In 2011, there was a tenfold increase in Asian Tiger Shrimp sightings along the U.S. Atlantic and Gulf of Mexico coasts.
Truly jumbo cannibal shrimp are invasive, but also tasty tells the critters can grow to 13 inches in length. Scientists are concerned because of the usual displacement of native species, but also because the tiger shrimp is very disease-prone. (They are nicknamed cannibal because they eat the native shrimp.) The MSN article says there is one positive aspect - they are good-eating.
The USGS website tells that during the 70s and 80s, the tiger shrimp were fish-farmed and like the dinosaurs of Jurassic Park, escaped into the wild. Wikipedia tells that the tiger shrimp is the most cultivated prawn species. (Currently though, there are no known tiger shrimp aquaculture facilities in the U.S.) Their susceptibility to disease is particularly alarming to scientists because they also pass along their diseases to the native population.
Scientists hasten to acknowledge there is no direct evidence that the shrimp have established a sustainable breeding population in U.S. waters. It possible the shrimp have drifted in tidal currents after being released from ship ballast waters. Fisherman however, are reporting much larger numbers, somewhat debunking the ballast water theory.