Collecting seashells is a favourite past time of Australians, young and old.

From the rare to the common, its easy to forget a living creature produced that shell found washed up on the beach.

Simon Grove, the senior curator of invertebrate zoology at the Tasmanian Museum and Art Gallery, said piles of shells were actually known as 'death assemblages'.

"Shells are the mortal remains of the living molluscs that used to live in them and actually made them," Dr Grove said.

"Once the mollusc dies, the shell rolls around on the seafloor, and if we are lucky they get washed up on our beaches."

Dr Grove, author of the Seashells of Tasmania, said in there were about 1,800 species of seashells in the island state, but there was a lot of missing data.

It was hard to say which shells were truly common or rare, he said.

Dr Simon Grove said molluscs were ancient creatures, dating back half a billion years. ( ABC Radio Hobart: Georgie Burgess )

"The ones that get washed up on the beach are just a small fraction of what's living out there," he said.

"The ones living out there that don't get washed up on the beach are hard to actually find in the first place, they may be common but we don't see them very often.

"Conversely, the ones that live on the seashore we find them all the time but that's a thin ribbon of habitat for these species to live so in a broader sense they are quite rare."

Dr Grove said molluscs were ancient creatures, dating back half a billion years.

He said there was no problem collecting empty shells on a beach (but keep track of National Park rules), but there were rules about collecting living shells.

The prized cowry

The umbilicated cowry, or wonder cowry is Tasmania's largest. ( ABC Radio Hobart: Georgie Burgess )

The cowry shell is the most coveted shell for beachcombers.

"I think they are generally everybody's favourite," Dr Grove said.

"There's something just very sensual about the shape and look of these beautiful shells.

"In Tasmania, we've got half a dozen species, we're blessed."

Dr Grove said perseverance was key to finding a cowry shell.

"They are never common so they are always the right rarity, so you have to look for them as you walk along the beach," he said.

"Don't give up on first attempts, keep looking."

Dr Simon Grove says the wonder cowry is his favourite shell. ( ABC Radio Hobart: Georgie Burgess )

He said cowrys lived under rocks and browsed over tiny invertebrate animals.

"Most of them aren't living very far offshore and when they die there's a chance they will get washed up as a dead shell," he said.

"They need rocks for living in, but currents can bring them to sandy beaches once the shell is dead."

Dr Grove said his favourite shell was Tasmania's largest cowry, the umbilicated cowry, or wonder cowry.

"I like to call it the belly button cowry, because if you look at the end it has a dimple like a belly button," he said.

"This species is only found in southern Australia, including Tasmania.

"In the northern part of its range, it's only in deep water, but down here in the south you can find them washed up on beaches because they live in shallow water because the water is that much cooler."

The pearly brooch

Pearly brooch shells are widespread in Tasmanian waters but not commonly found on the beach. ( ABC Radio Hobart: Georgie Burgess )

The brooch shell lives offshore in sandy, exposed environments and is endemic to south-eastern and south-western Australia.

While it is widespread in Tasmania, it is not commonly found beached.

"I just love these, these are some of the first shells I noticed on Tasmanian beaches when I arrived in the state back in 2001," Dr Grove said.

"They set me off on the course I'm still on.

"You can imagine them being used as brooches."

Dr Grove said when the first French explorers visited Tasmania, they did dredging in the d'Entrecasteaux Channel and dug up brooch shells.

"The naturalist on board immediately recognised them as a living fossil because he was aware of fossils of these things from millions and millions of years ago, but no-one knew they still existed."

The shell reportedly jumped off the boat, and the explorers had to search for another.

Beware the Japanese softshell clam

The Japanese clam shell is not wanted in Australia. ( ABC Radio Hobart: Georgie Burgess )

The Japanese softshell clam is from the Pacific and has only been recently detected in Australia at Orford on Tasmania's east coast.

Dr Grove said the shell was found in mud, and its presence was concerning.

"They are invaders," he said.

"Potentially it will compete with our native species, but it's a bigger worry than that.

"This is the first detection in the southern hemisphere, if it gets from here to anywhere else in the southern hemisphere, it could cause even bigger trouble."

Dr Grove said if the shell got to New Zealand it would out-compete similar-sized molluscs.

He said the shells were confined to the Orford Triabunna area, but other sightings should be reported to Biosecurity Tasmania.

Pen shell rarely beached intact

Pen shells, or razor clams, tend to appear after storms. ( ABC Radio Hobart: Georgie Burgess )

The large pen shell, or razor clam, is Tasmania's largest bivalve mollusc, and is found widely in Tasmania, as well as New South Wales and Victoria.

But, it's rarely found intact.

"You tend to get them after storms when the storm has churned up the soft sediment they're embedded in," Dr Grove said.

"The shells are actually quite flat, there's not a lot of space between the valves.