Abstract
Although the sperm of externally fertilizing fishes usually has a brief life span of up to a few minutes, this study showed that the sperm of the three‐spined stickleback Gasterosteus aculeatus moved for several hours in brackish water and up to at least 10 h in the presence of ovarian fluid. Three‐spined sticklebacks were able to spawn in waters ranging from full‐strength sea water to fresh water, an ability unusual among fishes. The influence of salinity on sperm motility was examined, using three‐spined sticklebacks from sea (salinity 30), brackish (5·5) and freshwater (0) populations. All three populations were found to have sperm with long motility periods in brackish water, lasting 165–270 min. Seawater three‐spined sticklebacks had sperm motile for up to 65 min in sea water, whereas sperm from fresh‐ and brackish‐water fish were quiescent in this medium. In fresh water, sperm from all three populations showed a very brief motility period, lasting