Sharks exhibit diverse reproductive strategies. While many species give birth to live young, a significant number lay eggs. This distinct method involves the development of young outside the mother’s body, housed within specialized protective casings.
Understanding Oviparous Reproduction
Oviparous reproduction in sharks refers to the process where the female lays eggs externally, allowing the embryo to develop outside her body. This contrasts with viviparous sharks, which give birth to live, fully formed offspring, and ovoviviparous species, where eggs hatch internally before live birth occurs.
All shark reproduction begins with internal fertilization, where the male uses specialized organs called claspers to transfer sperm to the female. After fertilization, the female’s body secretes a tough, protective casing around the fertilized egg. This encapsulated egg, often referred to as a “mermaid’s purse,” is then deposited by the mother into the marine environment. The developing embryo inside the egg case receives all its necessary nutrients from a yolk sac until it is ready to hatch.
Common Egg-Laying Shark Species
Approximately 40% of shark species are oviparous, primarily smaller, bottom-dwelling types. The catshark family (Scyliorhinidae) includes recognized egg-layers like the chain catshark, small-spotted catshark (also known as lesser spotted dogfish), and the nursehound (or greater spotted dogfish). These sharks typically inhabit coastal and deep-sea environments, with their egg cases often washing ashore. The brown catshark, for instance, lays one egg case at a time, with embryos sometimes developing for over two years in colder waters.
Horn sharks (Heterodontidae), including the Port Jackson shark and the California horn shark, are another prominent group of oviparous species. Port Jackson sharks are known to carry their egg cases in their mouths, possibly to find a secure hiding spot. California horn sharks lay about two eggs every 10 to 14 days during spring and summer, and their eggs have a distinctive spiral shape.
The zebra shark (Stegostoma fasciatum), despite its common name suggesting stripes, develops spots as an adult, but its young display striking zebra-like patterns. Female zebra sharks lay large eggs, typically between one and four at a time, attaching them to suitable underwater structures. Other oviparous sharks include bamboo sharks (Hemiscylliidae) and epaulette sharks (Hemiscyllium ocellatum), often found in shallow, tropical waters. Wobbegong sharks (Orectolobiformes) also belong to this egg-laying group.
The Unique Shark Egg Case
The shark egg case, often called a “mermaid’s purse,” is designed for protection. These cases are composed of a tough, leathery material, primarily collagen, which provides resilience against predators and harsh oceanic conditions. Their appearance varies significantly among species. Catshark egg cases, for instance, are often rectangular with long, curly tendrils at each corner. These tendrils serve to anchor the case securely to seaweed, coral, or other underwater structures, preventing them from being swept away by currents.
Horn shark egg cases are particularly distinctive, featuring a spiral flange resembling a corkscrew. This unique shape allows the mother to wedge the egg firmly into rocky crevices, offering added security. Zebra shark egg cases are large, measuring around 17 cm long, and are dark brown to purplish-black with hair-like fibers that help them adhere to the substrate. Inside this protective capsule, the embryo develops, nourished by its yolk sac, for a period ranging from several months to over a year, influenced by water temperature and species. Once fully developed, the shark pup uses a specialized “egg tooth” to emerge, leaving the empty case behind.