Identification of potential indicators for testis quality in swamp eels (Monopterus albus) through metabolomics analysis

The swamp eel (Monopterus albus) is a commonly cultured freshwater fish. Selection and cultivation of fecund broodstock is crucial for efficient artificial reproduction in this species. However, there are currently no guidelines for selecting high-quality males in M. albus. This study quantitatively investigated the sperm motility of male swamp eels during the breeding season by using computer-assisted sperm analysis system. Variability in the proportion and number of motile sperm was observed between individual males. The testes were grouped into high-quality and low-quality according to sperm quality. Morphometric parameters of the two groups were analyzed. We found that high-quality testes were longer (150 ​± ​27 ​mm verses 127 ​± ​19 ​mm) than low-quality testes. Besides, the proportion of testes with a dark appearance was slightly higher (58.3% verses 48.1%) in the high-quality group than in the low-quality group. Metabolomic analysis was conducted to compare the metabolite profiles of high-quality and low-quality testes. Glycerophospholipids, organic acids, glycerolipids, amino acids, sphingolipids, and nucleotides were the most abundant metabolites in the testes of swamp eels. Moreover, 135 differential metabolites were identified. Several nucleotide derivatives, including 5′-deoxyadenosine, 2′-deoxyadenosine, and ribosyl adenosine, were the most significantly enriched metabolites in high-quality testes. These metabolites are potential indicators for the testis quality in M. albus. Altogether, our work provides systematic and quantitative data about the physiological and biochemical characteristics of M. albus testis, which provides an evidential basis for the selection of high-fecundity parents to improve artificial reproduction efficiency of M. albus.