Cold currents and warm preferences: how temperature shapes the 3D locomotion of Niphargus longicaudatus.

Groundwater ecosystems play a pivotal role in global biodiversity and ecosystem functioning, yet they face increasing pressures from climate change. The amphipod genus Niphargus, a dominant taxon in European groundwater habitats, has shown evidence of broad thermal adaptability that challenges prevailing theories on narrow thermal niches in groundwater species. This study investigated the locomotory behaviour of Niphargus longicaudatus (Costa, 1851), a stygobitic amphipod, under habitat temperature (9 °C) and preferred temperature (15 °C) using 3D tracking techniques. Individuals at 15 °C displayed significantly higher average swimming speed, increased vertical occupancy, and greater trajectory tortuosity compared to those at 9 °C, despite spending a similar amount of time in movement. These behavioural shifts suggest metabolic adjustments enabling enhanced resource exploration at warmer temperatures. The findings are contextualized within the evolutionary history of the amphipod genus Niphargus, shaped by past climatic, geological and hydrological conditions, which may have selected for eurythermal traits in some lineages. These adaptations highlight potential to exploit habitats across a broad temperature range, not necessarily providing an advantage to N.longicaudatus due to the complex effects of climate change on groundwater ecosystems. This work underscores the importance of integrating behavioural, metabolic, and paleoclimatic perspectives in understanding the impacts of climate change on subterranean biodiversity and distribution.