Yes, some sharks lay eggs, but this oviparous method is only one of the varied ways these cartilaginous fish reproduce. The reproductive strategies across the hundreds of shark species are remarkably diverse. Unlike many bony fish that release a large number of unprotected eggs and sperm, sharks utilize internal fertilization and invest significant energy into producing a small number of well-developed young. Understanding shark reproduction requires looking closely at the different approaches they take.
Understanding Shark Reproductive Diversity
Shark reproduction is broadly categorized into three main classifications based on how the embryo develops and receives nourishment. The first is oviparity, where species lay eggs that develop and hatch outside the mother’s body. The embryo is entirely nourished by the yolk sac within the protective egg case.
The second method is ovoviviparity, where the fertilized eggs develop and hatch internally inside the mother’s reproductive tract. The young are born alive, but they receive no direct nutritional connection from the mother, relying instead on the yolk sac for sustenance. This strategy is sometimes also called aplacental viviparity.
The third category is viviparity, which involves a true live birth where the developing young are nourished by the mother through a placental connection. This method is similar to mammalian reproduction, as the mother transfers nutrients and oxygen via a structure analogous to an umbilical cord.
Sharks That Lay Eggs
Oviparous sharks represent a minority of all shark species but include groups such as Catsharks, Horn Sharks, and Carpet Sharks. After internal fertilization, the female deposits the fertilized egg, encased in a tough, leathery pouch, into the environment. This process is sometimes called extended oviparity.
The embryo develops completely outside the mother, relying solely on the large yolk sac provided within the case for months, sometimes over a year. The mother typically seeks out a sheltered location, such as rocky crevices or dense kelp beds, to affix the egg case. The Port Jackson Shark is famous for laying a uniquely spiral-shaped egg case.
Catsharks and Zebra Sharks are also common oviparous examples, producing oblong egg cases anchored to the substrate. The pup must emerge from the case as a fully formed miniature version of the adult, ready to fend for itself.
Anatomy of the Egg Case
The egg case of an oviparous shark is often colloquially known as a “mermaid’s purse” due to its distinct, purse-like or rectangular shape when found washed ashore. This protective capsule is composed of a tough, collagen-based fibrous protein, giving it a leathery texture. The material is highly durable, which is important since the embryo may spend many months inside while exposed to ocean currents and predators.
The shape of the egg case is species-specific, aiding in identification and providing specialized anchoring mechanisms. Catshark egg cases are typically purse-shaped with long, curly tendrils extending from the corners. These tendrils function to entangle the case onto sea plants or reef structures.
Horn sharks produce a distinctive corkscrew or spiral-flanged case designed to be screwed into a crevice for a secure fit. All cases possess small slits or openings that allow for the circulation of oxygenated seawater to the developing embryo inside.
Other Ways Sharks Reproduce
The majority of shark species do not lay eggs, instead opting for one of the two forms of live birth. The most common reproductive method among sharks is ovoviviparity, where the eggs hatch internally within the female’s uterus. Species like the Great White Shark and Nurse Shark use this strategy, with the pups developing from their yolk sac until birth.
In some ovoviviparous species, the developing pups may consume unfertilized eggs or even their siblings within the womb. This intrauterine feeding, known as oophagy or adelphophagy, results in very small litters of large, highly developed pups.
The most advanced method is viviparity, practiced by sharks such as Bull Sharks, Hammerheads, and Blue Sharks. The embryo first develops a yolk sac, which transforms into a placental structure that connects directly to the mother’s uterine wall. This placental connection allows for constant nutrient and gas exchange, resulting in a live birth analogous to that of mammals.