The sharks that have made Aliwal Shoal one of the most famous dive destinations on the planet are a source of constant fascination not only for our guests but for us, too. Even after leading baited dives day in and day out for the past two decades, we are still in thrall to our sharks. We are constantly learning new things about them, and each new thing we learn only serves to confirm what we’ve always known- that these are amazing creatures without which our ocean would be a much poorer place. They are the result of thousands of years of evolutionary perfection, and to celebrate that truth, we’d like to share just one amazing fact about each of the four species most commonly seen on and around the reefs of the Shoal.
Oceanic blacktip (Carcharhinus limbatus)
Oceanic blacktips are one of a just a few shark species that have been proven to be able to reproduce parthenogenetically. A reproductive technique usually reserved for plant and insect species, parthenogenesis literally translates as ‘virgin birth’- an appropriate name, as it describes the phenomenon whereby female blacktips can give birth without having sex. Also observed in bonnethead sharks, white spotted bamboo sharks and zebra sharks, the first instance of an oceanic blacktip reproducing in this way was recorded in 2008. In that year, a female blacktip named Tidbit living in Virginia Aquarium was discovered to be pregnant despite having had no opportunity to mate with a male blacktip since her capture as a pup from the wild eight years previously. DNA testing proved that Tidbit’s embryo was indeed the product of parthenogenesis, as it showed no male DNA at all. Parthenogenesis results in offspring that have exactly half of their mother’s DNA, can only produce female pups, and in blacktips at least, produces only one pup at a time.
Ragged-tooth (Carcharias taurus)
Ragged-tooth sharks, or raggies as we know them on the Shoal, also have a unique method of reproduction- one which in their case, is seen in no other species at all. This phenomenon is known as intrauterine cannibalism, and is an evolutionary tactic that ensures ragged-tooth pups are born equipped with the best chances of survival possible. Female raggies have two uterii, and when pregnant, each one has its own clutch of eggs which eventually hatch out to become tiny embryonic sharks. In each uterus, the most dominant embryo will feed on its brothers and sisters until it is the only one left, at which point it is strong enough and well-nourished enough to be completely independent from the moment of its birth. For this reason, raggies only ever produce two offspring (one from each uterus), each of which emerge fully developed and measure approximately a metre in length. Their unusual size makes ragged-tooth pups more likely to reach adulthood, and intrauterine cannibalism is therefore an effective (if grisly) example of the survival of the fittest.
Tiger (Galeocerdo cuvier)
Tiger sharks have earned themselves a reputation as the dustbins of the sea, thanks to the diversity of their diet. Depending on the size and maturity of the tiger in question, their feeding habits can include any number of species, ranging from fish to seabirds, to crustacea, large marine mammals and turtles. Their opportunistic feeding habits mean that they have even been known to consume strange, inedible objects like floating car tyres and licence plates- whilst in Hawaii, terrestrial animals including horses have been found among their stomach contents. Tiger sharks are able to enjoy such a varied diet because of their uniquely shaped teeth, which are the same on both their upper and lower jaws. Boasting both a sharp point and tiny serrated cusplets along their edges, tiger shark teeth and are designed to cut and saw simultaneously. The strength of tiger shark dentition is what allows them to cut through almost anything- including the bones of large marine mammals, and a turtle’s tough shell. The tigers’ incredible diet is also facilitated by the square shape of their heads, which allows them a better grip on their victim than the pointed snouts of other shark species.
Bull (Carcharhinus leucas)
As well as having the strongest bite of all recorded cartilaginous fishes, bull sharks (or Zambezi’s as they’re known locally) are unique for their ability to tolerate fresh and saltwater environments equally. Whilst they are apex predators of the marine world, bull sharks are just as comfortable in estuaries, rivers and even inland lakes. As well as being found in coastal waters in temperate and tropical areas the world over, bull sharks have been seen more than 4,000 kilometres up the Amazon River; in the lake of an Australian golf course; in America’s Mississippi River and even in Lake Nicaragua- while South Africa’s own Brede River is home to some of the planet’s biggest specimens. Amazingly for an ocean-going species, scientists believe that bull sharks are capable of living permanently in freshwater and that a lack of sufficient food would be the only limiting factor preventing them from doing so. The bull shark’s tolerance for freshwater is made possible through adaptations of its rectal gland, kidneys, liver and gills, which work together to balance sodium and chloride levels within the shark’s body depending on the salinity of the water around it.