Effects of winter length on survival, body condition, dispersal, and reproduction in aquatic invertebrate Notonecta undulata.

Background/Question/Methods

Many aquatic macroinvertebrates remain active under ice. However, it is generally unknown whether feeding during winter compensates for winter energy use, or if winter survival relies on internal energy reserves. Body condition coming out of winter has potential to affect important ecological processes such as dispersal and reproduction in spring. Shortening of ice coverage due to climate change could increase winter survival and spring body condition if feeding does not compensate for winter energy use. The backswimmer Notonecta undulata feeds at 4°C and is active under ice throughout the winter season. We tested the effects of winter length on the survival, dispersal propensity, reproduction, and body weight in N. undulata. We simulated long winter conditions by holding backswimmers at 4°C for 204 days, while individuals in the short winter treatment were held at 10°C for 30 days then moved to 4°C for 174 days. Backswimmers were fed and monitored for mortality weekly then monthly during winter simulation. After the simulation, all survivors were subjected to a dispersal assay. A subset of surviving individuals were paired (male with female) and monitored daily for egg output at 21°C until death. The other survivors were preserved for measurement of dry body weight.

Results/Conclusions

Initial analysis of overwinter mortality data shows a treatment by sex interaction, wherein males in the long winter treatment were more likely to die compared to males in the short winter treatment or females in either treatment (p=0.02). Time to first dispersal attempt was longer in the long winter treatment group in females(p=0.01) but not in males (p=0.68). Time to first egg laid was longer in the long winter treatment group (p< 0.01), but lifetime egg production did not differ between groups (p=0.45). Dry body weights of winter survivors did not differ significantly between treatment groups (p=0.66). Although we did not detect differences in body weights between the two groups, the lag in both time to dispersal and time to egg laying observed in the long winter group suggests that winter activity has energetic costs that are not met by feeding at 4°C. The sex-biased winter mortality is potentially a result of females being larger than males, with greater body fat reserves. Shorter winters could result in higher overwinter survival and better spring body condition for backswimmers and other winter-active aquatic invertebrates, with potential impacts on important ecological processes such as dispersal, reproduction, and population demographics.