1 | INTRODUCTION
The feeding habits of fish are reflected by their digestive organ, mainly the intestine. Scholars generally classify fish feeding habits as herbivorous, carnivorous, omnivorous and filter-feeders according to feeding method and food content. The intestine tract is the main site for digestion and nutritional uptake, which has been regarded as a key organ in fish nutrition (Kumar et al., 2005; Wang et al., 2018; Zhou et al., 2021). The fish digestive enzymes activities are closely related to the diet consumed and the ability of fish to digest and absorb different nutrients (Bakke et al., 2010; Liu et al., 2021). Evidently, previous studies found herbivorous fish such as Roho labeo (Labeo rohita ) and Japanese eel (Anguilla japonica ) had stronger amylase activity compared to carnivorous fish such as Great white catfish (Wallago attu ) (Agrawal et al., 1975) and rainbow trout (Oncorhynchus mykiss ) (Hidalgo et al., 1999). Therefore, the influence of feeding habits on digestive enzymes activities is beyond doubt.
The feeding habits of fish also affect digestive tract structure and intestinal microorganisms (Li et al., 2019; Meng et al., 2014; Valdes et al., 2018). Interference with intestinal morphology such as muscularis thickness and villi width affect nutrients absorption and intestinal microbiota (Limbu et al., 2018). Fish gut microbiota contribute to digestion and affect gastrointestinal tract development and overall growth of fish (Clements et al., 2014; Ghanbari et al., 2015). However, feeding habits (Larsen et al., 2014; Meng & Nie, 2019; Roeselers et al., 2011), which determines the feed composition consumed (Benson et al., 2010; Spor et al., 2011; Sullam et al., 2012) has been reported to shape microbial communities in fish (Larsen et al., 2014; Meng & Nie, 2019; Roeselers et al., 2011). Accordingly, diet has been reported as a dominant source of variation in the microbiota composition of rainbow trout (Desai et al., 2012; Ingerslev et al., 2014). The disruption in intestinal microbiota induced by feeding habit via diet usually affect digestive functions of the host through disturbance in bacterial digestive enzyme production (Ghanbari et al., 2015). Certain gut microbiota such as the cellulolytic enzyme-producing bacterial community, which were isolated from the intestinal tract of herbivorous fish species are known to metabolize a remarkable variety of substrates (Li et al., 2016) improving growth performance. Therefore, several studies have explored the manipulation of gut microbiota through diet to improve fish growth (Fan et al., 2021; Li et al., 2019; Pan et al., 2021). However, studies exploring the relationship between feeding habits, digestive enzyme activity, intestinal structure and gut microbial composition in fish are currently limited. Such a knowledge gap limits our knowledge on designing appropriate approaches for feed formulation and feeding practices in aquaculture.
China is currently the largest producer and consumer of cultured fish (FAO, 2022), including Grass carp (Ctenopharyngodon idellus ), a herbivorous fish (Liu et al., 2017) and mandarin fish (Siniperca chuatsi ), a carnivorous fish (Shen et al., 2021). Aquaculture production in China also include species such as Dianchi high-back crucian carp (Carassius auratus ), an omnivorous fish (Shi et al., 2017) and Kunming Schizothoracin (Schizothorax grahami ), a filter-feeding fish, which are endemic to Yunnan, China. These species are the main economic fish produced in Yunnan because of their nutritional value (Zheng et al., 2016). To ensure continued production of these species, knowledge on the influence of feeding habits on digestive enzymes, intestinal morphology and microbiota composition is needed for effective feed formulation. The present study compared the relationship between feeding habits, and digestive enzyme, intestinal morphology and intestinal microbiota of C. idella, S. chuatsi ,C. auratus and S. grahami as herbivorous, carnivorous, omnivorous and filter-feeding fish representative species, respectively. The results obtained provide a scientific basis for development of appropriate formulation of compound fish feeds.