Quantifying invasion dynamics: Quantitative polymerase chain reaction vs. droplet digital polymerase chain reaction in monitoring the alien invasive bloody-red shrimp Hemimysis anomala and its interaction with perch in Lake Geneva

Invasive alien species constitute a major threat to aquatic systems due to their potential impact on endemic biodiversity, ecosystem functioning, infrastructure, and possible sanitary issues. It is therefore crucial to obtain information on their presence, abundance, and distribution. The bloody-red shrimp Hemimysis anomala, which originated from the Ponto-Caspian area, has recently settled in Western European lakes, including Lake Geneva. Although divers have frequently reported the presence and development of this small crustacean over the last decade, no monitoring has yet been proposed. During a period of 2.5 yr, we tested and optimized an environmental deoxyribonucleic acid approach by comparing two polymerase chain reaction techniques, quantitative and digital droplet polymerase chain reaction, to assess the presence, abundance, and dynamics of the animal as well as that of a potential predator, the perch (Perca fluviatilis). We show and discuss the efficiency of the methods and reveal for the first time the seasonal dynamics of Hemimysis anomala at a selected site in Lake Geneva. We highlight, in agreement with diving observations, that the animal's abundance is high in winter and declines rapidly in early spring, concomitantly with temperature increase and perch presence.