Sharks exhibit a wide array of reproductive strategies, extending beyond giving birth to live young. While many marine species release vast numbers of eggs and sperm into the water, sharks engage in internal fertilization, a process that generally yields fewer, more developed offspring with a greater chance of survival. This diversity includes species that lay eggs that develop outside the mother’s body, offering insight into their complex life cycles.
Shark Reproduction Methods
Sharks employ three primary methods for reproduction: oviparity, ovoviviparity, and viviparity. Oviparity refers to egg-laying sharks, where a gland secretes a protective case around the fertilized egg, which is then deposited into the marine environment. The embryo develops entirely outside the mother, relying on the yolk within the egg for nourishment.
This distinguishes them from ovoviviparous sharks, which produce eggs that hatch internally, with the young then born live. In this method, embryos also receive nutrients primarily from a yolk sac while developing inside the mother’s uterus, though some species may also absorb additional fluids from the uterine lining.
Viviparous sharks, on the other hand, give birth to live young that develop inside the mother with a placental connection, similar to mammals. In these cases, the embryo receives direct nourishment and oxygen from the mother’s bloodstream through a yolk sac placenta. While ovoviviparity is the most common reproductive strategy among sharks, oviparity is observed in approximately 40% of all shark species. This egg-laying strategy is particularly common among bottom-dwelling sharks.
The Distinctive Shark Egg
The eggs laid by oviparous sharks are often encased in unique structures known as “mermaid’s purses.” These tough, leathery pouches are primarily composed of collagen, offering robust protection for the developing embryo inside.
The appearance of these egg cases varies considerably among species, ranging in shape from rectangular to spiral, or even purse-like. When first laid, the cases are typically soft and pale, hardening and darkening within a few hours.
Many shark egg cases feature specialized tendrils that anchor them to underwater structures like kelp or seaweed, preventing them from drifting.
For instance, Horn Sharks and Port Jackson Sharks lay distinctive spiral-shaped eggs that are often wedged into rock crevices, a design that helps secure them in place. The egg capsule also has small slits that facilitate water flow, allowing the developing shark to breathe.
Identifying Egg-Laying Sharks
Several shark species are known for their egg-laying behavior, each with unique egg characteristics. For example, Horn Sharks (Heterodontus francisci), found off western North America, lay corkscrew-shaped eggs.
These eggs, typically laid in pairs every 10 to 14 days, are pliable when deposited but quickly firm up. Their ridges resemble a screw, enabling a tight fit within rocks, which protects the embryo from predators and currents.
The Port Jackson Shark (Heterodontus portusjacksoni), native to Australian waters, also lays spiral-shaped eggs, often dark brown and 7-8 cm wide and 15 cm long. Like the Horn Shark, the female wedges these eggs into rock crevices.
Catsharks (Scyliorhinidae family) are another diverse group of oviparous sharks, with species like the Smallspotted Catshark laying eggs in pairs, each encased in a tough, leathery case with long, curly tendrils. These tendrils are used to attach the egg cases to seaweed or other marine vegetation.
Many catshark species, including the Brown Catshark, lay eggs with distinct shapes and tough, leathery membranes. Some carpet sharks, like bamboo sharks, also lay eggs, often with tendrils that anchor them to the seabed.
Development Within the Egg
Inside the protective casing, the embryo develops, sustained by a yolk sac. This yolk sac provides all necessary nutrition, shrinking as the embryo consumes it.
The embryo receives oxygen through small slits or pores in the egg case, with the developing shark sometimes moving its tail to increase water flow and enhance oxygenation.
The incubation period varies significantly among species, influenced by factors such as water temperature and genetic programming.
Catshark eggs, for example, typically incubate for 6 to 9 months. Horn Shark eggs can take between 6 to 10 months to hatch, while Port Jackson Shark eggs usually hatch after 10 to 12 months.
Once fully developed, a miniature shark, often 10-17 cm long, emerges. Empty egg cases may then wash ashore.