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Environmental Impacts of Marine Exotics

James T. Carlton


Marine exotic, or invasive, species are a global problem. These uninvited plants and animals:

  • compete with native species for space, food, and other resources
  • put a strain on the economy, such as commercial fisheries
  • can contribute to public health problems
  • impact recreational enjoyment by fouling the environment

May 2004

What are some examples of marine invasive species, or exotics, in the U.S.?

Most U.S. coastlines have been invaded by exotics.

Panne and Spartina alterniflora are aquatic nuisance species. Creative Commons: Sandy Richard.

Carlton: If we look around the coasts of the United States, it’s actually hard to find a piece of the coastline that doesn’t have a signature invasion, especially in bays and estuaries and, in fact, along some rocky coastlines. Invasive species, or exotics, are species that have been transported from one ecosystem to another. These exotics are also called aquatic nuisance species (ANS). Some examples in the U.S. are:

Some invaders can spread across a wide geographic range.
  • The very common Japanese eel grass, which has become very abundant in the estuaries of the Pacific northwest, was probably introduced accidentally with Japanese oysters.

  • In Willapa Bay, Washington, the Atlantic cord grass, Spartina alterniflora, occupies thousands of acres.

  • In San Francisco Bay we have well over 200 species or marine invaders, including the European green crab and the Chinese mitten crab. The European green crab, Carcinus maenas, for example, was introduced to California in 1989, and since then its population has expanded rapidly from Morro Bay to British Columbia.

Exotics can impact commercial fisheries.
  • Southern California’s harbors and marinas have scores of exotic species that just dominate the estuaries and the floats in marinas, on the pilings, and on the moorings.

  • The Gulf of Mexico has a major invasion of a Pacific Ocean jellyfish, Phyllorhiza punctata, or Australian spotted jellyfish. This giant species can weigh up to 25 pounds. These jellies may be capable of consuming the larvae and eggs of commercially important aquatic species.

  • Florida has a host of invasive species, where over 40 exotic fishes are reproducing in fresh water, such as the Mayan cichlid from Central and South America and the Asian swamp eel.

  • Chesapeake Bay, like San Francisco Bay, is well invaded; a particularly outstanding new invasion is a 6-inch tall snail, the rapa whelk, from Japan, that eats clams and oysters.

  • Long Island Sound, and much of New England, has scores more of invaders, such as the Asian shore crab that now dominates the seascape.

What about an example of a global problem?

Sea squirts are causing damage worldwide.

Carlton: We’re seeing distribution by ships of the same species around the world so that certain kinds of sea squirts, which are often large, gelatinous organisms, have been popping up in ports and harbors all over the world—Hong Kong, Sydney, Pearl Harbor, and San Diego—and in ports in the Altantic Ocean and the Mediterranean. Some of these sea squirts, which can be massive in size, can occur by the thousands of pounds on pilings and on floats. They are major fouling organisms. As a group they present a global problem.

Today, ballast water is a major means of transporting invaders.

What is the most common way of introducing marine exotics?

Carlton: There are a number of common ways to move organisms around the world. For centuries, ships and boats have been moving hundreds, maybe thousands, of species around in the world’s oceans. Organisms attach themselves to the hulls of these vessels. In modern times, ballast water (which is not bilge water) has been a major vector. We think that on any given day there may be more than 5,000 species in motion around the world via tens of thousands of ships.

What is being done to alleviate this transportation problem?

Carlton: Ballast water, as an example, has been the subject of a good deal of thought in recent decades as to what we can do to control human-mediated invasions in the sea. A number of countries around the world have established both voluntary and mandatory regulations to attempt to regulate ballast water. The challenge is what exactly should be done.

Exchanging ballast water in the open ocean is one way to dispose of invaders.

Right now the rule of thumb, the stopgap measure, is for a ship coming from a coastline, port, harbor, or estuary to exchange its water out in the middle of the ocean before it proceeds to a new port. The hope is that it has dumped organisms along with the water in the open ocean and it will arrive at its new port with organisms from the ocean but not from a foreign port. The measure has problems and challenges for both the safety and stability of the ship and also because the process is not completely effective.

Currently a lot of thinking is being devoted to how to treat the water, whether to heat it, filter it, UV it, or do something else with it so as to remove the living organisms there. Ballast water management is also a major initiative of the United Nations International Maritime Organization.

Exotics harm more than just the environment.

What kind of damage can marine exotics do to the environment?

Carlton: It’s a very long list; invasions harm not only the environment but also have industrial, social, recreational, and economic impacts. The list is long because so many hundreds of species have invaded the world’s coastlines.

  • A species can become the dominant organism in many communities, replacing and displacing native species.

  • In a direct sense, they impact recreation by creating huge fouling communities that need to be dealt with by recreational boaters, operators of marinas, those maintaining buoys, and so on.

The U.S. and Canada have spent hundreds of millions of dollars on eradicating zebra mussels.
  • There are certainly impacts on the economy. An example is the zebra mussel of the Great Lakes, Dreissena polymorpha. This small, fingernail-sized fouling mussel is native to the Black Sea of Asia. Since its introduction to North America in the 1980s, it has spread rapidly to all of the Great Lakes, the Mississippi River, and many other waterways. Hundreds of millions of dollars have been spent by the U.S. and Canada on the zebra mussel invasion.

  • The North American comb jelly collapsed the anchovy and other fisheries in the Black Sea, an industry worth hundreds of millions of dollars a year.

Bacterial exotics are a threat to human health.
  • In California, a tiny sabellid worm from South Africa has impacted the abalone aquaculture industry.

  • There are also human health impacts with the movement of bacteria, such as cholera, in ballast water.

  • So the list is long, almost as long as the number of species that are invaders.

Why is it that some exotics seem benign to the environment while others take over their new home?

No invasive species is benign; they all impact ecosystems.

Carlton: Some exotic species become abundant very quickly. But we’re not so sure that any species is benign. Every species entering a new environment that becomes established is using space, using food, and using other resources. So the question of “benign” may be more a human value and human impact question, rather than impact on the environment. If we have a small introduced crustacean, a small worm, or a small mollusk, which displaces or replaces a native species, that’s not benign. The invasive species may not have a direct first-order human impact on society, but hundreds of species have invaded which have probably vastly altered the biodiversity of American waters and the seascape.

Some invasive species multiply and spread quickly.

I don’t know if we can actually demonstrate that any exotic is benign, whether it’s terrestrial, freshwater, or marine. They all impact the environment as they establish themselves. Some of them become very abundant. Others spread more rapidly than others. It’s often a species-specific case. It has to do with their reproductive biology, the kind of habitat they occupy, their mechanisms of dispersal, the kind of food resources they need. Because of these and other variables, it’s often hard to predict whether a given species will spread rapidly or not unless we know that it’s the kind of species that spreads rapidly from its previous history, but even then there are surprises.

How can the general public help in the control of marine exotics?

The public should not release exotic aquatic pets into the environment.

Carlton: The public can help in two major ways: prevention and monitoring. The public is often involved advertently or inadvertently in the release of exotic species into the environment. Many exotics are sold to the public by aquarium stores, pet shops, nurseries, and even on the Internet. For various reasons, some people will actually release these into lakes, salt water, and public parks. These releases have led to some outstanding invasions. Just a basic awareness of protecting the environment by not releasing nonnative species is critical.

The public can help monitor invasions by bringing unknown species to be identified.

The other way the public can help is by monitoring the environment. This is very important to controlling invasive species. The public are often the first to notice new invasions. For example, there are millions of people walking the beaches and in our national forests compared to the relatively small number of biologists studying invasive species in the field. The public are the biologist’s eyes. When people notice some new animal or plant they’ve never seen before, they should bring it to the local nature center, the local museum, the local Sea Grant office, or the local college. Finding incipient, new populations will help tremendously in eradicating the species when their numbers are small. It’s a major management tool.

James T. Carlton, Ph.D., is professor of marine sciences at Williams College, in Williamstown, Massachusetts. Since 1989 he has directed the Williams–Mystic Program, the maritime studies program of Williams College and Mystic Seaport located in Mystic, Connecticut. His research encompasses two aspects of the history of life in the world’s oceans: worldwide biological invasions of nonnative species in coastal environments, and modern-day extinctions of marine organisms in sensitive marine and estuarine habitats. In 1996, Dr. Carlton was awarded the prestigious Pew Fellowship. He was the founding editor-in-chief of the journal, Biological Invasions. In 1999, Dr. Carlton was the first U.S. scientist to be awarded the federal government’s Interagency Task Force Award for his national and international work to reduce the impacts of exotic invasions in the sea. He received his doctorate from University of California at Davis. Carlton was interviewed at the 2004 AIBS Annual Meeting “Invasive Species: A Search for Solutions.”

Environmental Impacts of Marine Exotics

BioScience Article

“Aquaculture Production and Biodiversity Conservation.”
Aquaculture has some positive impacts on biodiversity, including that aquaculture often boosts natural production and species diversity. On the negative side, species that escape from aquaculture can become invasive in areas where they are nonnative. Read the abstract of James Diana’s January 2009, BioScience article, or log in to purchase the full article.

National Aquatic Nuisance Species Clearinghouse

North America’s most extensive technical library of publications related to the spread, biology, impacts, and control of the invasive marine and freshwater aquatic nuisance species in North America.

National Invasive Species Council, a U.S. Federal portal, provides information on state agencies and organizations with an interest in the prevention, control, or eradication of invasive species.

For Kids: marine detective game

Nab The Aquatic Invader is a fun, interactive game where players are detectives who must track suspects and arrest invading aquatic plants and animals.

International Maritime Organization

This organization of the United Nations is responsible for improving maritime safety and preventing pollution from ships. One of its initiatives is the Global Ballast Water Management Program.

Invasive Species…..What are they and why are they a problem?

The National Park Service is working to manage invasive species on park lands through a suite of national and local programs, each based upon the following strategies: cooperation and collaboration, inventory and monitoring, prevention, early detection and rapid response, treatment and control, and restoration. The site includes many invasive species resources, as well as links to individual national park websites.

National Ballast Water Clearinghouse

The Smithsonian Research Center provides information about “the synthesis, analysis, and interpretation of national data concerning ballast water management and ballast-mediated invasions.”

National ANS (aquatic nuisance species) Task Force

Information for boaters, fishermen, and aquarium owners on how they can help in the prevention of aquatic invasions.

American Littoral Society

If you live in a coastal community, get involved with the littoral society in your area. It is a great way to get outdoors and clean up the environment.

Community action

Get involved in eradicating the invasive species from your community! Click on the link on the left: What can I do about invasive species? to download a fact sheet (MS Word document).

Boaters: Help Prevent the Spread of Aquatic Plants and Animals

Fact sheet provides easy steps boaters can take to prevent spread of exotics when transporting watercraft.

For educators: classroom resources on marine exotics


Understanding Science