**1. Introduction**

Due to the various changes caused by the Earth's rotation and autotransfer, all organisms have biological clocks that are autonomous endogenous timing mechanisms within the organism that regulate its adaptation to exogenous rhythmic changes in light, temperature and other environmental factors [1,2]. The daily rhythmicity of animal behavior, physiology, metabolism and immunity is controlled by biological clocks that are genetically synchronized with environmental cycles and can maintain a 24 h rhythm even in the absence of environmental cues [3]. The biorhythmic center that controls periodic changes in biological functions is called the biorhythmic pacemaker [4]. At the molecular level, biological rhythms are regulated through feedback loops formed at the level of highly conserved transcripts [5]. The biological clock consists of biological clock genes and transcription factors involved in the transcription–translation feedback loop, including *BMAL1*, *CLOCK*, period genes (*PER1*/*PER2*/*PER3*) and cryptochrome genes (*CRY1*/*CRY2*) [2,6]. These genes are not only expressed and function in the cells of the biological rhythm centers, but are also present in all tissues and cells of the organism. Therefore, transcription factors have an important role in regulating circadian rhythms [7].

The physiology, metabolism and immunity of most fish are regulated by the biological clock [8]. Additionally, the liver, the main metabolic organ for lipids, is controlled by

**Citation:** Wang, W.; Hu, J.; Fu, Z.; Yu, G.; Ma, Z. Daily Rhythmicity of Hepatic Rhythm, Lipid Metabolism and Immune Gene Expression of Mackerel Tuna (*Euthynnus affinis*) under Different Weather. *J. Mar. Sci. Eng.* **2022**, *10*, 2028. https://doi.org/ 10.3390/jmse10122028

Academic Editor: Nguyen Hong Nguyen

Received: 12 November 2022 Accepted: 16 December 2022 Published: 19 December 2022

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

circadian rhythms [9]. Some studies have found that circadian rhythms exist in reptiles and birds [10,11]. Mammalian lipid metabolism also follows a circadian rhythm [12,13]. A correlation between metabolic pathways and circadian rhythms has also been found in studies of mice rhythmically oscillating [14]. Rhythmic gene expression for lipid metabolism in Atlantic bluefin tuna found in fish studies [15]. In recent years, studies have been conducted on the genetic correlation between mammalian immunity and daily rhythmicity. Studies show that mammals develop innate immune effects and daily rhythmicity during the feeding of mammals exposed to microorganisms associated with food [16]. However, there are fewer studies on the immune system and circadian rhythms in fish [17].

The mackerel tuna (*Euthynnus affinis*) is a species of tunas known as the eastern little tuna, skipjack tuna or kawakawa [18]. The production of mackerel tuna is entirely dependent on fishing, and a large proportion of mackerel tuna is 12–32 cm juveniles since the main fishing methods are seining and trawling. Although the production of mackerel tuna is still increasing, the Catch-MSY model estimates that it is currently overfished and will continue to obey the overfishing trend [19]. Therefore, from the perspective of marine resource conservation, it is necessary to carry out captive breeding of mackerel tuna. Currently, captive breeding of mackerel tuna has been reported only in Japan [18]. Our research team realized the land-based recirculating water culture of mackerel tuna [20]. Although the survival rate and better artificial domestication are guaranteed in artificial culture, there are still many problems. The situation of an unsynchronized circadian rhythm system of aquatic animals with the existing environment caused by the change of relative time under artificial culture conditions may lead to a state of stress and undesirable consequences, such as slow growth and reduction of disease resistance, which is a great obstacle to the development of artificial culture [21]. In this study, the mRNA expression levels of immune, metabolic and rhythm genes in the liver of mackerel tuna were investigated by RT-qPCR. The study aimed to elucidate the daily rhythm expression of lipid metabolism genes, immunity genes and rhythm genes in the liver of the mackerel tuna. This study is essential for this species to be cultivated in captivity to maintain its population and provides basic information to ensure the healthy, green and sustainable development of the mackerel tuna farming industry.

#### **2. Materials and Methods**
