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.