Threat beneath the waves: invasive marine species

With 18 poisonous barbs, the invasive lionfish wreaks havoc on marine ecosystems where it's been introduced - in the Atlantic, in the Caribbean and now, in the Mediterranean Sea. Its painful sting isn't necessarily deadly, but it's enough to scare off potential predators. In places like Florida, Cuba, Colombia and the Bahamas, governments have encouraged people to start killing or eating the fish.

The sea walnut, a kind of jellyfish, is originally from the east coast of the Americas. In the early 1980s, it turned up in the Black Sea and then the Caspian Sea, transported in the ballast water of cargo ships. It has since spread to the Mediterranean, North and Baltic seas. At 12 centimeters (5 inches) in length, it competes with fish for food and has led to the collapse of fisheries.

This common seaweed is one of two algae on the list of the world's 100 worst invasive species, as compiled by the IUCN. Widely used in home aquariums, the invasive plant was accidentally introduced into the Mediterranean in wastewater from the Oceanographic Museum at Monaco. It has spread over more than 13,000 hectares of seabed, preventing the growth of native seaweeds and other marine life.

The zebra mussel is a common sight on the shores of North America's Great Lakes - but that wasn't always the case. Originally from the Caspian and Black seas, the mollusk spread through canals in the 1800s and 1900s to the Baltic Sea, and by the 1990s to lakes and waterways in Canada and the US. It has become a nuisance, clogging water intake pipes and competing with local fish for plankton.

A very well-known species, the rainbow trout is now found around the world, having been introduced for sport fishing and commercial aquaculture. Its home is the West Coast of North America and cold-water tributaries, but it can now be found on every continent - where it out-competes and preys on native fish populations.

This barnacle is native to Caribbean waters, but was introduced - likely by cargo ships - to the Pacific in the 1970s. It reached Hawaii in the mid 1990s, and has become one of the most abundant organisms in harbors and bays throughout the Hawaiian Islands, according to Reef Resilience. The barnacle is seen as a threat to coral reefs, as they can prey on the soft inner tissues of coral polyps.