Remember learning about asexual and sexual reproduction in every single biology course you’ve ever taken? No matter what, teachers constantly bring up the two-fold cost of sex and the benefits associated with each mode of reproduction. Topics are glossed over and examples are given. The majority of examples for asexual reproduction are pulled from prokaryotic organisms or plants, simply because they occur most often.
What teachers don’t always mention is that asexual reproduction occurs in many animal species as well. Although most asexually reproducing animal species are land-dwellers, startling observations have been made that show marine animals can actually produce asexually as well. Even sharks have been known to reproduce asexually by a process known as parthenogenesis.
Countless cases of female sharks reproducing asexually have been made since an initial discovery in 2001 at Omaha’s Henry Doorly Zoo. The aquarium held three female bonnethead sharks that were collected in their juvenile phases three years before. These females had never been in the presence of male bonnethead sharks. One morning, unsuspecting aquarium workers were startled to find that a fully developed and free-swimming bonnethead pup randomly appeared in the tank.
This new and exciting information sparked attention in the reproductive patterns of other captive shark species. More discoveries were made for white-spotted bamboo sharks, black tip sharks, and zebra sharks.
These discoveries posed important questions for more research about shark reproduction in the wild. Unfortunately, the testability of this in the field is very difficult, because sharks are fast, strong, and hard to monitor.
On the one hand, some may speculate that parthenogenesis within wild species is inevitable, because it seems so common in captive populations. On the other hand, some suggest that parthenogenesis occurs because of the degree of stress captive sharks are exposed to. Sharks in aquariums are subject to close living quarters, distractingly loud aquarium-goers, harsh and unnatural light, and tons of electrochemical fields. None of these are present in natural environments.
With all of these different variables, it’s hard to determine whether parthenogenesis is human-induced or natural for shark species. So, what do you think? Do you think we’re causing sharks to adapt from their natural behavior? Or are we just lacking the technology to realize it happens in nature?
-by Echelle Burns
Holtcamp, W. (2009). Lone Parents: Parthenogenesis in Sharks. Bioscience, 59(7): 546-550.
Robinson, D.P., Baverstock, W., Al-Jaru, A., Hyland, K. and K.A. Khazanedari. (2011). Anually recurring parthenogenesis in a zebra shark Stegostoma fasciatum. Journal of Fish Biology. 79: 1367-1382.
Feldheim, K.A., Chapman, D.D., Sweet, D., Fitzpatrick, S., Prodöhl, P.A., Shivji, M.S. and B. Snowden. (2010). Shark virgin birth produces multiple, viable offspring. Journal of Heredity. 101(3): 374-377.