The diving response of a diving beetle: effects of temperature and acidification

Non-target effects of methoprene and larvicidal surface films on invertebrate predators of mosquito larvae

Mosquito larvicides are used across a variety of aquatic habitats, although when applied they likely affect other aquatic organisms. The removal or impairment of top insect predators via larvicides could be beneficial to mosquitoes by allowing their populations to rebound once pesticide levels dissipate. Our goal was to determine if two larvicide types, growth regulators (IGRs) and surface films (SFs), harm non-target aquatic insect communities, and if these chemicals influence the ability of predatory aquatic insects to regulate mosquitoes. We surveyed aquatic sites before and after IGR and SF-application, then compared changes in insect community structure. Evenness was lower in SF treated habitats, and when analyzing prey/controphic taxa only, evenness and diversity changed in untreated reference areas, suggesting that differences measured were due to other environmental factors, not larvicide presence. A field experiment was then conducted by exposing specific predatory aquatic insects to varying doses of IGRs and SFs and then placing them in mesocosms containing mosquito larvae. Surface films were directly lethal to adult dytiscids at recommended and high concentrations. Although we found no significant differences in mosquito emergence among all treatment levels, there was a trend of negative controls (no predator mesocosms) and SF-treated predators allowing the most mosquitoes to emerge compared to positive controls (predators not exposed to larvicides) and IGR-treated predators. Thus, these larvicides may have minimal effects on mosquito larvae predators, but the direct effects of surface films on insects that interact with the water's surface require further investigation.

Intraspecific evolution of sexually dimorphic characters in a female diving beetle can be promoted by demographic history and temperature

Previous studies have predicted that antagonistic intraspecific evolution of sexually dimorphic characters causing rapid speciation can be driven by demographic history and environmental variations. However, researchers have rarely examined this issue in the wild. Here, we examined intraspecific evolution of sexually dimorphic characters and its driving force by using a diving beetle, Acilius japonicus, which has very marked sexually dimorphic characters. Males with wider big suction cups could copulate with females with a higher success rate, whereas the mating durations of females with more hairs on their pronota were shorter. Females in a region with greater interpopulation genetic differentiation had more pronotal hairs. Considering that a previous study showed that less continuity among populations leads to a higher female cost of mating, this result suggests a greater female cost of mating in this region. Females at warmer sites also had more pronotal hairs. In light of the increase in O₂ consumption in warmer water, our result suggests that more pronotal hairs in females at warmer sites have been maintained to prevent prolonged underwater mating at higher O₂ demand. These findings suggest that demographic history and temperature can direct the evolution of sexually dimorphic characters related to sexual conflict in females.

The critical thermal maximum of diving beetles (Coleoptera: Dytiscidae): a comparison of subterranean and surface-dwelling species

Thermal tolerance limits in animals are often thought to be related to temperature and thermal variation in their environment. Recently, there has been a focus on studying upper thermal limits due to the likelihood for climate change to expose more animals to higher temperatures and potentially extinction. Organisms living in underground environments experience reduced temperatures and thermal variation in comparison to species living in surface habitats, but how these impact their thermal tolerance limits are unclear. In this study, we compare the thermal critical maximum (CTmax) of two subterranean diving beetles (Dytiscidae) to that of three related surface-dwelling species. Our results show that subterranean species have a lower CTmax (38.3-39.0°C) than surface species (42.0-44.5°C). The CTmax of subterranean species is ∼10°C higher than the highest temperature recorded within the aquifer. Groundwater temperature varied between 18.4°C and 28.8°C, and changes with time, depth and distance across the aquifer. Seasonal temperature fluctuations were 0.5°C at a single point, with the maximum heating rate being ∼1000x lower (0.008°C/hour) than that recorded in surface habitats (7.98°C/hour). For surface species, CTmax was 7-10°C higher than the maximum temperature in their habitats, with daily fluctuations from ∼1°C to 16°C and extremes of 6.9°C and 34.9°C. These findings suggest that subterranean dytiscid beetles are unlikely to reach their CTmax with a predicted warming of 1.3-5.1°C in the region by 2090. However, the impacts of long-term elevated temperatures on fitness, different life stages and other species in the beetle's trophic food web are unknown.

Transgenerational exposure to warming reduces the sensitivity to a pesticide under warming

Despite the increased attention for temporal aspects of stressor interactions and for effects of warming in ecotoxicological studies, we lack knowledge on how different exposure durations to warming may affect pesticide sensitivity. We tested how three types of exposure duration to 4 °C warming (acute, developmental and transgenerational exposure to 24 °C vs 20 °C) shape the effect of the pesticide chlorpyrifos on two ecologically relevant fitness-related traits of mosquito larvae: heat tolerance and antipredator behaviour. Transgenerational (from the parental generation) and developmental (from the egg stage) warming appeared energetically more stressful than acute warming (from the final instar), because (i) only the latter resulted in an adaptive increase of heat tolerance, and (ii) especially developmental and transgenerational warming reduced the diving responsiveness and diving time. Exposure to chlorpyrifos decreased the heat tolerance, diving responsiveness and diving time. The impact of chlorpyrifos was lower at 24 °C than at 20 °C indicating that the expected increase in toxicity at 24 °C was overruled by the observed increase in pesticide degradation. Notably, although our results suggest that transgenerational warming was energetically more stressful, it did reduce the chlorpyrifos-induced negative effects at 24 °C on heat tolerance and the alarm escape response compared to acute warming. Our results provide important evidence that the exposure duration to warming may determine the impact of a pesticide under warming, thereby identifying a novel temporal aspect of stressor interactions in risk assessment.

Gas exchange and dive behaviour in the diving beetle Platynectes decempunctatus (Coleoptera: Dytiscidae)

Many aquatic insects use bubbles on the body surface to store and supply O₂ for their dives. There are two types of bubbles: air stores, which store O₂ gained from air at the surface, and gas gills that allow passive extraction of O₂ from water. Many insects using air stores and gas gills return to the surface to replenish their bubbles and, therefore, their requirement for O₂ influences dive behaviour. In this study, we investigate gas exchange and dive behaviour in the diving beetle Platynectes decempunctatus that uses a sub-elytral air store and a small compressible gas gill. We measure the PO₂ within the air store during tethered dives, as well as the amount of O₂ exchanged during surfacing events. Buoyancy experiments monitor the volume of gas in the gas gill and how it changes during dives. We also directly link O₂-consumption rate at three temperatures (10, 15 and 20 °C) with dive duration, surfacing frequency and movement activity. These data are incorporated in a gas exchange model, which shows that the small gas gill of P. decempunctatus contributes less than 10% of the total O₂ used during the dive, while up to 10% is supplied by cutaneous uptake.

Evidence of morphological adaptation to life underwater: sternal keel affects swimming speed in giant water scavenger beetles (Coleoptera: Hydrophilidae: Hydrophilini)

Fundamentally, insects evolved on land and secondarily inhabited aquatic environments multiple times. To live underwater, aquatic insects have acquired enormously variable morphological, developmental, physiological, and ecological traits, such as gas exchange systems and swimming-related characteristics. Giant water scavenger beetles of the tribe Hydrophilini (Coleoptera: Hydrophilidae) are characterized by the presence of a sternal keel, which often extends posteriorly. Despite being a conspicuous morphological trait, its function remains unclear. Here, I verified two hypotheses: keel affects (1) submergence time following air replacement and (2) speed and oscillatory movement during forward swimming in Hydrophilus acuminatus Motschulsky, 1854. Submergence time was affected by body mass rather than keel removal; in other words, larger individuals replaced their gas gills more frequently. Keel removal reduced swimming speed by 12.5%. These observations support the second hypothesis, and are also consistent with previous speculations that the sternal keel is a key adaptation for swimming, but the results showed that the degree of oscillation was closely related to body mass but not to keel removal. Further studies are warranted to elucidate precise factors through which the presence of the keel increases swimming speed. Such studies would provide clues into understanding the associations among body size, swimming methods, and morphological traits.

Cutaneous respiration by diving beetles from underground aquifers of Western Australia (Coleoptera: Dytiscidae)

Insects have a gas-filled respiratory system, which provides a challenge for those that have become aquatic secondarily. Diving beetles (Dytiscidae) use bubbles on the surface of their bodies to supply O2 for their dives and passively gain O2 from the water. However, these bubbles usually require replenishment at the water's surface. A highly diverse assemblage of subterranean dytiscids has evolved in isolated calcrete aquifers of Western Australia with limited/no access to an air–water interface, raising the question of how they are able to respire. We explored the hypothesis that they use cutaneous respiration by studying the mode of respiration in three subterranean dytiscid species from two isolated aquifers. The three beetle species consume O2 directly from the water, but they lack structures on their bodies that could have respiratory function. They also have a lower metabolic rate than other insects. O2 boundary layers surrounding the beetles are present, indicating that O2 diffuses into the surface of their bodies via cutaneous respiration. Cuticle thickness measurements and other experimental results were incorporated into a mathematical model to understand whether cutaneous respiration limits beetle size. The model indicates that the cuticle contributes considerably to resistance in the O2 cascade. As the beetles become larger, their metabolic scope narrows, potentially limiting their ability to allocate energy to mating, foraging and development at sizes above approximately 5 mg. However, the ability of these beetles to utilise cutaneous respiration has enabled the evolution of the largest assemblage of subterranean dytiscids in the world.

Water Beetles as Models in Ecology and Evolution

Beetles have colonized water many times during their history, with some of these events involving extensive evolutionary radiations and multiple transitions between land and water. With over 13,000 described species, they are one of the most diverse macroinvertebrate groups in most nonmarine aquatic habitats and occur on all continents except Antarctica. A combination of wide geographical and ecological range and relatively accessible taxonomy makes these insects an excellent model system for addressing a variety of questions in ecology and evolution. Work on water beetles has recently made important contributions to fields as diverse as DNA taxonomy, macroecology, historical biogeography, sexual selection, and conservation biology, as well as predicting organismal responses to global change. Aquatic beetles have some of the best resolved phylogenies of any comparably diverse insect group, and this, coupled with recent advances in taxonomic and ecological knowledge, is likely to drive an expansion of studies in the future.

Snails from heavy-metal polluted environments have reduced sensitivity to carbon dioxide-induced acidity

Anthropogenic atmospheric CO₂ reacts with water to form carbonic acid (H₂CO₃) which increases water acidity. While marine acidification has received recent consideration, less attention has been paid to the effects of atmospheric carbon dioxide on freshwater systems—systems that often have low buffering potential. Since many aquatic systems are already impacted by pollutants such as heavy metals, we wondered about the added effect of rising atmospheric CO₂ on freshwater organisms. We studied aquatic pulmonate snails (Physella columbiana) from both a heavy-metal polluted watershed and snails from a reference watershed that has not experienced mining pollution. We used gaseous CO₂ to increase water acidity and we then measured changes in antipredatory behavior and also survival. We predicted a simple negative additive effect of low pH. We hypothesized that snails from metal-polluted environments would be physiologically stressed and impaired due to defense responses against heavy metals. Instead, snails from populations that acclimated or evolved in the presence of heavy metal mining pollution were more robust to acidic conditions than were snails from reference habitats. Snails from mining polluted sites seemed to be preadapted to a low pH environment. Their short-term survival in acidic conditions was better than snails from reference sites that lacked metal pollution. In fact, the 48 h survival of snails from polluted sites was so high that it did not significantly differ from the 24 h survival of snails from control sites. This suggests that the response of organisms to a world with rising anthropogenic carbon dioxide levels may be complex and difficult to predict. Snails had a weaker behavioral response to stressful stimuli if kept for 1 month at a pH that differed from their lake of origin. We found that snails raised at a pH of 5.5 had a weaker response (less of a decrease in activity) to concentrated heavy metals than did snails raised at their natal pH of 6.5. Furthermore, snails raised a pH of 5.5, 6.0, and 7.0 all had a weaker antipredatory response to an extract of crushed snail cells than did the pH 6.5 treatment snails.