MACHIAS, Maine — A group of students from the University of Maine-Machias made a bittersweet discovery this week. They found an Asian shore crab on Great Wass Island in Beals, the northernmost point where the crab has been sighted. The excitement of their discovery was tinged with disappointment, however, because the Asian shore crab is an invasive species that threatens Maine’s coastal ecosystem.

The four students — Katie Barvenik, Bennett Ellis, Jacob Snyder and Wesley Turner — were participating in a field study as part of their marine ecology class with professor Brian Beal. They were taking samples — finding and counting marine life — on the rocky shore adjacent to the Downeast Institute for Applied Marine Research and Education, of which Beal is research director.

It was a cold, windy Tuesday morning, and the students were working in hip waders and jackets. Ellis turned over a rock, and crabs scurried for cover.

Three of the students — Barvenik, Ellis and Snyder — discussed the discovery in a conference room on campus Thursday.

“Picked it up, the rock, there’s about six or eight crabs under there,” said Ellis. “I started grabbing a couple of the bigger ones that were crawling away. And Katie reached over and grabbed a few small ones in her hand and saw one of them was an Asian shore crab.”

She told the other students immediately that she found an Asian shore crab. Snyder told her she was wrong.

“I knew it immediately,” said Barvenik, who is from Portsmouth, N.H. “I knew what it was.” She recognized it because she has seen them before around Bath and also in southern New England.

“They’re pretty easy to distinguish,” added Ellis.

“Part of me was really excited,” said Barvenik. At the same time, however, she and the other students and Beal felt a sense of discouragement. “It’s cool to find it,” said Barvenik, “but it is an invasive species.

“It definitely was disheartening and exciting,” she added.

Beal, describing the study in his office on Friday, acknowledged the sense of “trepidation” at finding “another invasive species in this part of the world.”

At the time Beal thought the northernmost finding of the Asian shore crab was around Searsport. However, he checked with officials of the state Department of Marine Resources and learned the northernmost presence of the crab was established in Schoodic Peninsula, which is east of Mount Desert Island, in 2005. Great Wass Island is roughly about 27 miles further up the coast from Schoodic Peninsula. The Asian shore crab was first discovered in the U.S. in Cape May, N.J., in 1988 and since then has moved slowly north.

“It’s interesting,” said Beal, “because it seems to be following the same pattern as green crabs.” Both species appear to have migrated up against the net flow of Maine’s coastal waters, which flow in a southerly direction from Lubec to Kittery, he explained. The European green crab, another invasive species that is damaging Maine’s coastal ecosystem, and the Asian shore crab both have managed to “buck the tide” he noted.

The reason is most likely human intervention — accidental, said Beal. The Asian shore crab probably was conveyed into the region through the movement of marine products, he said. Crabs easily could have been transported in a pool of water in a boat or some other accidental means. The crab is very tiny, its shell only growing to about the size of a silver dollar. The one found by the students is about one inch.

“The point is, it certainly was in a natural environment for it,” said Beal, under a rock, in the shade, protected, surviving and feeding on the ecosystem.

The Asian shore crab feeds on plants and animals, noted Beal, who has witnessed its destruction of the shoreline ecosystem in southern New England. “That’s what makes it so well suited for any habitat,” the fact that it feeds on both plant and animal life.

The tiny invader is prey for lobsters, crabs, birds and other species, noted Beal. However, it reproduces prolifically, which is why it has spread.

“I’m not sure what we can do,” said Beal. The type of damage the invader potentially can cause is “kind of disheartening,” he said.