Lionfish (Pterois) were first spotted near Miami in 1985. The popular aquarium fish with the fluttery, polka-dot fins are native to the Pacific but have spread alarmingly fast through the Atlantic. They are now found as far north as Rhode Island and throughout the Caribbean and the Gulf of Mexico. Equipped with 18 venomous spikes, lionfish are aggressive and insatiable. Their stomachs can expand to 30 times their normal size, allowing them to binge on some 70 marine species. One study found that a single lionfish caused a 79 percent decline in the number of juvenile fish on a coral reef in just five weeks. U.S. Geological Survey fish biologist Pam Schofield calls them "a kind of biological pollution."

Asian tiger shrimp (Penaeus monodon) are just getting established in the U.S. Southeast, but given that they can be as long as a person's forearm, they're a memorable sight. The striped crustaceans hail from the Sea of Japan and Southeast Asia and were first seen in the United States in 2006, probably after an accidental release from a shrimp farm. They have now been found in coastal waters from North Carolina to Texas, and their size and appetite have fisheries managers worried. "The question is, does it eat more than the ecology can keep up with?" asks Pam Fuller, program leader for the USGS Nonindigenous Aquatic Species Program. 

An upside to these invasions is that tiger shrimp taste like lobster, and the mild, white flesh of the lionfish is low in mercury, full of omega-3 fatty acids, and great in ceviche. With no other predators available, humans had better step up to the table. "They're out there; they're going to stay out there," Fuller says. "That's the problem with these things—we can't recall them. The oceans are too vast."