Sharks have been dominating the oceans for over 400 million years, so it’s little wonder they’re sometimes referred to as the fossils of the sea.
Sharks existed on this planet before the first tree appeared, and they’re still here today, which proves just how resilient they are. We need to dig a little deeper into the life cycle of a shark if we want to understand how it’s has flourished for so long.
Understanding the shark’s life cycle also enables us to evaluate our own behavior and how it may be endangering one of our natural world’s most impressive and unique species.
What is the Shark’s Life Cycle?
A shark’s life cycle goes through four stages, beginning with fertilization before moving through incubation and gestation to the birthing process. Once born, sharks spend several years in the nursery before reaching sexual maturity and entering adulthood.
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Sharks usually live for between 20 and 30 years, but many reach maturity only once they reach 12 or 15. Prior to that, adolescent sharks spend much of their time close to their place of birth, only venturing into deeper waters at night when the hunting’s good.
The 4 Stages of a Shark’s Life Cycle
Stage One: Fertilization and Gestation
Most shark species have never been seen mating in the wild, so our understanding of the process is limited. We do know, however, that sharks reproduce using two different methods. Most species reproduce sexually for the majority of the time, but some are also capable of asexual reproduction.
When they reproduce sexually, they do so via internal fertilization, unlike the Osteichthyes fish species that rely on spawning or external fertilization.
Once a female shark reaches sexual maturity, she releases pheromones, or chemical signals, that let any males in the area know she’s ready to mate.
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George H. Burgess, director of the Florida program for shark research at the University of Florida, says, “Once those signals are out, the female literally has to fend off her suitors.”
Although Burgess maintains that the female has very little say in the matter, others suggest that in species like the sand tiger or spotted ragged-tooth, the females “choose which male(s) to mate with.” Not only that, but once the mating process is complete, the female can decide whether to use the sperm immediately or save it for a later date.
The actual fertilization process is, according to some, pretty kinky. The male bites onto the female’s fins and body to hold himself in place. He then inserts his clasper, located on the pelvic fin, into the female’s cloaca. In the frictionless ocean, this is no mean feat, which is why the male holds on for dear life, inflicting some painful love bites as he does.
That could be why some shark species have developed an alternative means of reproduction. Hound sharks, zebra sharks, bonnetheads, and blacktips can all produce young using parthenogenesis.
Parthenogenesis is a form of sexual reproduction in which an egg develops without being fertilized by sperm – the epitome of single motherhood!
In those shark species that depend on fertilization, the female will mate with numerous males to ensure they’ll reproduce. As sharks reach sexual maturity comparatively late in life, have long gestation periods, and only mate as infrequently as once every two to three years, they need to make sure every mating season is a success.
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If a female does mate with multiple males, the offspring in her uterus will have various fathers, which could explain why things get so violent in there.
As David Attenborough explains in the BBC nature documentary, Story of Life, “Inside each female, infant’s teeth are being put to good use, as the female’s two largest unborn pups slowly eat their siblings.”
It’s believed this macabre behavior is designed to ensure only the strongest survive. Once the embryos have finished eating their siblings, they start on their mother’s unfertilized eggs. This behavior is called oophagy and is practiced by several shark species, including the thresher and the shortfin mako.
Omophagy occurs only in certain species of viviparous shark. Around 70% of all shark species are viviparous, giving birth to live young. Some, however, use a mode of reproduction known as placental viviparity. A placenta joins the mother to the embryo, enabling the mother to provide the pup with all the nourishment it requires.
Other viviparous sharks practice aplacental viviparity, in which the shark lays eggs inside her own body. The embryo begins life there before hatching inside the female’s body. As there is no placenta, the embryo must rely on cannibalism and omophagy to survive.
A handful of species, including the zebra shark, epaullette, and swellshark, lay eggs externally, attaching them “to structures on the seafloor by tendrils to prevent them from floating away.” These eggs are laid in egg cases known as “mermaid’s purses,” the size and shape of which vary from species to species.
A female shark’s gestation period is a lengthy affair that lasts anywhere between five months and, for the unfortunate frilled shark, 3½ years.
Comparing the Gestation Periods of 10 Shark Species
|Species||Gestation Period||Mode of Reproduction||Number of Pups|
|Basking||12 – 36 months||Aplacental viviparity||± 6|
|Blacktip||± 10 months||Placental viviparity||4 – 11|
|Blue||9 – 12 months||Placental viviparity||25 – 100|
|Great White||± 12 months||Aplacental viviparity||2 – 12|
|Hammerhead||11 months||Placental viviparity||6 – 42|
|Lemon||10 – 12 months||Placental viviparity||4 – 17|
|Thresher||9 months||Aplacental viviparity||2 – 6|
|Tiger||15-16 months||Aplacental viviparity||30 – 35|
|Whale||Unknown||Aplacental viviparity||Up to 300|
|Zebra||5 – 6 months||Oviparity||± 10|
The Reproductive Anatomy of the Shark
Stage Two: Birthing and Baby Sharks
As the table above illustrates, sharks give birth to anywhere between 2 and 100 pups in a single birthing. The mothers have very little to do with their offspring once they’re born. They quickly abandon the nursery area, and their babies, as soon as they emerge.
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At birth, shark pups vary dramatically in size. The gargantuan whale shark, which reaches lengths of up to 40 feet as an adult, gives birth to pups that are just 16 to 24 inches long. Great white pups, on the other hand, are already between 3.5 and 5-feet long when they emerge.
Baby sharks, or pups, are born with innate survival knowledge and a full set of teeth, some of which have already been put to good use in utero.
Although the mother heads off to deeper waters, the pups stay near their place of birth for months, if not years, enjoying the comparative safety and abundance of food offered by the shallow waters.
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Scalloped hammerhead shakes are born in shallow areas close to the coast, where they stay until they reach three years of age. Studies indicate that they will venture out of their safety zone or “core area” under the cover of darkness, but tend to stay close to home during the day.
Lemon sharks similarly remain in the lagoon they were born in, honing their hunting skills for several years before exploring deeper waters.
Stage Three: Growth and Young Sharks
Although young sharks grow quickly in the first few years of their lives, this soon slows down, and some species only reach maturity at the age of 30. Studies show that the whale shark grows at about 20 to 30cm a year initially, after which the female’s growth rate slows down.
Male whale sharks reach sexual maturity at about 30 years old, at which time they measure, on average, around 25ft or 8m long. Females, on the other hand, only reach sexual maturity at 50, by which time, they measure approximately 45ft or 14m in length.
Mark Meekan, a marine biologist at the Australian Institute of Marine Science, believes that because a female whale shark can give birth to up to 300 pups at one time, they need that extra size to accommodate them all.
Great white sharks do much of their growing in utero, but it still takes them years to reach their full potential. A study, published in Marine and Freshwater Research in 2015 “suggested males take 26 years to reach sexual maturity.” For females, it’s even longer, and they are only ready to reproduce once well into their 30s.
Great whites grow at around the same rate as whale sharks. As they reach maturity, their growth rate slows but never stops, which is how sharks like Deep Blue manage to reach lengths of 20 feet or 6m.
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Other shark species, like the hammerhead, reach sexual maturity sooner, with the males being ready to breed at the age of 6. Again, the females take a little longer and grow a little bigger. A female scalloped hammerhead is sexually mature only once it reaches 6 feet in length, by which time’s she’ll be around 14 years old.
Smaller species, like the diminutive dwarf lantern shark, are just 2.2 to 2.4” long at birth and are sexually mature once they reach a length of just 6.3 to 6.9”. It takes them four to five years to grow this big, suggesting that they grow at a rate of approximately one inch per year.
Stage Four: Adult Sharks
Most shark species, including the dwarf lantern shark, have a life expectancy of 20 to 30 years, although research shows that the Greenland shark can live for over 500 years.
Using radiocarbon dating to analyze the shark’s eye lens, marine biologists estimate that one female Greenland shark was “as “young” as 272 or as old as 512.”
Greenland shark is a member of the sleeper shark family and is believed to be the longest-living vertebrate on Earth.
Researchers have used similar techniques to establish the life expectancy of other shark species.
In the Fifties and Sixties, the testing of nuclear devices led to a “rapid rise in radiocarbon in the ocean.” Using this as a timestamp, researchers count the “growth bands” sequentially deposited in the vertebrae of the great white and estimate their age accordingly.
Before this ground-breaking study, great whites were thought to live to approximately 23 years of age. It’s now known that they can live for up to 50 years longer than that. Similar studies performed on the whale shark indicate that this species also lives well past its centennial year, achieving a “longevity of ~130 years.”
The Life Cycle of Sharks
The Life Cycle of a Basking Shark
The basking shark is one of the largest shark species, reaching up to 12m or nearly 40’ long and weighing as much as 6,000kg. Basking sharks can live for up to 50 years, assuming they’re not predated upon by humans or killer whales.
Basking sharks spend summers in the British coastal waters before migrating south in the winter. Male basking sharks reach sexual maturity at around 8 years, while the females take a little longer, waiting until they reach between 8.1 and 9.8m in length at the age of approximately 11 years old.
While very little is known about the reproduction of the basking sharks, it’s believed that the females leap high out of the water as part of their courtship.
The basking sharks’ breeding season takes place between May and July, during which time the females “leap meters out of the water, then land, elaborately creating an outward surge of water.” Scientists believe that the females use this breaching behavior to indicate to the males that they are ready to mate.
Basking sharks reproduce through internal fertilization, and both males and females have multiple mates. To nourish her babies, the female basking shark produces “approximately 6 million small eggs averaging 2 mm in diameter.”
As the fertilized embryos develop, they feed on the unfertilized eggs in their mother’s uterus and, by the time their born, measure around 5 to 6’ or 1.5m to 2m long. The basking sharks’ gestation period can last up to three years, after which they’ll give birth to up to six pups.
Juvenile basking sharks have a “long, hook-like snout” which is thought to make in utero and early feeding easier by increasing the flow of water through the mouth. This characteristic feature disappears rapidly during its first year of life.
While the location of basking sharks’ birthing and nursery grounds is unknown, there is evidence to suggest that they “utilize Massachusetts coastal waters for secondary nursery habitat.”
The Life Cycle of a Tiger Shark
The tiger shark is the fourth-largest shark species and can measure over 5 m in length when fully grown. Larger females weigh as much as 200lb or 900kg, while the males remain slightly smaller.
It takes a male tiger shark approximately seven years to reach sexual maturity, by which time he’ll measure between 7 to 9’ in length. Females take an extra year, maturing when they reach 8 to 10’.
To mate, tiger sharks use internal fertilization. Once fertilized, the large yolks within the female’s eggs provide the growing embryos with their first few meals. In the later stages of development, when there is no more yolk available, the embryo switches to uterine fluid as a source of nutrition. This process is called embryotrophy.
The tiger shark’s gestation period is between 14 to 16 months, after which the female gives birth to anywhere between 10 to 80 pups. These pups measure around 20 to 30” long at birth but grow rapidly during their first year, with the females reaching lengths of approximately 40” by the time they’re one.
Juvenile tiger sharks are natural predators from birth, feasting on coastal fishes and invertebrates as soon as they’re born. They spend their early years in protected bays and estuaries, which offer some protection against other sharks and predators.
At birth, and as juveniles, the tiger shark’s characteristic stripes or bars are distinctive. As they mature, these bars become less noticeable but remain partly visible throughout their lives.
These predatory sharks have a life expectancy of around 15 to 20 years, although there are reports of one individual living to the age of 50.
The Life Cycle of a Great White Shark
The great white is already nearly 5’ long when it’s born and, by the time it reaches adulthood, can measure up to 20’ long when fully grown. The average female weighs approximately 680–1,110 kg, while the male remains slightly smaller at 522–771 kg.
The gestation period of great whites lasts for around 14 months or more, and this slow reproduction process makes this species particularly vulnerable to extinction. Females can only give birth every other year and produce just five or so pups per pregnancy. To make matters worse, “many baby sharks do not survive their first year.”
Once born, the juvenile great whites immediately leave their mothers and, according to researchers at the UK’s Nottingham Trent University, “select certain waters as ‘training grounds’ where they can safely learn to hunt larger prey.”
A study of the great white population off Mossel Bay in South Africa suggests that juveniles utilize the bay due to its “sheltered conditions and abundance of food.” These conditions “increase their growth and development and help them to avoid predation, competition, and harassment from larger sharks.”
For many years, researchers believed that female great whites reached sexual maturity at between 7 and 13 years of age. A study published in the journal Marine and Freshwater Research in 2014, however, concluded that females may be as old as 33 before they’re ready to reproduce and that the males “take 26 years to reach sexual maturity.”
A shark’s life cycle is long and slow, beginning with a long gestation period, and continuing for up to several hundred years. Shark pups vary dramatically in size but nearly all take many years to reach sexual maturity.
The long-drawn-out reproductive process and lengthy gestation periods, make these species vulnerable to extinction, and many, including the whale shark, are already endangered.
An in-depth understanding of the shark’s life-cycle can help us better understand its vulnerabilities, and protect the habitats it utilizes during its early years.
Nicky is a British adventurer and animal lover who spends her time exploring the natural world and writing about her experiences. Whether on horseback, underwater, running, hiking or just standing with a fishing rod in hand, she embraces everything her adopted home of South Africa has to offer.