Sharks, ancient inhabitants of the world’s oceans, exhibit diverse reproductive strategies. Unlike many fish that release eggs and sperm into the water, sharks employ internal fertilization, leading to varied ways of nurturing their young. These range from laying eggs in protective cases to giving birth to live young.
Mating and Fertilization
Shark reproduction begins with internal fertilization, a process where the male transfers sperm directly into the female’s reproductive tract. Male sharks possess specialized organs called claspers, which are extensions of their pelvic fins. During copulation, one clasper is inserted into the female’s cloaca, and sperm travels along a groove into her oviduct, where fertilization occurs.
Males often bite the female’s back, flanks, or pectoral fins to hold position during this process, sometimes resulting in visible wounds. Female sharks in some species, such as blue sharks, bull sharks, and tiger sharks, have evolved thicker skin in these areas, sometimes twice as thick as the males’, to withstand these mating bites. Females can also store sperm for extended periods, allowing for delayed fertilization.
Egg-Laying Sharks
Some shark species are oviparous, meaning they lay eggs that develop and hatch outside the mother’s body. These eggs are encased in a tough, leathery capsule often referred to as a “mermaid’s purse.” The embryo inside the egg case is nourished by a yolk sac.
These distinctive egg cases come in various shapes, including spiral, purse-like, or rectangular with tendrils. The tendrils or sticky filaments serve to anchor the egg case to underwater structures like seaweed or rocks, preventing them from being swept away by currents. Oviparous sharks typically include bottom-dwelling species such as horn sharks, catsharks, Port Jackson sharks, and some bamboo sharks. The development period within the egg case can range from several months to over a year, depending on the species and water temperature.
Live Birth from Internal Eggs
The most common reproductive strategy among sharks is ovoviviparity, sometimes called aplacental viviparity. In this method, the eggs hatch internally within the mother’s oviduct or uterus, and the embryos continue to develop inside her body. The young receive initial nourishment from their yolk sacs.
There is no direct placental connection between the mother and the developing pups in ovoviviparous species. Once the yolk supply is depleted, embryos may obtain additional nutrition by consuming unfertilized eggs produced by the mother, a practice known as oophagy. In some cases, such as with sand tiger sharks, the developing pups may even consume their siblings within the uterus, a phenomenon called adelphophagy. Examples of ovoviviparous sharks include great white sharks, tiger sharks, nurse sharks, and mako sharks. Pups are born live and fully formed, often in sheltered nursery areas, ready to fend for themselves.
True Live Birth
Viviparity, or true live birth, is another significant reproductive strategy found in sharks, resembling the reproductive process in mammals. In viviparous species, embryos develop inside the mother’s uterus and receive continuous nourishment directly from her bloodstream through a placental connection. This connection forms when the yolk sac attaches to the uterine wall, creating a yolk sac placenta.
The placenta facilitates the transfer of nutrients and oxygen from the mother to the developing pups and removes waste products. This method ensures that the young are born well-developed and prepared for independent life. Viviparous sharks give birth to live young that are essentially miniature versions of the adults. Species that exhibit true viviparity include bull sharks, hammerhead sharks, blue sharks, and lemon sharks.
Unusual Reproductive Strategies
Beyond the primary modes, sharks display some exceptional reproductive variations. Parthenogenesis, often called “virgin birth,” is a rare form of asexual reproduction where an egg develops into an embryo without fertilization. This has been observed in some female sharks, including bonnethead, blacktip, and zebra sharks, often in captive environments where males are absent. While providing a means of reproduction in isolation, parthenogenetic offspring often have reduced genetic diversity and may face developmental challenges.
Another unusual strategy involves the feeding behaviors of embryos within the womb. Oophagy, the consumption of unfertilized eggs, provides supplementary nutrition. This occurs in sharks like the shortfin mako and bigeye thresher. Adelphophagy, a more extreme form of intrauterine cannibalism, involves the strongest embryos eating their weaker siblings in the uterus. This brutal competition is notably seen in sand tiger sharks, where typically only one or two large pups survive from a litter.