Intestinal characteristics and microbial community composition of rapid‐growing golden‐backed carp ( <i>Cyprinus carpio</i> var. <i>Jinbei</i> ) in rice fields

The golden-backed carp (Cyprinus carpio var. Jinbei) is a fish species that thrives in rice fields. Some populations of this species are known to demonstrate rapid growth; however, the underlying mechanisms and characteristics of rapidly growing groups remain unclear. To investigate these mechanisms, offspring from a single pair of golden-backed carp parents, which were of similar initial body size and morphological specifications, were stocked in a rice field and reared for 93 days. Histological, enzymatic and high-throughput sequencing methods were used to compare differences in intestinal histological morphology, digestive enzyme activity and intestinal microbial communities among different growth groups. Histomorphologically, villus height and the villus-height-to-crypt-depth ratio significantly differed among the three growth groups, whereas villus width, crypt depth, muscle layer thickness and goblet cell abundance did not vary. Amylase and trypsin levels in the rapid-growth group were significantly higher than those in the moderate- and slow-growth groups. The gut microbiota in the rapid-growth group was characterized by the presence of the phyla Firmicutes (41.40%), Proteobacteria (35.33%), Clostridia (16.21%) and Actinobacteria (3.52%). At the genus level, the dominant taxa included Streptococcus (31.22%), Bartonella (29.45%) and Cetobacter (16.21%). In rice fields, golden-backed carp with different growth rates exhibit different survival strategies. The adjustment of survival strategies results from the combined effects of digestive tract morphology, key digestive enzyme activity and specific intestinal microbial communities. This study further demonstrates the adaptability of golden-backed carp to paddy-field culture, providing a reference for improving the efficiency of ecological farming and breeding systems in paddy fields.