Different fish have different sperm head shapes, such as embolic, spherical or oval. The neck is extremely short or not obvious; The tail has flagella. Fish sperm tails and nuclei are arranged in a straight line, and some tails and nuclei are arranged at a certain angle. Sperm size varies from species to species. Chondrichthyes have the longest sperm, such as stingrays, which are 215μ m long. However, the sperm of bony fish is short, such as perch, which is 20 μ m.
Because semen has the functions of protecting, nourishing, balancing osmotic pressure, agglutinating and even anesthetizing sperm, sperm is inactive in semen and keeps for a long time, but after entering water, it is activated by oxygen in water and becomes active immediately. When sperm moves in water, a small amount of energy is used for movement, while most energy is used for osmotic pressure adjustment. Freshwater fish sperm can only adjust osmotic pressure in low osmotic pressure environment, that is, prevent protoplasm from absorbing water from the environment. However, in high osmotic pressure environment (such as high salinity seawater), it can not play a regulatory role, that is, it can not prevent protoplasm from losing water. Therefore, in hypertonic seawater, the protoplasm of freshwater fish sperm will condense due to water loss and lose the ability of exercise and fertilization. But some fish sperm, such as tiger fish, can adjust osmotic pressure in both seawater and fresh water, so they can reproduce in both seawater and fresh water.