VISION Magazine Publication – Small Wonders
The aquarium at the Burj Al Arab hotel in Dubai is small; in fact you could be forgiven for not knowing it’s there. The only indication that anything aquatic is going on behind the scenes are the mesmerizing tanks of tropical fi sh that line each side of the escalator from the lobby to the first floor; that and a round tank in the Al Mahara restaurant which contains so many different brightly coloured fish in a reef-like environment, that trying to hold a conversation with anyone in there is like trying to talk to a child playing a computer game. Should you ever find yourself in Al Mahara look carefully into the tank and you’ll see two greyish white sharks covered in black spots with a cylindrical body shape, a broad head at one end and a rudder-like tail fi n at the other. They are zebra sharks (also known as leopard sharks), mother and daughter; the subject of an astonishing discovery by the Burj Al Arab’s team of marine biologists and the focus of their scientific paper on parthenogenesis or asexual reproduction. This may be a small team of marine biologists working in a small aquarium but they are punching above their weight with the discovery that the female zebra shark reproduced without a mate. Parthenogenesis has only been confirmed in a few other species of shark, namely blacktip and bonnethead sharks, which both give birth to live young (livebearing – ie not egg-laying), and the egg-laying bamboo shark.
The phenomenon has never been observed in the egg-laying zebra shark before, making this finding an international fi rst for a genetically confirmed parthenogenic birth for this type of shark and only the fourth time it has been seen in sharks at all. “This is the only aquarium in the world that has managed to [observe and] exhibit a parthenogen pup,” points out Warren Baverstock, the aquarium’s Operations Manager. “Breeding zebra sharks is hard in itself. Even without the parthenogenesis this would be interesting,” adds David Robinson, Assistant Operations Manager and principal author of the scientific paper. Back in 2007 a scientific paper was published in the US confirming that a bonnethead shark which had been kept in isolation from a mate had given birth to a pup. Genetic analysis proved the pup to be a clone of the mother. The zebra shark at the Burj Al Arab had also been kept in isolation from a possible mate and so when she produced eggs in 2006, the marine biologists didn’t think anything of it and discarded them. However, the publication of the paper on the bonnethead shark prompted Baverstock and Robinson to keep the eggs their zebra shark produced in 2007. “It was a bit of a long shot,” says Robinson, “because that was a live-bearing shark, completely evolutionarily distant from our shark which is egg-laying.” But the long shot worked out and the aquarium has been producing live pups since those first eggs were kept in 2007. It has not been easy, though, and the fact that asexual reproduction is an extreme form of inbreeding meant that there were many abnormalities, as Robinson explains: “When a shark releases an egg, it splits into four polar bodies; in parthenogenesis one of the polar bodies fuses back with the mother so that all the offspring are genetically identical to the mother but not to each other. It depends which part of the egg fuses back. If the mother is carrying a bad allele [one of the alternative forms of a gene, found at the same place on a chromasone] and mates sexually it may well be overridden, but if she reproduces asexually it’s amplified.”
Even with the tender care of Abdulkareem Vettan, who is in charge of the husbandry of the pups, there have been problems with nutrition. It’s difficult for the team to know whether these problems are genetic or developmental, and the data collection and research are ongoing. The findings of this research are important for marine biologists all over the world who are interested in understanding shark reproduction and evolution. As Robinson points out: “It shows that evolutionarily speaking, when they move back to a common ancestor, it is probable that all sharks across the whole range of sharks can do this.” Dr Aaron Henderson from the department of Marine Science and Fisheries at Sultan Quaboos University in Oman, says: “It also elicits a number of interesting questions such as, do females only revert to parthenogenesis when there are no males available? What are the implications for genetic diversity and fitness? What are the physiological processes giving rise to parthenogenesis? The questions are endless.” As are the future research possibilities, the next stage of which is discovering whether these parthenogen pups can reproduce. The scientific community is particularly excited by the fact that this shark has produced litters of pups parthenogenetically over successive years, because up until now they didn’t know this was possible. It means that this species can reproduce without the input of a male if it has to and suggests that rather than being a one-off response to isolation when in captivity, sharks could, potentially, be doing this in the wild. Most importantly, it means that this shark is able to keep its genes within the genome in an effort to survive. Jennifer Schmidt, Associate Professor of Biological Sciences at the University of Illinois, Chicago points out: “As shark numbers continue to decline there may be more and more populations facing breeding pressures of this type. An increasing rate of parthenogenesis could have both helpful and harmful effects on a dwindling population, maintaining the number of individuals in the short term, while altering sex ratios and reducing genetic diversity in the long run.” With this exciting discovery, the marine biologists at the Burj Al Arab are keen to work with others and share their research. Baverstock and Robinson say they can see a point in the future when they will be twinning with aquariums around the world, sharing knowledge, and continuing to contribute to the global understanding of fish biology.