Invasive Species

Every year, the U.S. “celebrates” Invasive Species Week.  Why do they take an entire week to talk about plant and animal species that take up residency in non-native habitats?  Not all immigrants stir up trouble, but those defined as invasive marine species can put ecosystems, economies and human health at risk.

Invasive marine species overtake native species, rapidly out-competing local fauna or flora. They can alter the entire local ecology, leading to the collapse of fisheries. Exotic algal species can also pose a risk to human health by contaminating seafood. The economic impact of invasive species account for $1.4 trillion in damage annually. In the United States alone, 42% of Threatened and Endangered Species are at risk due to invasive species.

Influx of Invaders

No matter what ocean you live near on this blue planet, the waters that surround you are home to marine invaders. Human activity, increased merchant shipping and related water pollution are chief culprits in the spread and impact of marine invasive species.  Some of these aliens hitchhike on ship hulls or in cargo ship ballast water, while others are intentionally released by well-meaning but misguided aquarium owners. Here are five of the top alien marine invaders.

1. Lionfish (Pterois volitans): No Room for this Roomba of the Reefs

In the top ten most invasive fish in the world, Pterois volitans vacuums the reefs of native fish from juveniles to mature reef fish of all kinds. (Dense lionfish populations can consume up to 460,000 reef fish per acre per year!)  Long-lived, temperature resilient, depth tolerant, highly reproductive and with no natural predators in the invaded range, lionfish have become the scourge of the western Atlantic, Caribbean and Gulf of Mexico. Originally from the Indo-Pacific, lionfish were introduced off the coast of South Florida by the aquarium trade in the 1980’s.

Difficult to control this seemingly unstoppable reef nemesis with trap or traditional fishing methods, local scuba divers have organized ongoing lionfish dives to physically remove them. Divers use spears and special lionfish containment devices to avoid being stabbed by the venomous lionfish spines. An upside is that lionfish are delicious. Until a better predator emerges to balance the ecosystem, humans are taking responsibility for “eradication” efforts with many restaurants adding lionfish to their menus. New technology is on the horizon, with NOAA designing a lionfish trap and a number of tech startups developing underwater robot technology to vacuum up these pervasive predators.

2. Green Crab (Carcinus maenas): The Green Glutton

Photo Credit: Emily Falcigno

This European crab is bad news for the shellfish industry and is the prime suspect in the shutdown of the commercial clam harvest in Maine. A voracious predator that consumes worms, clams, oysters and other mollusks of its size or smaller, one crab can take out three dozen small mussels a day. European cargo ships brought the green crab to the east coast of America in the 1800’s through seafood shipments and ballast water. These crabs thrive in their invasive range that now includes both coasts of the U.S. and Canada, southern South America, Asia, Australia, South Africa, and Japan. Management strategies include chemical control, and trapping and removal programs. Some states have also tried netting juvenile clams to protect them from these voracious predators. There is a new movement afoot to turn green crabs into a viable caviar fishery and as the new soft shell crab on the culinary block.

3. Northern Pacific Seastar (Asterias amurensis): Billion Dollar Beauty

Photo Credit: Flickr

Named one of the world’s 100 most invasive species, the Northern Pacific seastar has a massive appetite. Crustaceans, mollusks, bivalves and echinoderms are no match for this pretty predator.  Originating from the North Pacific off the coasts of China, North Korea, South Korea, Russia and Japan, this destructive seastar can now be found in southern Australia, the U.S. and Europe. With few predators to keep them at bay, Northern Pacific seastars are credited with an estimated billion dollar loss in the shellfish industry in Tasmania and devastating the shellfish industry in Japan.  For now, people rely on physical removal and the use of traps to control their populations.

4. Killer Algae (Caulerpa taxifolia): Habitat Destroyer and People Poisoner

Photo Credit: Alexandre Meinesz

Another aquarium escapee, this strain of seaweed is native to the Indo-Pacific. It forms dense meadows that crowd out native algae and seagrasses, restricting food and habitat for marine life and causes immense ecological harm. Fish can’t spawn or find food, native plants can’t grow, and other organisms (like starfish and anemones) can’t survive. Not only do they beat out edible flora, Caulerpa also produces toxins that herbivorous fish can’t eat. Fish that can eat Caulerpa build up all these toxins in their bodies. The fish then becomes too poisonous for people to eat.

An attractive and hardy algae, Killer Algae was propagated for the aquarium trade, where it became a favorite of aquarium hobbyists around the world. The seaweed escaped public and private aquariums in California, Japan, Australia, and Monaco through tank dumping into local waterways. It then spread widely in the Mediterranean.  Almost impossible to get rid of, it was eradicated in California at considerable cost using herbicides and covering the affected areas with tarps.

5. Sea Walnut (Mnemiopsis leidyi): Battle of the Comb Jelly

Photo Credit: Daniel Zupanc

The invasion of the Black Sea by the Sea Walnut is one of the best-documented examples of marine invasive species introduced through ballast water.  Another member of the top 100 most invasive species, this ctenophore (not a true jellyfish) arrived on ships from the American Atlantic coast in 1982. It ate up both zooplankton, the food of commercially important fish in the Black Sea, and the eggs and larvae of the same fish species.  With no enemies in their new home, the non-stinging jellies propagated at an alarming rate. By the mid-1990s, they accounted for 90% of the total biomass in the Black Sea. A voracious eater and reproductive menace, the Sea Walnut contributed to the near collapse of local anchovy, scad and sprat fisheries and an increase in toxic red algae blooms.

This comb jellyfish subsequently spread to the Asov Sea where anchovy fisheries – already under stress from pollution and overfishing – completely collapsed. Dolphin numbers in the Black and Azov Seas also dropped dramatically, as the fish they used to feed on disappeared. The sea walnut entered the Caspian Sea, causing similar havoc including the reduction of oxygen in the sea. The sea walnut has also been discovered in more open bodies of water like the the Mediterranean, Baltic, and North Seas.

A natural enemy that preys almost exclusively on the sea walnut, the Beroe ovata comb jelly was introduced to take down the enormous populations of sea walnut.  The reduction of sea walnut has been brought to a low enough level that the Black Sea ecosystem is showing growing signs of recovery.

By Laurie J.Wilson