BY Susana E Navajas
Founder & Director, Save The Sharks
You’ve most likely seen it all over social media, “Shark Gives Virgin Birth,” or something of the sort. However, the reality is far more complex and interesting that just lazily giving it the title of ‘virgin’ birth. It’s called parthenogenesis, and it’s as fascinating as it sounds. Parthenogenesis, Greek for ‘virgin birth’, is the “…production of offspring without fertilization by a male…” (Robinson et al., 2011). To further clarify it, there is also automictic parthenogenesis which is a type of asexual reproduction “…characterized by the production of an egg through meiosis, followed by the fusion of the ovum with one of its sister polar bodies, producing a diploid zygote with elevated homozygosity compared to its mother.” (Robinson et al., 2011).
Whoa. In more common terms, after cell division has happened, instead of using one male cell and one female cell, two female cells are used and fused together to create a zygote, which has elevated homozygosity (basically having zero genetic diversity) as opposed to heterozygous (which has both male and female). As if sharks weren’t cool enough, right?
The first ever well studied case of a parthenogenesis in sharks was with Bonnethead sharks in captivity. It was “…significant because the well-documented capture history of these sharks is inconsistent with sperm storage by the mother as the probable explanation.” (Chapman et al., 2007) It was improbable that they were capable of sexual activity prior to the capture, so any theories of sperm storage are void. Due to these sharks being very well studied, and the births being well documented, it provided the “…first evidence for asexual reproduction in the most ancient jawed vertebrate lineage.” (Chapman et al., 2007)
Chapman describes the moment he saw the results as a watershed moment. “Up until then, we logically assumed male and female mate and they give birth,” he says. “It was one of those just great eureka moments you have in science, when bam, you see this picture come out of nothing, when you see something that just completely surprises you.” (Holtcamp, 2009)
Upon further studies, they found that since a male was not used at all, only female offspring were produced, which is consistent with automictic parthenogenesis. One of the main proposals for why parthenogenesis takes place is due to the fact that in certain situations, “…female sharks have difficulty encountering suitable mates.” (Chapman et al., 2007)
Furthermore, these documented cases may be expected to “…result in both a substantial loss of genetic variation…” and it could also “…result in unexpectedly high inbreeding, loss of genetic variation and changes in founder contribution.” (Hedrick, 2007) Since these female sharks do not have male mate counterparts, they learn to adapt, however, certain adaptations may contribute to genetic limitations because there just isn’t enough genetic “material.”
There was no information on whether this occurred in other chondrichthyan species or in the wild. “The big question was, was this a fluke, or is this unique to hammerheads [a recently derived lineage of elasmobranchs]?” Chapman said. “As it turns out, not even a couple weeks after the first paper was released, there was a newspaper report of another shark potentially doing this, a blacktip. This time I didn't bet any Guinness on it.” (Holtcamp, 2009)
Bonnethead sharks were the first verified case, and the second case of parthenogenesis in sharks were found within Blacktip sharks, in captivity. Due to there being several cases, this could indicate that “small bodied sharks have evolved parthenogenesis as a means to avoid reproductive failure in situations when males are scarce within isolated habitat patches since small shark species tend to have more limited dispersal capabilities than larger species.” (Chapman et al., 2008) Which is further backed up by the fact that almost all of the cases have involved egg-laying species that have small adult size bodies (less than 2m in length).
Parthenogenesis has only been documented for sharks in captivity thus far, but one of the theories for sharks in the wild possibly exhibiting the process is that “…the widespread population collapses occurring for many sharks due to overexploitation may increase the expression of automictic (also known as automixis) if females have difficulty finding mates at low population densities and significant numbers of their ova are left unfertilized.” (Chapman et al., 2008) Simply put, if females cannot find males with whom to mate, they begin this asexual reproductive process.
Within these two documented cases, the females have only produced a single offspring, and none have lived more than three days. Enter the White-Spotted Bamboo shark. In 2010, a genetically verified case of automictic parthenogenesis was verified, and the offspring survived for 5 years or more. What makes this sharks’ case most interesting is that it was “…purchased by the hobbyist as an egg case and was hatched and reared with no contact with other bamboo sharks.” (Feldheim, et al., 2010) Seven eggs in total, within a 6-year period, were deposited and four of them developed all female embryos.
Prior to the Bamboo shark, both documented cases were within oviparous (egg laying) species, but once the Bamboo had verified parthenogenetic offspring, the taxonomic variation became diverse because they are placental viviparous (birth giving) species. In addition, the Bamboo shark provides “…the first evidence in sharks that multiple offspring can be produced, and that they can live for long periods.” (Feldheim, et al., 2010)
“Females actually will continuously lay eggs whether they have a male or not. Normally people would assume these are infertile eggs and throw them in the garbage,” Chapman said. It appears eggs from this species (bamboo sharks) seem to undergo parthenogenesis quite commonly. “What actually happens is a female lays 20 to 30 eggs, and 1 to 3 develop,” says Chapman, who is on the trail of genetically confirming parthenogenesis in these sharks. (Holtcamp, 2009)
Then, in 2011, another verified and documented case of a shark producing embryos and pups in the absence of a male was reported. It was a Zebra shark and “…a total of 15 pups were produced, over a period of four consecutive years.” (Robinson et al., 2011) Now having four different species of sharks being documented for having parthenogenesis reproduction, “…it is of general biological interest to determine how widespread and common it is among sharks.” (Chapman et al., 2008).
Again, with the Zebra shark, all of the embryos were female, which pairs up with parthenogenetic reproduction. With this case, like all of the others, the question of whether any male genetic material was used, as “several species of shark are known to store sperm for several months after copulation” (Robinson et al., 2011) Also, like all of the other cases, through DNA testing, it was shown that no paternal contribution took place. Therefore, “this finding is thought to provide the first genetically confirmed successive virgin birth for chondrichthyans as well as the fourth verified case of parthenogenesis in sharks and the first for the family Stegostomatidae.” (Robinson et al., 2011)
It’s been well established that aquarists believe more species are capable of parthenogenesis, but since larger species could easily eat the smaller newborn pups, it is hard to tell. Given that parthenogenesis does not seem to be a rare occurrence for sharks in captivity, it can lead to questions about how common it may or may not be for sharks in the wild. A concern, however, with species in the wild doing this is the status of genetic variability. Due to the fact that the pups are half clones of the mother, genetic diversity in shark populations could be reduced. With less diversity, comes physiological and anatomical defects, which make it harder for them to properly function in the wild.
The fact that shark numbers are declining worldwide, could result in species resorting to parthenogenesis as a primal form of survival. This, of course, has not been properly verified. Other negative effects noted could be that “…shark fisheries often intensively fish a particular location which also increases the chance that all of one sex could be wiped out in a particular region.” (Holtcamp, 2009)
Parthenogenesis has a further disadvantage for sharks: Through sexual reproduction, sharks can deliver up to 15 pups per litter; with parthenogenesis, in every case only one pup has been delivered. With egg-laying species, only a few develop from a clutch. So, while parthenogenesis is an extremely interesting process, it may not have the best positive effects on shark populations. The best thing to do, would be to further observe, document, and verify these cases within captivity and see what can further be learned from sharks, and their spectacular adaptive qualities.