Reduced input of photosynthetically produced oxygen during the night affects diel shifts in DO concentration. Fluctuations of dissolved oxygen (DO) concentration on the other hand, are mostly negligible on land but can be subject to pronounced rhythms in water bodies. The large heat capacity of water and other factors, such as rainfall or vertical mixing, can affect daily temperature cycles ( Hut et al., 2013 Häfker and Tessmar-Raible, 2020). In deep and/or turbid water bodies, ambient light levels may reach values where day and night are indistinguishable. Light and temperature, reliable terrestrial diel time cues ( Hut et al., 2013), can be less predictable underwater. Many aquatic habitats are characterized by rhythmic changes in environmental conditions, although these may differ from terrestrial cycles. The predictable nature of these cycles has made it possible for organisms to adapt to, and anticipate, environmental change in order to optimize the use of resources and environmental conditions ( Häfker and Tessmar-Raible, 2020). Rhythmic behavior is driven by cyclical (abiotic) shifts in environmental conditions, such as the day/night cycle, the lunar cycle, and seasonal changes, e.g., in temperature and rainfall. This study establishes a new technology to record EODs in the field and provides a window into the largely unknown behavior of mormyrids in their natural habitat.įrom the morning songs of birds to the seasonal migrations of salmon, rhythmic patterns of behavior are ubiquitous in nature across all taxa of animals ( Wulund and Reddy, 2015 Häfker and Tessmar-Raible, 2020). We hypothesize that being dark-active and tolerant to hypoxia increases the resistance of these fish against predators. The fact that fish showed pronounced nocturnal activity patterns in the laboratory and in the open areas of their habitat, but not under floating vegetation, indicates that light intensity exerts a direct effect on their activity. Nocturnal increase of movement range coincided with diel declines in DO concentration to extremely low levels. In the wild, fish preferred structurally complex habitats during the day, but dispersed toward open areas at night, presumably to forage and interact. Under laboratory conditions, both species showed increases of activity and exploration behavior that were closely synchronized to the onset of the dark phase. We tracked the EOD and swimming activity of two species of mormyrid weakly electric fishes ( Marcusenius victoriae and Petrocephalus degeni) over diel cycles in the laboratory, and we recorded EODs and environmental dissolved oxygen (DO) concentration and temperature over several months in a naturally hypoxic habitat in Uganda. Weakly electric fishes provide an excellent opportunity to overcome some of these challenges because they generate electric organ discharges (EODs) to sense their environment and to communicate, which can be detected non-invasively. To understand animal ecology, observation of wildlife in the natural habitat is essential, but particularly challenging in the underwater realm. 2Department of Biology, McGill University, Montréal, QC, Canada. ![]() 1Behavioral Physiology, Institute of Biology, Humboldt-Universität zu Berlin, Berlin, Germany.Stefan Mucha 1†, Franziska Oehlert 1, Lauren J.
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