Tarsal asymmetry, nutritional condition, and survival in water boatmen (Callicorixa Vulnerata)

Mayfly emergence production and body length response to hydrology in a tropical lowland stream

Background

Hydrological impacts on aquatic biota have been assessed in numerous empirical studies. Aquatic insects are severely affected by population declines and consequent diversity loss. However, many uncertainties remain regarding the effects of hydrology on insect production and the consequences of energy transfer to the terrestrial ecosystem. Likewise, sublethal effects on insect morphology remain poorly quantified in highly variable environments. Here, we characterized monthly fluctuation in benthic and emerged biomass of Ephemeroptera in a tropical lowland stream. We quantified the proportion of mayfly production that emerges into the riparian forest. We also examined the potential morphological changes in Farrodes caribbianus (the most abundant mayfly in our samples) due to environmental stress.

Methods

We collected mayflies (nymphs and adults) in a first-order stream in Costa Rica. We compared benthic and adult biomass from two years’ worth of samples, collected with a core sampler (0.006 m²) and a 2 m²-emergence trap. The relationship between emergence and annual secondary production (E/P) was used to estimate the Ephemeroptera production that emerged as adults. A model selection approach was used to determine the relationship between environmental variables that were collected monthly and the emergent biomass. To determine potential departures from perfect bilateral symmetry, we evaluated the symmetry of two morphological traits (forceps and forewing) of F. caribbianus adults. We used Spearman’s rank correlation coefficients (ρ) to examine potential changes in adult body length as a possible response to environmental stress.

Results

Benthic biomass was variable, with peaks throughout the study period. However, peaks in benthic biomass did not lead to increases in mayfly emergence, which remained stable over time. Relatively constant mayfly emergence suggests that they were aseasonal in tropical lowland streams. Our E/P estimate indicated that approximately 39% and 20% (for 2002 and 2003, respectively) of the nymph production emerged as adults. Our estimated proportion of mayfly production transferred to terrestrial ecosystems was high relative to reports from temperate regions. We observed a strong negative response of F. caribbianus body length to increased hydrology (Spearman: ρ = −0.51, p < 0.001), while slight departures from perfect symmetry were observed in all traits.

Conclusion

Our two years study demonstrates that there was large temporal variability in mayfly biomass that was unrelated to hydrological fluctuations, but potentially related to trophic interactions (e.g., fish predation). Body length was a good indicator of environmental stress, which could have severe associated costs for mayfly fitness in ecosystems with high temporal variation. Our results highlight the complex ecological and evolutionary dynamics of tropical aquatic insects, and the intricate connection between aquatic and terrestrial ecosystems.

Fluctuating asymmetry and feather growth bars as biomarkers to assess the habitat quality of shade coffee farming for avian diversity conservation

Shade coffee farming has been promoted as a means of combining sustainable coffee production and biodiversity conservation. Supporting this idea, similar levels of diversity and abundance of birds have been found in shade coffee and natural forests. However, diversity and abundance are not always good indicators of habitat quality because there may be a lag before population effects are observed following habitat conversion. Therefore, other indicators of habitat quality should be tested. In this paper, we investigate the use of two biomarkers: fluctuating asymmetry (FA) of tarsus length and rectrix mass, and feather growth bars (average growth bar width) to characterize the habitat quality of shade coffee and natural forests. We predicted higher FA and narrower feather growth bars in shade coffee forest versus natural forest, indicating higher quality in the latter. We measured and compared FA in tarsus length and rectrix mass and average growth bar width in more than 200 individuals of five bird species. The extent of FA in both tarsus length and rectrix mass was not different between the two forest types in any of the five species. Similarly, we found no difference in feather growth between shade coffee and natural forests for any species. Therefore, we conclude our comparison of biomarkers suggests that shade coffee farms and natural forests provide similar habitat quality for the five species we examined.

Strong nonlinear selection against fluctuating asymmetry in wild populations of a marine fish

Theoretical links between fluctuating asymmetry (FA) and fitness have led many to use FA as a proxy for average fitness. However, studies examining whether asymmetry actually correlates with individual fitness in wild populations are relatively rare and often use simple measures of association (e.g., correlation coefficients). Consequently, the pattern of selection on asymmetry in the wild is seldom clear. We examined selection on FA of pectoral fin morphology in two wild populations of a marine fish (the kelp perch; Brachyistius frenatus). As expected, variance in signed FA in each initial sample was significantly greater than that found in the surviving population, indicating selection against FA. Our estimate of the fitness surface confirmed perfect symmetry as the phenotypic optimum and indicated strong, nonlinear selection against asymmetry. No difference in the form of selection was detected between populations. However, the level of FA in the initial samples varied among populations, leading to an overall difference in the level of selective mortality. Our results suggest that selection on asymmetry in wild populations may be strongly nonlinear, and indicate that the demographic costs of asymmetry may play a substantial role in the dynamics of populations.

Lizards from urban areas are more asymmetric: using fluctuating asymmetry to evaluate environmental disturbance

The increase in human activities that leads to wildlife decline and species extinction poses an urgent need for simple indicators of environmental stress in animal populations. Several studies have suggested that fluctuating asymmetry (FA) can be an easy, direct measure of developmental instability because it is associated to environmental stress and, as such, it can be a useful indicator of population disturbance. We examined three different morphological traits in urban and rural populations of the common wall lizard (Podarcis muralis) to test whether anthropogenic disturbance causes an increase in FA. Compared to rural populations, urban ones showed higher levels of FA in all analyzed traits, thus providing evidence that FA can respond to anthropogenic disturbance. However, we also found significant differences in FA among traits, where femoral pores and subdigital lamellae, traits with a functional relevance, were more stable developmentally compared to supracilliar granules which have no evident function. Unsigned FA [abs(right-left)] exhibited significant, but weak, positive correlations among traits, indicating that developmental noise does not have a uniform effect across characters and thus questioning the view of developmental stability as an organism-wide property. The degree of signed FA (right-left) was more similar between structurally associated traits, possibly as an outcome of morphological integration. In conclusion, our results demonstrate that FA can be a reliable indicator of disturbance provided that it is analyzed on multiple traits simultaneously and examined at the population level.

Effects of parental radiation exposure on developmental instability in grasshoppers

Mutagenic and epigenetic effects of environmental stressors and their transgenerational consequences are of interest to evolutionary biologists because they can amplify natural genetic variation. We studied the effect of parental exposure to radioactive contamination on offspring development in lesser marsh grasshopper Chorthippus albomarginatus. We used a geometric morphometric approach to measure fluctuating asymmetry (FA), wing shape and wing size. We measured time to sexual maturity to check whether parental exposure to radiation influenced offspring developmental trajectory and tested effects of radiation on hatching success and parental fecundity. Wings were larger in early maturing individuals born to parents from high radiation sites compared to early maturing individuals from low radiation sites. As time to sexual maturity increased, wing size decreased but more sharply in individuals from high radiation sites. Radiation exposure did not significantly affect FA or shape in wings nor did it significantly affect hatching success and fecundity. Overall, parental radiation exposure can adversely affect offspring development and fitness depending on developmental trajectories although the cause of this effect remains unclear. We suggest more direct measures of fitness and the inclusion of replication in future studies to help further our understanding of the relationship between developmental instability, fitness and environmental stress.

Neonatal exposure to short days and low temperatures blunts stress response and yields low fluctuating asymmetry in Siberian hamsters

Fluctuating asymmetry (FA) refers to small, non-directional deviations from perfect bilateral symmetry in morphological characters. Individuals with low FA presumably either developed in a relatively stable environment and/or were better able to buffer against developmental stressors. The present study investigated the effects of seasonal factors measured by day length and ambient temperature manipulations on the development of bilateral characters and concomitant changes in stress responses. Siberian hamsters were exposed to either long days (16 h of light per day) or short days (8 h of light per day) combined with either standard temperatures (21+/-2 degrees C) or low temperatures (8+/-2 degrees C) on the day of birth until weaning. Cortisol concentrations at baseline and following acute restraint stress, and FA values were measured in adulthood. Females reared in winter-like conditions with short day lengths and low temperatures had low FA and low cortisol concentrations following restraint stress compared to other females. Females reared in long day lengths and standard temperatures had the highest rate of increase in cortisol concentrations after restraint among other female groups. No group effects were observed in males regarding day length and temperature manipulations. Baseline and post-restraint cortisol concentrations were higher in females than males for all groups except in animals reared in short day lengths and low temperatures. Our results suggest that winter-like conditions during neonatal period evoke hyposensitivity to stress in adult females and this blunted response to stress is a key factor in achieving ideal growth patterns.

On Adaptive Accuracy and Precision in Natural Populations

Adaptation is usually conceived as the fit of a population mean to a fitness optimum. Natural selection, however, does not act only to optimize the population mean. Rather, selection normally acts on the fitness of individual organisms in the population. Furthermore, individual genotypes do not produce invariant phenotypes, and their fitness depends on how precisely they are able to realize their target phenotypes. For these reasons we suggest that it is better to conceptualize adaptation as accuracy rather than as optimality. The adaptive inaccuracy of a genotype can be measured as a function of the expected distance of its associated phenotype from a fitness optimum. The less the distance, the more accurate is the adaptation. Adaptive accuracy has two components: the deviance of the genotypically set target phenotype from the optimum and the precision with which this target phenotype can be realized. The second component, the adaptive precision, has rarely been quantified as such. We survey the literature to quantify how much of the phenotypic variation in wild populations is due to imprecise development. We find that this component is often substantial and highly variable across traits. We suggest that selection for improved precision may be important for many traits.

Food fights in house crickets, Acheta domesticus, and the effects of body size and hunger level

Animals often compete directly with conspecifics for food resources, and fighting success can be positively related to relative resource-holding power (RHP) and relative resource value (i.e., motivation to fight). Despite the ease of manipulating resource value during fights over food (by manipulating hunger levels), most studies have focused on male fighting in relation to gaining access to mates. In this study, pairwise contests over single food items were used to examine the effects of being the first to acquire a resource, relative body mass, relative body size (femur length), and relative level of food deprivation (i.e., hunger) on competitive feeding ability in male and female house crickets, Acheta domesticus. Only when the food pellet was movable did acquiring the resource first improve fighting success. When the pellet was fastened to the test arena, increased relative hunger level and high relative body mass both increased the likelihood of a takeover. However, the effects of body mass disappeared when scaled to body size. When the attacker and defender were equally hungry, larger relative body size increased takeover success but, when the attacker was either more or less hungry, body size had little effect on the likelihood of a takeover. Thus fight outcomes were dependent on an interaction between RHP and motivational asymmetries and on whether the resource was movable or stationary. Contest duration was not related to the magnitude of morphological differences between opponents, suggesting that assessment of fighting ability may be brief or nonexistent during time-limited animal contests over food items.

Is there a relationship between fluctuating asymmetry and reproductive investment in perch (Perca fluviatilis)?

Fluctuating asymmetry (FA), or random deviation from perfect bilateral symmetry, is often used as an indicator of perturbed development. Several studies attempt to correlate FA with components of individual fitness or population viability. In this study we test for a correlation between FA and four fitness traits in female Eurasian perch (Perca fluviatilis) inhabiting acidified or non-acidified lakes. Three bilateral meristic characters were counted on each side of the fish: number of gill rakers on the lower first branchial arch, number of gill rakers on the upper first branchial arch, and number of pectoral-fin rays. An asymmetry index summarizing the numbers of asymmetric characters per fish was also calculated. Four traits related to fitness were measured: gonad dry mass, egg mass, gonadosomatic index, and fecundity. There were significant differences in FA among the five perch populations for the characters number of pectoral-fin rays and number of upper gill rakers, and also for the FA index. Asymmetry was generally greater in perch living in acidified lakes than in those in non-acidified lakes. However, there was no significant correlation between FA and any of the four fitness-related traits within populations. Therefore, asymmetry in the traits measured here may not be a good indicator of individual fitness in perch.

Tarsal asymmetry, trait size, and extreme phenotypes in a sexually size-dimorphic water boatman Callicorixa vulnerata

Fluctuating asymmetry (FA) in morphological traits can vary with the size of characters and the mode of selection acting on them. In a previous study, mid-leg tarsal FA (tarsal spine number and tarsal length) in the water boatman Callicorixa vulnerata was inversely related to fitness in both sexes. The mid-legs of water boatmen are used to cling to bottom substrate during underwater feeding and if under stabilizing selection, extreme phenotypes are predicted to exhibit elevated FA. In this study, it is shown that water boatmen with large or small mid-leg tarsal traits tend to have higher levels of tarsal FA than modal phenotypes, possibly because of increased homozygosity, poor genomic balance, or greater stress during development in extreme phenotypes. This relationship suggests selection against asymmetry per se will indirectly impose stabilizing selection on trait size. Regression analyses revealed that the relationship between FA and trait size differed between the sexes and was best described by a U-shaped distribution in females but by a relatively flat, negative linear association in males. These results indicate possible directional selection on male tarsal traits. Alternatively, they suggest associations between FA and trait size do not always reflect the mode of selection acting on a trait.