So, what is parthenogenesis?
Parthenogenesis is defined as the asexual reproduction of a female organism. This process is initiated, usually, by lack of access to a male of the same species, prompting the formation of a female gamete without the need of fertilization. This “virgin birth” can produce one or more offspring that are genetically identical to the mother. Parthenogenesis has been observed in many different elasmobranch species, such as blacktip sharks, bonnetheads, smalltooth sawfish, zebra sharks, white spotted bamboo sharks, and dusky smooth hounds, and even skates, but mostly in individuals confined within in a single-sex aquarium situation. When scientists and aquarists alike first observed this miracle, it was speculated to be the result of sperm storage within the females, but many of these sharks were captured as juveniles and housed alone in an aquarium for an upwards of 10 years with no interactions with males of the same species.
Okay, cool…but how does it work exactly?
Elasmobranchs specifically undergo automictic parthenogenesis, which is when the female’s body is undergoing a reproductive cycle that mimics that of sexual reproduction. Inside the uterus, one egg is formed, accompanied by three polar bodies. If one of the polar bodies is genetically identical to the mother, it fuses with the egg, creating a fertilized ovum, and eventually a single, genetically-identical offspring. It is not fully understood what ecological factors trigger parthenogenesis, but it is hypothesized that lack of male sharks within the environment may be a cause. There is little to no documentation of parthenogenesis of sharks in the wild, but as shark populations are increasingly declining, it is theorized to become more prevalent.
Unfortunately, parthenogenesis is certainly not the cure for the declination of shark populations. Most sharks produce anywhere from 6-40 pups, averaging about 15 per event, dependent upon the species. With automictic parthenogenesis, females will only produce 1, maybe 2 pups at one time. Also, because the pups are genetically identical to the mother, the risk for zero genetic diversity within the population increases. Offspring that are not genetically diverse tend to be weaker and more susceptible to diseases and predation, and ultimately do not survive very long, even within aquarium environments which tend to be more stable and have better surveillance. This is not always the case, as there is one documented case of a captive female white spotted bamboo shark (C. plagiosum) delivering a clutch of 20-30 egg cases, with just a few hatching, and being viable for almost five years.
Scientists still do not have all of the information about parthenogenesis in elasmobranchs, as more information is being analyzed continuously to better understand why and how this process occurs. Properly-managed and conservation-focused aquariums are to thank for allowing scientists to better understand these processes in a controlled environment, as this could be key in better understanding shark reproduction habits in nature. A better understanding of elasmobranch reproduction and biological processes only helps bridge gaps in research, which only aids in the conservation and preservation of these critical organisms and their environments.
Chapman DD, Shivji MS, Louis E, Sommer J, Fletcher H, Prodöhl PA. Virgin birth in a hammerhead shark. Biol Lett. 2007 Aug 22;3(4):425-7. doi: 10.1098/rsbl.2007.0189. PMID: 17519185; PMCID: PMC2390672.
Dudgeon CL, Coulton L, Bone R, Ovenden JR, Thomas S. Switch from sexual to parthenogenetic reproduction in a zebra shark. Sci Rep. 2017 Jan 16;7:40537. doi: 10.1038/srep40537. Erratum in: Sci Rep. 2017 Apr 07;7:45881. PMID: 28091617; PMCID: PMC5238396.
Holtcamp, Wendee, Lone Parents: Parthenogenesis in Sharks, BioScience, Volume 59, Issue 7, July 2009, Pages 546–550, https://doi.org/10.1525/bio.2009.59.7.3
Photos: David Clode - Unsplash