Christine Ann Parisek, Michael P Marchetti, Matthew R Cover

Lake and stream fauna are frequently studied, yet surprisingly little is known about ecological and evolutionary dynamics of species that inhabit both lentic and lotic habitats. There are few examples of species co-occurring in the different habitat flow types, which raises questions on how this may impact their ability to adapt to changing climatic conditions. The aquatic insect Limnephilus externus Hagen (Trichoptera: Limnephilidae) is widely distributed in lakes of the Nearctic and Palearctic regions; in our study area of the northern Sierra Nevada mountains (California, USA), larval stages of this species co-occur in connected lakes and streams. We examined larval body and case morphology, interspecies phoretic associations, and the mitochondrial DNA cytochrome c oxidase I (COI) gene among lake and stream populations of L. externus. Further, we begin to explore potential morphological differences in distinct L. externus haplogroups. We observed differences between lake and stream populations in abundance, phenology, some aspects of body and case morphology, and abdominal mite presence, indicating that lakes and streams may yield distinct ecological phenotypes for the species. We also observed distinct regional differences in caddisfly body condition and sturdiness of case construction, as well as distinct communities of micro-invertebrates associated with the caddisfly and cases. Lake-stream L. externus did not show genetic divergence; however, three potentially distinct haplogroups were present across the research sites, as well as in sequences from North America and Canada which were imported from BOLDSYSTEMS. Limnephilus externus appears to exhibit wide geographic range and low geographic sequence structure which could account for the species’ large variation in phenology and morphology at the lake-stream level. As the Sierra Nevada faces warming temperatures, reduced snowpack, and flow cessation, sensitive high elevation species will face potentially detrimental consequences. Aquatic insect life history and phylogenetic structure provides valuable insight into the ecological and evolutionary dynamics that influence the adaptability of aquatic fauna to climatic change.