This Easter Australians will be hunting for a different kind of egg as CSIRO, Australia’s national science agency, urges citizen scientists to venture out and discover egg cases washed up on Australia’s shores.
What are egg cases? Well, also known as mermaid’s purses, they’re the tough leathery capsule that protects the developing foetuses of oviparous chondrichthyans – egg-laying sharks, skates, and chimaeras.
CSIRO has launched the Great Eggcase Hunt, an initiative of the UK-based charity The Shark Trust, to help collect data for scientists who are studying the taxonomy and distribution of these animals.
Citizen scientists will be able to record sightings of egg cases underwater or washed up on beaches via the Shark Trust citizen science mobile phone app, or through the project website. The app helps identify the egg species and includes information about sharks,
Helen O’Neill, CSIRO Australian National Fish Collection biologist, says this will help scientists discover what the egg cases of different chondrichthyans look like, with some species’ egg cases still unknown.
“Egg cases are important for understanding the basic biology of oviparous chondrichthyans, as well as revealing valuable information such as where different species live and where their nurseries are located,” says O’Neill.
Eggs this Easter are weirder than normal
Wait a second, isn’t it well-known that sharks give birth to live young? And baby sharks can even eat each other while still in the womb?
Yes! A whole bunch of them are indeed do, including mako sharks, great white sharks, and hammerhead sharks.
But chondichthyes – fishes with skeletons made of cartilage – have some of the most diverse reproduction strategies found in vertebrates (animals with a spine). This can include parthenogenesis (no father), multiple paternity (more than one father of the litter), adelphophagy (baby sharks predating each other in the womb) and, of course, various modes of egg laying (oviparity).
And like a shopaholic mermaid’s dream come true, egg cases come in an incredible diversity of shapes and colours; they range in size from about 4 to 25 centimetres and come in hues from cream and butterscotch to deep amber and black.
While some egg cases are smooth and minimalist, others have unique physical characteristics like ridges, keels, or curling tendrils that help anchor them to kelp or coral. Port Jackson sharks even have spiral shaped eggs that allow the mother to pick them up with her mouth and screw them into rocks and crevices to stop them from being washed away.
And all of these unique morphologies are helpful in the science of describing and naming species: taxonomy.
Incubation times vary depending on the species but can range from a few months to years, though by the time egg cases are found on beaches they rarely contain live embryos.
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“Egg cases found washed up on beaches have likely already hatched, died prematurely due to being washed ashore or been predated on by creatures like sea snails, who bore a hole in the egg case and suck out the contents,” says O’Neill.
“At the Australian National Fish Collection, we are matching egg cases to the species that laid them. We borrow egg cases from other collections, museums and aquariums around the world and use our own specimens collected from fish markets and surveys at sea or extracted from the ovaries of preserved specimens in our collection.”
The Great Eggcase hunt began in the UK in 2003, and this year are celebrating 20 years and the identification of more than 380,000 individual egg cases from around the world.
“We’re really excited to be partnering with CSIRO to officially launch this citizen science project in Australia and to be able to expand the Shark Trust’s egg case identification resources,” says Senior Conservation Officer at The Shark Trust, Cat Gordon.
“There’s such a diversity of species to be found around the Australian coastline, and with a tailored identification guide created for each state, they really showcase the different catsharks, skate, horn sharks, carpet sharks and chimaera egg cases that can be found washed ashore or seen while diving.”
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