Blood cell identification and hematological analysis during natural sex reversal in rice field eel (Monopterus albus)

Abstract Hematology can provide valuable information for the evaluation of fish health. Rice field eel (Monopterus albus), a typical protogynous hermaphrodite fish, is famous for a fairly high nutritive value as a popular farmed fish in some Asian country, especially in China. Basic characteristics and reference values of hematology is poorly understood in M. albus, which hinder disease prevention and breeding in aquaculture. In this study, cell types, changes and typical parameters of peripheral blood were identified and systematically analyzed in M. albus during natural sex reversal. Erythrocytes, leukocytes and thrombocytes were distinguished, meanwhile, their cellular and ultrastructural features were analyzed among female, intersex and male individuals. The number and area of erythrocyte in males was significantly higher than that in both females and intersex. After detecting the serum estradiol and 11-keto testosterone level, changes of erythrocyte are the same as the trends among female, intersex and male in 11-keto testosterone level. Interestingly, dividing erythrocyte was found in the peripheral blood among the three types individuals, more dividing erythrocytes were found in intersex. The differential leukocyte count (DLC) revealed predominance of lymphocytes, followed by neutrophils and monocytes, and showed significant differences among female, intersex and male. Our findings revealed the types, cellular and ultrastructural features, and dynamic changes of blood cell during sex reversal in M. albus. It provides hematological reference for breeding, disease prevention and control in protogynous hermaphrodite fish, especially M. albus.