Shark pre-birth battles

It's a shark-eat-shark world and in some species, shark embryos cannibalise their siblings in the womb...
22 June 2021

Interview with 

Deborah Foote, The University of Queensland

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Let’s take a peek at how sharks begin their lives. These fish have three main ways to make baby sharks, one is by laying eggs and another is by giving birth to live young. Some species, like grey nurse or sand tiger sharks, use a mixture of the two and it can get brutal: baby sharks eat each other inside their mother! To dive deeper into how sharks reproduce, Charlotte Birkmanis spoke with Deborah Foote.

Charlotte - With everything going on, life is hectic at the moment. So how about a beach walk? Walking along the beach, you kick something with your toe. After the initial surprise wears off and you stop the dog from trying to eat it, you see a tough, dark brown spiral, about eight centimetres wide and 15 centimetres long. You dust the sand off it, stop the dog from trying to eat it again and realise it's an egg case from a Bullhead shark. And although they look quite different from the traditional idea of an egg, these empty egg cases occasionally wash up all over the world. This gets you starting to think about sharks, and of course a certain tune comes to mind... There is so much more to these creatures of the deep than just their teeth, and we have so much more to learn about them, but let's start from the beginning.

Charlotte - Where do baby sharks come from? Egg-laying is used by a small number of shark species. These eggs are often palm sized and come in a variety of colours, shapes, and textures, depending on the species of the shark. We often call these mermaid purses. Some shark species carry the young in a similar way to us, with the baby in a womb attached to its mother with a placenta, but other shark species do something in between. They produce eggs, but don't lay them. The eggs develop inside the mother and are nourished by an egg yolk until they're ready to hatch, and then they're born live. Let's find out about one species that has one of the most fascinating starts to life, though I might be a bit biased because I'm a shark scientist! For them, life starts with a cannibalistic pre-birth blood battle. It's a shark eat shark world out there.

Deborah - In Australia we call them Grey Nurse sharks, but they're also known as Spotted Ragged Tooth sharks or Sand Tiger sharks in other places in the world.

Charlotte - That's Deborah Foote from the University of Queensland

Deborah - And the pups will hunt and consume their brothers and sisters while in utero.

Charlotte - Now that is some intense family rivalry going on right there! But there's also some competition going on at the genetic level as well.

Deborah - The mother will mate with multiple partners, the pups could have different dads. Only one pup ends up being born from each uterus.

Charlotte - Yeah, you heard that right, Grey Nurse sharks have more than one uterus - they have two!

Deborah - Because they have eaten the other brothers and sisters, so by the end of it they've essentially had one father for their offspring. The first pup that hatches, so the biggest one, is the most likely to be the surviving pup because they can eat its small brothers and sisters, which are usually the ones that are from other dads.

Charlotte - But the food supply doesn't end there - after they're finished snacking on their siblings, their mother keeps the canapes coming by supplying a virtual conveyor belt of unfertilised eggs to nourish their offspring. This gives them quite the advantage when they're out in the world and have to start fending for themselves.

Deborah - They're born quite large, so up to about a metre total length, which is quite big if you're wanting to avoid being eaten by bigger fish. They've had practice already feeding themselves by eating eggs and other embryos in the womb.

Charlotte - They can also swim in the womb and even go from one uterus to the other, this may help them access those little hard to reach eggs and embryos.

Deborah - One of the reasons that this mode of embryonic nourishment has developed - so being able to feed itself inside, practice swimming inside - is that then they're at a competitive advantage when they get to the outside world because they already know how to feed themselves, they're already well-practiced at swimming and because they're quite big, they're less likely to be eaten by other predators.

Charlotte - Although somewhat scary looking Grey Nurse sharks are not aggressive. But the pups? They sure are feisty!

Deborah - The researcher that discovered that Grey Nurses eat eggs while in the womb to nourish themselves had cut open a dead pregnant female shark and was reaching inside to have a look inside the female reproductive organs, felt like he had had a bite on his finger. It wasn't enough to puncture the skin, and of course then when he opened up further and had a look at the pups, he realised that they had quite well advanced dentition. So a lot of baby sharks, before they're born, have a sheath over their teeth, but that is not the case in Grey Nurse. They have nice sharp teeth for eating those eggs and the other embryos as required during gestation.

Charlotte - And this pregnancy can go on from nine to twelve months, that's a long time for the Mum to be carrying around pups squirming around and eating each other inside her. So the next time you see a shark gliding around, or watch one on TV, think of the investment that the mother has gone into to get it this far

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