Emma Bernard, a fossil fish expert at the Museum , explains the reasons behind the unusual shape. There are more viviparous shark species - which bear live young - than sharks that lay eggs. But throughout Earth's oceans, viviparity occurs in a variety of forms. Whale sharks Rhincodon typus are the largest species of shark. Although these animals produce eggs, they don't lay them. Instead, the young hatch while still in the female's body and are born as miniature adults.
This is known as ovoviviparity. In , a paper documented the case of a whale shark that was dubbed a 'megamamma'. The animal, which was harpooned off the coast of Taiwan, was found to be carrying around embryos. Many of the sharks had already hatched from their egg cases, ready to be released into the ocean.
Hammerhead sharks are viviparous, giving birth to live young. Although the young initially rely on a yolk sac, this eventually becomes a placenta-like structure.
In some species, the female will produce unfertilised eggs, which are eaten by embryos. This is known as oophagy 'egg eating' and occurs in species including the shortfin mako Isurus oxyrinchus and bigeye thresher sharks Alopias superciliosus. Embryos of other sharks survive by feeding on their smaller siblings. This is called intrauterine cannibalism or sometimes as embryophagy 'embryo eating'.
This is known to occur in sand tiger sharks Carcharias taurus. Oophagy occurs in shortfin mako sharks, a species in the same order as great white and goblin sharks.
Placental viviparity occurs in some species of shark - once the yolk sac has been depleted, it attaches to the uterine wall, acting as a pseudoplacenta. Sharks may have one or two dorsal fins that act to stabilize the shark during swimming. The second dorsal fin is usually smaller than the first dorsal fin and is located posteriorly toward the tail to the first larger dorsal. Stability is the main function of the anal fin for sharks that have one, other sharks may lack this fin.
It is located on the ventral bottom side between the pelvic and caudal fins. The caudal fin is also called the tail fin. This fin is responsible for propelling the shark through the water as it swims. Do sharks lay eggs or give live birth?
Egg cases from nurse shark. There are oviparous egg-laying species and viviparous live-bearing species. The embryos are nourished by a yolk-sac inside the egg capsule.
Viviparous species can be separated into two categories: placental having a placenta, or true connection between maternal and embryonic tissue , or aplacental lacking a placenta. Among the aplacental species, there are those whose embryos rely primarily on a yolk-sac for nutrition during gestation and those that consume yolk-filled, unfertilized egg capsules oophagy. There is even one species, the sandtiger Carcharias taurus , in which the two largest embryos that were fertilized first, consume the other embryos of the litter adelphophagy.
It has also been suggested that the young of some viviparous species may be nourished by uterine secretions during some part of gestation, but this has not been conclusively documented. There is no parental care after birth among viviparous sharks. What is a mermaid's purse? This name is given to the egg cases of many sharks and skates. This tough, protective purse-shaped egg case contains one fertilized egg. Shark species that utilize this mode of reproduction include the swell shark, dogfish, and angel sharks.
How do sharks reproduce? All sharks have internal fertilization. Mating has been observed in relatively few species of sharks, but both hormonal and behavioral cues are likely involved. The female is typically passive as the male bites and grasps her with his teeth to hold on during copulation.
Depending on species, sperm may or may not be stored in the female prior to fertilization of the oocyte, or ovum. During ovulation, the female releases oocytes from the ovary. Then, these oocytes are fertilized by sperm, and the fertilized ova are encapsulated in an egg case in a specialized organ called the nidamental or shell gland.
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. 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! Considerably smaller in size, scalloped hammerhead sharks Sphyrna lewini average around 3m in length and kg 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 months gestation and there can be up to 30 in a litter, although the average is Scalloped hammerheads have a relatively slow rate of growth, with the male taking years to mature.
Where better to start than in a shark nursery? One female, caught off the coast of Taiwan, was found to be carrying pups and, most astonishingly of all, at different stages of development.
Somehow, every kid seems to know this song, but scientists actually know very little about where and when sharks give birth. The origins of these famous baby sharks are still largely a mystery. Many of the large iconic shark species — like great whites, hammerheads, blue sharks and tiger sharks — cross hundreds or thousands of miles of ocean every year.
Scientists have struggled to figure out precisely where and how often sharks mate, the length of their gestation, and many aspects of the birthing process. Until very recently, the technology to answer these questions did not exist.
But marine biologist James Sulikowski, a professor at Arizona State University and my research mentor, changed that. He developed a new satellite tag called the Birth-Tag with the help of the technology company Lotek Wireless.
He has no stake in the company. Using this new satellite tag, our team is working to uncover where and when tiger sharks give birth and is demonstrating a proof of concept for how scientists can do the same for other large shark species. The Birth-Tag is a small, egg-shaped device that we insert into the uterus of a pregnant shark where it will remain dormant and hidden among the fetal sharks throughout pregnancy.
This kind of tag has never before been used on sharks, but similar implanted tags have been used to figure out the birthing locations of terrestrial mammals, such as deer, for decades with great success.
When a tagged mother shark gives birth, the tag will be expelled alongside the babies and float to the sea surface. Once it senses dry air, the tag transmits its location to a passing satellite, which then sends that location and time of transmission back to our lab.
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