On a sandy beach in a state park in Ketchikan, a group of local beachcombers encountered something ominous: shells of two invasive European green crabs, shed as part of the creatures’ growth process.

That discovery, made during a June 6 beach survey that was part of a class held by the University of Alaska Southeast Ketchikan campus, led to more in the community.

It makes Ketchikan the newest known Alaska beachhead in a northward…