Salinity is associated with habitat use and mitochondrial lineage diversity in halophilic Ephydra flies (Diptera: Ephydridae)

Abstract

Brine flies of the genus Ephydra (Diptera: Ephydridae) are ubiquitous extremophile animals whose larval forms are able to thrive in high-stress environments, including hypersaline and high carbonate waters. Despite their historical and ecological importance, little is known about their evolutionary history. Here, we focus on an understudied population of Ephydra living in former industrial salt evaporation ponds of the San Francisco Bay Area (SFBA), a region which has undergone significant environmental changes in the past century. We observed and collected Ephydra specimens and took salinity measurements of waters they inhabited during a 6-month period in 2024. We found that E. millbrae and E. gracilis demonstrate strong salinity preferences. We also analyzed DNA from Ephydra spp. specimens from distant sites and a timespan of ~40 years, generating the most comprehensive gene genealogy of the group to date based on a single mitochondrial locus. Our analysis of phylogenetic signals suggests that salinity envelopes are influenced by evolutionary relationships in Ephydra. This study provides new information about the natural history of extremophile flies in the SFBA and fills in current gaps in our knowledge about the evolutionary history of the genus more broadly.