Shark eggs, often found washed ashore, are intricate, protective casings for a developing shark embryo. Their diverse appearance fascinates beachcombers and scientists, offering insight into the early life stages of these ancient marine animals.
Diversity in Shark Reproduction
All sharks utilize internal fertilization. While many shark species give birth to live young (viviparity or ovoviviparity), only a minority lay eggs externally. Approximately 40% of shark species are oviparous, meaning they deposit eggs that develop and hatch outside the mother’s body.
This egg-laying strategy is observed in specific shark families, including horn sharks, catsharks, swellsharks, bamboo sharks, and zebra sharks. These species typically lay their eggs in locations that offer some protection, such as rocky crevices, kelp forests, or among seaweed. This placement helps shield the vulnerable embryos from predators and strong currents.
Characteristics of Shark Eggs
Shark eggs, colloquially known as “mermaid’s purses,” are encased in tough, leathery capsules made primarily of collagen protein strands. The term originated from folklore, as sailors imagined these odd, purse-like objects, found on beaches, to be lost mermaid handbags. These capsules come in a variety of shapes, sizes, and textures, each adapted to the specific environment and species.
For instance, horn shark eggs are distinctively spiral or corkscrew-shaped, allowing the female to wedge them securely into rocky crevices, preventing them from being dislodged by currents or predators. Catshark eggs are often rectangular or purse-shaped, featuring long, curly tendrils at their corners. These tendrils function as anchors, enabling the egg case to attach firmly to marine vegetation like seaweed or corals.
Swellsharks lay green or amber-colored, flattened eggs that are sometimes flask-shaped and possess tendrils for attachment to kelp or reefs. The size of these egg cases can vary, often palm-sized, typically 4 to 12 centimeters long, though some species produce larger ones. Their coloration usually ranges from yellowish to brown or black, which aids in camouflaging them within their marine habitats.
Life Within the Egg
Inside the protective casing of a shark egg, a single embryo develops, sustained by a large yolk sac. This yolk sac provides all necessary nutrients for growth, similar to how a chicken egg nourishes its chick. The duration of embryonic development within the egg case varies considerably among species and is influenced by environmental factors, particularly water temperature.
Gestation periods can range from a few months to over a year. For example, swell shark eggs typically hatch within 9 to 12 months, while horn shark eggs take between six and nine months to develop. The spiny dogfish, in contrast, has one of the longest known gestation periods among vertebrates, lasting up to 24 months.
Throughout this period, the tough, leathery egg case provides a robust defense against physical damage and many predators. As the embryo grows, it consumes the yolk, and in some species, such as catsharks, the developing shark can be observed moving inside the translucent egg case. Once fully developed, a miniature, self-sufficient shark emerges from the egg case. After hatching, the young shark receives no parental care and must immediately navigate its marine environment independently.