Extremely low-frequency magnetic fields (ELF-MF) up to 100 μT exhibit impacts on physiological processes, including heart function. The mechanisms underlying the influence of these fields on fish heart rates remain insufficiently explored. We assumed that the direct impact of ELF-MF with a frequency close to the heart rate could entrain oscillatory processes responsible for autonomously maintaining heart rhythm in zebrafish embryos. Embryos' heart rates ranged from 1.44 to 3 Hz depending on age, and ELF-MF with frequencies precisely matched, 10% higher, or lower than the heart rate were applied. Additionally, embryos experienced ELF-MF with amplitudes varying by an order of magnitude. Almost all tested ELF-MF induced an increased heart rate effect. This effect was the most pronounced when the exposure occurred earlier during ontogenesis. Fields with frequencies close to the heart rate did not entrain cardiac contractions in zebrafish embryos. A significant negative correlation between heart rate increase and ELF-MF frequency was observed for ELF-MF with amplitudes of 1.98–3.2 μT and 46.8 μT but not 30 μT. Probable molecular mechanisms underlying these effects are discussed in terms of magnetic influence on radical pairs within biochemical oscillating processes.

The species Moina micruraKurz, 1875 (Crustacea: Cladocera), which is new for the Vistula Lagoon of the Baltic Sea, was recorded there for the first time. Over the past years, the species has formed...

Zebrafish (Danio rerio) are widely used in behavioral neuroscience and ecotoxicology to study stress-related responses. Traditional plus maze paradigms rely on arm preference, with arms differing in perceived safety. Here, we propose an alternative approach using a plus maze with identical arms, where stress is assessed based on movement patterns rather than preference. Fish explored the maze for 20 min under baseline conditions, followed by exposure to hyperosmotic stress (5‰ salinity). Behavior was quantified using automated video tracking, including locomotor activity, spontaneous alternation, and transition patterns between arms. Salinity stress did not significantly affect overall locomotor activity or spontaneous alternation. However, it markedly altered movement structure. The proportion of circular transitions decreased significantly. Most notably, transitions between adjacent arms declined, while transitions between opposing arms increased to nearly 50% of total entries. The runs test analysis further indicated an increased tendency toward repetitive movement sequences under stress conditions. These behavioral changes were associated with significantly increased whole-body cortisol levels. The findings identify a shift toward opposing-arm transitions as a potentially sensitive and previously underappreciated behavioral correlate of stress in zebrafish. This metric may complement conventional behavioral endpoints and offers particular promise for automated high-throughput applications in ecotoxicology and preclinical research.