International Journal of Molecular Zoology 2024, Vol.14, No.3, 166-181 http://animalscipublisher.com/index.php/ijmz 175 7.3 Invertebrates Invertebrates, including insects and marine organisms, also exhibit circadian rhythms that regulate various aspects of their physiology and behavior. The molecular mechanisms underlying these rhythms are remarkably conserved across different species, with core clock genes such as Clock, Bmal1, Period, and Timeless playing central roles (King and Takahashi, 2000; Van Gelder, 2003). In Drosophila, for example, the circadian clock consists of interlocked feedback loops involving these genes, which generate rhythmic patterns of gene expression and behavior (Van Gelder, 2003). Circadian rhythms in invertebrates are essential for coordinating activities such as feeding, mating, and locomotion. In Drosophila, the circadian clock regulates daily patterns of activity and rest, ensuring that the flies are active during the day and rest at night (Van Gelder, 2003). Similarly, marine invertebrates, such as the pico-eukaryotic alga Ostreococcus tauri, possess circadian clocks that regulate their photosynthetic activities and other metabolic processes (O’Neill et al., 2010). Interestingly, recent studies have shown that non-transcriptional mechanisms, such as protein oxidation, also play a significant role in sustaining circadian rhythms in these organisms, highlighting the evolutionary conservation of circadian timekeeping mechanisms (O’Neill et al., 2010). In summary, circadian rhythms are a fundamental feature of animal life, regulating a wide range of physiological and behavioral processes across different taxa. The molecular mechanisms underlying these rhythms are highly conserved, yet they exhibit remarkable flexibility to adapt to the specific needs and environmental conditions of each species. Understanding these mechanisms provides valuable insights into the evolutionary significance of circadian rhythms and their impact on animal health and behavior. 8 Case Studies 8.1 Circadian rhythms in nocturnal animals Nocturnal animals exhibit unique adaptations in their circadian rhythms to thrive in environments with minimal light. The master clock in the suprachiasmatic nucleus (SCN) of the hypothalamus plays a crucial role in synchronizing these rhythms with the external light-dark cycle. Interestingly, the SCN's responsiveness to light is maximal during the night for both nocturnal and diurnal species, indicating a shared mechanism despite their opposite activity patterns (Mistlberger and Skene, 2004). In nocturnal mammals, behavioral arousal during their resting period can act as a potent non-photic synchronizing cue, affecting the SCN's function. This feedback mechanism involves several brain nuclei and neurotransmitters, ultimately altering the molecular function of SCN pacemaker cells. The circadian system's sensitivity to arousal stimuli varies between nocturnal and diurnal species, with nocturnal animals showing reduced light resetting when aroused during their sleep period (Mistlberger and Skene, 2004). Moreover, some nocturnal animals can exhibit prolonged intervals of activity with attenuated or no overt circadian rhythms without apparent ill effects. This phenomenon is particularly observed in herbivores, animals in polar regions, and during specific life-history stages such as migration or reproduction. The underlying mechanisms suggest that some circadian pacemakers continue to measure time even in the absence of overt rhythms, indicating a potential for chronobiological plasticity (Bloch et al., 2013). 8.2 Impact of circadian disruption in captive animals Circadian disruption in captive animals can lead to significant physiological and behavioral changes. For instance, studies on mice have shown that sleep restriction can lead to an 80% reduction in circadian transcripts in the brain and profound disruption of the liver transcriptome. These changes include a significant reduction in the circadian regulation of transcription and translation, as well as core clock genes in peripheral tissues (Goede et al., 2018). In zebra finches, constant light exposure (LL) during development resulted in weight gain, lipid accumulation in the liver, and disrupted circadian gene expression. Despite these disruptions, some birds maintained rhythmic activity, indicating a dissociation between behavior and clock gene rhythms. This suggests that diurnal animals
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