Grey Nurse sharks

 

See below to read all about Reproductive techniques of sharks. This article was published in Dive log Australia in July 2013. www.divelog,net.au

 

gns and me Grey Nurse shark

Marine Biology of sharks
Shark reproductive techniques

Text and photos by Mike Scotland

 

Sharks have an extraordinary natural history. They display incredible variety in their reproductive techniques.  There are five broad methods of reproduction that vary in complexity from simple to complex and bizarre amongst the 400 species of sharks.

It is wise to remember that sharks are fish. Most fish are bony. Sharks and Rays are cartilaginous.

Everyone knows that fish lay eggs. Snapper, for example produce millions of eggs. They are spawned into the water, fertilised and drift off into the plankton to fend for themselves. This technique is inefficient because most of the eggs are lost to predators. The vast majority of the eggs simply provide food for countless hungry plankton feeding creatures.

Fish parents generally have little to do with caring for the eggs. One exception is the mouth broodering Cardinal fish. Male fish scoop up the eggs and protect them by holding them inside their mouths. Sharks are more advanced when it comes to reproduction!

 

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 produce the renowned spiral corkscrew shaped egg. The egg case is extremely tough and is impenetrable to most predators.

Imagine a Wobbygong that is trying to eat a Port Jackson shark egg. It faces a very serious and probably life threatening problem. Spiral shaped egg cases are designed to fix itself very firmly into narrow spaces. The egg would plug up the throat of the shark and apart from being uncomfortable, it would eventually starve. Wobbygongs would quickly vomit out the egg and never try to eat one again! I also suspect that the tough egg case is quite indigestible. If it did get swallowed, it would plug up the digestive canal, killing the shark.

 

 

Port Jackson sharks migrate up to eight hundred kilometres during late winter to join mating aggregations. These Port Jackson ‘parties’ last for over a month. I have counted 140 sharks on a single dive at Bowen Island one August. They were stacked up two and three deep resting after another heavy night of PJ activity. What exactly is a Port Jackson party really like? What is their fore play and mating ritual? It happens under cover of darkness. It is probable that no one has ever witnessed exactly what they do. What a great opportunity for someone to document the nocturnal behaviour of PJ’s.

The mating behaviour of what is probably one of the most successful life forms on the planet in terms of evolution after the Crocodile and the Turtles remains a mystery. Port Jackson sharks have remained virtually unchanged for one hundred million years. For an evolutionary biologist, this means that whatever environmental pressures that Port Jackson shark face, they have survived without the need to change and adapt. They are so well built that they have reached a pinnacle of perfection. The humble Port Jackson shark is the ‘ideal’ in terms of its survival skills and its ability to survive. Do not be fooled by its low intelligence and slow behaviour. They have been perfected in nature’s great scheme of life and do not need any improvements for their lifestyle.

How do sharks mate?

 

As with all sharks and Rays, reproduction is strictly internal. Male sharks have a pair of intro-mittent organs called claspers. These are modified pelvic fins. The inner edge has developed a tubular structure and is 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 to 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 point for you to consider. Shark skin is made of denticles and is used as sandpaper by South Pacific Islanders. I have had a close look at shark’s claspers, usually when a wobbygong has its head stuck in a crevice and it 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 of the Wobbygongs’ claspers.) The upper side is smooth soft skin. It looks sandpapery but only on the lower half lengthways.

There are millions of opportunistic feeders in the sea such as Sabre Tooth Blennies. Around Sydney, they swim amongst schools of Eastern Hula fish. Their special trick is to dart out at an unsuspecting fish and to use its massive sabre tooth fangs to bite a chunk of flesh off them. Then they scurry off into their hidey hole. The stunned fish has a painful hole in its side.

How safe are male shark’s claspers from such biting fish?  How do they stop fish from biting at their claspers?

This must be a very annoying problem for male sharks. I think the fact that the lower length of the clasper is covered with the sandpapery denticles may offer the shark some protection. Well, we have another mystery in the world of sharks! This then makes for a problem for the female sharks during mating.

Female cloacas therefore must have some kind of adaptation to the sandpapery claspers of the males. Does the internal lining of the cloaca have sandpapery lining? Or is it smooth? Just how does sandpaper on sandpaper work in the act of sexual intercourse in sharks? It’s a fine line between pleasure and pain.
There’s a fraction too much friction.

Male sharks bite the female for foreplay and also during sexual intercourse. It does look brutal. It would be prudent for us to not impose our own simplistic judgements onto shark behaviour. Sharks have had 300 million years to sort out what they do.

The fact is that we need to learn a lot more to understand the why’s and wherefore’s of shark mating. Female sharks are often much larger than males. In the case of the Tiger shark, females are bulkier and far more powerful than males. She is a magnificent display of power! She has rules in her world! Whatever she does in the reproductive process is in no way an act of submission to the males. She is in control of her biology. No male could match the power of a female adult tiger shark! The apparently violent mating process that sharks display may be just that. Obviously, our superficial judgements do not reflect on what is really happening.

Live birth is termed Vivaporous.

 

All the other sharks give birth to live young. That is, babies are born from the mother alive. There are four types of live birth.

Ovovivaporous …. 60% have live birth from an egg.

 

Tiger sharks for example, lay eggs which are retained within the two uteri. The foetus grows over a period of about sixteen months. It gets all of its nourishment from the egg yolk. Baby Tiger sharks can be up to sixty centimetres long. On average, forty one 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 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.

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 Ovovivaporous …. Live birth from an egg.

Yolk Sac Placenta

 

Hammerhead sharks and Whaler sharks have taken this to the next level. Eggs are laid. 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 a success.

 

Intra-uterine cannibalism.

The most bizarre and amazing technique of development with the womb of sharks is Intra-uterine cannibalism.

There are two forms of Intra-uterine cannibalism.
The first is the Egg Eating Embryos. This is the next level of sophistication up from the the Yolk Sac Placenta of Whaler sharks. This technique is when the ovaries produce a continuous supply of eggs to feed the developing embryo.

 

The baby shark can only feed after it has developed teeth. Once, the foetus has developed a mouth and teeth, the mother can feed it with the extra eggs. Baby shark is being 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.)

 

Finally, there is the most gruesome form of all, displayed in the Grey Nurse. The largest foetus eats its smaller siblings. After cannibalising its brothers and sisters inside the womb, it develops into a super baby. There are powerful evolutionary advantages for this form of nourishment of the baby sharks. Firstly, it removes opposition. Monster baby has one chance at life but it has stacked the deck in its favour. Secondly, the single baby when born is a fully developed giant. It is powerful and well able to take care of itself in the wild from day one. The evolutionary pressure here is the high mortality of baby sharks. Remember the Tiger shark has up to eighty babies. Most are eaten by Cod, Groper and sharks in the first few weeks. Bigger babies solve this problem!

Detailed knowledge of marine life gives us a much greater appreciation of the sea. It improves your enjoyment of diving. After all, when I look back on the day I finished my open water scuba course, I hardly knew anything.

 

 

  1. Egg laying       Oviparous

 

Live birth        2. Eggs retained and babies born live             Vivaporous
3. Eggs retained and babies born live             Yolk sac placenta
4. IntraUterine cannibalism     4a Oophagy eggs fed to foetus
4b Siblings eaten by biggest baby.