Lionfish

Lionfish in the Atlantic are termed invasive species: a non-native organism that has intruded into an area and may have serious detrimental effects on native organisms, the local economy and human health.
Lionfish are voracious predators and are taking the already threatened Caribbean reefs by storm. Lionfish are non-selective feeders, and with virtually no natural enemies in the tropical western Atlantic they've invited themselves to an all you can eat seafood buffet. Lionfish have been observed consuming 20 small fish in a 30-minute period and prey up to 2/3rd of their own length. Impressively, their stomachs can expand up to 30 times their normal size after a meal. Mark Hixon et al (2009) determined that a single lionfish can reduce juvenile fish populations by 79% in just 5 weeks.

Samples of lionfish stomach contents in the western Atlantic have shown that they consume more than 50 different species, many of which are overfished and diminished to already critical levels (Gupta, 2009). Given this extreme rate of feeding lionfish are out-competing native predators for their food sources, as well as reducing fish populations through direct predation.

Not only are they dangerous to the fragile ecosystems, but they can inflict an extremely painful sting to humans, not usually deadly, but it can make you quite sick.