Most fishes are reproducing externally in the water, and this requires a broad set of evolutionary adaptations that will allow to support the whole process, including the specific structure of the gametes or features which support a new organism while developing in the water. These adaptations include in particular the evolution of a dense protective shield around the egg, which in most fish species has only one minute opening, called the micropyle, allowing the penetration of the spermatozoa through it.

Fertilization in the water entailed the appearance of a sophisticated system of spermatozoa motility initiation and further support of propagation using the “power” of external factors, e.g., ionic content or osmolarity of the medium servings as triggers. Many sea invertebrate organisms, which are broadcast spawners, use the water to spread the chemical signals from their eggs to the male gametes, allowing them to find each other. The scientists have uncovered the amazing membrane-associated system of receptors, channels and other molecules, making possible the precise guidance of the spermatozoa on its way to the egg in these marine invertebrates.

Freshwater fish are quite unique among all externally fertilizing organisms due to the specific conditions of the environment in which they live and reproduce, In particular, one of the features is the extremely low osmolarity, which acts extremely negative on the cells. This conditions make the need for a specific support of cell encounters even more apparent. The eggs of many externally fertilizing freshwater fish species are released into the external milieu surrounded by a coat of ovarian fluid which contain various ions, proteins, amino acids, sugars, etc. in proportions being ideal for supporting and protecting eggs and sperm against the harmful effect of freshwater.

There are data that show the positive effect of ovarian fluid or substances released by the eggs on the behavior of male gametes and finally the outcome of fertilization. Some of the research results show the biased choice of genetic material from a specific parent caused by the presence of ovarian fluid, e.g. resulted from the enhanced sperm motility traits of particular male etc. The specific mechanisms of this selection in externally fertilizing fish are still unclear, which makes further research in the field highly promising.

The members of the Laboratory of Reproductive Physiology are studying the phenomena of post-copulative female control over gamete encounter believing that new knowledge in this area will contribute not only to the fundamental physiology of reproduction but also to the optimization of artificial reproduction technologies.

More information can be found in the following article: Kholodnyy, V., Gadêlha, H., Cosson, J., Boryshpolets, S., 2020. How do freshwater fish sperm find the egg? The physicochemical factors guiding the gamete encounters of externally fertilizing freshwater fish. Reviews in Aquaculture 12: 1165–1192.

 

Hypothetic scheme of how sperm guidance and selection may occur in freshwater fish reproduction.
Trapping of carp spermatozoa by the ovarian fluid. You can see the video here.