect of temperature on sperm swimming performance. However, warm‐acclimated fish had sperm with higher swimming speeds across all temperatures. The absence of phenotypic changes in the critical thermal limits of sperm or thermal dependence of sperm swimming performance is surprising given that whole‐organism traits in G. holbrooki generally show improved performance after exposure to novel environments. As such an inability to thermally adjust gamete function may be widespread among other organisms, we urge biologists to investigate the generality of this result.

Correlated behaviour and stress physiology in fish exposed to different levels of predation pressure

1. Natural selection can generate correlated suites of phenotypic traits by acting independently on physiological and behavioural characters or on mechanisms that exert pleiotropic actions. 2. Current theory, supported by artificial selection studies, suggests that physiological and behavioural stress responses are at least partially under genetic control and covary in a predictable manner. Indeed, physiological mechanisms such as hormonal stress responsiveness may underlie variation in behaviour, including consistent behaviours described as temperament or personality, with bolder, more exploratory and active individuals being less hormonally responsive to stressors. 3. This relationship, however, has yet to be demonstrated in natural populations. We investigated the relationship between hormonal and behavioural stress responsiveness in multiple natural populations of a tropical freshwater poeciliid fish, Brachyrhaphis episcopi, that experience different levels of predation pressure and hence encounter different rates of stressful events. Predation can impose a strong selection pressure, and living with a high risk of predation is known to select for specific behavioural traits. 4. We quantified variation in stress responsiveness via cortisol release rates (exp. 1) and behaviour in an open field test followed by cortisol release rates (exp. 2). Populations exposed to high levels of predation were consistently more exploratory and active and had lower release rates of cortisol in response to a stressor than conspecifics sampled at sites with few predators. 5. However, this difference in stress responsiveness was only apparent after fish had experienced the mild stress of behaviour testing (in exp. 2), which resulted in elevation of cortisol levels. The relationship between hormone release and behaviour was also not apparent within populations once independent factors were controlled for, highlighting the importance of factors such as size and sex on individual variability. 6. This study demonstrates that the relationship between hormonal and behavioural stress responsiveness can result from natural selection pressures, such as that imposed by predation.

A comparison of two methods to assess audience-induced changes in male mate choice

Abstract Multidirectional communicative interactions in social networks can have a profound effect on mate choice behavior. Male Atlantic molly Poecilia mexicana exhibit weaker mating preferences when an audience male is presented. This could be a male strategy to reduce sperm competition risk: interacting more equally with different females may be advantageous because rivals might copy mate choice decisions. In line with this hypothesis, a previous study found males to show a strong audience effect when being observed while exercising mate choice, but not when the rival was presented only before the choice tests. Audience effects on mate choice decisions have been quantified in poeciliid fishes using association preference designs, but it remains unknown if patterns found from measuring association times translate into actual mating behavior. Thus, we created five audience treatments simulating different forms of perceived sperm competition risk and determined focal males’ mating preferences by scoring pre-mating (nipping) and mating behavior (gonopodial thrusting). Nipping did not reflect the pattern that was found when association preferences were measured, while a very similar pattern was uncovered in thrusting behavior. The strongest response was observed when the audience could eavesdrop on the focal male’s behavior. A reduction in the strength of focal males’ preferences was also seen after the rival male had an opportunity to mate with the focal male’s preferred mate. In comparison, the reduction of mating preferences in response to an audience was greater when measuring association times than actual mating behavior. While measuring direct sexual interactions between the focal male and both stimulus females not only the male’s motivational state is reflected but also females’ behavior such as avoidance of male sexual harassment.

Ecology and evolution of poeciliid fishes

Ecology and evolution of poeciliid fishes

Monophyly of Heterandriini (Teleostei: Poeciliidae) revisited: a critical review of the data

The systematics and taxonomy of poeciliid fishes (guppies and allies) remain poorly understood despite the relative importance of these species as model systems in the biological sciences. This study focuses on testing the monophyly of the nominal poeciliine tribe Heterandriini and the genus Heterandria, through examination of the morphological characters on which the current classification is based. These characters include aspects of body shape (morphometrics), scale and fin-ray counts (meristics), pigmentation, the cephalic laterosensory system, and osteological features of the neurocranium, oral jaws and suspensorium, branchial basket, pectoral girdle, and the gonopodium and its supports. A Maximum Parsimony analysis was conducted of 150 characters coded for 56 poeciliid and outgroup species, including 22 of 45 heterandriin species (from the accounted in Parenti & Rauchenberger, 1989), or seven of nine heterandriin species (from the accounted in Lucinda & Reis, 2005). Multistate characters were analyzed as both unordered and ordered, and iterative a posteriori weighting was used to improve tree resolution. Tree topologies obtained from these analyses support the monophyly of the Middle American species of "Heterandria," which based on available phylogenetic information, are herein reassigned to the genus Pseudoxiphophorus. None of the characters used in previous studies to characterize the nominal taxon Heterandriini are found to be unambiguously diagnostic. Some of these characters are shared with species in other poeciliid tribes, and others are reversed within the Heterandriini. These results support the hypothesis that Pseudoxiphophorus is monophyletic, and that this clade is not the closest relative of H. formosa (the type species) from southeastern North America. Available morphological data are not sufficient to assess the phylogenetic relationships of H. formosa with respect to other members of the Heterandriini. The results further suggest that most tribe-level taxa of the Poeciliinae are not monophyletic, and that further work remains to resolve the evolutionary relationships of this group.

ECOLOGY AND EVOLUTION OF POECILIID FISHES ‐ Edited by J. P. Evans, A. Pilastro and I. Schlupp

ECOLOGY AND EVOLUTION OF POECILIID FISHES ‐ Edited by J. P. Evans, A. Pilastro and I. Schlupp

The mate choice brain: comparing gene profiles between female choice and male coercive poeciliids

Genes that mediate mate preferences potentially play a key role in promoting and maintaining biological diversity. In this study, we compare mate preference behavior in two related poeciliid fishes with contrasting behavioral phenotypes and relate these behavioral differences to gene profiles in the brain. Results reveal that one poeciliid fish, the Northern swordtail, exhibits robust mate preference as compared to the Western mosquitofish, which utilizes a coercive mating system. Female swordtails display no significant difference in association time between male- and female-exposure trials, whereas female mosquitofish spend significantly less time associating with males relative to females. Furthermore, the preference strength for large males is significantly lower in female mosquitofish relative to swordtails. We then examine expression of three candidate genes previously shown to be associated with mate preference behavior in female swordtails and linked to neural plasticity in other vertebrates: neuroserpin (NS), neuroligin-3 (NLG-3) and N-methyl-d-aspartate receptor (NMDA-R). Whole brain gene expression patterns reveal that two genes (NS and NLG-3) are positively associated with mate preference behavior in female swordtails, a pattern opposing that of the mosquitofish. In mosquitofish females, these genes are downregulated when females express biases toward males yet are elevated in association with total motor activity patterns under asocial conditions, suggesting that the presence of males in mosquitofish species may inhibit expression of these genes. Both gene expression and female behavioral responses to males exhibit opposing patterns between these species, suggesting that this genetic pathway may potentially act as a substrate for the evolution of mate preference behavior.

Dietary niche overlap in sympatric asexual and sexual livebearing fishes Poecilia spp.

The present study investigated the spatiotemporal patterns in trophic resource use in a system of a gynogenetic poeciliid fish, the Amazon molly Poecilia formosa, and its sexual congeners the sailfin molly Poecilia latipinna and the Atlantic molly Poecilia mexicana using gut contents analysis. No statistically significant differences in trophic resource use were found between sexual and gynogenetic species, but gut contents varied significantly across sites and over time. In addition, variation in trophic morphology (i.e. gut length) was significant across sites but not species, and laboratory experiments indicated that gut length is phenotypically plastic. Overall, trophic differentiation between coexisting asexual and sexual Poecilia appears to be minimal, and it is unlikely that niche differentiation contributes to a stable coexistence of the two reproductive forms.

Divergent evolution of male aggressive behaviour: another reproductive isolation barrier in extremophile poeciliid fishes?

Reproductive isolation among locally adapted populations may arise when immigrants from foreign habitats are selected against via natural or (inter-)sexual selection (female mate choice). We asked whether also intrasexual selection through male-male competition could promote reproductive isolation among populations of poeciliid fishes that are locally adapted to extreme environmental conditions [i.e., darkness in caves and/or toxic hydrogen sulphide (H2S)]. We found strongly reduced aggressiveness in extremophile P. oecilia mexicana, and darkness was the best predictor for the evolutionary reduction of aggressiveness, especially when combined with presence of H2S. We demonstrate that reduced aggression directly translates into migrant males being inferior when paired with males from non-sulphidic surface habitats. By contrast, the phylogenetically old sulphur endemic P. sulphuraria from another sulphide spring area showed no overall reduced aggressiveness, possibly indicating evolved mechanisms to better cope with H2S.

Assessment of Two Nonnative Poeciliid Fishes for Monitoring Selenium Exposure in the Endangered Desert Pupfish

We assessed the suitability of two nonnative poeciliid fishes—western mosquitofish (Gambusia affinis) and sailfin mollies (Poecilia latipinna)—for monitoring selenium exposure in desert pupfish (Cyprinodon macularius). Our investigation was prompted by a need to avoid lethal take of an endangered species (pupfish) when sampling fish for chemical analysis. Total selenium (SeTot) concentrations in both poeciliids were highly correlated with SeTot concentrations in pupfish. However, mean SeTot concentrations varied among fish species, with higher concentrations measured in mosquitofish than in mollies and pupfish from one of three sampled agricultural drains. Moreover, regression equations describing the relationship of selenomethionine to SeTot differed between mosquitofish and pupfish, but not between mollies and pupfish. Because selenium accumulates in animals primarily through dietary exposure, we examined fish trophic relationships by measuring stable isotopes (δ 13C and δ 15N) and gut contents. According to δ 13C measurements, the trophic pathway leading to mosquitofish was more carbon-depleted than trophic pathways leading to mollies and pupfish, suggesting that energy flow to mosquitofish originated from allochthonous sources (terrestrial vegetation, emergent macrophytes, or both), whereas energy flow to mollies and pupfish originated from autochthonous sources (filamentous algae, submerged macrophytes, or both). The δ 15N measurements indicated that mosquitofish and mollies occupied similar trophic levels, whereas pupfish occupied a slightly higher trophic level. Analysis of gut contents showed that mosquitofish consumed mostly winged insects (an indication of terrestrial taxa), whereas mollies and pupfish consumed mostly organic detritus. Judging from our results, only mollies (not mosquitofish) are suitable for monitoring selenium exposure in pupfish.

VARIATION IN OFFSPRING SIZE WITH BIRTH ORDER IN PLACENTAL FISH: A ROLE FOR ASYMMETRIC SIBLING COMPETITION?

Asymmetric sibling competition arises when siblings with different competitive abilities share a limited resource. Such competition occurs in species with postnatal parental care and may also occur when mothers provision embryos between fertilization and birth (matrotrophy). We hypothesized that the combination of matrotrophy and the simultaneous provisioning of embryos in different stages of development (superfetation) leads to asymmetric competition between sibling embryos. Moreover, we expect the intensity of this competition to increase with the level of superfetation as high levels of superfetation result in greater temporal overlap between broods. This hypothesis predicts that offspring from early broods, which predominantly compete with less-developed siblings, will be larger at birth than offspring from later broods, which experience competition from more and less-developed siblings. Data on offspring size at birth from two populations of the highly matrotrophic fish, Heterandria formosa, and similar studies of poeciliid fish spanning a range of life histories are consistent with our hypothesis. Together these results suggest that sibling competition is a direct consequence of the evolution of matrotrophy and superfetation in poeciliid fish.