Mandarin fish (Siniperca chuatsi) is highly sought after for its delicious taste and rich nutritional value (Song et al., 2017; Li et al., 2019). With a thriving market and annual farming production exceeding 400,000 tons in China (2022), mandarin fish aquaculture significantly contributes to the country’s food security (Li et al., 2017). Mandarin fish have a very peculiar feeding habit, as they only accept live prey fish and refuse dead prey fish or artificial feed in the wild (Yi et al., 2013). Traditionally, mandarin fish have been fed with live prey fish. However, this method presents several challenges. The supply of live prey fish is unreliable, and the prey fish might be a source of pathogens or parasites (Tao et al., 2007), potentially jeopardizing the health of farmed fish. These limitations hinder the sustainable development of mandarin fish aquaculture. Exploring nutrient metabolism and related signaling pathways is essential in order to achieve precise nutrient regulation and meet the demand for fish health. Meanwhile, the level of domestication of mandarin fish in the juvenile stage has been greatly improved (Liang et al., 2001). Therefore, mandarin fish can be farmed with an artificial diet in recent years (Li et al., 2015). According to incomplete statistics, approximately 1/10 of mandarin fish in China have been fed with artificial diets. However, studies into the precise feed nutrition of mandarin fish have just started, and there are great differences in the nutritional composition of artificial diets from different companies. Moreover, concerns have emerged regarding its potential impact on fish health and potential farming failures during the feeding process.

The liver plays a central role in regulating lipid and glucose metabolism throughout the body (Watt et al., 2019). In mammals, metabolic imbalances leading to abnormal accumulation of lipids and glycogen in the liver can trigger the production of proinflammation cytokines, ultimately contributing to chronic liver inflammation (Badmus et al., 2022). Similar observations have been made in several fish species, such as large yellow croaker (Larimichthys crocea) (Fang et al., 2021), blunt snout bream (Megalobrama amblycephala) (Dai et al., 2019), spotted seabass (Lateolabrax maculatus) (Guo et al., 2023), and largemouth bass (Micropterus salmoides) (Zhao et al., 2024). In these cases, unsuitable artificial diets disrupt hepatic metabolism and induce oxidative stress and inflammation. However, the specific molecular mechanisms by which artificial diets affect mandarin fish remain unclear.

Therefore, this study aimed to investigate the effects and the underlying mechanisms of artificial and natural diets on the metabolism and immune response in the liver of mandarin fish. By designing an artificial diet formulated to completely replace live prey fish, we sought to establish a theoretical basis for optimizing mandarin fish feeds.

In the past decade, advancements in mandarin fish feed nutrition research (Li et al., 2023; Xie et al., 2023) and improved domestication techniques for juveniles (Guan et al., 2020; Lu et al., 2023) have opened the door for using artificial diets in mandarin fish aquaculture. However, feeding artificial diets might affect the health status of mandarin fish and increase the probability of farming failure. The underlying mechanisms of these effects required further investigation.

This study therefore employed liver transcriptome analysis of mandarin fish fed with an artificial or natural diet to explore the dietary effects on metabolism and immunity. The transcriptome analysis revealed significant changes in the metabolism-related pathways after feeding different diets, particularly those involving lipid and carbohydrate metabolism. These findings were consistent with a previous study demonstrating altered transcript levels of several genes related to fatty acid biosynthesis pathway in hybrid mandarin fish fed with an artificial diet (Guan et al., 2020). The transcriptome analysis revealed that artificial diets affected the Toll-like receptor signaling pathway and NOD-like receptor signaling pathway. These receptors are responsible for recognizing and initiating the response of the immune system to defend against infectious pathogens (Dasari et al., 2008; Chu and Xu, 2020). Dysregulation of these receptors, along with their downstream signaling pathways, may lead to uncontrolled inflammation or pathogen transmission (Cook et al., 2004). According to the results of transcriptome analysis, in this study we systematically investigated the effects and mechanism of artificial diets on hepatic lipid and glucose metabolism and immunity response in mandarin fish.

In the present study, the artificial diets increased hepatic lipid accumulation by promoting FAS activity, which may be attributed to a potentially higher fat content in the artificial diets than in live prey fish. However, mRNA expression of lipid synthesis was inhibited in fish fed with an artificial diet, suggesting a potential feedback regulatory mechanism to protect against lipotoxicity. The results were similar to our previous study where artificial diets induced excessive TG accumulation in the muscle of mandarin fish (Fang et al., 2024). The regulation of lipid metabolism in an intertissue manner needs further study. In addition, previous studies have demonstrated that unsuitable artificial diets induced hepatic de-novo lipogenesis (Pang et al., 2021) and inhibited lipid secretion (Cao et al., 2019), leading to abnormal lipid accumulation. These results collectively indicated that artificial diets disrupt lipid metabolism in mandarin fish, leading to abnormal lipid accumulation in the liver.

Carbohydrates are an important non-protein energy source that are commonly incorporated into artificial diets (Hatlen et al., 2005). However, the ability of fish to utilize dietary carbohydrates varies, especially in carnivorous species, such as largemouth bass (Liu et al., 2017) and yellowtail kingfish (Seriola lalandi) (Booth et al., 2013). Carnivorous fish are characterized by continuous postprandial hyperglycemia after feeding with a high-carbohydrate diet (Stone, 2003). In the present study, the proportion of carbohydrates in artificial diets is higher than in live prey fish. The artificial diets induced excess glycogen accumulation in the liver by activating the gluconeogenesis pathway. However, the activity of PK and mRNA levels of genes was related to the glycolysis pathway, which might be a feedback regulation of the fish to decrease hepatic glycogen accumulation to some extent. These results indicated that the artificial diets disrupted glucose metabolism in the liver of mandarin fish.

Reactive oxygen species (ROS) are primarily generated in the mitochondria (Thannickal and Fanburg, 2000). While normal levels of ROS play a role in cellular functions (Lee et al., 2012), excessive production of ROS under stress conditions can induce oxidative stress, a highly toxic state for cells (Cichoż-Lach and Michalak, 2014). Previous studies have found that increased levels of hepatic lipid and glycogen will generate more substrates entering mitochondrial respiration, leading to increased production of ROS (Styskal et al., 2012; Sun et al., 2020). In the present study, long-term feeding of artificial diets significantly decreased the transcription level of antioxidative stress-related genes and the enzymatic activities of antioxidant defense systems and increased MDA content in the liver. MDA is an important biomarker of oxidative stress (Ding et al., 2015). Artificial diets significantly increased the hepatic MDA level in mandarin fish hybrid (Siniperca chuatsi ♀ × Siniperca scherzeri ♂) (Li et al., 2017). Thus, artificial diets destroyed the balance of oxidation and antioxidation, leading to oxidative stress in the liver. In addition, oxidative stress is largely thought to negatively affect lipid and glucose homeostasis, which might further aggravate metabolic homeostasis disorder (Sekiya et al., 2008; Zhang et al., 2024).

Numerous studies have found that oxidative stress induces inflammation by activating multiple pathways. The excessive ROS activate NF-κB, leading to TNF-α secretion (Anderson et al., 1994). ROS activate NOD-like receptor protein 3 (NLRP3) inflammasome, resulting in the release of more proinflammatory cytokines (Schroder and Tschopp, 2010; Zhou et al., 2011). In the present study, the artificial diets upregulated the il-1β and ifn-γ transcriptional levels. Previous studies have shown that the activities of alkaline phosphatase and lysozyme in the serum of mandarin fish hybrid fed with an artificial diet were lower than those in the serum of fish fed with a natural diet, which indicated that the artificial diets reduced the immunity of fish (Li et al., 2017). Collectively, the artificial diets induced inflammation in the liver of mandarin fish. Supplementation with suitable additives is an important way to improve the health of fish. For example, bile acid alleviated hepatic lipid accumulation and oxidative stress induced by high-fat diets (Ding et al., 2020). Supplementation with exogenous bile acids improved intestinal immune function in grass carp (Peng et al., 2019). Chinese herbal additives have been proven to have positive effects on improving fish health (Ji et al., 2021; Xia et al., 2022). Thus, in our future study, we will study several additives that can mitigate the issues caused by the replacement of live prey fish with artificial diets.

The artificial diets disrupted hepatic lipid and glucose metabolism, leading to oxidative stress and inflammation, thus affecting the health status of mandarin fish. Thus, the effects of artificial diets replacing live prey fish for the feeding of mandarin fish need further study, especially in metabolism, oxidative stress, inflammation, and additives.