How do sharks mate?
MsC Marine BiologyUnlike bony fish, which release large quantities of eggs and sperm simultaneously into the water, sharks have developed internal fertilisation as their mode of reproduction. Their lifestyle strategy is to grow slowly, have a longer period of maturation to adulthood and produce a few offspring (called ‘pups’), each with a naturally high chance of surviving! This is a slow method of breeding, with pregnancies (or ‘gestation periods’) averaging between nine and 12 months – and in some sharks lasting several years. Like other members of the animal kingdom, many shark species do not give birth every year, opting instead to rest between litters.
There is a big difference between sharks and other animals that have adopted this strategy, though. Whereas the others, including mammals, tend to also exhibit strong parental care and nurturing and will even fiercely protect their offspring, that isn’t the case for elasmobranchs at all. Once a pup has been born, it is all on its own!
How does fertilisation happen?
Internal fertilisation means that, just as in mammals, the male shark and female shark must physically come together so that the male can pass his genetic material (sperm) into the female for her eggs to be fertilised. How does he do this? With his clasper! Similar to a penis, a clasper is an external appendage found on male sharks (and stingrays, skates and chimaeras) that is designed to deliver sperm when inserted into the female’s cloaca. However, unlike a true penis, it is not an independent appendage but an extension of the pelvic fin.
What does shark mating look like?
It is thought that when a female shark is ready to mate, she gives out chemical signals, or pheromones, to let any males in the vicinity know. Once she has been found by a male, mating involves a lot of biting, thrashing and rolling around! Often the male has to bite the female’s back, flanks and fins in order to get into a position to mate successfully. For this reason, some female sharks have evolved to have skin that is almost twice as thick as the males’ skin in these spots! Most shark species have never been observed mating in the wild.
Fun fact: not all sperm is used at once! Some female sharks, like the brownbanded bamboo shark, can store sperm for future use – a captive one in California gave birth to a pup at least three and a half years after she could have mated! Some females also mate with more than one male and use different sperm to fertilise their eggs, so the pups in the same litter can have different fathers.
What happens after mating?
After an egg has been fertilised, the embryo’s organs and main systems will start to develop. As the embryo grows, it receives nutrients from its surroundings in a multitude of ways. Embryos that are dependent on yolk are called ‘lecithotrophic’, whereas those nourished by both yolk and their mother are called ‘matrotrophic’.
Did you know that some elasmobranchs, like stingrays, feed on the yolk and then later become dependent on histotroph (‘uterine milk’) produced by their mothers? The nutrients this milk provides to the pups isn’t well studied.
What kind of reproduction is seen in sharks?
There are more than 500 species of sharks and they have remarkably diverse lifestyles. Some species are oviparous (egg-laying) and some are viviparous (live-bearing). However, there are three main types of shark reproduction:
Oviparity
Oviparity is when the female shark lays egg cases and buries them in mud or sand, or wedges them into a structure (such as a rock cluster, coral or a kelp stalk). Some egg cases – they are sometimes known as ‘mermaid’s purses’ – even have tendrils that help to anchor them securely. Typically laid in pairs, the tough and leathery egg cases are generally made of a fibrous protein, collagen. While each one usually houses a single embryo, in some species there are multiple embryos per case. Water in the surrounding environment can penetrate the egg case and thus deliver oxygen to the developing embryos, and any excrement (body waste) from the embryos can seep out too.
The nourishment available to oviparous pups – zebra sharks, for example – is limited, so these pups tend to be relatively small when they leave the egg cases. Once all the yolk has been used up, it’s time for the pup to break out of the egg case and brave the ocean world! As it grows bigger, it begins to wriggle and rotate around in the limited space of the case until it pushes itself out and is born. Fun fact: a shark pup in an egg case will stop moving to avoid being detected by a predator!
Viviparity
There are two categories of viviparous sharks: placental (having a placenta, or true connection between maternal and embryonic tissue) and aplacental (lacking a placenta).
Placental viviparity
Sharks that are viviparous and have a placenta – like bonnetheads – produce offspring much as mammals do. The shark pups live off the placenta until they are ready to be born. They even have an umbilical cord located between their pectoral fins that delivers nutrients and oxygen from their mother’s bloodstream; this leaves them with a little belly button when they are born, although it heals quite quickly. Once the pup is ready to be born, it moves into the cloacal chamber and out through its mom’s cloaca into the ocean. Most pups are born tail first, but some come out head first!
Aplacental viviparity
Aplacental viviparity, which is sometimes known as ‘ovoviviparity’, is a mode of reproduction used by some elasmobranchs – such as eagle rays – where they produce eggs, but don’t lay them. Instead, the eggs develop and hatch inside the mother’s body. This strategy occurs in a diversity of animals, including certain insects, fish, lizards and snakes!
These egg cases are not as thick as the egg cases of oviparous sharks and they remain flimsy inside the mother’s uterus. The embryos lack a placental connection and therefore aren’t nourished from their mother. Instead, they live on their yolk supply! However, once the pups are born they may take advantage of some extra sustenance available: unfertilised eggs that are continually deposited into the uterus. This is known as oophagy (‘egg eating’) and it occurs in species like the shortfin mako and the bigeye thresher.
As well as swimming around the uterus they hatched in, the little sharks can switch between their mother’s multiple uteruses! Doing this opens up the possibility for them to eat not only unfertilised eggs, but also their smaller siblings. The ragged-tooth shark is famous for this practice, which is called intra-uterine cannibalism.
Parthenogenesis
A less common type of shark reproduction is parthenogenesis, which is when a female shark gives birth to pups without her eggs being fertilised by male sperm. There have been a few cases of this confirmed in captive sharks! It isn’t an ideal way to reproduce, but it can be useful when there aren’t any males around.
What are shark nurseries?
To improve their pups’ chance of survival, some sharks give birth, or lay their eggs, in nurseries. These areas tend to be in a protected environment (like mangroves), their waters are warm and shallow, and they have a good food supply but few predators. The small sharks learn how to hunt here without having to worry about becoming a meal for other animals – like bigger sharks! Once they are big enough, they leave the safety of the nursery.
