Among fish there are very different systems of reproduction. Many species have external fertilization: the female lays the eggs and the male waters them with his sperm. However, among the ‘chondrichthyans’, a group to which sharks and rays belong, reproduction is more intimate.
The male and female shark have slight anatomical differences perceptible to the naked eye, that is, they present sexual dimorphism . The main difference lies in the pelvic fins, where the copulatory organ of the male is located.
The reproductive system of the shark
Unlike mammals and other animals, male chondrichthyans do not have a penis. Instead, they have fleshy, ribbed appendages on the last pair of fins—the pelvic ones—called pterygopodia or claspers .
During maturity, the claspers harden and articulate from the base of the fin. They are structures only present in males, so they are the best way to differentiate them from females. For their part, these have ventral grooved openings, in front of the pelvic fins, which receive the claspers during mating. The rest of the reproductive system is internal.
In males, the testicles are very advanced, at the height of the pectoral fins, just above the liver. The vas deferens are long, reaching the seminal vesicles . The so-called alkaline glands also flow into them, which raise the pH of the fluid and provide electrolytes that maintain sperm motility. Finally, the seminal vesicles open into the sperm sacs, where the sperm is stored in structures called spermatophores, and from there into a single receptacle, the urogenital papilla, which opens into the animal’s cloaca.
The female apparatus is externally less spectacular, but internally no less interesting. The ovaries are located, like the testicles of males, at the height of the pectoral fins, surrounding the esophagus. Eggs are produced there and travel backwards through the oviduct. In this duct are the nidamentary glands , which form the protective coverings of the eggs. Further back are the uterus, two, and both join in the cavity of the vagina, which leads to the cloaca.
Chondrichthyan copulation is quite particular. The male anchors one of his claspers in the female’s prepelvic slit, and uses the other clasper to transfer his spermatophore to the female’s cloaca. The claspers’ channels are connected to structures called siphons , skin sacs under the pelvic fins, which can pump high-pressure seawater.
The male, in this way, places a spermatophore at the outlet of his cloaca, and pumps a powerful jet of seawater with the siphon, with which he propels the spermatophore under pressure into the female’s reproductive system.
If the female has eggs ready to be fertilized, the sperm will meet them in the oviduct. If they are not, the sperm will remain alive in a safe place, protected by the nidamentary glands. Once the eggs are fertilized, they are stored in the uterus until they mature.
Shark eggs have a characteristic and unmistakable appearance, with a soft and fibrous shell, they are shaped like a bag, with tendrils at both ends , very different from the oval eggs of reptiles or birds. They can often be found already empty on the beaches; they are colloquially called mermaid bags .
The females usually use vertical structures in the environment, such as rocks, algae or corals, to deposit the spawn. With the egg emerging from the cloaca, the female begins to swim in circles around the structure, causing the tendrils to entangle and adhere to it. Once achieved, the female moves away so that the egg can finally come out of the cloaca and hang on the structure. It is ready to lay the next egg; They are usually pairs.
The structures end up becoming large egg masses , from different females and at different stages of development. When the embryo inside it has grown enough, —a time that changes depending on the species—, the egg hatches and the little sharks are born with free and autonomous life, fully capable of swimming and feeding themselves.
A significant proportion of chondrichthyans give birth to live young. This can happen in two ways: ovoviviparity , or viviparity .
An ovoviviparous shark has eggs, like a normal oviparous shark, which develop in the female’s uterus. During development, the young depend on the stored yolk for nutrition. However, hatching occurs inside the uterus, the young are born alive , and the empty remains of the egg are later expelled to the outside.
However, many species of chondrichthyans—as many as 420 species of sharks and rays—are truly viviparous , like most mammals. In these cases, the egg never forms; the fertilized embryo is implanted in the uterus, and develops entirely inside it. It is nourished by the mother’s blood , and is transferred from the wall of the highly vascular uterus to the yolk sac that surrounds each embryo.
Castro, JI 2013. A primer on shark reproduction for aquarists.
Pratt, H. 1979. Reproduction in the blue shark, Phionace glauca. Fishery Bulletin-National Oceanic and Atmospheric Administration, 77, 445-470.
Rumfelt, LL 2014. Shark Reproduction, Immune System Development and Maturation. In Immunobiology of the SHARK. CRC Press.
Solomon, EP et al. 2013. Biology ( 9th). Cengage Learning Publishers.
Wourms, JP 1981. Viviparity: The Maternal-Fetal Relationship in Fishes. American Zoologist, 21(2), 473-515. DOI: 10.1093/icb/21.2.473