Shark cannibalism has been in the news recently due to the footage shot by the Bazaruto Centre for Scientific Studies (BCSS) showing a half-eaten oceanic blacktip shark still swimming and hunting off the coast of Mozambique. Having been caught by the researchers, it was in the process of being released when a group of sharks, including bull sharks, attacked it. Although it managed to escape, it died twenty minutes later.
Ask any marine biologist and they have probably had a shark taken off the line by another, bigger shark. Dr E. W. Gudger published a paper in 1932 entitled Cannibalism Among the Sharks and Rays, where he lists tiger, hammerhead and ‘mackerel’ sharks as known perpetrators of shark-on-shark attacks. His work recounts stories from as far back as 1867, where sharks were caught with stomachs full of other elasmobranchs, or where scientists witnessed an attack, as in the 1918 case of a female hammerhead eating four others whilst caught in a net together at Cape Lookout.
But whilst we might be horrified by the thought of eating someone of our own species, shark cannibalism is as old as sharks themselves and could be one of the reasons why they are still here.
For some sharks it all begins whilst still in the womb with what is known as intrauterine cannibalism. A pregnant lamniform shark (such as thresher, mako, great white and porbeagle sharks), will continue to produce eggs long after mating as a means of supporting her unborn young. These eggs are consumed by her developing embryos in a process known as oophagy (meaning egg-eating), but given that they are unfertilized, it is not exactly cannibalism as we would know it. The same cannot be said for the second form of intrauterine cannibalism, known as embryophagy.
First identified by the biologist Stewart Springer in 1948 and researched in more detail in a study in 1983, it is also known as adelphophagy: to eat one’s brother.
Although a female sand tiger shark (Carcharias taurus) can carry up to ten embryos, she will only ever give birth to two live young, one from each of her two uteri. She will mate more than once, storing the sperm until she is ready to produce pups, and male sand tiger sharks will often guard her against other males, possibly as a way of ensuring that his sperm fertilizes the eggs before any other. This is particularly important in a species that engages in intrauterine cannibalism. Embryos will hatch when about 49-63mm long with fully formed teeth and an innate knowledge of how to use them. It takes them until they are 100mm before they are strong enough, but then they will begin to hunt, not just oophagy, but also targeting the unhatched eggs and the weaker, smaller embryos. The study found evidence of damaged egg casings and even an embryo that had four smaller embryos in the pharynx section of its throat. It concluded that ‘by the time the embryo reaches a length of 227-340mm, … it will have consumed its intrauterine competitors.’ Only the largest, most aggressive and determined will survive the 9-12 months in the womb.
The father has a big part to play in deciding who this will be. Using microsatellite DNA profiling, a 2013 study was able to show that in 60% of their sampled sharks, the two dominant pups were full siblings from the same father, meaning that ‘competition and sexual selection can still occur after fertilization’. By 2018, researchers also realised that shark embryos can swim from one uterus to the other allowing dominant embryos from one male to cancel out their rivals whichever womb they are in.
As grisly as it may be, there is an evolutionary upside to this. The sand tiger shark pups that emerge are larger than they would have been had they shared the womb with four other embryos, and they are healthier thanks to their rich, sustained diet. Coupled with their size is the fact that they are born having already learnt how to hunt and consume their prey, and this gives the pup a distinct advantage in surviving its early years. Recent research into the extinct Otodus megalodon (also a lamniform shark) has concluded something similar: that intrauterine cannibalism may have been one of the reasons why their pups were already two meters in length when born, giving them a colossal advantage, even over some adult sharks.
The fact that the dominant embryos are usually from one father ensures that only the strongest males will have pups, eliminating weaker specimens in the gene pool. There are also benefits for the mother. Female sharks are often injured during mating, and, marine biologist, James Gelsleichter believes that intrauterine cannibalism allows the female to be submissive to any subsequent mate without it risking the need for her offspring to come from the dominant male.
Life for a baby shark does not get easier on birth. Young pups are a natural and easy prey for larger predators, which accounts for why species such as lemon and blacktip reef sharks give birth in shallow waters, inaccessible to larger sharks on the hunt for easy pickings. For the pups, however, it is not just stranger-danger that poses a risk. Siblings still prove a threat and will have no qualm in eating their weaker brothers or sisters. Worst still, filial cannibalism is also practiced by sharks, meaning that mothers will eat their own young (something also performed by cute fluffy bunny-rabbits!). Fossilized excrement from an Orthacanthus shark that lived during the Carboniferous Period 300 million years ago, contained the teeth of juvenile sharks of the same species, indicating that this is nothing new. As the apex predator in freshwater swamps, it is assumed that they resorted to eating their own young when other resources ran out. It is written in the DNA of an adult shark to save itself.
It’s not surprising then that, with this start, sharks will continue to feed on each other as adults. This weeds the weaker fish from the gene pool, and they have an ability to pick up on signals emitted by a shark who is in trouble. This may be what ultimately decided the fate of the blacktip in Mozambique. On being caught it would have released a distress signal, attracting the attention of other sharks – when they arrived it was still attached to the line, trapped and unable to outmanoeuvre the predators. For the bigger sharks, it was an easy kill.
The evolutionary upshot of all this remains the survival of the fittest and biggest. Cannibalism both before and after birth has its advantages: it gives new-born pups a better chance of surviving, clears weaker specimens from the gene pool and ensures that the DNA from the stronger males dominates the lineage. Calling them cannibals makes it sound as if there is something unnatural in what they are doing – there is not. In their world, eating to survive is their primary drive and anything that is in the water is fair game. If that happens to be another shark, their brother or even their own young, then so be it.