Sharks, ancient inhabitants of our oceans, exhibit diverse life strategies, including varied reproductive methods. While many marine creatures release vast numbers of eggs, some sharks employ a specialized approach: laying eggs. This unique aspect of their biology highlights their adaptations and protective measures for their young.
Understanding Shark Reproduction
Shark reproduction differs significantly from most bony fish, as all sharks use internal fertilization. This leads to three primary strategies. Oviparity involves females laying eggs externally, encased in protective coverings. The developing embryo relies on the yolk for nourishment until hatching. Viviparity, similar to mammals, involves live birth. Pups receive nourishment directly from the mother, often through a placental connection, and are born fully formed. Ovoviviparity involves eggs hatching inside the mother, with young born alive. Embryos are nourished by their own yolk sacs or other internal sources, not directly by the mother.
The Egg-Laying Sharks
Several shark species are known for their egg-laying behavior, primarily bottom-dwelling types. These oviparous species include horn sharks, various catsharks, and swellsharks, each with unique adaptations for protecting their developing embryos. Over 250 known shark species are oviparous, comprising about 40% of all shark species.
Horn sharks (Heterodontus) are known for their distinctively shaped eggs. Found along the eastern Pacific, they lay spiral-shaped egg cases up to 5 inches long. Female horn sharks wedge these corkscrew-like cases into rocky crevices or seaweed, using the egg’s shape to secure it from predators and currents, ensuring it remains safe during months-long development.
Catsharks (family Scyliorhinidae) are a large group of oviparous sharks. Found worldwide in coastal and deep-water environments, they produce rectangular egg cases with tendrils at each corner. Females use these tendrils to anchor the egg case to seaweed, coral, or other underwater structures, allowing secure embryo development. Some catsharks, like the small-spotted catshark, can lay up to 240 eggs per year, often in pairs, with pups hatching after 5 to 11 months.
Swellsharks (Cephaloscyllium ventriosum) are named for their ability to inflate their bodies with water as a defense mechanism. These eastern Pacific sharks lay amber or greenish flattened egg cases, usually two at a time. Their egg cases also feature tendrils that help them attach to kelp or rocky reefs. Development for swellshark embryos ranges from 9 to 12 months, influenced by water temperature.
Anatomy and Development of Shark Eggs
The eggs laid by oviparous sharks are commonly called “mermaid’s purses” due to their unique appearance. These protective egg cases are not hard like bird eggs but are leathery and composed primarily of collagen. Their shapes vary significantly between species, from the corkscrew design of horn shark eggs to the rectangular pouches with tendrils seen in catsharks and swellsharks. These distinct shapes and features aid in anchoring the eggs to the seafloor, preventing them from being swept away by currents.
Inside the casing, the developing shark embryo is sustained by a yolk sac, which provides nutrients for growth. The egg case often has small slits or pores to facilitate water flow, allowing oxygen to reach the embryo. Hatching time varies depending on the shark species and environmental conditions, particularly water temperature, ranging from a few months to over a year. Once developed, the shark pup emerges as a miniature adult, equipped to survive independently, as oviparous sharks provide no parental care after the eggs are laid. Empty egg cases are often found washed ashore, providing evidence of shark development.