Acquisition and Loss of Potential for Motility of Spermatozoa of the Japanese Eel Anguilla Japonica

To date, cultivated male Japanese eels have not matured sexually in captivity under normal conditions. However, spermatogenesis and spermiation can be induced by the injection of gonadotropins. During this study, most males spermiated after the fifth or sixth weekly injection of human chorionic gonadotropin (HCG; 1 IU/g BW/wk) and the milt weight gradually increased as the number of injections increased. Motility of spermatozoa (percent motility after dilution with 450 mM NaCl) from HCG injected males showed significant individual differences and periodical changes after each weekly injection of HCG. Motility of spermatozoa in Japanese eel milt was found to have an intimate relationship between the pH of the milt and concentration of potassium in the seminal plasma. Motility of spermatozoa could be regulated by changes in the ionic constituents of the isotonic incubation media before dilution with a hyperosmotic solution. The percent motility of spermatozoa in eel milt increased significantly after incubating for 60 min in isotonic artificial seminal plasma (ASP), which consists of NaCl + KCl + CaCl 2 + MgCl 2 + NaHCO 3 buffered with TAPS-NaOH at pH 8.1, and with Ca, Mg free-ASP. Motility, however, decreased rapidly in K free-ASP and in HCO 3 free-ASP. These results indicate that acquisition and loss of the potential for motility of eel spermatozoa can be altered by changing the potassium and bicarbonate ion concentrations of the incubating medium irrespective of the initial potential for motility. The methods to obtain good quality spermatozoa described in this experiment should compensate for low volume of milt at the time of artificial fertilization.