Sharks are a group of chondrichthyan fishes or cartilaginous fishes belonging to the superorder of selaquimorphs, that is, those that have a "shaped shark". As its name indicates, this type of fish has a skeleton made up of cartilage and only the jaw is bony.
There are many orders of selaquimorphs, so in this article on our site we will focus on some of the most studied, such as the sharks of the Order Carchariniformes and the Order Lamniformes.
So, we will see how white sharks reproduce and many other sharks. Also, we will solve the question of whether sharks are mammals, due to a reproductive strategy that some of these species follow.
Shark reproductive strategy
From different perspectives there are various ways of carrying out reproduction, all with the same goal, to perpetuate the species. Sharks are the largest fish in the seas and oceans and, together, they form a group of more than 100 species, each with a different reproduction strategy but which can be grouped into three different types:
Oviparous Sharks
oviparity is the reproductive strategy by which animals lay eggsIn oviparous species of sharks, the fertilized ova are encapsulated in an egg case and deposited in the external environment. All the nutrients that the embryo will require are inside this egg capsule. No species of pelagic shark, that is, one that lives free in the ocean far from the coast, is oviparous.
Aplacental ovoviviparous sharks with oophagia
Sharks of the order lamniformes, such as the thresher shark or the salmon shark, exhibit aplacental viviparity with embryonic oophagia. This means that they are ovoviviparous animals, the development of the embryo occurs within the mother's uterus but in a very different way from how a placental mammal would. In this case, only the right ovary is functional. After copulation occurs and the eggs are fertilized, they are individually packaged into capsules, called blastodisc capsules These capsules migrate to the uteri (two), where they have place development.
In the first phase of gestation, the embryos are nourished by the yolk of the yolk sac that is inside the capsule. Once the yolk has been exhausted, the embryos emerge from the capsule, engulfing it and, during this phase, feed on the unfertilized ovules (oophagia) that the mother has continued to produce during gestation. Consumption of these nutritional capsules causes the embryonic stomachs to distend, which is why they are often referred to as "yolk stomachs."
Towards the end of gestation, the female stops producing eggs, and late-stage embryos rely on yolk digestion in the stomach for energy until birth. Being aplacental means that there is no placental connection between the fetuses and the mother in these species. The reproduction of the white shark is poorly understood, but the few data that exist indicate that it must follow this reproductive strategy.
Placental ovoviviparous sharks
Sharks of the Order Carchariniformes, specifically of the genera Carcharhinus and Prionace are all species placental viviparousAs in the previous case, the only functional ovary produces ovules that, once fertilized, are encapsulated in individual eggs and migrate to the uterus where development will take place. Early in development, embryos are nourished by the yolk stored in the egg, but when the yolk is depleted, the empty yolk sac forms a placenta-like connectionwith the maternal uterine wall, which becomes highly vascularized (appearance of many blood vessels.
This "pseudoplacenta" is different from the placenta of placental mammals, but functions as a true placenta, providing nutrient and probably gas exchange between the maternal and fetal systems. The embryos will depend on this placenta to overcome the final phase of development inside the mother's womb. Just before birth, this connection breaks and the little sharks reabsorb the rest of the sac. Newborns will have a small navel-like scar
Shark reproduction
As you may have verified, the reproduction of these animals is very different between the different species, therefore, the times in the gestation of the sharks will also be different, in fact, some do not even have a period of gestation, because being oviparous, the development of the embryos will take place outside the mother's body.
For sharks that are ovoviviparous, the gestation time varies between 9 and 22 months, depending on the species, they could even reach 24 months of gestation. These data are not exact, since studying these animals in their natural environment is certainly complicated.
It is also not known whether there is a specific breeding or heat period for each species, although the data collected so far indicate that they can breed at any time of the year.
In this video you can observe the mating ritual and copulation of white tip sharks:
How to tell the difference between a male and a female shark?
The sexual maturity of sharks is reached when the reproductive organs are fully developed, something that is not noticeable from the outside, for Therefore, to know if an individual is an adult or we must not look at its size which, of course, varies according to the species.
For example, sexual maturity in male Alopias superciliosus or thresher sharks is reached when the animal measures between 270 and 288 centimeters, this size corresponds to the age of 9 or 10 years old Females reach sexual maturity when they measure between 300 and 355 centimeters, that is, when they are between 12 and 13 years oldold.
To know if a shark is male or female at a glance, we must look at the vent area or anal area. In fish, both the reproductive, urinary and excretory systems pour their fluids into the cloaca and from there to the outside. In females, next to the cloaca we will see the cloacal fins (if the species has them, otherwise we will only see a small opening).
The males, on both sides of the cloaca, have an appendage called pterygopodium These appendages have an organ inside them called siphon that fills with water before copulation and empties inside the female along with the sperm.
In the image below we can see a) Female sexual organ (or cloaca) of the shark; b) Male sexual organ (or claspers); c) Shark in neonatal stage with open umbilical orifice; d) Juvenile shark of the year with the umbilical hole partially closed.
