Intraspecific variation in diet, growth, and morphology of landlocked Galaxias maculatus during its larval period: the role of food availability and predation risk

Food availability and predation risk have been shown to affect phenotypes during early life history of fishes. Galaxias maculatus, a small fish widely distributed around the southern hemisphere, clearly exhibits a complex trade-off between feeding and predation avoidance during growth over the larval period. We studied the effect of different environmental variables on diet, growth, mortality, and morphology through field surveys and data revision in the literature for limnetic G. maculatus larvae in five oligotrophic lakes of Patagonia. Both number of food categories and prey ingested by larvae were directly related to zooplankton density. Larval growth rate was related with zooplankton density and temperature. Lakes with high zooplankton densities and low predation risk had larvae with deeper bodies and shorter caudal peduncles, while in lakes with less food and high predation risk larvae were slender with shallower bodies and longer peduncles. Food availability and predation risk seem to operate on the swimming performance of G. maculatus larvae through the slenderness of the body and the length of the caudal peduncle. The observed phenotypic variation in growth and morphology could be a key feature that has allowed this species to successfully colonize a wide variety of environments in the southern hemisphere.     

Fecundity,autogeny, and the larval environment of the pitcher-plant mosquito,Wyeomyia smithii

Summary Pupae and fourth instar larvae of a southern (30°N, Alabama, USA) population of Wyeomyia smithii Coq. (Diptera: Culicidae) were collected from pitcher plants. Adults which emerged were maintained without food then dissected to determine their egg clutch size. Among females which matured eggs, fecundities were negatively correlated with larval densities in individual pitchers. The mean autogenous fecundity of the overwintering generation did not differ from a summer sample. Adults unable to mature eggs comprised 6–39% of samples, depending on whether collected as pupae or fourth instar larvae. Fecundity was negatively correlated with time to adult eclosion among larvae maintained on unrenewed pitcher contents in the laboratory.Cohorts from this population were reared in artificial containers from egg hatch to adulthood at a single density and a superior or inferior diet. On the superior larval diet, all females survived to reproductive age, and all but one (>99%) produced eggs autogenously. On the inferior diet, survivorship to adult eclosion was significantly less, a high proportion of females died before reaching reproductive age, and only 19% of survivors matured eggs without blood. Protracted larval development induced by the inferior diet did not influence the probability of autogeny among females that survived to reproductive maturity.The relationship between larval environment and reproductive strategies is contrasted across the geographic range of W. smithii. Bloodfeeding occurs among southern populations where density dependent constraints on preimaginal growth are constantly severe. The loss of hematophagy among northern populations may have been facilitated by periods of density independent larval growth.Institute of Food and Agricultural Sciences, University of Florida, Experimental Station Journal Series No. 3645     

Feeding and fate of wild larval razorback sucker

The razorback sucker (Xyrauchen texanus) is disappearing throughout its native range in the Colorado River basin of western North America. The largest remaining wild population in Lake Mohave, Arizona-Nevada, has shown no recruitment since the 1950s. Although annual spawning is successful and larvae are seasonally abundant, no juveniles have been collected in recent decades. To evaluate the potential role of food availability in determining fate of larvae, fish and zooplankton samples were taken in 1985 from the reservoir and an adjacent, isolated backwater in which larvae were naturally produced. Food availability and primary dietary constituents were similar in both habitats. Reservoir larvae selectedBosmina spp. (Cladocera) and apparently avoided Copepoda, while larvae from the backwater selectedBosmina, but avoided Rotifera. Larvae from both places showed evidence of selection for certain sizes of zooplankters, but preferred sizes differed between habitats. These differences were neither attributable to larval size nor zooplankton community structure. Nutritional factors such as type, number, or size of available foods do not explain disappearance of larval razorback suckers from Lake Mohave, since larvae survive to far greater ages and size in the backwater. Predation by introduced fishes appears a significant cause of larval mortality.     

Effects of zooplankton availability and foraging mode on cannibalism in three dragonfly larvae

Summary Cannibalism is likely to operate as a form of population control in dragonfly larvae. I performed aquarium experiments to investigate the effect of foraging activity and zooplankton availability on cannibalism in three dragonfly larvae. Large Cordulia aenea larvae showed low activity, and large Leucorrhinia dubia larvae showed high activity irrespective of zooplankton availability. In contrast, large Coenagrion hastulatum larvae changed from high activity in the absence to low activity in the presence of zooplankton. Small Cordulia aenea larvae were active in the absence of large conspecifics irrespective of zooplankton availability. In the presence of large conspecifics they showed a reduced activity when zooplankton were present. Small L. dubia larvae showed high activity and small Coenagrion hastulatum larvae low activity irrespective of pressence or absence of zooplankton and large conspecifics. In all three species cannibalism was highest in the absence of zooplankton. In the absence of zooplankton cannibalism was low in Coenagrion hastulatum compared to the other two species. On the contrary, in the presence of zooplankton, cannibalism did not differ between the three species.     

An experimental study of population regulation in the salamander,Notophthalmus viridescens dorsalis (Urodela: Salamandridae)

Summary The roles of density-dependent larval survival and cannibalism of larvae as potential mechanisms of population regulation in the newt (Notophthalmus viridescens dorsalis) were evaluated in laboratory and field experiments. In laboratory containers, adults cannibalized larvae and large larvae cannibalized smaller larvae. In artificial ponds, larval survival did not depend on initial larval density. No cannibalism could be demonstrated in the complex environment, although the experiment was powerful enough to detect an ecologically relevant difference in survival. Adult growth was negatively correlated with the final biomass of larval newts, suggesting that the two life stages competed for resources. Larval growth rates were negatively correlated with final larval density, suggesting that larvae competed with each other. The proportion of larvae that became sexually mature at age 7 months (paedomorphs and adults that skipped the eft stage) varied inversely with larval density. Therefore, the potential regulatory mechanisms identified in this study are competition within and between life stages.     

Individual responses to population size structure: the role of size variation in controlling expression of a trophic polyphenism

We investigated how size structure affects development of alternative larval phenotypes in Arizona tiger salamanders, Ambystoma tigrinum nebulosum, by testing the hypothesis that population size structure per se is a significant component of an individual's environment. Larvae of this subspecies exhibit one of two feeding phenotypes; typical larvae eat zooplankton and macroinvertebrates and cannibalistic larvae feed primarily on conspecifics. Previous laboratory experiments showed that larval density positively affected expression of the cannibalistic phenotype. In this study we tested the hypothesis that size variation among larvae also serves as a cue triggering development of the cannibalistic phenotype. We report laboratory experiments and field observations showing that both an individual larva's position in a size distribution and the amount of size vaiation among larvae serve as cues stimulating development of cannibalistic larvae. Larval density and population size structure provide a larva with an indication of the abundance and vulnerability of potential conspecific prey. Size variation among larvae, in turn, appears to be influenced by larval density. Thus, a complex relationship exists between larval density, population size structure, and the frequency of cannibals within a habitat.     

The relationship among egg size,density and food level on larval development in the wood frog (Rana sylvatica)

Summary Although inter- and intraspecific variation in egg size among amphibians has been well documented, the relationship between egg size and fitness remains unclear. Recent attempts to correlate egg size intraspecifically with larval developmental patterns have been equivocal. In this study the development of larvae derived from large eggs and small eggs, from a single population in Maryland were compared under a range of food levels and larval population densities. Both food level and density had significant effects on the length of the larval period and size at metamorphosis. However, the response among larvae derived from different egg sizes was not additive. At low densities and high food levels, larvae from small eggs had longer larval periods and a larger size at metamorphosis than larvae derived from large eggs. In contrast, at high densities larvae from small eggs had longer developmental periods but were smaller at metamorphosis than larvae from large eggs. In addition, larvae from small eggs were more sensitive to density irrespective of food level. These results suggest that optimal egg size is correlated with environmental factors, which may explain the maintenance of both geographic and within population variation in egg size commonly observed in amphibians.     

Effects of crowding on populations of Aedes albifasciatus larvae under laboratory conditions

Aedes (Ochlerotatus) albifasciatus(Macquart 1838) (Diptera: Culicidae) is a neotropical flood water mosquito, incriminated as the main vector of the western equine encephalitis virus, and which affects beef and milk production in central Argentina. The short time required to hatch and develop from egg to adult, usually in temporary pools, suggests a strategy which allows for exploitation of transient pools, thus evading predation and interspecific competition. Under these conditions intra specific competition could represent a major density-dependent source of larval mortality, but the relative importance of density-dependent regulation of mosquito populations has generated controversy. Therefore we examined the effects of larval density on basic population characteristics of Ae. albifasciatusin the laboratory. Larvae were obtained by synchronous hatching of eggs laid by field-trapped females. Emerging larvae (L1) were used to build cohorts of different initial densities, kept in plastic trays with 400 ml of distilled water, and food supplied daily during the first 10 days (0.1 g per larvae day^–1). Age-specific development time and specific and relative mortality were estimated, and their relation to initial larval density was assessed through linear and non-linear regressions and correlation analysis. First hatching was registered 3 h after flooding the eggs. Higher levels of pre-adult mortality were detected in groups with higher densities. Specific mortality and average time to enter a stage of L1 to L3 could directly be related to initial larval density, but no significant relations were found for L4 and pupae. Results suggest that crowding could be a factor capable of regulating the density of natural populations of Ae. albifasciatus.     

Nutritional availability and larval density dependence in Aedes aegypti

1. Density dependence is the effect of density on population growth. Density dependence is an aggregate term for a suite of complex interactions between animals and their environment. 2. Mechanistic studies of density dependence in mosquito ecology are sparse, and the role of environmental factors is poorly understood. 3. Two empirical study designs were compared to consider the interaction between nutritional availability and density in Aedes aegypti. First, larvae were fed per capita. Second, larvae were fed a fixed amount of food unadjusted for the number of individuals; therefore, at higher densities, individuals received less per capita. 4. Survivorship, wing length, and development rate were lower at high densities when larvae were fed a fixed, unadjusted amount of food. The opposite was observed when food was adjusted per capita, suggesting that high densities may be beneficial for larval development when per capita nutrition is held constant 5. These results demonstrate that negative associations between Ae. aegypti larval density and larval development are a manifestation of decreased per capita nutrient uptake at high densities. 6. Population regulation is a proportional response to environmental variability in Ae. aegypti. Increased survivorship at high densities when larvae were fed per capita demonstrates that nutritional availability is not the only mechanism of density dependence in mosquitoes. Further studies should characterise density dependence in mosquitoes by using mechanistic study designs across diverse environmental conditions.     

Experimental simulations of climate change induced mismatch in oak and larval development rates impact indicators of fitness in a declining woodland moth

Consequences of climate change-driven shifts in the relative timing of spring activities of interacting species are insufficiently understood, especially for insects. We use a controlled experiment which simulates a trophic mismatch scenario in which lepidopteran larvae predominately feed on older leaves due to foliage developing faster than larvae growth rates. As a case study our experiment uses Orthosia cerasi, which is a widespread but declining woodland moth whose UK declines appear to be driven by warming temperatures. In the control experiment larvae are fed young oak Quercus robur leaves (bud burst stages six and seven), whilst in the treatment newly emerged larvae are fed young leaves but then gradually transition to feed on older leaves (post bud burst stage seven). We assess impacts on duration of the larval stage, pupal size and overwintering duration and survival. Larvae in the phenological mismatch treatment had a longer larval period, and smaller and lighter pupae. Larval diet did not carry over to influence emergence dates as earlier pupation of control larvae was balanced by an equivalent increase in the duration of the pupal stage. Increased time spent as larvae could increase predation rates from avian predators, whilst slowing the seasonal decline in food availability for those bird species. Reduced pupal size and weight are indicators of lower fecundity in emerging adults. Notably, we find that adults emerging from the mismatch treatment exhibited greater rates of abnormal vestigial wing development, which is likely to further reduce fitness. Trophic mismatches in which caterpillars have reduced availability of young leaves may thus contribute to the population declines observed in many woodland moth species due to increased mortality at larval stages, and adverse effects of early life conditions that reduce the reproductive success of emerging adults.     

Crowding-triggered phenotypic responses alleviate consequences of crowding inEpirrita autumnata (Lep., Geometridae)

Summary CrowdedEpirrita larvae had shorter larval periods than, and similar pupal masses to, their solitary siblings when reared on low quality diets. When fed on high quality diets, pupal masses of crowded larvae were lower than in singletons, and there was no difference in larval period. Because changes in food availability (absolute shortage, induced resistance in foliage) are caused by high larval densities in the field, crowding-triggered phenotypic changes may helpEpirrita to overcome detrimental consequences of high larval density. Pupal period was longer in crowded larvae than in singletons and crowded adults emerged later than their solitary siblings. Eggs of late emerging moths eclosed late in the ensuing spring, which coincides with delayed leaf flush in the year after defoliation. The reason for the faster growth of crowded individuals on poor diets was higher intake albeit less thorough processing of food in crowded, but not in solitary, larvae. On good diets solitary individuals tended to consume more than crowded larvae but there was no difference in processing. Predicted differences of host plant use between stealthy and opportunistic types of herbivores (sensu Rhoades 1985) were generally found between solitary and aggregated larvae on poor but not on good diets. The group response could not be explained by benefits to the group although the assumptions of Wilson's model of group selection were satisfied.     

Feeding habits of larval Mirogrex terraesanctae (Steinitz, 1952) in Lake Kinneret (Israel) II. Experimental study

Interactions between the larvae of Mirogrex terraesanctae (Steinitz, 1952) in Lake Kinneret, Israel, and their zooplankton prey were studied experimentally. Prey species preference and size selectivities were measured. Larvae were hatched in the lab from eggs collected in the field, and fed different food items in various concentrations. The food items included lake zooplankton, algae, and commercial pellets. It was shown that small, first feeding larvae (7–8.5 mm SL) prefer small bodied zooplankters (< 180 µ). The effect of these food sources on larval growth was measured. It was found that larval Mirogrex grew at a higher rate when fed zooplankton prey sized from 63 µ–250 µ. Food items smaller than 63 µ, larger than 250 µ and Scenedesmus sp., produced less than optimal growth rates. The importance of Mirogrex feeding habits and their potential influence on the Kinneret ecosystem is considered.     

Does energy availability predict gastropod reproductive strategies?

The diversity of reproductive strategies in nature is shaped by a plethora of factors including energy availability. For example, both low temperatures and limited food availability could increase larval exposure to predation by slowing development, selecting against pelagic and/or feeding larvae. The frequency of hermaphroditism could increase under low food availability as population density (and hence mate availability) decreases. We examine the relationship between reproductive/life-history traits and energy availability for 189 marine gastropod families. Only larval type was related to energy availability with the odds of having planktotrophic larvae versus direct development decreasing by 1% with every one-unit increase in the square root of carbon flux. Simultaneous hermaphroditism also potentially increases with carbon flux, but this effect disappears when accounting for evolutionary relationships among taxa. Our findings are in contrast to some theory and empirical work demonstrating that hermaphroditism should increase and planktotrophic development should decrease with decreasing productivity. Instead, they suggest that some reproductive strategies are too energetically expensive at low food availabilities, or arise only when energy is available, and others serve to capitalize on opportunities for aggregation or increased energy availability.     

Effects of temperature and food supply on the growth of whitefish Coregonus lavaretus larvae in an oligotrophic peri‐alpine lake

The relative roles of temperature and food availability on the seasonal and daily growth of whitefish Coregonus lavaretus larvae were investigated in the oligotrophic peri‐alpine Lake Annecy, France. During the spring from 2004 to 2007, surface water temperature and density of potential zooplankton prey were monitored, and 2688 larvae were caught and measured. In addition, the daily growth of 130 larvae was estimated retrospectively by investigating the microstructure of their otoliths. Temperature played the predominant role in controlling both seasonal and daily growth of early larvae. In contrast, the abundance of Mesocyclops leuckarti and larval density was only slightly correlated to larval growth, suggesting no food limitation nor strong interindividual competition over the study period. Overall, these findings run counter to concerns about potential food limitation, but sound a warning about the potential impact of climate change on fish ecology and fisheries management.     

Effects of an invasive plant transcend ecosystem boundaries through a dragonfly-mediated trophic pathway

Trophic interactions can strongly influence the structure and function of terrestrial and aquatic communities through top-down and bottom-up processes. Species with life stages in both terrestrial and aquatic systems may be particularly likely to link the effects of trophic interactions across ecosystem boundaries. Using experimental wetlands planted with purple loosestrife (Lythrum salicaria), we tested the degree to which the bottom-up effects of floral density of this invasive plant could trigger a chain of interactions, changing the behavior of terrestrial flying insect prey and predators and ultimately cascading through top-down interactions to alter lower trophic levels in the aquatic community. The results of our experiment support the linkage of terrestrial and aquatic food webs through this hypothesized pathway, with high loosestrife floral density treatments attracting high levels of visiting insect pollinators and predatory adult dragonflies. High floral densities were also associated with increased adult dragonfly oviposition and subsequently high larval dragonfly abundance in the aquatic community. Finally, high-flower treatments were coupled with changes in zooplankton species richness and shifts in the composition of zooplankton communities. Through changes in animal behavior and trophic interactions in terrestrial and aquatic systems, this work illustrates the broad and potentially cryptic effects of invasive species, and provides additional compelling motivation for ecologists to conduct investigations that cross traditional ecosystem boundaries.     

Bi- and trivoltine complex life cycles in a Kanto (Japan) population of a wild bruchid Kytorhinus sharpianus

The partial trivoltinism and overwintering of Kytorhinus sharpianus Bridwell (Coleoptera: Bruchidae) was studied in the Kanto district, Japan. The later in the summer eggs were laid by the first-generation adults, the higher was the incidence of larval diapause in the second generation. The incidence of diapause also fluctuated between years, influencing the abundance of third-generation larvae. A relatively large proportion of third-generation larvae did not attain the diapause stage by the beginning of winter. The diapause development of larvae in diapause was completed by mid-January. Immature larvae of the third generation also overwintered and emerged as adults in the spring.     

The effects of food level and conspecific density on biting and cannibalism in larval long-toed salamanders, Ambystoma macrodactylum

Previous studies have examined abiotic and biotic factors that facilitate agonistic behavior. For larval amphibians, food availability and conspecific density have been suggested as important factors influencing intraspecific aggression and cannibalism. In this study, we examined the separate and combined effects of food availability and density on the agonistic behavior and life history of larval long-toed salamanders, Ambystoma macrodactylum. We designed a 2Ƕ factorial experiment in which larvae were raised with either a high or low density of conspecifics and fed either a high or low level of food. For each treatment, we quantified the amount of group size variation, biting, and cannibalism occurring. Additionally, we examined survival to, time to and size at metamorphosis for all larvae. Results indicated that differences in both density and food level influenced all three life history traits measured. Moreover, differences in food level at which larvae were reared resulted in higher within-group size variation and heightened intraspecific biting while both density and food level contributed to increased cannibalism. We suggest that increased hunger levels and an uneven size structure promoted biting among larvae in the low food treatments. Moreover, these factors combined with a higher encounter rate with conspecifics in the high density treatments may have prompted larger individuals to seek an alternative food source in the form of smaller conspecifics.     

Variability in condition and growth of Atlantic cod larvae and juveniles reared in mesocosms: environmental and maternal effects

Standard length, dry mass and RNA : DNA ratio measurements of 3876 Atlantic cod Gadus morhua larvae and juveniles from 26 families of recruit (fish during their first year of spawning) and repeat spawners (fish which were in their subsequent spawning season) reared in two mesocosms (2500 and 4400 m³) under semi-natural conditions were analysed over a period of 10 weeks using microsatellites. Larvae from recruit spawners were significantly longer and heavier at hatch and throughout the 10 weeks. RNA : DNA ratios from recruit spawner offspring were only significantly higher at week 1. The smaller (2500 m³) mesocosm was characterized by low plankton density during the transition from endogenous to exogenous feeding followed by a higher density during the metamorphosis period (weeks 4 and 5), with the reverse pattern evident in the 4400 m³ mesocosm. Patterns of larval growth followed patterns of zooplankton density. Significant differences in RNA : DNA ratios between the mesocosms at all comparable sampling dates were found and within each mesocosm individual fish exhibited a wide range of growth and condition responses under the same environmental conditions. RNA : DNA ratios as a function of size differed in the amount of variability between mesocosms, indicating that the higher food density led to a higher proportion of well-conditioned larvae in the first 3 weeks. Food availability probably has a major role in determining offspring growth and condition, with limited effects due to maternal effects in cases where the broodstock females are approximately of similar size and condition.     

Intraspecific competition in ant-lion (Macroleon quinquemaculatus) larvae in the field

The number of larvae of the pit-digging ant-lion Macroleon quinquemaculatus at a food-poor site in Tanzania increased between May and October 1983 and then remained at high density until the following rainy season in March. Food availability was low and small larvae were most likely to suffer food shortage. Hunger level varied within and between instars: hunger was greatest after moults and only third-instar larvae weighing more than 100 mg were well fed. A simple model showed that exploitation competition was asymmetric and most likely to affect small larvae, by removing small prey. Exploitation was estimated to reduce food intake by 30–60%. Within-site differences in growth rate and size were associated with differences in larval density. Small larvae were much more likely to move than large larvae; the latter tended to occupy the periphery of the habitat. The reduction in food intake experienced by small larvae was a consequence of interference competition, which was the main factor operating at this site.