Monty Schloss was boating when a mud crab with "three hands" landed in his net.
"This one had an additional pincer, so the top and bottom jaw hanging off of one of the other two," he said.
Mr Scholl said his dad scooped the crab out of the net near Tin Can Bay in Queensland's Wide Bay region.
"Just a straight 'you beauty' moment, that's more claws, more meat," he told ABC Radio Brisbane.
"He was pretty thrilled; I was pretty thrilled to be a part of it."
He said not only was it an exciting catch, but it was also delicious.
"We ate it, so, I can confirm, it definitely did exist. My stomach agreed with that," he said.
"Boiled a pot of water, break apart, just straight meat, no seasoning. Mud crabs are beautiful."
He said they had never seen a crab like it before but believed it was not a genetic mutation.
"It's from an injury when they regrow their claws or just through the moulting process and mistake in their code at that time," he said.
"Unfortunately, we can't breed them to have more claws, it sounds like it's just by chance when they get injured or attacked."
'Unfortunately it's been eaten'
Bribie Island aquatic animal health specialist Ben Diggles said the crab's additional claw was "very unusual".
"Mud crabs are quite interesting creatures," Dr Diggles said.
"They can throw their claws off and then grow them back under normal circumstances."
But he said he did not think that was the case with this crab.
"Usually, we see common deformities in mud crabs and other crabs related to them growing back damaged claws or damaged legs," he said.
"This is not like that at all.
"It indicates more it's the gene expression that codes the development of claw, has been interfered with," he said.
Dr Diggles said he would have liked to have the chance to investigate the rare crab and perhaps solve the mystery of its third claw.
"I understand that people involved actually have processed the crab and eaten it which is unfortunate," he said.
"I'm pretty sure the scientists around here ... would've loved to have had a closer look at that one."
There is the possibility it was caused by an injury, Dr Diggles said.
"There is some science on these sorts of deformities, and it has been able to be reproduced in laboratories when crabs or lobsters have been injured in certain way," he said.
"It seems the injury has to occur during a particular stage of the moult cycle, and it has to be a particular type of injury, and from that the crab tries to regrow the affected area.
"And over time, over subsequent moult cycles, the injured claw actually grows into another claw and it's at right angles to the original claw."
