Changes in external osmolality and ionic composition affect Megaleporinus obtusidens sperm motility

•Osmolality reduction triggers Megaleporinus obtusidens sperm motility.•Activating solution osmolality affects the percentage of motile sperm and velocity.•Composition of the activating solution influences sperm swimming path.•Intermediate osmolality extends motile phase and retains a high percentage of motile sperm. Understanding the effects of environmental factors on sperm motility characteristics can increase artificial reproduction efficiency in species that do not spawn naturally in captivity, such as Megaleporinus obtusidens. This study evaluated the effects of the osmolality (25, 85, 145, 205, 265, and 325 mOsm kg−1) and composition of activating solutions (NaCl, KCl, or fructose) on the percentage of motile sperm, and the swimming velocity and path straightness of M. obtusidens spermatozoa. The concentrations of major ions in the seminal fluid were also assessed and Na+ (74.46 mmol L−1), K+ (37.24 mmol L−1), and Cl− (114.29 mmol L−1) were the most abundant. When the activating solution was hypertonic (325 mOsm kg−1) compared to the seminal fluid (293 mOsm kg−1), sperm motility was completely inhibited. A wide range of osmolalities that initiated sperm motility were identified for all three solutions. Both, the percentage of motile sperm and the motility duration were reduced (P 50%) up to 40 s after activation and the motile phase lasted for >50 s, regardless of the activating solution composition. Over the postactivation time, the curvilinear velocity and straightness were similar (P > .05) for fructose and NaCl solutions, whereas KCl solutions induced a higher (P