Port Jackson sharks lay spiral-shaped eggs that look like large screws.

The Port Jackson shark, or Heterodontus portusjacksoni, is a member of the bullhead shark family found primarily in the coastal waters of southern Australia. Unlike many shark species that give birth to live pups, this species is oviparous, meaning it lays eggs. The unique spiral shape of the egg case is an evolutionary adaptation that allows the mother to physically screw the egg into tight rock crevices or reefs using her mouth.This anchoring mechanism is crucial for survival in the high-energy surge zones where these sharks often breed. By securing the egg deep within a crack, the mother ensures it will not be swept away by powerful tides or easily extracted by predators like large fish or other sharks. The egg case itself is made of a tough protein called collagen, which hardens upon contact with seawater to form a protective shell.Research conducted by marine biologists at Macquarie University has shown that these embryos develop for nearly a full year before hatching. During this time, the embryo relies entirely on a large yolk sac for nutrients. Once the shark hatches, it is approximately 25 centimeters long and fully independent. The discarded spiral casings are frequently found by beachgoers along the Australian coastline, where they are commonly referred to as 'mermaid's purses' due to their unusual and ornate appearance.