Sperm motility in Gasterosteiform fishes : The role of salinity and ovarian fluid

Abstract: In externally fertilising fishes, various factors in the surrounding environment may influence the viability of the sperm and eggs, thus determining the success of reproduction. In this thesis, the influence of salinity and ovarian fluid on sperm motility has been investigated in three Gasterosteiform fishes.The three-spined stickleback, Gasterosteus aculeatus, has successfully invaded fresh water and differs from most other fishes in its ability to spawn in waters of all salinities. Our results show that sperm of the three-spined is strongly stimulated by the ovarian fluid surrounding the eggs. In fresh water, where the period of motility is only about a minute, ovarian fluid prolongs motility to last several hours. We show that this effect is due to fluid’s ionic content and that the fluid remains in the nest, surrounding the eggs, for a prolonged time due to the proteins and/or other macromolecules in the fluid. Our results explain how successful spawning can occur in fresh water despite that it takes several minutes for all three-spined stickleback eggs to be fertilised. The stimulating effect of ovarian fluid may be one of the factors that have enabled the originally marine three-spined stickleback to colonise fresh water.The fifteen-spined stickleback, Spinachia spinachia, is exclusively marine. We found their sperm motility to be good in seawater, reduced in brackish water, and non-existent in freshwater. The presence of ovarian fluid made no difference in any salinity, a factor that might have contributed to their inability to colonise fresh water. Being regarded as a primitive member of the stickleback family, the lack of response to ovarian fluid suggests that this is not a primitive trait among the sticklebacks.The male straight-nose pipefish, Nerophis ophidion carries the eggs attached to its ventral surface. Fertilisation has previously been suggested to be external, but our results show that their sperm are not motile in seawater alone, but in a mixture of seawater and ovarian fluid. This result, together with the finding of some sperm in the female genital area, and that the sperm head is elongated, suggests that the fertilisation in the straight-nose pipefish occurs in close proximity with the eggs and ovarian fluid. This could then explain why the straight-nose pipefish has minute testes and complete confidence of paternity.