A crab with the dubious honour of being dubbed one of the 10 most unwanted species in the world was found in Hamilton Harbour Monday, and officials have no idea how it got there.

An adult Chinese Mitten Crab was captured by Royal Botanical Gardens workers in a barrier between the harbour and Cootes Paradise, the marsh that feeds into the harbour's west end. The extensive barrier has a series of traps meant to keep invasive carp out of the marsh, but let other types of fish pass back and forth unhindered. The crab was caught in one of those traps.

"It's hopefully a one-off," said said Tys Theysmeyer, head of natural lands at the RBG. "It's not the sort of thing you expect when you're not by the ocean."

The crabs, which are about 8 cm long, are usually found in salt water. Biologists consider them to be an invasive species because the crustaceans can reproduce rapidly, clog up fishing gear and speed up erosion by digging holes in riverbanks.

They first hitched a ride in ships to Europe from China way back in 1912, and have since invaded San Francisco Bay. According to Fisheries and Oceans Canada, the Mitten Crab was first reported seen in the Great Lakes in 1965, and occasionally shows up in Lakes Ontario and Erie.

It appeared in the St. Lawrence River in 2004.

Officials aren't totally sure how the crab ended up in the water at the RBG, but Theysmeyer noted the animal is a popular food item in China.

"It could have been that someone just flipped it in the water," he said.

"We're not terribly worried – if we come up with several more, then we'll start to worry."