The first nudibranch species Cobb ever spotted—Gymnodoris aurita—is also known as a strawberry slug. It’s around 3.5 inches long when full-grown. Georgette Apol / Alamy ...

But the sea slug (scientists call it a “nudibranch”), has been making headway in New England waters. Since 2017, it’s been found off the coast of Cape Ann, Massachusetts.