© ABCSpider crabs shells
Local residents have reported the orange crabs, around the size of a human hand, have washed up on Raspins Beach at Orford in recent days.
Recreational Fishing Tasmania's Don Paton says he would like to get to the bottom of the event.
"Whether there's some viral infection that might have caught in the ocean or whether it's a natural phenomenon they actually do at certain times of the year," he said.
"It's amazing to me to think that over the last 30 or 40 years, or longer, that I've been around looking at the beaches up here most of my life, I've never seen a phenomenon like it."
Marine biologist Karen Gowlett-Holmes says it is likely the shells are washing up after the crabs have mated at sea.
"Spider crabs and most crabs can only mate when the female has just moulted her shell," she said.
"If you have a look at the shells that wash up, you generally find that it is just the shell, there's no actual meat inside it.
"The entire shell is moulted, down to the coating over the shell and the eyeballs; if you get hold of the bit that has the eyes, it's actually empty inside."
Raspins Beach is one of five being considered for Keep Australia Beautiful's national clean beach award and was assessed yesterday.
Judge Avril Bones says the naturally occurring event will not affect her assessment.
"I saw yesterday the work [the community] are doing to fence off the shorebird nesting which is much more significant for the judging of the beach than the crabs washed up," she said.
The winner will be announced in July.
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