Shark eat Shark World

Let's get this straight, it really isn't a dog eat dog world out there. Dog cannibalism just doesn't happen that often. For sharks on the other hand, cannibalism is a bit more of a regular occurrence; in fact, there is one species in which every shark becomes a cannibal before even being born.

That species is the sand tiger shark (also known as the grey nurse shark or spotted ragged-tooth shark).

Sand tiger sharks definitely look scary enough for that title, but don't worry they're unlikely to attack humans unprovoked and their mouths are too small for a fatal bite. Image credit: R.A. Killmer (Flickr)

For their very first meal sand tiger sharks eat their brothers and sisters in what can best be described as a real life Hunger Games.

Enter the arena

Just like with many other things in life, this story begins in a uterus. What's unusual though, is that this uterus is an arena for one of natures most violent 'games'.

The game begins at fertilisation. The sharks' first mission is to grow, not necessarily to become the biggest, but just to be the first shark to be big. But why on earth does it matter who's big first?

Well the answer is simple. Once they're big enough they can start removing their competition. That's right, all's fair in love and war (although this scenario seems to involve a lot more war than love). They won't even wait for a fair fight, as soon as they're done with the food their mother provided them (the egg yolk) they can start chowing down on their siblings before they can fight back.

This is known as 'embryophagy', or also 'adelphophagy' (literally meaning "brother eating"). It may sound horrific but it's actually rather nutritious, and once they're done feasting on their siblings, there are plenty more unfertilised eggs for them to gorge on (this is a surprisingly common strategy among sharks).

But how on earth do they eat so early in life? Well these embryos come packed to the brim with sibling-munching teeth. Yep, imagine an embryo sprouting rows of razor-sharp slashers. It's a horrifying concept that sounds like it could come directly from a Stephen King novel.

The teeth of the embryo may not be as developed as the adult teeth shown here, but this gives a slight idea of the nightmarish sight you'd find in this 'arena'. Image credit: Travis (Flickr) 

At the end of it all, a victor emerges from each uterine horn, each one 1 metre long and packed full of teeth. Ready to go straight into the world as a fish-chomping machine.

But why?

Probably the most important thing to understand here is that children are costly. I'm not meaning that sharks have their own form of child support, I mean it quite literally in terms of resources. It takes a lot of nutrients to build a baby, especially if those babies are coming out of you as 1 metre long capsules of muscle and teeth.

The other important factor to consider is the reproduction beforehand. Female sand tiger sharks aren't exactly very picky when it comes to choosing mates. As a result, her eggs are often fertilised by multiple males. She doesn't want to be stuck wasting energy on some wimpy children from the weakest father, she only wants the best.

This strategy solves both of her problems in one fell swoop.

She'll supply the nutrition necessary for their initial growth. But she has fifty children growing inside her, and they grow to about a third of her body length before they're born. Unsurprisingly she doesn't want to supply enough food for fifty babies that size, so instead she provides it to none of them. As soon as they've grown enough to start eating each other they can get the rest of their nutrition from her "failed experiments".

And what better way to create the best offspring than to apply one of the most rigorous forms of natural selection ever. Fifty go in but only two come out. If they really do have good genetics then they should grow quickly and they should have the aggressiveness to do what needs to be done to survive. It's brutal, but it's definitely effective.

So as you can see it really should be called a 'sand tiger shark eat sand tiger shark' world... You know what, scratch that thought it takes way too long to say. I think we'll just have to stick with the (admittedly fictitious) 'dog eat dog' world.

It's a shark eat shark world, but I still think sharks are pretty cool animals...and they obviously have amazing dance moves. Image credit:ElementOf-Loyalty (Deviantart)

You can read more about sand tiger sharks here and here.