DIVE LOG Australasia

Baby Tiger sharks can be up to sixty centimetres long. On average, 41 pups are born. The egg would have to be the size of a basketball to create such a big baby. It stands to reason that there must be a supplementary source of nourishment for the developing embryonic tiger shark. The way it is done is the uterus secretes a nutritive fluid called ‘uterine milk’ which bathes the eggs. Nutrients are absorbed into the yolk and supplement the feeding of the foetus. It is not really milk but a simple food source. The egg is not laid as in Port Jackson sharks. The egg is held within the uterus and develops entirely from the yolk and the uterine milk inside. To put this in an easy way to understand, you could imagine a hen which does not lay the egg. Rather, it retains the egg until the chick is fully developed. The chick then pops out fully formed. This method is known as Ovo-vivaporous …. Live birth from an egg. YOLK SAC PLACENTA Hammerhead and Whaler sharks have taken this to the next level. Eggs are laid inside the uterus. The foetus begins its development from the egg yolk. Blood vessels develop from the wall of the uteri and grow into the egg yolk. It is not exactly a placenta but it functions to replenish

nutrients to the foetus via the egg sac. It is called the Yolk Sac placenta. This is getting biologically sophisticated for a mere egg laying fish. However, there is more! The fact that sharks have been evolving for hundreds of millions of years has meant that they have developed even more ingenious strategies to ensure that the next generation is far more likely to be a success. Recent research has found that some sharks use two or more forms of egg nourishment inside the womb. Baby sharks use the yolk first, then the yolk sac placenta develops and finally Intra-uterine milk assists with egg growth. INTRA-UTERINE CANNIBALISM. The most bizarre and amazing technique of development with the womb of sharks are the two forms of Intra-uterine cannibalism. TWO FORMS OF INTRA-UTERINE CANNIBALISM • 1. Egg Eating Embryos The first is the Egg Eating Embryos. This is the next level of sophistication up from the Yolk Sac Placenta of Whaler sharks. This technique, known as Oophagy, is when the ovaries produce a continuous supply of eggs to feed the developing embryo.

This Pygmy Ornate wobbegong from Port Stephens is so heavily pregnant that she could barely move.

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DIVE LOG Australasia #408 - October ‘24

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