What do we know about the sex life of sharks?

Sharks have an extraordinary natural history and display incredible variety in their reproductive techniques. Amongst the 400 species of shark their five broad methods of reproduction vary from simple to complex, and the bizarre. (Elasmodiver.com suggests there are now close to 500 species.)
Everyone knows fish lay eggs, with millions of them spawned into the water, fertilised, and drifting off into the plankton where the vast majority simply provide food for countless, hungry plankton feeding creatures.
Fish parents generally take little care of their eggs but sharks are far more advanced!

40% of sharks lay eggs. They are Oviparous.
Forty per cent of sharks lay eggs which develop in the typical fashion. The embryo feeds on the yolk and develops within the egg without any help from the mother. Port Jackson sharks in Australia produce the renowned spiral corkscrew shaped egg the case of which is extremely tough and impenetrable to most predators.

Port Jackson sharks migrate up to 800 kilometres during late winter to join mating aggregations to what I call PJ parties. They last over a month. One August I counted 140 sharks on a single dive at Jervis Bay, stacked up two and three deep, resting after another heavy night of activity.

What exactly is a Port Jackson shark party like? What is their foreplay and mating ritual? Because it happens under cover of darkness probably no one has ever witnessed exactly what they do. What a great opportunity to document their nocturnal behaviour.

In fact the mating behaviour of sharks, one of the most successful life forms on the planet in terms of evolution after the crocodile and turtles, remains a mystery!
Port Jackson sharks have remained virtually unchanged for 100 million years, which means, whatever environmental pressures they have faced, they have survived without the need to change and adapt. They are ideal as they are in terms of their ability to survive.

How sharks mateThe reproduction of all sharks and rays is strictly internal. Male sharks have a pair of intro-mittentorgans called claspers, which are modified pelvic fins. The inner edge of these has developed a tubular structure able to transfer a sperm packet inside the female’s cloaca.  

Claspers are not penises. The do not have any erectile tissue. Instead, they stand at right angles for copulation. The tip of the clasper unfolds to reveal clasper spurs which help prevent the clasper from coming out. (Water is a spermicide and has to be prevented from entering the cloaca so it does not destroy the sperm).

Here is an interesting thought. Shark skin is made of denticles and used as sandpaper by South Pacific Islanders. I have had a close look at shark claspers, usually when a Wobbygong Shark has its head stuck in a crevice and cannot turn around. The Ornate Wobbygong seems to have rough shark skin on the lower half of its clasper along its entire exposed length (see the photograph). The upper side is smooth soft skin. It looks sand papery but only on the lower half lengthways.

I think the lower length of the clasper being covered with the sandpapery denticles may offer the shark some protection.  But it represents a mystery since it makes a problem for the female during mating. Female cloacas must also have some kind of adaptation to the sandpapery claspers of the males.  Does the internal lining of the cloaca also have a sandpapery lining? Or is it smooth? Just how does sandpaper on sandpaper work in the act of sexual intercourse in sharks?  There’s a fraction too much friction.

Male sharks bite the female during foreplay and also during sexual intercourse. It looks brutal but sharks have had 300 million years to sort out what they do, and we need to learn a lot more to understand the whys and wherefores of shark mating.

Female sharks are often much larger than the males. In the case of the Tiger Shark, females are bulkier and far more powerful, and a magnificent display of strength! No male could match the power of a female adult Tiger Shark!  The apparently violent mating process sharks display maybe just that, and obviously our superficial judgements do not reflect on what is really happening.

Live birth is termed VivaporousAll other sharks give birth to live young; that is, babies are born from the mother alive. There are four types of live birth.

1.  Ovovivaporous ...60% give live birth from an egg
Tiger Sharks for example, lay eggs which are retained within their two uteris. The foetus grows over a period of about 16 months, initially getting all its nourishment from the egg yolk. Baby Tiger Sharks can be up to 60 centimetres long and on average, 41 pups are born, and to create such a big baby the egg would have to be the size of a basketball! It stands to reason they must have a supplementary source of nourishment. 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 to supplement the feeding of the foetus. It is not really milk but a simple food source, and the egg is not laid as for Port Jackson sharks, but held within the uterus as it develops entirely from the yolk and the uterine milk.

2. Yolk Sac Placenta - Live birth from an egg
Hammerhead Sharks and Whaler Sharks have taken birth to the next level. Eggs are laid inside the uterus, and the foetus begins developing from the egg yolk with blood vessels developing from the wall of the uteri and growing into the egg yolk. It is not exactly a placenta but functions to replenish nutrients to the foetus via the egg sac. This is called the Yolk Sac Placenta – quite biologically sophisticated for an egg laying fish.
There’s more! The fact that sharks have been evolving for hundreds of millions of years has meant they have developed even more ingenious strategies to ensure the success of the next generation.

3. Intra-uterine cannibalismThe most bizarre and amazing technique within the womb of sharks is Intra-uterine cannibalism. It comes in two forms.
3a Egg Eating Embryos

The first is the Egg Eating Embryos, a further level of sophistication up from the Yolk Sac Placenta. When the foetus has developed a mouth and teeth, the mother is able to feed it with a continuous supply of eggs. The baby shark is fed solids within the womb! This so-called Oophagy occurs in Thresher sharks, Makos, Tawny Nurse Sharks, Basking Sharks and Great White Sharks. (Oophagy means egg eating).

3b Full intra-uterine cannibalism
Finally, there is the most gruesome form of all, displayed in the Grey Nurse, where the largest foetus eats its smaller siblings. After cannibalising its brothers and sisters inside the womb, it continues to consume eggs as it develops into a super baby.

This form of nourishment provides powerful evolutionary advantages. First, it removes opposition. Monster baby has one chance at life but has the deck stacked in its favour. Second, the single baby when born is a fully developed giant, powerful and well able to take care of itself in the wild from day one.
Remember the Tiger Shark has up to 80 babies, most eaten by cod, groper, sea snakes and sharks in the first few weeks. Bigger babies solve the problem!  

Footnote: Recent research on Manta Rays by Andrea Marshall of the Marine Megafauna Foundation has discovered that they are ovo-vivaporous. The male has more teeth than the female and uses them to bite the female’s left wing to hold her for mating purposes. Male claspers show considerable damage after mating as does the internal lining of the female’s claoca.‍

Mike Scotland, B. Sc. (Maths & Zoology) was a biology and marine biology teacher in Sydney before retiring to a life given over to diving. He has made over 5800 dives during 41 years and a PADI instructor since 1982. Though he has made dozens of trips throughout the Pacific he says his favourite place is still Sydney for its marine life.