24 Feb An Eggstraordinary Shark Tale
PJ Shark – John Turnbull FlickR
Did you know that some sharks lay eggs? Researchers report that around 40% of all sharks in the world lay eggs, while other shark species give birth to live young.
Baby PJ Shark – John Turnbull
One of those species is the Port Jackson Shark, Heterodontus portusjacksoni (Meyer, 1793), named after Port Jackson (Sydney Harbour) in Sydney. This shark has a distinctive blunt head that has a spine in front of both dorsal fins. It also has harness-like markings that cross the eyes, run along the back to the first dorsal fin, then cross the side of the body. This pattern makes them easy to identify. Only the Heterodontidae sharks have the combination of no anal fin and spines on the leading edge of the two dorsal fins.
The family Heterodontidae has eight species, all in the genus Heterodontus, three of which are found in Australian waters. These are the Port Jackson Shark, the Zebra Horn Shark, Heterodontus zebra, and the Crested Horn Shark, Heterodontus galeatus. All are oviparous, which means that the females lay eggs.
Crested horn shark egg – Aust Museum
PJ Shark Egg – Kate Bunker credit
The Port Jackson shark forages for food at night when their prey is most active. They eat sea urchins, molluscs, crustaceans and fish. They are not large compared to other species, growing to around 1.65m and they live for about 30 years.
The egg case is a tough, dark brown spiral about 7 cm to 8 cm wide and 15 cm long. It is soft when first laid by the female. She uses her mouth to wedge the egg case into a rock crevice where it hardens, and from which one young shark emerges after 10-12 months. The Crested Horn Shark has a similar-looking egg case with the addition of long twisted tendrils on the bottom end. These are often attached to seaweed.
The baby sharks, or pups, that emerge are only 25cm long.
Another interesting fact about the Port Jackson Shark is that they have the ability to eat and breathe at the same time. This ability is unusual for sharks, many of which need to swim with the mouth open to force water over the gills. The Port Jackson Shark can pump water into the first enlarged gill slit and out through the other four gill slits. By pumping water across the gills, the shark does not need to move to ‘breathe’. It can lie on the bottom for long periods of time, a behaviour that is observed at breeding time.
It is very common to see the Port Jackson shark egg casings washed up on beaches so next time you’re on a coastal holiday and wandering along the beach, keep an eye out for them and other interesting shark egg casings.
Video by Thierry Rakotoarivelo. https://www.youtube.com/watch?v=4NxhpNipztk