Why is the reproduction of fishes involved in great obscurity?

Because the element in which they reside conceals from us the actions which they perform, so that we are unable to point out, with certainty, the uses of the different organs, or the functions which they exercise. Even in the days of Aristotle, the difference in the mode of reproduction between the cartilaginous and the osseous fishes had been observed ; and although many accurate observations have been made by modern zootomists, much still remains to be done, both in the field of observation and dissection.

In reference to the reproductive system, fishes may be divided into two classes. Thus, some have the sexes distinct, while in others they are united. - Fleming.