Reproduction in sharks

In this section we will learn about reproduction in sharks and their relatives the skates, rays, and Chimeras. Hi, my name is Dr. Christine Dudgeon and I’m an expert in the ecology and evolution of marine fishes. based at the University of the Sunshine Coast, Australia. Sharks and their relatives, skates, rays and chimeras, are a special group of fishes who skeletons are made of cartilage. They have been around in various forms for over 450 million years, which is a really long time, especially for vertebrate animals. That is animals with a backbone. One of the reasons I’ve been so successful is because they have evolved various ways to reproduce, that is to make baby sharks. The first step in reproduction is mating.

Sharks need to come together in the vast ocean environment, and many species will aggregate at predictable times of the year for this to occur. Some species have courtship rituals. One of the most elaborate is found in manta rays, several males will form a line behind a female and a courtship train, and the female will put them through their paces. She undertakes all sorts of acrobatic manoeuvres and the remaining male that has managed to keep up with her gets the opportunity to mate. Similar to us mammals and quite different to bony fish sharks have internal fertilisation, that is they have sex.

Sharks are either male or female from birth and they don’t change throughout their lives, you can visually tell the difference. Females have a single body opening called a cloaca, which is used for both reproduction and excretion, similar to what you find in birds and reptiles. Males, however, have specialised reproductive organs called claspers. These are modified pelvic fins and essentially look like two penises. They have them from birth, but they become functional when the males reach maturity, which can be anywhere between a few years and decades depending on the species. To have sex, the male and female sharks have to come together.

As sharks don’t have opposable thumbs to keep the female close, the male shark will often bite onto the female and then insert one clasper but into her cloaca and eject seminal fluid into the oviduct. This mating behaviour has been filmed in a range of species and there is other evidence of mating behaviour too. There are many sightings of large female white sharks with bite marks on their head and fin area. Female sharks have much thicker skin than males, which is a likely adaptation to these types of Love Bites.

After the egg is fertilised inside the female the embryonic sharks develop in a multitude of ways, including those that lay eggs known as oviparity through various forms of live birth viviparity, egg laying or oviparity is mostly found in smallest sharks, rays and chimeras. Several eggs are laid over a breeding season with over 100 in a year for some species. The egg cases are usually quite tough to protect the embryo inside it. They come in a few different shapes such as the mermaid purse. Here are some examples. These ones are from Epaulette sharks are different types of carpet sharks, as well as a corkscrew that we find in the different horn sharks.

Some shapes like this corkscrew enable the eggs to be wedged into crevices and others have sticky tendrils or hairs to attach the egg to rocks, corals, and other substrates. The embryo is nourished by the yolk in the egg and once the yolk is used up, the pup hatches out of the egg as a miniature version of the adult. The rest of the sharks and rays are viviparous, that is they give live birth but the way the embryo is nourished varies. In some sharks and rays embryos are only nourished by an egg yolk but develop inside the mother. Some species will still have internal egg cases while others do not.

As the embryos are also only nourished by a yolk the pups are small, and after they finish the yolk inside the mother they then born out into the world. One of the most extraordinary examples of this is the whale shark, the largest fish in the sea. A pregnant female was caught off the waters in Taiwan in 1995 and nicknamed mega mama, she was found to have 304 embryos inside her some still an egg cases and some that had hatched out. The female was 10.6 metres in length, but the largest of the hatchlings were only 60 centimetres long. This is about 1/20 the size of the adult. Some sharks and stingrays give the embryo more nourishment than just the yolk.

In stingrays, the embryo receives uterine milk, which is a highly nutritious secretion. This extra nutrition enables the embryo to grow quite large. The Devil Rays only have one pup every breeding season and these can reach approximately one quarter the size of the mother at birth. Several shark species provide the embryos extra nutrition using a placental type organ. This is known as placental viviparody. These include the ground shark species, such as the bull shark and the blacktip shark. The embryos in a few shark species receive extra nutrition in extraordinary ways. In the Mako Shark the largest embryos feed on the unfertilized eggs in the uterus known as oophagy.

Embryos and the grade nurse shark, also known as the San Tiger Shark take this even further, in a developmental form known as “intra uterine cannibalism”. The largest embryo in each uterus feeds not just on the unfertilised eggs but also the fertilised eggs and any smaller developing embryos. How’s that for sibling rivalry? The mothers give birth to two babies at the most one in each uterus, and these babies are quite large at around one metre approximately 1/3 of the adult size. After eggs are laid and pups are born, the shark babies are on their own, there is no more parental care.

However, compared with the very small planktonic larvae of bony fish and invertebrates, sharks have few offspring, but they are large and well developed because of those extra nutrients provided during embryonic development, and this gives them a good chance at survival. Some species come back to the same place year after year and give birth or lay eggs. And these places are known as nurseries. Shark nurseries tend to be shallow areas full of food, where the young sharks have some protection from larger predators. If all of that is not amazing enough, sharks have one more trick when it comes to reproduction.

Over the last 10 years aquarists and scientists have reported that female sharks and rays and about 10 species are able to produce babies without any input from males. This is a special process of asexual reproduction known as parthenogenesis, which translates directly to virgin birth. Most examples come from female sharks raised in aquaria that never had contact with a male sharks since birth, and started producing viable pups when they reached maturity. A rare case is that of a female sharks switching from making pups with a male partner to producing pups parthenogenetically. This was recorded in a captive Indo Pacific leopard shark, also commonly known as zebra shark.

She had produced dozens of pups with her male partner, and because they were too successful, the male was removed from the tank. Within three years, she was producing live pups by herself, it was possible to tell that these pups were asexually produced by examining their DNA. They only had the DNA of their mother, while the older pups had the DNA from both parents. Like us, sharks have xy sex determination, where females have 2x chromosomes. All the parthenogenetic offspring were female is they did not receive any Y chromosome DNA from a male parent. Parthenogenesis has only been documented from one shark and Ray species in the wild the swordfish with the rest of the reports coming from aquaria.

It is still a mystery how often sharks reproduce this way and why they do it. It may be a way that sharks are able to colonise new areas and persist until potential mates turn up. Parthenogenesis and all these different ways that sharks have evolved to reproduce and provide the best start in life for their offspring are just some of the many reasons that sharks have been so successful over the last 450 million years.