The Five Threats
How does slow reproduction affect sharks and rays?
All life on earth has evolved for survival. Whether a living organism is in the form of a bacterium or a tree, a mushroom or a shark, the way it lives and reproduces has been gradually tuned and adapted to suit conditions at the time. When conditions change, the organism changes with them.
How slow reproduction affects sharks and rays
The way that sharks and rays live has been very successful. For 420 million years, sharks of some sort have been on earth. Their survival strategy is to live a long time, grow slowly, get big, mature late in life and have fewer offspring – but they invest a lot of time and energy in the pups they do produce, which means that each individual stands a good chance of surviving. Sharks’ reproduction in general is a slow process, but even within this group there is variation. The dogfish shark is one of the slower species, with a pregnancy that can last for up to two years, whereas the blue shark has on average 25–50 pups a year (although a litter size of 134 pups has been recorded). This strategy has worked well for sharks, but it is starkly different to the lifestyle of animals that have evolved to live fast and die young. Sardines, for example, live for a short time, grow quickly, stay small, mature early and have many offspring in which they invest little. This has worked well for them too. However, when a species is put under pressure from threats such as fishing, habitat loss, climate change and persecution, those that live fast and die young, like sardines, are more likely to survive than species that live slow and die old, like sharks. Why is this? Sharks have always been high on the food chain, which means that they are not ‘designed’ to handle high levels of predation – by other marine species or by humans. When their numbers decline, they are slow to recover because of their natural reproduction methods. If the threat continues and there is no time for recovery, the numbers are likely to continue to dwindle.
Left alone, sharks have been able to persist in a changing world for millions of years. But now they are being pushed to their limit. Their biological characteristics are such that they can’t support losses from large-scale fishing – or from other causes, particularly when those causes are combined, as they are now. If we want to ensure that sharks are not pushed to their biological limit, it is important to understand their lifestyle characteristics – and that is why the Save Our Seas Foundation funds research into shark and ray reproduction. Sharks need help from us to ensure that they aren’t pushed to their biological limit. Research, policy development, management measures, effective enforcement and awareness all contribute to achieving this goal.
Frequently asked questions about slow reproduction in sharks:
How do sharks mate and reproduce?
This is a difficult question to answer, as many species have never been observed or scientifically recorded mating in the wild. Studying mating and courtship behaviour is particularly tricky, as they don’t mate often and when they do, it probably takes place in the open ocean, in deep, murky waters. Most of what we do know comes from videos captured by chance or from the few, very well-studied populations whose mating grounds are known and accessible to humans (like the nurse sharks of Dry Tortugas National Park in Florida).
In general, there are two types of reproductive strategy in nature. You can either go ‘quantity over quality’ and have lots and lots of low-quality young, in the hope that just by sheer numbers some will survive. Many bony fish and other marine species use a reproductive strategy known as spawning, which is when they release masses of gametes (reproductive cells or sperm and eggs) into the water column in the hope that they meet and the eggs are fertilised. But sharks and rays, collectively known as elasmobranchs, are different. They opt for the second life strategy: to invest in fewer, higher-quality offspring. They are generally long-lived, slow to reproduce and have relatively small broods. Unlike fish, they accomplish fertilisation internally, meaning the male and female must physically come together and the egg is fertilised inside the female. Males have external reproductive organs called claspers, which are inserted into the female via her cloaca. The male then releases sperm inside the female, which fertilises the egg.
Courtship and mating take many different forms. In almost all species, the male bites the female’s pectoral fins, either to get her interest or to hold onto her (sharks don’t have hands, so mating in water can be a bit tricky!). In some species, it is thought that males sometimes induce a hypnotic state in females, called tonic immobility, by turning her upside down, which makes it easier to mate with her. In other species, sharks swim together during mating. Scalloped hammerheads are thought to engage in some kind of ‘freefall’ together, where a mating pair stop swimming and drop down to deeper water. And, incredibly rare sightings of great whites mating suggest that they roll around together on the surface!
While all sharks reproduce sexually, some species have shown evidence of reproducing asexually. Parthenogenesis – which translates as ‘virgin birth’ from Greek – is where females can produce offspring without a male contribution. It’s been recorded in a few species, including bonnethead, blacktip and zebra sharks. Sounds weird, but a great survival strategy – if all the males die out, it’s not a big problem!
Do sharks give birth or lay eggs?
Both! With more than 1,000 species of sharks and rays, there is a huge diversity of reproductive strategies. But there are three main types:
Oviparous species produce eggs. The eggs are generally protected in a tough, leathery case, but the cases look different, depending on the species. Some have curly tendrils, whereas others have ‘horns’ or sticky mucus filaments. All these appendages serve as ways to attach the egg case to seaweed, kelp or other forms of substrate and stop it from floating off into the open ocean, where it is more vulnerable to predators. This is a more ‘hands-off’ form of parenting, although the female may spend a long time laying and tending to her eggs to make sure they’re firmly fixed in place. Once that’s done, however, she’ll leave them to the mercy of the elements. The egg case has everything the shark pup needs to survive: food, nutrients and protection. Once the pup is big enough, it will emerge as a mini-me of the adult and begin its own adventure into adulthood.
Viviparous species give birth to live young. This is the most resource-intensive method, as the shark pup develops inside the female, getting nutrients from the placenta – just like mammals. Hammerhead and bull sharks are among the species that are viviparous. Once the pups are born, however, they are left to fend for themselves. They do not stay with the mother for their young life, unlike many mammals and birds.
Ovoviviparous species are a mixture of the two. The female produces eggs, but rather than lay them, she carries them inside her body until they are ready to hatch. The egg has a thinner casing than that of oviparous species, as they don’t need a tough casing to protect them from the outside world. The young actually hatch inside the mother and are then effectively born live. Sharks that are known to do this include the sand tiger and basking sharks.
How often do sharks reproduce?
The frequency of mating varies greatly between species, from as much as once a year (as in blue sharks) to once every other or even several years, with a ‘resting phase’ in between. Some mate year-round (e.g. the spadenose shark), whereas others prefer certain times of the year, known as the ‘mating season’. That being said, there is still much we don’t know about the mating habits of many species. For example, the basking shark – even though it is the second largest species in the world – has never been scientifically recorded mating and we have no idea where its mating and breeding grounds are.
How long does it take a shark to reproduce?
Again, for many species we still don’t know – but we do know there is a huge diversity. We also know that most sharks are long-lived and reach sexual maturity – the age at which they can produce offspring – at a much greater age than other species of fish. The copper shark, for instance, becomes sexually mature between 13 and 19 years (for males) and at 19 or 20 years (for females). One of the longest lived species of shark, the Greenland shark, remains shrouded in mystery, but scientists think a female does not produce young until it’s 150 years old!
The time it takes for the young to develop, from conception to birth (also known as the gestation period) varies greatly between species – from a matter of months to years. The spiny dogfish, for example, has a gestation period of 24 months; that’s two years!
Are sharks’ reproductive strategies successful?
As animals go, sharks and rays are extremely successful in evolutionary terms. They’ve been around, in some form, for millennia – much, much longer than the human species! But their strategies of being long-lived and slow to reproduce and investing in fewer but higher-quality offspring have also made them extremely vulnerable to exploitation. As many species take 10+ years to reach sexual maturity, many individuals are caught before they are able to produce young. It takes the population much longer to recover from fishing pressure.
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Swatipriyanka Sen, et al., 2018, Reproductive strategy of spadenose shark, Scoliodon laticaudus Muller and Henle, 1839 along north-eastern Arabian Sea, Wiley Online Library.
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Julius Nielsen, et al., 2016, Eye lens radiocarbon reveals centuries of longevity in the Greenland shark (Somniosus microcephalus), Science.
Lisa J. Natanson, et al., 2017 Gestation period and pupping seasonality of female spiny dogfish (Squalus acanthias) off southern New England., EBSCO.