The zebra shark, scientifically known as Stegostoma tigrinum, is a distinctive species of carpet shark found across the Indo-Pacific. Unlike many of their relatives, zebra sharks reproduce by laying eggs. This egg-laying strategy involves depositing specially constructed protective casings that allow the embryo to develop externally.
The Zebra Shark Identity
The common name of this species often causes confusion because the shark undergoes a dramatic change in appearance as it matures. Juveniles exhibit dark brown and white stripes, which is the reason for the name “zebra shark.” As the shark grows up to 2.5 meters, the stripes transform into a pattern of dark spots on a pale background. This spotted adult pattern often leads to the species being called a “leopard shark,” which can be misleading as that name applies to a different species. Zebra sharks are bottom-dwelling and nocturnal, spending daylight hours resting on sandy flats or coral reefs in tropical waters up to 62 meters deep.
Reproductive Strategy: Oviparity
The zebra shark employs an oviparous reproductive strategy, meaning the female lays eggs that develop and hatch outside of her body. This mechanism is one of three main reproductive modes found in sharks, though it is less common among larger species.
After internal fertilization, the female forms the protective egg case. She typically releases these eggs in batches, sometimes laying up to four eggs at a time over several months. Egg-laying is not dependent on a specific season and is often observed year-round in captivity. Female zebra sharks have also demonstrated the ability to reproduce asexually through parthenogenesis, where an embryo develops without male fertilization, though this is rare and mostly observed in captivity.
Anatomy of the Egg Case
The zebra shark egg is a tough, leathery capsule often referred to as a “mermaid’s purse.” This rectangular case is composed of collagen protein, giving it a fibrous yet sturdy feel designed to protect the developing embryo within. It is relatively large for a shark egg, measuring up to 17 centimeters long, 8 centimeters wide, and 5 centimeters thick. The color is typically dark brown or purplish-black, helping it blend into the marine substrate. A distinctive feature is the presence of adhesive, hair-like fibers along its sides. These sticky tendrils are crucial for anchoring the egg, allowing the female to securely entangle the capsule onto hard structures like coral, rocks, or kelp on the seafloor, preventing it from being swept away by currents.
Incubation and Hatching
Once the egg case is deposited and secured, the embryo begins its external development. The embryo is sustained entirely by a large yolk sac contained within the capsule, which acts as its sole source of nourishment. The length of the incubation period is variable and dependent on water temperature, but it generally lasts between four and six months. In controlled environments, the average incubation time is around 152 days. During this time, the tough casing provides protection while allowing oxygenated seawater to be absorbed. When the pup is ready, it pushes its way out of the egg case, emerging as a fully formed miniature shark. Hatchlings are relatively large, typically measuring between 20 and 36 centimeters long, and are immediately independent.