In 30 years of studying the Susquehanna River, Brian Mangan looked deeply into just about all of its critters — bugs and slugs, snails and fish, natives and exotics.

But the Kings College biologist had never given much thought to crayfish. Then one day six years ago, on a hunt for invasive clams, he walked toward a boat ramp in Halifax, PA, and hundreds of russet-colored, lobster-like creatures swam away from him.

It was then that Mangan first became aware that the rusty crayfish, a native of the Ohio drainage basin that is spreading to other watersheds — was here — and probably here to stay.

"I had never seen a concentration like that before," said Mangan, who directs the Wilkes-Barre college's environmental program. "I just wondered, oh, my gosh, what are animals in these concentrations doing to the (native) population?"

In 2008, he began a study that confirmed his fears. Of the 11 sites he surveyed, from the New York border to Harrisburg, the invasive rusty crayfish was found in about half of them. In all, Mangan collected 800 crayfish, both natives and nonnatives.

Until Mangan's work and other efforts from Lock Haven University biology professor Ted Nuttall and zoologist Dave Lieb, who trained at Penn State, the only comprehensive study of crayfish in Pennsylvania occurred in 1905. The researcher who conducted it focused mostly on the streams in the western part of the state. Considering their importance to the ecosystem, it's surprising that crayfish have been so overlooked. They are a major food source for brown trout, bass and the hellbender salamander.

Mangan wanted to get a baseline before the rusty crayfish spread throughout the native crayfish's range and the native disappeared altogether. The rusty crayfish is larger and more aggressive than the native crayfish species. Like natives, rusty crayfish eat algae, small worms, detritus and bugs that stay along the bottom. But they also eat the eggs of native crayfish and take over the natives' habitat — usually under rocks in cold streams.

Animals that feed on native crayfish don't tend to eat the rusty crayfish. That has led to concerns about the health of hellbenders in particular, an indicator species that is highly sensitive to changes in water quality, habitat and diet.

"Wherever there are rusty crayfish, there are no natives. Where there are rusty crayfish, there are no hellbenders," said Peter Petokas, a biologist at Lycoming College who is documenting the salamanders' health.

Mangan is also interested in the crayfish's level of mercury, which he suspects the animals are passing along to the larger fish that eat them, and that humans, in turn, consume. Kings College has a direct mercury analyzer that Mangan and Petokas use to test the crayfish. They hope to re-test their sites next year to see if the mercury levels are changing.

Scientists believe that anglers accidentally introduced the rusty crayfish a couple of decades ago. The crayfish were a popular bait, although they are no longer used in most places because of their invasive nature. Fishermen likely threw the bait in the water when they finished, a gift to the fish that turned out to be not so thoughtful.

The Pennsylvania Fish and Boat Commission has banned the sale, purchase, possession, propagation and transport of live rusty crayfish. But the invasive may have already done its damage.

Lieb, a zoologist, has studied crayfish for the Western Pennsylvania Conservancy and the state's Fish and Boat Commission for several years. When he returned after four or five years to spots where the rusty crayfish was introduced, he told the Williamsport Sun-Gazette, he found did not find any natives.

To study the crayfish, Mangan had to first make his own traps. He couldn't afford the $25 commercial ones on his shoestring budget, and they likely wouldn't be of much use because they wouldn't catch the smaller ones. Using the mesh that masons use to attach stucco to walls, Mangan and his students made 100 traps, which they soaked in the river for 48 hours. Tuna-flavored cat food attracted the crayfish.

Mangan's work, along with Lieb's, is adding to scientists' baseline knowledge of crayfish, a keystone species in streams because so much depends on them. In the last decade, Lock Haven's Nuttall has also contributed, with an online key to Pennsylvania crayfish.

But with few exceptions, most biologists didn't know much about them. Now, that is changing because the health of the ecosystem depends on it.

"We all cut them apart in Bio 101, but you can probably count on two hands who the big names are in crayfish taxonomy," Mangan said. "But now, we have no choice. We really have to pay attention to these."