Without pretending to be an exhaustive explanation, we are going to see in a brief and didactic way one of the most unknown and curious characteristics of the elasmobranchs. Unlike bony fishes, where all are oviparous and their fertilization occurs externally (eggs are expelled in large quantities into the water and the males release their sperm, producing fertilization in the water), in the case of sharks and rays, fertilization is always internal through a process of copulation.

Elasmobranchs have 3 basic forms of reproduction. Depending on the species, they can be divided into oviparous and viviparous species (these are further divided into placental and placental viviparous).

Oviparous

They reproduce by means of eggs. The fertilized egg is protected by a horny capsule that is expelled to the outside. In most species, the female leaves the egg fixed by means of filaments to algae or gorgonians. The embryo feeds on the yolk sac (a food reserve fixed to its body), connected to its digestive system, which will be consumed as it develops. Once it has fully developed, the individual breaks the egg and goes outside. Some examples of this type of reproduction would be the smallspotted catshark (Scyliorhinus canicula), the nursehound (Scyliorhinus stellaris) and some skates like the undulate ray (Raja undulata), for example.

Aplacental viviparous (also known as ovoviviparous)

In this case, the eggs are incubated inside the female for one or two years (depending on the species) and are expelled to the outside when they are perfectly developed. Once the egg has been fertilized, a kind of membrane that surrounds it is secreted and can hold up to four individuals, being stored in the uterus. The embryos develop by feeding on the yolk sac and secretions generated by the female until they are fully developed, when they will be expelled to the outside by means of contractions. In some species such as the porbeagle (Lamna nasus), when the yolk sac is consumed, oophagy can occur, i.e. those embryos that are more developed can feed on unfertilised eggs rich in yolk. Intrauterine cannibalism can also occur, where the strongest and most developed embryos can feed on the smaller and weaker ones. This is actually an advantage, as it helps the mother to ensure the survival of the strongest embryos with significant energy savings. The shortfin mako (Isurus oxyrinchus) would be another example of this reproductive typology.

Placental viviparous

In the first stages, the embryo feeds on its yolk sac, but as this ends, the sac becomes vascularized and fuses with the wall of the uterus and the embryo absorbs the nutrients supplied by the mother directly through the blood by means of an umbilical cord, in a similar way to the placenta of mammals, although it would not be the same structure. Once their development is complete, they are expelled to the outside world. Some examples of this type could be the blue or blue shark (Prionace glauca) or the hammerhead shark (Sphyrna zygaena).