Warming causes higher bilateral asymmetry in aquatic hemipterans from geothermal streams

Increase in temperature associated with climate change may alter community structure and disrupt developmental stability in ectothermic organisms. Using a natural geothermal gradient (20.6–40.3 °C) in Neotropical streams in central-western Brazil, we investigated how warming influences abundance, richness, body size, and fluctuating asymmetry (FA) in aquatic hemipterans. We sampled 18 streams and analyzed 285 individuals belonging to 12 species or morphotypes (Belostomatidae and Naucoridae). Species richness decreased significantly with increasing temperature, whereas total abundance showed no overall trend; however, species-level responses revealed marked reductions in Limnocoris burmeisteri De Carlo, 1967 in warmer streams. Adults were less frequent at high-temperature sites, suggesting potential constraints on development or persistence under thermal stress. Contrary to predictions of metabolic theory, no general reduction in body size was detected at the community level, although L. burmeisteri exhibited significant decrease in total and standard length along the thermal gradient. In contrast, fluctuating asymmetry increased consistently with temperature, particularly in structures related to locomotion and flight, indicating greater developmental instability under warming conditions. Species-specific analyses further revealed response strategies: Belostoma anurum (Herrich-Schäffer, 1848) maintained body size but showed increased asymmetry, whereas L. burmeisteri exhibited both size reduction and pronounced morphological instability. Our results demonstrate that thermal stress in tropical freshwater systems may reduce species richness and increase sublethal developmental instability, even when changes in body size are not widespread. Incorporating intra-individual morphological metrics, such as fluctuating asymmetry, may improve the detection of early warming effects on aquatic predator communities in Neotropical ecosystems.