For several years while teaching at the University of Vermont, Joe Roman, a conservation biologist, challenged students to an unusual exercise. Design an animal ideal for the role of marine invader, he’d say, a creature with the natural traits to colonize a territory not its own and, in the relentlessly competitive scrum of the wild, establish itself sturdily for the long term. Roman has spent decades studying how species travel the globe, multiply beyond pioneering toeholds to something like ubiquity and change ecosystems. His classroom exercise invariably landed as a living example on one little-known but astonishingly widespread member of the animal kingdom: the European green crab. Small, fertile, rugged and fueled by an expansive appetite, the species, he says, is “an exemplary invader, a perfect invader.”

European green crabs originally hail from the northeastern Atlantic Ocean and the southwestern Baltic Sea. Roman ticked off traits that helped them conquer much of the world. As omnivores, scavengers and cannibals, they sustain themselves on almost any organic food. They have a high fecundity, with females releasing as many as 185,000 eggs a year. They survive in water temperatures from freezing to 86 degrees Fahrenheit and tolerate sweetwater zones where salt meets fresh. Moreover, adult European green crabs can live 10 days or more out of water. Taken together, these characteristics explain why they were first documented along the United States coast in 1817 and continue their tour of the temperate world.

My curiosity about European green crabs began after witnessing their near indestructibility beside my own home. Our family fishes commercially in New England, where we grow or catch a substantial portion of our food. About 15 years ago, my children and I began capturing bushels of European green crabs for use as fertilizer. The process was simple. We harvested crabs in baited wire-mesh traps in an estuary in southern Rhode Island. Every so often we’d transfer bushels into coolers of tap water, where we expected they would die, as lobsters quickly do when immersed in fresh water. We’d then dump their limp carcasses into compost piles and cover them under a foot of decomposing leaves, vegetable scraps and manure.

One day not long after one of these deposits, a neighbor arrived at our yard’s edge. He seemed bemused, and he asked if I knew anything about the crabs picking their way across his property. He had discovered the exploded chitin exoskeleton of an itinerant crustacean after hearing a thunk under his lawn mower. Yes, I admitted, I did. Upon inspecting the compost, we found that an untold number of crabs we thought dead had unearthed themselves and walked off. Over the next week we found more — hiding in shade, crouched under purple dead-nettle, pressed against the base of a shed. A murder of crows feasted on them. Crabs kept turning up.