Have you ever wondered how shells are birthed?

By Cathy Finch 18 January 2021
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Looking like something straight out of Alien, this egg casing is a nursery for hundreds of precious seashells.

While on an afternoon walk around North West Island on the Great Barrier Reef, Brisbane couple Isabel Stubbs and Dan Crowther discovered something remarkable.    

“We went for a walk around the island and found this washed up on the beach after last night’s rough weather,” Isabel says holding out a goopy-looking mass. “It’s an egg casing of the baler shell! We only knew what it was because we found something similar here a couple of years ago and a National Parks ranger identified it for us.  

“The casing we found today is full of little babies in all stages of life, from embryos to smaller shells, and larger shells at the far end. Initially we popped it back in the water, but it washed up again on this afternoon’s tide, so we decided to bring it back to camp to show anyone who is interested. We’ll try to put it deeper into the ocean now, so hopefully the shells will continue to develop.”

“I have been coming to the island for 30 years,” says Dan, “and this is only the second time I’ve seen such an egg casing. There’s always something new to learn here.”

Adult northern baler shell and its muscular, slug-like foot. Image credit: Cathy Finch

A couple of years ago, on my last visit to North West island, I found a huge adult baler shell at low tide while doing a reef walk. It was extraordinarily heavy with animal and magnificent to see alive and well on our reef.  It’s wonderful to think this could be a breeding ground for them.  

The baler shell is a large marine mollusc that can reach up to 45cm in length and has a strong, muscular, slug-like foot, which manoeuvres the animal along the sea floor. It also uses it to smother its food catch. They are carnivorous and prey on other molluscs such as tritons and scallops.

A juvenile baler shell, straight from egg casing. Image credit: Cathy Finch

The eggs of the baler shell are normally found attached to rock or other shells, laid layer by layer to form a translucent, hard sponge-like casing in a mesmerising spiral with holes where the shells develop. The casing is hollow, which ensures there is water flow and circulation around the eggs.  They can contain more than 100 developing shells. Juveniles develop and crawl away straight from the egg, immediately caring for themselves.   

There are two species of baler shell, the northern baler (Melo amphora) and the southern baler (Melo miltonis). The shells of this mollusc were highly valued by Aboriginal people, who used them to store water. The name came about because early Europeans recorded Aboriginal people bailing out their canoes with these huge shells.

The shells of balers have attractive patterns. However, even more striking is the brown slug-like foot, which has intricate patterns of white lines that resemble an Aboriginal painting. Image credit: Suzanne Long

The southern baler is distributed from the Houtman Abrolhos Islands in Western Australia to South Australia. The northern baler is found from the Houtman Abrolhos north around the WA coast and across the Northern Territory coast to Queensland.