Aquarius Remigis Research Articles

Predation risk, food deprivation and non-random mating by size in the stream water strider, Aquarius remigis

Non-random mating by size (NRMS) plays a central role in the study of sexual selection and the evolution of mating systems. Theory suggests that NRMS should be influenced by conflicting demands (e.g., predation risk, hunger); few experimental studies, however, have addressed these effects. We used a factorial experiment to examine the influence of predatory green sunfish and food deprivation on NRMS in male and female stream water striders, Aquarius remigis. As predicted by theory, food deprivation decreased the large-male mating advantage. The influence of predation risk, however, went against existing theory; that is, risk increased the large male mating advantage. The degree of large-male mating advantage was negatively related to a measure of the rate of male harassment of females. A behavioral mechanism that can explain these patterns emphasizes the contrasting effects of different competing demands on male harassment rates, female resistance and the role of male size in overcoming female resistance. Females usually resist male mating attempts. Successful mating occurs when males overcome female resistance. If harassment rates (of females by males) are low, larger males have a mating advantage over smaller males perhaps because females resist heavily and thus only larger males can overcome female resistance. If, however, male harassment rates are very high, female resistance might be swamped; mating should then be more random with respect to male size. Food deprivation increases gerrid activity and thus increases harassment rates which should then reduce NRMS. In contrast, risk decreases gerrid activity, thus decreasing harassment rates and increasing NRMS. Females did not show significant NRMS. Females did, however, show a pattern of change in NRMS that is consistent with male choice for larger females.

GENETIC POPULATION STRUCTURE AND LEVELS OF GENE FLOW IN THE STREAM DWELLING WATERSTRIDER, AQUARIUS (=GERRIS) REMIGIS (HEMIPTERA: GERRIDAE).

Gene flow, in combination with selection and drift, determines levels of differentiation among local populations. In this study we estimate gene flow in a stream dwelling, flightless waterstrider, Aquarius remigis. Twenty-eight Aquarius remigis populations from Quebec, Ontario, New Brunswick, Iowa, North Carolina, and California were genetically characterized at 15 loci using starch gel electrophoresis. Sampling over two years was designed for a hierarchical analysis of population structure incorporating variation among sites within streams, streams within watersheds, watersheds within regions, and regions within North America. Hierarchical F statistics indicated that only sites within streams maintained enough gene flow to prevent differentiation through drift (Nm = 27.5). Above the level of sites within streams gene flow is highly restricted (Nm ≤ 0.5) and no correlation is found between genetic and geographic distances. This agrees well with direct estimates of gene flow based on mark and recapture data, yielding an Ne of approximately 170 individuals. Previous assignment of subspecific status to Californian A. remigis is not supported by genetic distances between those populations and other populations in North America. Previous suggestion of specific status for south-eastern A. remigis is supported by genetic distances between North Carolina populations and other populations in North America, and a high proportion of region specific alleles in the North Carolina populations. However, because of the high degree of morphological and genetic variability throughout the range of this species, the assignment of specific or subspecific status to parts of the range may be premature.

The origins of allometry: size and shape polymorphism in the common waterstrider, Gerris remigis Say (Heteroptera, Gerridae)

Changes in size, whether ontogenetic or phylogenetic, tend to be associated with changes in shape. This allometry can arise through two different evolutionary mechanisms: (1) selection acting primarily on overall size may be associated with changes in shape because of physiological and mechanical constraints or differential responses of different body components; or (2) selection acting primarily on shape (on the size of specific body components) may be associated with changes in overall size because of genetic correlations, and thus correlated responses, of other body components. To assess the relative importance of these two mechanisms, shape polymorphism is examined along two axes of size dimorphism (sex and wing morphology) in the common waterstrider, Gerris remigis Say. Eight measurements were made of body and appendage components of 234 adults, from three independent populations. Univariate and multivariate analyses reveal that both sexes and wing morphs differ significantly in size and shape. Shape differentiation along the two axes of size dimorphism is found to be dissimilar, partially independent of size, and strongly correlated with the ecological specialization of the various morphs. These observations suggest that selection is acting directly on shape, and thus that allometry in this species primarily reflects shape-mediated changes in size (mechanism 2), rather than size-mediated changes in shape. The role of developmental processes in facilitating this shape differentiation is discussed.

The Influence of Starvation and Predators on the Mating Behavior of a Semiaquatic Insect

Few experimental studies of mating behavior have examined mating behavior shifts in response to ecological factors (e.g., predators, starvation). We conducted experiments designed to: (1) examine multiple trade—doffs between predator avoidance, feeding behavior, and copulating duration in a semiaquatic insect (Microvelia austrina); and (2) address the influence of altered copulation duration on male reproductive success. To examine multiple trade—offs, we used a 2 x 2 factorial design where we measured the amount of time that mating microveliids remained in tandem, for starved and fed microvellids in the presence and absence of predaceous water striders (Gerris remigis). We also looked at the effects of presence vs. absence of green sunfish (Lepomis cyanellus) on microveliid tandem duration. To address the impact of premature pair separation on male mating success, we separated paired microvellids in field populations and measured the time required for each sex to find new mates. To examine the effects of local sex ratio on remating times of males and females, we quantified the sex ratio of field populations via periodic censuses and correlated local sex ratio with remating times. Finally, to address oviposition rates while pairs are in tandem, we measured female oviposition patterns relative to tandem duration for poorly fed and well—fed females. Both Starvation and gerrid presence significantly reduced tandem duration. As a result of male—biased sex ratios, males that prematurely separated from females often faced long search times before securing a new mate. Females oviposited at a constant rate while in tandem. A simple computer simulation incorporating the above observations suggests that reductions in tandem duration can decrease fertilization rates.

Fitness consequences of food-based territoriality in water striders, Gerris remigis

Fitness consequences of food-based territoriality in water striders, Gerris remigis

Facultative estivation in the water strider Gerris remigis

Marked individuals of the water strider Gerris remigis estivated facultatively during summer in damp areas underneath stream-bed rocks when a pool in a small stream in New York twice went dry.

Foraging in groups of water striders (Gerris remigis): effects of variability in prey arrivals and handling times

In many species, socially subordinate individuals frequently remain in a group despite their lower priority of access to food and mates. This is expected to occur when the net benefits of staying in the group exceed those of a solitary existence. Analogously, territorial foragers surrounded by conspecific competitors are faced with tradeoffs as to patch tenacity. In either case, spatio-temporal variability in resource availability directly affects behavioral patterns and payoffs, particularly of individuals with low priority of access to the resource. However, such individuals may take advantage of natural environmental fluctuations in resource supply in cases where dominants are preoccupied. This arises regularly if two resource items cannot be handled simultaneously and if a second item arrives before handling of the first has been completed. I advance a heuristic model that predicts that foraging or mating success of individuals with low priority of access to resources may increase both with higher variance in inter-arrival times of the resource (given the same mean) and with an increase in the average handling time of the resource. I tested both predictions with two associations of individually marked, naturally foraging water striders (Gerris remigis) in the field. In natural streams individual water striders tend to occupy consistent positions that they may defend, resulting in priority of access to prey items floating downstream for individuals further up front. I manipulated the variance in prey inter-arrival times given the same mean, and the prey handling times by offering larger prey. The outcome was in qualitative agreement with the predictions of the model.

Fitness consequences of foraging success in water striders (Gerris remigis; Heteroptera: Gerridae)

Fitness consequences of foraging success in water striders (Gerris remigis; Heteroptera: Gerridae)

Sexual dimorphism and predaceous feeding habits of the waterstrider Gerris remigis Say (Heteroptera: Gerridae)

The feeding habits of Gerris remigis were examined in a southeastern Ontario stream. Gerrids with prey items were collected from two pools along a 15-m length of the site. The size distribution of potential prey available in the surface drift during the survey was estimated. Gerrid prey consisted mainly of terrestrial insects; coleopterans, dipterans, homopterans, and hymenopterans represented the predominant orders. The median size of prey taken by all gerrids was significantly larger than that available in the surface drift. A sexual dimorphism in gerrid body size was found. The length of prey taken did not differ between the sexes; however, the diet of the larger bodied females consisted of prey with a greater median body width than the diet of smaller males. Females also took a greater size range of prey. These results support a size-dependent predation model which predicts that the upper size range of prey that can be successfully captured is correlated with predator size.

An Experimental Study on the Effects of Predation Risk and Feeding Regime on the Mating Behavior of the Water Strider

Influence du risque du a la presence de predateurs et a au jeune subit sur le taux d'accouplement et sur sa duree

Sperm competition in the water strider, Gerris remigis

Sperm competition occurs in the giant water strider. In serial mating experiments using normal and sterilized males, the last male to mate was able to displace, on average, 65% of the sperm of the previous male. Success of second males was, however, highly variable but can be partially explained by relative copulation lengths of the two males. Although there was a tendency for long copulations by the first male to be followed by long copulations by the second, second males could only obtain the ‘last male’ advantage if their copulations were long relative to those of first males. A long latency period occurred between genital contact and the effective transfer of sperm as no viable eggs were produced until copulations lasted at least 15 min. Once sperm transfer began, however, virtually the entire sperm load was delivered instantaneously to the female. Females continued to lay fertile eggs for 24 days after a single copulation, and although this period increased to 30 days after two matings, the difference was not statistically significant. Although the incompleteness of sperm displacement suggests that alternative copulatory strategies among males should be common, the existence of a long latency period before sperm transfer should not only reduce this tendency, but also increase female choosiness.

The effects of operational sex ratio and food deprivation on copulation duration in the water strider (Gerris remigis Say)

A North American water strider, Gerris remigis (Hemiptera) copulates for prolonged periods of time, apparently beyond the time necessary for insemination (Rubenstein 1984; Wilcox 1984; Clark 1987), I tested predictions from two different hypotheses of the adaptive significance of this behavior. Manipulations of the operational sex ratio in artificial “streams” revealed that water striders copulate significantly longer when the sex ratio is male-biased. This result is consistent with the hypothesis that prolonged copulation functions as a type of contact mate guarding, which reduces sperm competition by preventing the female from remating prior to oviposition. The finding is also consistent with the hypothesis that copulation is prolonged to enhance female foraging efficiency. In a second experiment, sex ratio and total population density were held constant, and the period of food deprivation of females was manipulated. Contrary to the predictions of the female foraging hypothesis, “starved” females did not copulate longer than “fed” females. Of the two hypotheses considered, only the copulatory guarding hypothesis explains both results. This does not mean, however, that there is no selective benefit from the enhanced foraging efficiency resulting from prolonged copulation. Nor does it exclude the possibility that this benefit has been necessary for the evolution of prolonged copulation in water striders.

SEXUAL SELECTION FOR HOMOGAMY IN THE GERRIDAE: AN EXTENSION OF RIDLEY'S COMPARATIVE APPROACH.

Ridley (1983) proposed that three factors are necessary and sufficient for the development of positive assortative mating (homogamy) in natural populations. These factors are 1) a correlation between male size and success in competition for mates (intrasexual selection), 2) a correlation between female size and fecundity, and 3) prolonged pairing. In the presence of these three factors, selection will favor males that select large (and thus more fecund) females as mates, thus leading to positive assortative mating. Although this simple hypothesis is generally supported by data across taxa (Ridley, 1983), the evidence for the Insecta is equivocal at best. In this paper, I test this hypothesis using three species of waterstriders (Gerridae; Hemiptera). The three factors and homogamy are measured in several samples from each species. Homogamy is predicted and observed in one species (Gerris remigis) and is not predicted and not observed in the other two (G. comatus and G. buenoi). These results support Ridley's hypothesis and suggest that the processes leading to homogamy in the insects do not, in general, differ from those dominant in other taxa. The possible influence of even low levels of homogamy on genetic variance within populations and the difficulties of assessing such levels on the basis of single samples are discussed.

Adaptive significance of wing dimorphism in the absence of dispersal: a comparative study of wing morphs in the waterstrider, Gerris remigis

ABSTRACT. Gerris remigis Say (Hemiptera; Gerridae) is primarily apterous, but populations with up to 33% macropters have been reported. The macropters seldom fly, and field studies have revealed no detectable differences between wing morphs in movement or survival at any time of year. In this paper, life history traits of macropterous and apterous G. remigis are compared in an attempt to determine the mechanisms responsible for the maintenance of macroptery in this species in spite of the very low flight capacity and infrequent flight of macropters. Development time, proportion breeding without diapause, and overwinter survival do not differ between morphs. However, pre‐diapause macropterous females have a significantly shorter pre‐oviposition period than apterous females. In contrast, post‐diapause macropters begin reproducing later than apters, and have a lower cumulative fecundity. These results suggest that macropters may be at a selective advantage in warm habitats which favour pre‐diapause reproduction, but that apters should be favoured in the preferred, cool, lotic habitats. However, crossing and rearing experiments indicate that wing morphology is primarily environmentally controlled in this species, and that the heritability of wing morphology is low, at best. In light of this, the relative impacts of purely phenotypic (environmental) variation, random effects, and the observed fitness differences on the maintenance of macroptery in this species are discussed.

Dispersion and spatial orientation ofGerris remigisin response to water current: a comparison of pre‐ and post‐diapause adults

ABSTRACT.The movements and orientation of adultGerris remigisSay (Hemiptera; Gerridae) were examined in an artificial stream to determine the mechanisms by which these stream‐dwelling waterstriders compensate for and/or avoid downstream drift, and to compare the movement tendency of pre‐ and post‐diapause adults.G. remigisrespond to current by turning to face directly into the current (rotational orientation), intermittently rowing against the current (translational orientation), and moving onto land to avoid the current. These rotational and translational responses enableG. remigisto maintain their positions in currents =S10 cm s‐1. However, avoidance increases with current speed, and few animals are found where currents exceed 10 cm s‐1. These observations correlate well with the observed distribution of adultG. remigisin a natural stream.Comparisons between life stages reveal that post‐diapause adults move more, and are more susceptible to drift than pre‐diapause adults. Post‐diapause adults also avoid even slight currents (<5 cm s_1) by seeking land, while pre‐diapause adults do not show a significant preference for land until currents exceed 10 cm s‐1. These data suggest that previously reported differences in the movement tendency of pre‐ and post‐diapause adults in the field may reflect physiologically‐based behavioural differences between these two life stages rather than merely direct responses to environmental cues.

The group size – flight distance relationship in water striders (Gerris remigis)

When food availability is constant, the distance from which first instar water striders (Gerris remigis Say) initiate escape from approaching (potentially cannibalistic) adults initially rises and then falls with increases in group size. This result cannot be explained by the currently accepted many-eyes hypothesis, which predicts a monotonically increasing relationship between flight initiation distance and group size, resulting from the increased probability of predator detection by larger groups. The results are consistent with an alternative hypothesis, according to which the preferred flight distance results from a trade-off between the costs and benefits of flight and declines with increasing group size due to risk dilution. The gerrids are constrained from achieving their preferred flight distance when tested as individuals, and the adult is able to approach more closely before being detected.

Does alary dimorphism imply dispersal dimorphism in the waterstrider,Gerris remigis?

ABSTRACT.The hypothesis that wing dimorphism reflects dimorphism for dispersal ability is tested in two populations of stream‐dwelling waterstriders,Gerris remigisSay, in southern Quebec, Canada.Movements were assessed directly by recaptures of marked adults, and indirectly by comparisons of residence times and patterns of disappearance of macropterous and apterous morphs. Sampling was done weekly, over a period of 26 months, and 4828 adultG.remigiswere individually marked during this time.Movements of >100 m were very rare for both morphs. The movement patterns of the two morphs were very similar, and differed only over the winter, when no macropters moved upstream. This latter observation suggests that macropters may be less successful than apterous individuals at moving overland, or rowing against a current at this time of year.Residence times and patterns of disappearance of the two morphs were very similar during all seasons.These results suggest that macropterousG.remigisdo not, in general, disperse using any means unavailable to the apterous morph, and that macropters in this species should not be simply categorized as long‐distance dispersers.

Comparative ecology of Gerris remigis (Hemiptera: Gerridae) in two habitats: a paradox of habitat choice

This study compares the population ecology of Gerris remigis (Hemiptera: Gerridae) in two habitats: a cool mountain stream, representing the characteristic habitat of this species, and the atypical habitat of a small, warm pond. Neither habitat supported breeding populations of any other gerrid species. The populations on the two sites were found to differ significantly with respect to population dynamics, dispersal, phenology, and wing dimorphism. Overall, the population on the pond was more productive than that on the stream, indicating that G. remigis can be highly successful in a warm, lentic habitat, in the absence of competing species. However, in spite of this success, adult G. remigis actively avoided the lentic site. Thus habitat choice in this species cannot be mediated solely by interspecific competition. Comparisons of the two sites suggest that current, temperature, and (or) shelter from direct sunlight and wind may act as proximate cues for habitat choice in this species. The striking differences between the two study populations are also evidence of a very high degree of phenotypic plasticity, which allows G. remigis to respond to differences in habitat characteristics on a very fine scale.

A test of the hypothesis of compensatory upstream dispersal using a stream-dwelling waterstrider, Gerris remigis Say.

This paper reports the results of 2 experiments designed to determine if adult Gerris remigis (Hemiptera, heteroptera), a stream-dwelling waterstrider, tend to disperse preferentially upstream, as predicted by the colonization cycle hypothesis summarized by Muller (1982). Markrecapture observations and experimental removals were used to assess the distance and direction of movement of adult G. remigis along a small mountain stream, over a full year. These experiments indicated that adult G. remigis show a significant upstream bias in movement distance, but not in numbers of animals moving. This upstream bias is characteristic of pre-reproductive, sexually immature adults of both sexes, and occurs primarily in association with movements to and from diapause sites. Although the existence of a significant upstream bias in movement distances tends to support the colonization cycle hypothesis, the data from the removal experiments clearly show that upstream dispersal is not sufficient to compensate entirely for downstream drift.

Male copulatory guarding enhances female foraging in a water strider

In the laboratory, females of Gerris remigis foraging singly after being separated from a copulating male averaged 16 times longer to repel male copulatory attempts compared to repelling time when copulating (60 trials). In<10 min after separation, all females either became immobile at the edge (in 34% of all trials), or recopulated and either continued to forage (61%) or swam to the edge and became immobile (5%). Copulating females captured 85% of imitation prey, but single females, harassed by males, captured only 32% of prey.