Tough and tiny snails that threaten to crowd out native species by hogging all the food have shown up in a Colorado creek for the first time, state wildlife biologists said Tuesday.

New Zealand mudsnails, native to the Southern Hemisphere, were found in Boulder Creek. How they got there is a mystery, since their nearest known location was in northeastern Utah, at least 200 miles away.

"They can cause a lot of damage. These snails are highly adaptable and reproduce in such great numbers that they can actually lock up the available nutrients in an ecosystem," said Peter Walker, senior fish pathologist for the state Division of Wildlife.

"Simply put, they can cover the stream bed and displace the other invertebrates that would normally be feeding a large variety of aquatic wildlife," he said.

The snails are just 2 to 5 mm long — so small they cannot be skimmed from waters and can even pass unscathed through a fish's digestive tract.

They can spread while clinging to wildlife, boats, boots, waders, nets and other fishing gear.

They can survive several days out of water and a wide range of temperatures. They reproduce by giving birth to well-developed clones, so one snail can start a colony in a stream or river.

The snails have also been found in Montana, California, Arizona, Oregon, Wyoming — including Yellowstone National Park — and in Lake Ontario between New York and southeastern Canada.

The snails are in a relatively small stretch of Boulder Creek and the Division of Wildlife hopes to keep them from spreading.

"We are very concerned about the potential negative impacts these little snails could have on trout streams in our state," said Eric Hughes, chief of fisheries for the Division of Wildlife.