Fish are the most numerous and diverse group of vertebrates. They occupy all types of aquatic habitats differing in salinity (from fresh to sea water), depth (from surface to nearly eight kilometers depth), and temperature (from -2 to 44 °C). The structure of fish reproductive system as well as their mode of reproduction are equally diverse. In contrast to external fertilization in most modern actinopterygian fishes, internal fertilization in cartilaginous fishes was retained from common ancient ancestor of fishes, reptiles, birds, and mammals. The same evolutionary origin of internal fertilization in extant chondrichthyan fishes and other groups of vertebrates predicts common traits in the structure and function of germ cells.

In frame of the Czech Science Foundation project (No. 16-03754S “The evolution of sperm capacitation: pioneering study in taxonomically isolated cartilaginous fish”), the researchers of our faculty in close collaboration with the members of Neotropical Ichthyology Laboratory, Universidade Estadual Paulista “Júlio de Mesquita Filho” (UNESP), Ilha Solteira, Brazil have studied some aspects of sperm biology in ocellate river stingray Potamotrygon motoro (representative of freshwater cartilaginous fish, see Fig. 1).

For the first time for cartilaginous fishes, sperm motility parameters were evaluated, and we found that stingray spermatozoa differ a lot from actinopterigian sperm in terms of structure, physiology and motility characteristics. They are unusually elongated cells, morphologically more similar to spermatozoa of birds and amphibians rather than to actinopterigians’ ones (see Fig. 2). They acquire motility in the male reproductive tract and can increase their velocity by contact with the uterine fluid in the female reproductive tract, a process similar to epididymal sperm maturation and capacitation well known in mammals, while possessing a lipid composition mostly similar to birds. The 3D motion of spermatozoon flagellum, giving rise to a screw-like progression of this gigantic cell, is also complicated and under our specific attention now (see video). All described characteristics are very specific among the animal kingdom, and we are fascinated by these unusual motile cells and looking forward to the next steps in understanding their functioning.

Detailed information can be found in the following articles:

- Dzyuba, V., Shelton, W.L., Kholodnyy, V., Boryshpolets, S., Cosson, J., Dzyuba, B., 2019. Fish sperm biology in relation to urogenital system structure. Theriogenology 132: 153–163.
- Dzyuba, V., Ninhaus-Silveira, A., Kahanec, M., Veríssimo-Silveira, R., Rodina, M., Holt, W.V., Dzyuba, B., 2019. Sperm motility in ocellate river stingrays: evidence for post-testicular sperm maturation and capacitation in Chondrichthyes. Journal of Zoology 307: 9–16.
- Dzyuba, V., Sampels, S., Ninhaus-Silveira, A., Kahanec, M., Veríssimo-Silveira, R., Rodina, M., Cosson, J., Boryshpolets, S., Selinger, M., Sterba, J., Dzyuba, B., 2019. Sperm motility and lipid composition in internally fertilizing ocellate river stingray Potamotrygon motoro. Theriogenology 130: 26–35.

Ocellate river stingray.

Scanning electron micrographs of ocellate river stingray and rainbow trout (right top) spermatozoa. See huge difference in total spermatozoon size and head length.

Spermatozoa of ocellate river stingray swimming in seminal fluid. Negative phase contrast video-microscopy.