Shark eggs, often found washed ashore and sometimes called “mermaid’s purses,” offer a unique glimpse into the diverse reproductive strategies of these marine animals. While many marine creatures lay eggs, shark eggs possess distinctive characteristics that aid in the development and survival of the young. These capsules provide protection for the developing embryo within the ocean environment.
The Varied Forms of Shark Eggs
The physical appearance of shark eggs varies considerably among species, reflecting different adaptations for survival. Many are commonly known as “mermaid’s purses” due to their resemblance to small, leathery pouches. These typically rectangular cases often feature tendrils or horn-like projections at their corners, which function to anchor the egg to underwater structures like seaweed, rocks, or the seafloor, preventing them from being swept away by currents. The size of these egg cases can range from approximately 5 to 25 centimeters in length.
Another distinct form is the spiral or corkscrew-shaped egg, notably laid by species such as horn sharks and Port Jackson sharks. This auger-like design allows the female shark to wedge the egg firmly into crevices and rocky substrates on the ocean floor. This wedging provides stability against strong currents and helps protect the developing embryo from predators. The irregular ridges of the spiral also contribute to camouflage, helping the egg case blend into its surroundings.
Shark egg cases are composed of a tough, leathery material made of collagen-like proteins, providing a durable barrier for the embryo. The texture can range from smooth and glossy to rough and fibrous, which aids in camouflage and adhesion to different surfaces. Their coloration typically ranges from yellow to dark brown or black, helping the egg case blend with the marine environment and avoid detection by predators.
From Egg Case to Hatchling
Once fertilized, the female shark deposits the egg case in a secure marine location. Typical sites include attachment to seaweed, kelp forests, rocky reefs, or buried in sand. Placement often depends on the egg case’s design; tendril-equipped cases tangle into vegetation, while spiral ones wedge into rocks. This initial placement is generally the extent of parental care, as eggs develop independently.
Inside the protective egg case, the developing embryo is nourished by a rich yolk sac, which provides all necessary nutrients for growth. As the embryo develops, it gradually absorbs the yolk, transforming into a miniature shark. Egg cases are designed with small openings or respiratory slits that facilitate gas and water exchange, allowing the embryo to breathe.
The incubation period for shark eggs varies significantly by species and environmental conditions, especially water temperature. This period can range from a few months (e.g., 6-9 months for some catsharks) to over a year (e.g., 9-12 months for horn sharks). Once fully developed and the yolk sac absorbed, the young shark, or pup, emerges from the egg case, ready to navigate the ocean independently.
Oviparous Sharks: The Egg-Layers
The term “oviparous” refers to sharks that lay eggs, distinguishing them from other shark reproductive strategies. Approximately 40% of known shark species reproduce through oviparity, where eggs are fertilized internally but laid outside the mother’s body to develop. This differs from ovoviviparous sharks, where eggs hatch inside the mother, and viviparous sharks, which give birth to live young with a placental connection.
Examples of oviparous sharks include horn sharks, swellsharks, catsharks, bamboo sharks, zebra sharks, and wobbegong sharks. These are typically bottom-dwelling species that secure their eggs to the seafloor. The tough, leathery egg case protects the developing embryo from predators and environmental challenges during its extended developmental period before hatching.