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Cavefish are vertebrates living in extreme subterranean environments with no light, temperature changes, and limited food. Circadian rhythms in these fish are suppressed in natural habitats. However, they can be found in artificial light–dark cycles and other zeitgebers. The molecular circadian clock has its peculiarities in cavefish. In Astyanax mexicanus, the core clock mechanism is tonically repressed in the caves due to the overactivation of the light input pathway. A lack of functional light input pathway but rather the entrainment of circadian genes' expression by scheduled feeding were revealed in more ancient Phreatichthys andruzzii. Different evolutionarily determined irregularities in the functioning of molecular circadian oscillators can be expected in other cavefish. The unique property of some species is the existence of surface and cave forms. Along with the ease of maintenance and breeding, it made cavefish a promising model for chronobiological studies. At the same time, a divergence of the circadian system between cavefish populations requires the strain of origin to be indicated in further research. [ABSTRACT FROM AUTHOR]

Simple Summary: Most physiological processes are subject to biological circadian rhythms maintained by a complex cascade of biochemical events. The circadian rhythmicity of behavior allows organisms to use energy and resources optimally under changing environmental conditions. To that end, endogenous circadian rhythms are synchronized with external pacemakers (zeitgebers), especially daily changes in illumination. In the 1960s, it was assumed that, in addition to this primary photic cue, animals can use diurnal geomagnetic variation as a secondary zeitgeber. Earlier research found that slow magnetic fluctuations can affect some behavioral endpoints of circadian rhythms by modulating an organism's physiological state. However, no direct experiments to test such an entrainment of biological clocks by artificial magnetic fields were performed due to the technical difficulty of eliminating natural geomagnetic variation. For the first time, we carried out such tests in a fully controlled magnetic environment using zebrafish as a research model. The experimental treatments included various light/dark cycles and continuous illumination coupled with pre-recorded natural geomagnetic variations. The obtained results indicate that slow magnetic fluctuations can entrain endogenous rhythmical activity in vertebrates. Probably, cryptochromes play a key role in this process. This research provides promising opportunities for the magnetic control of circadian processes, e.g., correcting circadian dysfunctions. In the 1960s, it was hypothesized that slow magnetic fluctuations could be a secondary zeitgeber for biological circadian rhythms. However, no comprehensive experimental research has been carried out to test the entrainment of free-running circadian rhythms by this zeitgeber. We studied the circadian patterns of the locomotor activity of zebrafish (Danio rerio) under different combinations of light regimes and slow magnetic fluctuations, based on a record of natural geomagnetic variation. A rapid synchronization of activity rhythms to an unusual 24:12 light/dark cycle was found under magnetic fluctuations with a period of 36 h. Under constant illumination, significant locomotor activity rhythms with 26.17 h and 33.07 h periods were registered in zebrafish exposed to magnetic fluctuations of 26.8 h and 33.76 h, respectively. The results reveal the potential of magnetic fluctuations for entrainment of circadian rhythms in zebrafish and genuine prospects to manipulate circadian oscillators via magnetic fields. The putative mechanisms responsible for the entrainment are discussed, including the possible role of cryptochromes. [ABSTRACT FROM AUTHOR]

The quagga mussel Dreissena rostriformis bugensis is a highly invasive species. It plays an important role in benthic communities, influencing their structure and functioning. Two morphs of this mussel have been described: a shallow-water (i.e. the typical) morph and a deep-water morph, profunda. Currently, profunda has been found in several water bodies only within the nonnative range of D. r. bugensis. In North America, the profunda morph is widespread and abundant in the Laurentian Great Lakes. In Europe, profunda was found for the first time in 2009 in the Cheboksary Reservoir, which is located on the central part of the Volga River. A 2016 search for profunda in the four deepest Volga reservoirs (Cheboksary, Kuybyshev, Saratov and Volgograd) failed to find this morph even at the site where it was found in 2009. Traditional and outline-based morphometric analyses showed that only the shallow-water morph individuals inhabited studied sites at depths of 25–33 m. The present study revealed that morphological differences between the typical and profunda morphs from both the European and North American ranges were accompanied by limited genetic differences. Microsatellite analysis showed only minor differences in allele frequencies between morphs sampled in 2019 from the Cheboksary Reservoir; these differences were not significant after correction for null alleles. High phenotypic and ecological plasticity of D. r. bugensis may facilitate its invasion success. [ABSTRACT FROM AUTHOR]

Over recent decades, changes in zebrafish (Danio rerio) behaviour have become popular quantitative indicators in biomedical studies. The circadian rhythms of behavioural processes in zebrafish are known to enable effective utilization of energy and resources, therefore attracting interest in zebrafish as a research model. This review covers a variety of circadian behaviours in this species, including diurnal rhythms of spawning, feeding, locomotor activity, shoaling, light/dark preference, and vertical position preference. Changes in circadian activity during zebrafish ontogeny are reviewed, including ageing‐related alterations and chemically induced variations in rhythmicity patterns. Both exogenous and endogenous sources of inter‐individual variability in zebrafish circadian behaviour are detailed. Additionally, we focus on different environmental factors with the potential to entrain circadian processes in zebrafish. This review describes two principal ways whereby diurnal behavioural rhythms can be entrained: (i) modulation of organismal physiological state, which can have masking or enhancing effects on behavioural endpoints related to endogenous circadian rhythms, and (ii) modulation of period and amplitude of the endogenous circadian rhythm due to competitive relationships between the primary and secondary zeitgebers. In addition, different peripheral oscillators in zebrafish can be entrained by diverse zeitgebers. This complicated orchestra of divergent influences may cause variability in zebrafish circadian behaviours, which should be given attention when planning behavioural studies. [ABSTRACT FROM AUTHOR]

We describe an impact of the geomagnetic field (GMF) and its modification on zebrafish's orientation and locomotor activity in a plus maze with four arms oriented to the north, east, south and west. Zebrafish's directional preferences were bimodal in GMF: they visited two arms oriented in opposed directions (east–west) most frequently. This bimodal preference remained stable for same individuals across experiments divided by several days. When the horizontal GMF component was turned 90° clockwise, the preference accordingly shifted by 90° to arms oriented to the north and south. Other modifications of GMF (reversal of both vertical and horizontal GMF components; reversal of vertical component only; and reversal of horizontal component only) did not exert any discernible effect on the orientation of zebrafish. The 90° turn of horizontal component also resulted in a significant increase of fish's locomotor activity in comparison with the natural GMF. This increase became even more pronounced when the horizontal component was repeatedly turned by 90° and back with 1 min interval between turns. Our results show that GMF and its variations should be taken into account when interpreting zebrafish's directional preferences and locomotor activity in mazes and other experimental devices. [ABSTRACT FROM AUTHOR]