Sand tiger shark

A tale of two uteri

If there ever was a fish with a confusing name, it must be the sand tiger shark Carcharias taurus. The confusion arises first, because these sharks also go by other common names. Specifically, they are often referred to as grey nurse shark. Having more than one common name isn’t necessarily all that uncommon (!), nor is it necessarily confusing – many species are known by more than one name. But in this case, there is already a tiger shark (Galeocerdo cuvier) and a nurse shark (Ginglymostoma cirratum), neither of which are the species we are concerned with here. And things get worse! Because scientists’ usual trick to deal with confusion over common names (switching to scientific names) is only partly helpful for sand tiger slash grey nurse sharks: unfortunately, their scientific Latin name, Carcharias taurus, translates to ‘bull shark’. Alas, there already is a bull shark (Carcharhinus leucas)! Bottom line: people are terrible at naming sharks, and anyways, these fish are exciting for their biology, not their linguistic tangle.

The cue to what is cool about sand tiger sharks is indeed in that other name of theirs – the fact that they are nursing their young. I do not know about you, but the word ‘nursing’ sounds to me like ‘being looked after’, ‘being taken care of’. I will let you be the judge of whether that is indeed what sand tiger sharks are all about or whether their biology gives the term ‘nursing’ a rather sinister new meaning.

alas, this equation is completely unrelated to the gruesome reality of sand tiger (grey nurse) shark reproduction

The name was likely chosen because little sand tigers aren’t all that little when they are released into the world. In many sharks, offspring are forced to fend for themselves, often already from before birth (eggs are just deposited and left in the care of the ocean). Sand tiger sharks differ drastically from this: their babies can be more than a meter long when they first have to stand on their own two feet; or rather swim with their own two (or so) fins – either way, they appear pretty well nursed when they can first be observed. And what is more, sand tiger sharks do not produce the hundreds or more offspring per spawning we know from many other cartilaginous fishes. No, they give birth to only two offspring at a time. Now that sounds suspiciously like the reproductive pattern of a mammal, does it not? Obviously, though, sand tiger sharks are no mammals. So what is going on? Do they have some form of pseudo-placenta like some other life-bearing fish? The answer is no – they have a nightmarish form of reproduction. Sand tigers engage in a behaviour termed ‘intrauterine cannibalism’. Behind this somewhat strange term hides sand tiger sharks’ dark secret: each and every one of them starts life guilty of multiple murders: sand tiger sharks that live to be born devour their siblings while still in their mother’s uterus. If you have siblings, you will have likely thought ‘I could kill them’ at one point or another. Sand tiger shark babies do not leave it at that – they go all in when it comes to sibling squabbles.

Siblicide is not particularly rare in the animal kingdom: many birds and mammals kill their younger siblings to gain access to more food; in many insects and fish anything is fair game to developing young, including their own brothers and sisters; and in some frogs the mother provides unfertilised eggs to her young to feed them (this is the reason why some poison-dart frogs are called Oophaga which roughly translates to ‘egg eater’). There are two factors that make the sand tiger shark case particularly interesting, though: first, their cannibalism is obligatory, meaning that it always occurs. In most other systems, siblicide only happens when conditions aren’t perfect – when there is plenty of food around, siblings get along just fine. Not so in sand tigers: there is no other food for them but their brothers and sisters. Second, they are polygamous and the babies feast on fertilised eggs and already hatched siblings. Often in nature, it does not matter to the parents which of their young survives – genetically, they are of the same value (no, you are not special to your parents compared to your sibs. At least not genetically). Or parents may avoid potential conflict over who gets eaten by not bothering to fertilise eggs destined to be fed to their hungry sproglets. To sand tiger dads, however, it matters a great deal whether their sperm help create the fastest growing baby shark (dub dub du dub du, baby shark dub dub du dub du… other parents will understand).

Like this, but only one survives! Let the Hunger Games begin!

Second place is first devoured. This cruel race to cannibalism occurs in both of the female’s uteri, hence litter size in sand tiger sharks is always 2, despite the female producing hundreds of eggs. By the way, the mother couldn’t care less about whose sperm win – it will always be her babies that are born. Ultimately, though, she will want sons that themselves have good sperm that make for fast-developing offspring. This ensures her genes win the race to first cannibal in future generations. So the potential dads really must do everything they can to have their sons and daughters eat everybody else. A female may mate with several males, but two at best get to propagate their DNA. The others only contribute to making more delicious food for the winners’ offspring. You may have many questions now, and so do I: how does this all works out? Is it important to be a female’s first partner? Does the female have a hand (or fin) in deciding when sperm get access to eggs? Can she manipulate whose sperm do get dibs? We don’t know yet.

for some reason, no one wants to be a sand tiger shark gynaecologist

As with basically all sharks, the actual reproductive behaviour of sand tigers is poorly understood and little documented. A lot of room for fascinating future research. So what do you think? Are grey nurse sharks loving parents or have they taken a page out of Rupert Murdoch’s book on ‘how to pin your children against each other for your own maximum entertainment and benefit’?