The secret lives of baby sharks

Enigma of the Great White

The great white shark Carcharodon carcharias is easily one of the most recognisable species of shark – and yet knowledge of them is incredibly scant. This is truly remarkable, considering these creatures can be more than 6m long and weigh over 2,000kg. However, they’re incredibly adept at hiding themselves, mainly due to their colouration, which makes them difficult to make out from above and below – and they’re also known to retreat to very deep water, where they’re almost impossible to track.

Their behaviour is hard to predict, too, as they adopt seemingly random pathways across the oceans. These differ between males, females, and juveniles: some will hug the coastline, some choose to stay in the wider ocean, with no consistency. As a result, scientists aren’t even sure how many white sharks exist, but it’s agreed they’re a vulnerable species with their numbers decreasing.

Remarkably, they’ve never been documented mating or giving birth, however it’s thought they go to deep waters in the Pacific Ocean to mate. Their gestation period is estimated to be around 12 months, but very little is known about where the females deliver the pups. This giant of the deep guards its secrets well!

Nursery time

Considerably smaller in size, scalloped hammerhead sharks Sphyrna lewini average around 3m in length and 100kg in weight. They give birth in coves, shallow areas along the coastline where food is abundant (mainly crustaceans) and which also provides shelter from larger predators that lurk in the open water. The pups are born after 9-12 months gestation and there can be up to 30 in a litter, although the average is 17. Once delivered, they’re on their own, learning to hunt in the shallows for three years or so, before braving the wider ocean.

Scalloped hammerheads have a relatively slow rate of growth, with the male taking 6-9 years to mature. They’re currently classified as “critically endangered” on the IUCN Red List, so the conservation of these sharks has never been more important. In part, their population has been reduced due to many getting caught in gillnets and suffocating, but it’s also thought they only breed every second year, rather than annually. All this means it’s vital that scientists continue to follow and study these incredible animals to ensure numbers are being accurately tracked. Where better to start than in a shark nursery?

Eggs-traordinary

There is truly no end to a shark’s extraordinary reproductive methods. Whale sharks Rhincodon typus are ovoviviparous, meaning they give birth to live young, but the embryo’s development occurs inside an egg, which nourishes it. The embryo then emerges from the egg while still inside the mother and remains there until it’s ready to be born. One female, caught off the coast of Taiwan, was found to be carrying 300 pups and, most astonishingly of all, at different stages of development. This led to the amazing theory that female whale sharks can store a male’s sperm, choosing when her eggs are fertilised. Talk about an independent mum!

Early developer

Sharks have long been put into the ‘scary’ category, along with other animals such as snakes and spiders, but is this really fair? These incredible creatures are often maligned simply due to the majority of the species being of the predatory persuasion; an important part of any ecosystem.

In some cases, such as baby sand tiger sharks Carcharias taurus, this predatory behaviour starts early. It’s a survival of the fittest from day one for them: they compete in the womb, which results in the strongest frequently eating their siblings. Intrauterine cannibalisation, to use the correct term, means the 100mm long sand tiger shark embryos in the womb will feed on their fellow brothers and sisters until only the two biggest and strongest are left. It’s not just embryos these baby sharks eat, but also the mother’s unfertilised eggs which provide extra nourishment before they’re born. They then emerge as stronger and healthier hatchlings who, as a result of their size, are less likely to be preyed upon.

Tide of life

Shark parenting certainly focuses on tough love. Lemon sharks Negaprion brevirostris, named for their pale-yellow skin that camouflages them in sandy waters, have their young in mangrove forests or lagoons. The pregnant females will wait for high tide before swimming into the forest to give birth, not a phrase you hear every day. As soon as the baby sharks are born, however, the mothers return to the deep ocean, leaving the pups to fend for themselves in the shallower water. They’re able to shelter in the mangrove and are small enough to swim deep into the labyrinth of roots, hopefully remaining safe from larger predators (including larger sharks of their own species). The juvenile lemon sharks stay in the lagoon for about two years, honing their hunting skills, before swimming out to sea on the high tide. Following in their mother shark’s footsteps so to speak.

Single parent

Zebra sharks Stegostoma fasciatum, found in the Western Pacific and Indian oceans, have been known to have “virgin births”. This phenomenon, called parthenogenesis, means giving birth at the onset of maturity without mating. Parthenogenesis is more commonly found in plants, some lizards and chickens. Studied in captivity, scientists at first thought that the zebra sharks were storing sperm, but the offspring were found to only bear their mother’s DNA. Researchers believe that zebra sharks may be able to adapt their reproductive strategy to environmental circumstances, such as the lack of a mate, which is especially interesting given that zebra sharks are endangered. Researchers are now monitoring whether the offspring born of parthenogenesis will be able to bear offspring from mating.

Featured image © Ed Charles | Silverback Films 2018