All the truth about sharks or do they really lay eggs?

Do-Sharks-Lay-Eggs

Sharks are one of the most fearsome inhabitants of the ocean. Lack of bone tissue and gill covers, structural features of their scales and many other features attest to their ancient origin. According to the paleontological data, the age of fossil remains of the first sharks is 350 million years. However, despite apparent primitivism, shark are one of the most perfect predatory ocean fishes. Over a long period of their existence, sharks have perfectly adapted to life in the water and now they successfully compete with bony fishes and marine mammals.

Now we have cleared everything up – sharks are fish, fishes lay eggs, that is why sharks are supposed to lay eggs too! So, is that really so? In fact, the truth is actually much more complicated than a simple yes/no answer. Sharks are a unique creation of nature.  As we have already mentioned, they have undergone a million years of evolution, which have adapted their reproductive system to many challenges.

It should be noted that sharks are cartilaginous fishes. And, like all cartilaginous fish, they breed by means of internal fertilization. However, the reproduction process can greatly vary among different shark species. In the first instance, it concerns the way in which shark pups come into this world. Now, here comes the most interesting part about sharks.

How do fishes give birth?

The majority of bony fishes is dioecious. The females have a large ovary in the body cavity, where the eggs are developing. The males have a pair of long testes of similar size. During the breeding season, the testes are filled with thick white liquid – milt. Milt contains millions of sperm cells or spermatozoa. Genitals open outwards on the ventral side of the genital pore of the body. With the maturation of germ cells, the fish reproductive instinct starts manifesting itself. At this time, they move to places that are more favorable for the development of their offspring. Some species flock from the sea into rivers, while others, on the contrary, leave the rivers and swim to the seas. The complex instinctive behavior of fish during the breeding season is called spawning. During this period, a female fish ejects huge amount of eggs that will soon be fertilized by a male. Of course, the majority of these eggs will be lost somewhere in the depth of oceans or will be feasted upon by other fishes. In such a way, only a small percentage of eggs will develop into baby fishes.

How do sharks give birth?

For a long time, the sexual life of sharks was poorly studied. After all watching them is dangerous to health, and as well technically difficult. Scientist even assumed that sharks were the first animals who used internal fertilization to exchange their genetic material. It was typical for fishes, mollusks and especially plants to reproduce either with the help of external fertilization or by division. On the one hand, it was confirmed that the armored fishes, which existed on Earth long before sharks, also had sex. On the other hand, they became extinct long ago, while sharks still exist, so we can say that they have the greatest sexual experience among all other organisms living on Earth.

So, there are three main ways in which sharks reproduce and give birth to baby sharks: viviparity, ovoviviparity (when embryos develop inside eggs that remain in the mother’s body), oviparity  (embryos develop in eggs laid in the water).  Despite the fact that sharks have different modes of reproduction, all they are characterized by internal fertilization. To ensure this, the pelvic fin of shark males has evolved into a kind of claspers. These claspers represent a pair of organs that function similarly to a mammal penis. The claspers are external and are used to deliver sperm into the female cloaca. In worth noting that claspers are also able to anchor a male to female. However, males are also not shy about using their sharp teeth to hold the female down during mating. Some shark species are able to store the sperm inside for a very long time, and use it for oocyte fertilization when they see the opportune moment.

Fertilized eggs of oviparous sharks go down through the ovarian tube, then pass through the special glands where they acquire hard shell. After that, eggs are laid at the bottom. The majority of modern shark-like fishes are ovoviviparous. And that means that the fertilized egg remains in the posterior part of the oviduct until the shark gives birth. Little ovoviviparous sharks are born fully developed.

Embryo development in viviparous sharks also takes place inside the mother’s body. Embryos receive nutrition with the help of placenta. The number of pups in viviparous sharks is relatively small: from 2-4 to 4-5 babies. Newborn sharks are usually 1-1.5 meters long and are fully prepared for independent living.

Sharks of all kinds have fewer offspring in comparison to other fishes. However, baby sharks are much better developed. To reproduce in safety, sharks carefully choose special places called nursery areas. The water of the nursery is usually warmer and full of small fish, which will serve as the meal for the newborn. In such a way, sharks ensure the survival of their pups.

Oviparous sharks

Near 40% of all shark species are oviparous. Such form of reproduction is considered primitive. Catsharks (sharks of the family Scyliorhinidae), some species of carpet sharks (zebra and whale sharks), a few species of dogfish sharks and all the 10 kinds of ancient and rare bullhead sharks order, bamboo sharks, Port Jackson sharks belong to oviparous type.

Oviparous sharks lay eggs, which are structurally similar to the eggs of birds or reptiles. However, shark eggs do not have the usual oval or round shape – they are covered with a tough, leathery membrane and are bizarrely shaped. Nevertheless, these are the most ordinary eggs, from which small sharks will hatch eventually. Like any other eggs of oviparous animals, shark eggs contain nutritious yolk that serves as a food for the embryo and allows it to develop successfully. The incubating process of eggs takes a lot of time, sometimes even more than a year. A newly hatched shark pup is fully capable of taking care of itself.

Mermaid’s or devil’s purses

Sometimes the sea casts shark eggs ashore, where people mistake them for some fantastic sea creatures. In ancient times, people called these findings mermaid’s purses, because they were not able to explain the origin of these strange objects.

The fact is that shark eggs have very interesting, bizarre form, which makes it seem like they were created by humans, beautiful mermaids or demonic beings. An egg itself is immersed in a thin capsule filled with collagen. Each side of the egg has a slit for better ventilation and oxygenation. They often have square or rectangular shape with something resembles horns and long tendrils. Egg capsule is very tough, so some sharks manage to push them into rock crevices not leaving any chances for other predators to find and eat the eggs. The eggs have less yolks comparison to eggs of sharks with a more complicated reproductive system. That’s why the pups also turn out to be smaller.

Of course, sharks do not plan to frighten naïve beachgoers and as a rule, they leave their fertilized eggs on the seabed, where they have to stay perfectly safe until the moment of hatching. Each egg  usually contains one embryo, however sometimes there can be more. Sadly, by the hatching time the strongest pup usually eats its brothers and sisters.

Viviparous Sharks

Viviparity is a method of reproduction when the embryo develops in the body of a female and receives all necessary nutrients directly from the mother’s body just like mammals do. The connection between an embryo and the female organism does not form immediately. At an early stage of its development, an embryo gets all rich nutrients from the yolk sac of a fertilized egg. Then, the yolk sac gradually transforms into a placenta, which provides an embryo with the connection to its mother. From this moment, an embryo starts receiving all nutrients and oxygen through the umbilical cord.

We cannot claim that this method is very widespread among modern shark species. It is known that viviparity is inherent to such ancient shark species as Greenland shark. Only the one-tenth of the known to science sharks including all kinds of gray sharks and some hammer-headed sharks exhibit viviparity. In fact, all viviparous sharks belong to the Carcharhiniformes order. Although, not all representatives of this order are viviparous.

The gestation period is very long, because the embryo needs enough time to develop into a strong pup able to stand up for itself. The birthing process is no different from the birthing process of the most mammals. As a result, big, strong, healthy and sturdy pups come into the world and instinctively swim away from their mother. After all, they do not need her care and they also have no wish to become her next meal, as well.

Shark viviparity is the only example of placental viviparity among cold-blooded vertebrates.

Ovoviviparous sharks

The majority of modern shark species (about 25%) is ovoviviparous. That means that females do not lay eggs on the ocean bottom. Instead, they carry them inside till the pups are hatched in their womb. Accordingly, the embryos have no placental connection with their mother and get nourishment from the yolk. Although, eggs of ovoviviparous sharks bear slight resemblance to the mentioned above mermaid’s purses, they are not as hard and are covered in thin substance.

By the way, the hatched pups can stay inside their mother for some time. It allows the mother shark to protect her pups from being eaten by predators. As a result, the pups come to this world well-developed and adapted to the independent existence. Due to this, they have better chances to survive than oviparous shark pups. The female shark’s  body can supply its eggs with more oxygen.

It is worth mentioning, that the exact period of offspring bearing of ovoviviparous sharks is still unclear to scientists. It is believed that it could vary from several months up to two years (spiny dogfish), which is one of the longest periods of pregnancy among all vertebrates. Such shark species as whale sharks, angel sharks, and spiny dogfish exhibit this mode of reproduction.

Oophagy and cannibalism

Oophagy is the process when pups that hatched first eat unfertilized eggs with the help of so-called temporary teeth. This phenomenon can occur only in the womb of ovoviviparous sharks and these are the majority of modern shark species (bigeye thresher, porbeagle sharks, and shortfin mako). An embryo does not require food only at the early stage of its development, as it gets nutrition from the yolk sac. However, in some cases, pups are not born immediately after hatching out of the egg and, of course, they need a large quantity of nutritious food for their further development.

Staying in the uterus (part of the oviduct), pups are not able to eat anything other except what is available in this narrow and dark place. That is why they have no choice but to eat unhatched siblings (this process is called cannibalism) or newly formed eggs. The organism of ovoviviparous females produces infertile eggs in excess specifically to provide the larger pups with food. Of course, only the first hatchling (or second and third in the best-case scenario) can feed itself by eating its brothers and sisters. Due to such high-calorie diet, a young shark is born sufficiently large and well-developed, able to feed itself and attack any prey of a suitable size.

Intrauterine cannibalism (IC)

Some species, such as sand tiger sharks have two uteri. That is why, the first two pups, each in its uterus can eat its siblings. Such process was called intrauterine cannibalism, its contracted form –  IC. Usually, when an embryo reaches about ten or more centimeters, it seeks out its siblings and kills them. Such little cannibal will soon grow very large by feeding on egg cases of the dead embryos. It ingests so many egg cases that it has a swollen abdomen called a “yolk stomach”. For example, sand tiger pups can sometimes reach about one-third of their adult size (nearly one meter). Shark pups are born or hatched as an exact replica of adult sharks.

Parthenogenesis or sharks’ immaculate conception

Researchers have proven that sharks are able to reproduce with the help of parthenogenesis. Parthenogenesis or asexual reproduction is the females’ capability to create and grow pups without male participation. In other words, it means that an embryo develops from an unfertilized egg. In nature, parthenogenesis is inherent in 70 species of animals. Such type of reproduction provides living organisms with the opportunity to survive under conditions of gender imbalance. Generally, pathogenesis is common for primitive life forms, although it is pretty common among some species of birds, lizards, and fish.

Why do sharks need parthenogenesis?

First of all, it should be noted that cases of this phenomenon were recorded only in sharks in captivity. However, it is assumed that such virgin reproduction helps sharks to keep their minimum number under the extreme conditions. For example, in the period of natural disasters when the population is reduced to critical numbers and the threat of extinction is palpable.

Asexual reproduction is a very rare phenomenon. That is why, when it occurs, an exhausting battery of test is conducted immediately. Such tests necessarily include paternity test to exclude the possibility of fertilization from the previous mating.

In conclusion, it could be said that asexual reproduction is a measure of last resort. As genetic diversity is the key purpose to produce healthy and strong younglings and thus ensure the survival of species. Various genetic defects could be avoided only by mixing parents’ DNA. Parthenogenesis, in turn, will inevitably lead to the emerging of weak and unsaturated gene pool.

Sharks face the risk of total extinction! Do not stay indifferent!

Unfortunately, currently shark protection is one of the most topical issues. It was estimated that nearly 11 shark species may go extinct in the next 50 years, including Mako, hammerhead, fox and great white shark. Meanwhile, these predators reproduce very slowly – the pregnancy period can last up to 2 years, and large pelagic species reach puberty at 8-9 years. However, it is very important to understand that sharks are top terrestrial predators that play an important role in maintaining the balance of the ecosystem.

Summarizing all the mentioned above, the answer to the question “Do sharks produce eggs?” – Yes, they do produce eggs. “Do sharks lay eggs?” – Yes and no, some species lay eggs, but the majority of them doesn’t!