Elasmobranch reproduction and embryology. It's just a topic that has always been interesting. Sharks, rays, and skates are all rather closely related. But we have skates- which lay eggs (oviparity); rays- which give live birth, via internal egg incubation (ovoviviparity); and sharks- which, depending on the species, lay eggs, incubate the eggs, or experience a more "mammalian" pregnancy known as vivparity. And then, there is the intrauterine cannibalism that is known in the species Carcharias taurus, where the fetal sharks are still developing, the yolk sacs have been depleted, so the fetal sharks eat the others within the uterus, resulting in the birth of two offspring (sharks have two uteri, so one from each).

And then there is the parthenogenesis phenomenon, that we only know about because of captivity. Certain sharks- notably bamboo sharks- if absent of a mate, can trigger a response that causes ova to mature and develop into a clone of the mother. This is also known to happen in lizards.

And a large number of female elasmobranchs can also store the sperm of males after copulation to fertilize their eggs for years.

It's just a fascinating topic all around.