4 | DISCUSSION
Feeding habit reflects directly the
digestive ability of fish to
different nutrient components. The ability of fish to digest and utilize
different nutrients in feed is affected by the structure of digestive
tract, the secretion of digestive
enzymes and the composition and diversity of intestinal
microbes.
This study explored the relationship
among feeding habits of omnivorous, carnivorous, herbivorous and
filter-feeder fish species and
digestive physiology, intestinal morphology, and intestinal microbial
diversity. We also predicted the functions of microbiome from the four
fishes. We found clear differences
in the enzymes activities of the four fish species depending on their
feeding habits. Evidently, the carnivorous fish (S. chuatsi ) had
higher trypsin and pancrelipase activities in the hepatopancreas and
enteropeptidase in the intestine than herbivorous (C. idella ),
omnivorous (C. auratus ) and the filter-feeder fish (S.
grahami ). The variations in digestive enzymes are caused by the
different feeding habits (Xu et al., 2011). Accordingly, the activities
of trypsin, intestinal protease and pancreatic lipase were roughly
carnivorous > omnivorous > herbivorous,
reflecting the feeding on animal materials with high protein and lipid
requiring secretions of related enzymes to digest them. Similarly, Liu
et al. (2014) reported higher protease activity in carnivorous fish than
omnivorous and herbivorous fish.
However, the intestinal lipase activity in this study was higher in
herbivorous and omnivorous fish species than filter-feeder and
carnivorous fish species. Contrary, Parrizas et al. (1994) reported
higher lipase activity in carnivorous fish than herbivorous and
omnivorous fish. The higher lipase activity in stomachless fish is
probably due to the relationship between intestinal tissue structure and
digestive enzymes (Pan et al., 1996). In our study, the mandarin fish
(S. chuatsi ) represents a carnivorous fish with a stomach, grass
carp (C. idellus ) and Dianchi high-back crucian carp (C.
auratus ) are typical herbivorous and omnivorous species, respectively,
without stomach and, Kunming Schizothoracin (S. grahami ) is a
filter-feeder fish with an enlarged sac between eosophagus and
intestinal tract, which can secrete digestive fluid and perform some
stomach functions. Therefore, the higher intestinal lipase activity in
the stomachless fish was due to secretions of the enzyme from the
intestine, which performs some functions of the stomach. Interestingly,
this study found higher pancreatic amylase and intestinal amylase
activities in herbivorous than carnivorous species. Similarly, Li et al.
(2012) found higher amylase in herbivorous than omnivorous and Liu and
Zhang (2001) reported higher amylase activity in omnivorous fish than
carnivorous fish. The higher amylase activity in herbivorous fish is
related to the utilization of carbohydrates in the feed. Similarly,
omnivorous fish can use higher carbohydrate levels than carnivorous fish
(Li et al., 2015a).
This study found significant differences in intestinal villi height,
villi width and muscularis thickness among the four fish. These
variations are due to the morphological and structural characteristics
of the fish gut reflected by the different feeding habits (Liu & Zhang,
2001; Zeng & Ye, 1998). The intestinal microvilli of carnivorous fish
had many branches and complex structures. The complexity of the
microvilli (villi length and villi width) and muscularis thickness, was
in the order carnivorous > herbivorous >
omnivorous. The intestinal
structure accommodates, transports and digests feed and absorbs
nutrients. The height, width and complexity of intestinal microvilli
increase the surface area for digestion and absorption of digested
nutrients (Sun et al., 2019). The muscularis thickness is composed of
smooth muscle, which promotes the movement of food in the intestine
through rhythmic relaxation and contraction. The thickness of the muscle
layer directly reflects the contraction and peristalsis ability of the
intestine. Accordingly, strengthening intestinal contraction and
peristalsis is an effective means to increase feed digestion and reduces
chyme circulation (Bian et al., 2021). Generally, carnivorous fish have
shorter intestines (Day et al., 2014). Accordingly, carnivorous increase
the complexity of the intestinal structure to reduce circulation rate of
chyme and enhance absorption of nutrients. Therefore, the complexity of
the intestinal structure is adapted
to the feeding habits of fish so as to achieve fully absorption of
nutrients.
The intestinal microflora of vertebrates plays an important role in host
nutrition, (Liu et al., 2016;
Valdes et al., 2018). Many studies have shown that dietary feeding
habits (Miyake et al., 2015; Zhou
et al., 2021) and host species (Li et al., 2019; Youngblut et al., 2019)
are the main factors affecting the
gut microbiota of fish. This study also found that the diversity of fish
gut microbiota was significantly affected by feeding habits and host
species. The diversity of gut microbiota species and abundance determine
the stability of the host gut microbiota associated with host nutrition
(Kuang et al., 2020). The community diversity of the four fish species
followed the order omnivorous > herbivorous >
filter-feeder > carnivorous. Contrary, Li et al. (2015b)
reported higher bacterial diversity in the gut of the filter-feeding
than herbivorous. It has been reported that the higher the Shannon
index, the better the stability of the bacterial community and the
better the digestion of nutrients (Zhang et al., 2019).
The results of this study showed Proteobacteria and Firmicutes phyla as
typical dominant flora in the gut of the four fish species.
Proteobacteria and Firmicutes are two phyla typical dominant flora in
the intestine of many fish, such as Oncorhynchus mykiss(Ingerslev et al., 2014), Nibea coibor and Nibea
diacanthus (Li et al., 2019), Megalobrama terminalis (Liu et
al., 2021), Micropterus salmoides (Zhou et al., 2021) andSymphysodon haraldi (Zhang et al., 2021). The four fish species
had variations in abundant of microbiota species. The different
symbiotic bacteria carried by fish species may be caused by the
selective enrichment of different microorganisms due to variations in
feeding and host species. A previous study indicated that during
evolution, hosts tend to acquire suitable environmental bacteria by
recognizing adhesion mechanisms on the cell surface (McFall-Ngai, 2015).
In addition, the differential enrichment of specific flora under
different feeding conditions may also
adapt to the function of the flora,
for example, Bacillus species and Cetacea species are
potential candidates for probiotics (Larsen et al., 2014),Pseudomonas species produces vitamin B12, andFusobacterium species produces butyrate (Zhou et al., 2019).
Accordingly, the carnivorous gut was dominated by A. johnsonii ,A. lückii and P. stutzeri bacteria species, which may
contribute to the digestion of proteins, while Bacteroidesspecies were dominant in herbivorous fish gut may help the host to
digest cellulose. The presence of these microbiome in the fish species
are useful in host nutrition. Indeed, the intestinal microbes of the
carnivorous (S. chuatsi ) showed higher protein digestion
potential and lower carbohydrate digestion potential, while gut microbes
of herbivorous fish (C. idellus ) showed lower protein and high
carbohydrate digestion, consistent with their feeding habits.