The high sperm density, together with the short spermatozoa swimming time, makes European eel sperm manipulation and assessment for quality difficult. Two diluting media (K15 and K30) previously designed for Japanese eel sperm were tested. After 24 h, European eel sperm showed significant reduction in the percentage of motile spermatozoa after activation and different motility parameters (VAP, angular velocity; VCL, curvilinear velocity; VSL, straight line velocity; BCF, beating cross frequency), concluding that these media are not suitable to preserve the sperm of this species. After a hormonal treatment to induce spermiation, sperm volume, density and motility were recorded at weekly samplings. The variation of the osmolality (325-330 mOsm kg )1 ), pH (8.4-8.6) and the ionic composition (concentration of Na + , K + , Mg 2+ and Ca 2+ ) of the seminal plasma were registered. Physio-chemical results were related with sperm quality throughout the treatment, to determine which must be the suitable characteristics of one extender for the sperm of this species, and to find the best conditions to obtain suitable cryopreservation media for European eel sperm. K + concentration increased, while Ca 2+ and Mg 2+ concentrations showed a progressive reduction in correlation with the sperm quality improvement. Na + showed a decreasing, but not significant tendency. P1 and P2 freezing media were designed considering the physio-chemical parameters as well as the ionic composition shown by the best quality sperm samples, and then compared with the previously described solutions, TNK and K30. Sperm quality was determined, checking the percentage of motile spermatozoa and motility parameters using computer-assisted sperm analysis (CASA) software. Samples were frozen after dilution (1:5, 1:20, 1:100) in different freezing media supplemented with 10% dimethyl sulfoxide (DMSO). After thawing, samples frozen with low dilution ratio (1:5) in TNK and P1 media showed higher, although not significant, spermatozoa survival (35.5±14.5 and 36.6±6.7%). The addition of L-a-phosphatidylcholine to the media seems to have a positive effect, as reported in the Japanese eel.
