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.

"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.

"Unfortunately, we can't breed them to have more claws, it sounds like it's just by chance when they get injured or attacked."

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.

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."