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.

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The deepwater profunda morph of quagga mussel Dreissena bugensis was found for the first time in the European part of its range. The mussels of this morph were found in the Cheboksary Reservoir situated in the midstream of the Volga River (Russian Federation). Traditional and geometric morphometrics confirm the similarity of studied specimens to profunda mussel described from the Great Lakes of North America. In the Cheboksary Reservoir the deepwater mussels live at depth of 26.5 m on sandypebbled substrate at conditions of high water velocity ([0.5 m/s), i. e. in the conditions unusual for American profunda. This fact reflects evident ecological plasticity of this morph. Discovery of the deepwater morph of quagga mussel in the European part of its range indicates that possibility of realization of the deepwater phenotype is inherent to this species. It is suggested that quagga mussel may possess two alternative developmental pathways that could be realized in appropriate conditions. Presumably, certain depth and/or water pressure may serve as signaling factors for activating the ''deepwater'' developmental pathway. The presence of deepwater and shallow-water morphs is very important adaptive feature for sedentary organisms such as dreissenids that are unable to select habitat actively since this feature allows for successful colonization of both deep and shallow water habitats.