One of the joys of moving to Australia is its wildlife - and, for me, most especially its sealife.
Snorkelling and diving has been an absolute delight. And seeing the sharks (the safe ones) is a great part of that pleasure.
Today I want to tell you about Grey Nurse Sharks.
They are also pretty seriously endangered. Due to their mean looks and size, it used to be assumed that they were a danger to swimmers and were so heavily fished and speared in response, that they were the first shark species to gain protected status.
But today I want to focus on their babies, 'cos Grey Nurse Sharks are totally unique in an aspect of their reproduction and its definitely worth sharing.....
Grey Nurses are viviparous.
But in both cases the same problem arises - how to feed the developing embryo? In all cases there needs to be enough food for the embryo to grow enough to fend and hunt for itself - no sharks care for their babies after hatching/birth. Now egg layers put food in the egg (the yolk) to feed the developing embryo and that's pretty much their only option, but in viviparous sharks there are lots of options - some do the yolk thing too, just the baby embryo consumes it inside the protective uterus, others have a type of placenta (or a weird placental milk) to feed the developing young.
Truly the weirdest of them all are the oophagous ones. These crazy beasts of the sea do the whole internal fertilisation thing, then embryos start to develop in the uterus using a bit of supplied yolk to keep them going, and quickly they start to grow TEETH. The mum-to-be then releases unfertilised eggs into the uterus as food for these toothy, little embryos to gobble up! Ah sweet!
"Kids! Eggs for breakfast!"
Now that is, in my book, pretty weird, but the weirdest oophagous creature,
the weirdest viviparous, oophagous, cartilaginous creature of them all has got to be....
the Grey Nurse Shark! (remember her? this is a fun fact about her)
Twenty or more eggs are fertilised and travel to two uteruses (uteri?). There, they are fed on unfertilised eggs for a while
AND THEN
the largest embryo in each uterus KILLS and EATS each of its siblings!! Thus providing further nourishment!
This feast of brother/sister omelette provides a lot of energy for growth - the 2 "babies" that are finally born are over 1m long (1/3rd of mums size) and already practiced at the hunt!
"Kids! Your brothers and sisters for breakfast!"
This charming process is known as "intrauterine cannibalism" and may seem harsh but does make evolutionary sense - only the fittest will leave the womb, and there is no waste!
The only other case of sibling rivalry I have come across that is close to this extreme is in Tasmanian devils where 20+ rice grain size embryos are born, but there are only 4 teats in the pouch. So the race is on and only the fastest 4 can survive!
Next time you argue with your brother or sister, just be glad you're not a shark!
Happy Biology Fun Fact Tuesday!