The reproductive system of the female shark!
by Christopher N. Owen
This is part two of my series of posts on shark reproduction. The first part covered the male reproductive system. Here I cover the female reproductive system, following the egg cell from formation to maturity, and then discuss adaptations of live-bearing and egg laying sharks. r
Egg cells (but not the complete egg) form in the ovary. Depending on the species, only one ovary may be functional, and it will be expanded across the center of the shark to fill the space of the opposite ovary as well. The ovary is lodged in the epigonal organ. Egg cells leave the ovary through the front end through the ostium and into the tubular oviducts. Oviducts are bilaterally paired, even in species with only one ovary. They soon curve around to direct eggs toward the back of the shark.
There is an expansion in the oviduct called the oviductal gland. The oviductal gland is responsible for the formation of leathery shell tissue in the egg-laying species. It also stores sperm in both egg-laying and live-bearing species.
The egg cells then pass through a region of the oviduct called the isthmus, which leads to the uterus. Like the oviducts, there are two uteri, one on each side of the shark. The two uteri merge near the rear of the shark, where they lead to the cloacal chamber, which opens to the exterior of the shark. (The cloacal chamber has several tubes leading to it, including the uterine canal, rectum, and urinary papillae).
During this journey from the ovary to the outside world, the egg cell will form an embryo. In egg-laying sharks, the embryo will not develop into an animal capable of survival in the outside world until some time after the eggs are laid. However, live-bearing sharks give birth to much more developed, independent young.
There is an evolutionary tradeoff between egg-laying (oviparity) and giving birth to more developed live young (viviparity). All sharks are descended from an oviparous ancestor. Many larger species, however, later developed viviparity. Because the young of viviparous species are more developed, they are more likely to survive after being born, but a direct consequence of having more developed young is that those young are larger. This decreases the number of young that will fit in the mother's uterus, thus making viviparity an advantage only for larger species.
Eggs cases have highly specialized shapes that help to increase their survival rate. Many are flattened and tapered at the ends so that they can be easily wedged into crevices. Some also have extensive filaments projecting from the case, which the mother can tangle around various objects. A few are even threaded, like screws, so that the mother can twist them tightly into position.
This concludes the parts of shark reproduction I care about the most. Now that you know how sharks reproduce, why not form some shark babies of your own?
Source: Klimley, P. A. (2013). The Biology of Sharks and Rays. The University of Chicago Press, Chicago, IL and London, England.
