Life on Mars, if it is there, probably isn’t advanced enough to take over Earth, so we probably won’t be seeing little green men with laser pistols any time soon. But hobbyists can help defeat some real aliens that do pose a real threat to the world’s oceans; some of them are even little and green!

Almost any hobbyist with access to the news knows that reefs are in danger from threats such as pollution, warming oceans and overfishing. A less well-known danger, however, is receiving increasing coverage lately: invasive species. This threat is of particular concern to aquarists because the damage these species cause can sometimes be directly attributed to hobbyists. A recent editorial by Calado and Chapman (2006) in the journal Marine Pollution Bulletin described some risks posed by invasive species attributed to the aquarium industry and entire workshops and symposia are devoted to this increasingly important and historically extensive subject.

Before we can proceed, it is important to present definitions of both alien and invasive species as it pertains to the aquarium industry. Typically, the definition of an alien species is "a species introduced to an area by the actions of man." The National Invasive Species Council defines an invasive species as “an alien species whose introduction does, or is likely to, cause economic or environmental harm or harm to human health” (USA, 1999). Alien species that become invasive species pose a significant threat to diversity. It has even been estimated that about 40% of threatened or endangered species in the U.S. are in peril due to habitat degradation, competition or direct predation as a result of invasive species (Pimental et al, 2001).

Photo 1: Red Lionfish (Pterois volitans), an invasive species in the Bahamas and Florida waters, is a concern based on its voracious appetite and protective spines. Photo courtesy of Paula Whitfield, NOAA Beaufort Laboratory.

To understand the environmental danger of invasive species, we need to realize that no species exists in isolation. All organisms are subject to disease, and predators exist for almost every stage in their life. These predators and diseases are important factors that limit species' population growth. Introducing organisms to new habitats allows them to “cheat” at the population game, at least temporarily. While disease and predation continue to constrain the native species, the introduced species often find themselves in environments with no predators or diseases that have evolved to target them. The introduced species have the advantage of being freed from some of the most important factors that limit population size and growth. With this competitive advantage, introduced species can create problems for native species in a number of ways (at which point an introduced species becomes an invasive species).

In their 2004 report, Padilla and Williams name invasive species as the second leading cause of extinctions. It might be hard to imagine how a piece of alga or a few larvae from a dime-sized mussel could possibly cause extinctions, but some invasive species spread in their new environment so well that they can create what is called a monospecific habitat: an environment dominated by a single species that lowers biodiversity and reduces the space available to native species. In the case of plants, algae and sessile animals, uncontrolled growth often leads to the overgrowth of native organisms and the homogenization of habitats. Not only are the outcompeted organisms eliminated from the area, but any other organisms that depended on those species for food or shelter may be driven out as well.

Invasives can also indirectly affect native species through their resource use, or by indirectly affecting other species that directly interact with the native species. If an invasive population grows large enough, it can competitively exclude native species that occupy similar environmental niches. Free from natural predators, even small invasive populations may monopolize resources because they can exploit them at times when native species are forced to hide.

Photo 2: Ilyanassa obsoleta is an invasive species that has entered the aquarium trade and should never be released into the wild, because it can monopolize resources. More information can be found here. Photo by Mike Greenemeier.

When predators or diseases are introduced into new areas, they often have a strong competitive advantage. Because the native species have not recently encountered the invasive species in their evolutionary histories, they have limited defenses against them. They may not even recognize the new predators as a threat. Native species have never had to develop a resistance to exotic diseases, so what may be a fairly benign disease in its natural habitat can become a plague in its new habitat. European explorers and traders brought several diseases such as smallpox and malaria to the new world, with disastrous consequences for the natives. Similarly, it’s believed that a still unknown pathogen was introduced to the Caribbean in the 1980s that subsequently wiped out nearly 95% of the sea urchin Diadema antillarum within a year and caused decade-scale phase shifts within the entire basin (Phinney et al, 2001).

Although the ecological damage invasive species cause is immense, they also have a huge economic impact. It has been conservatively estimated that each year invasive species cost America $120 billion dollars (Pimentel, 2005). The actual number is likely quite a bit higher, though, because marine species and some important species such as kudzu aren’t included in the figure due to unreliable data. To put that number into perspective, with $120 billion you could match the combined budgets of the U.S. Army and NASA, and still have a few billion left over for that nano tank you’ve been planning (DoD, 2006; NASA, 2006)!

Most of the cost associated with invasive species results from lost productivity. Farmers see reduced yield due to introduced pests and diseases. In industry, encrusting organisms such as zebra mussels and tunicates foul intake pipes and ships’ hulls, reducing efficiency and eventually requiring downtime for cleaning. Additional money is spent repairing property damage from invasive species. Gulf Coast fishermen are not only experiencing reduced catches due to invasive jellyfish, but huge masses of the jellies are clogging and damaging their nets, necessitating expensive repairs (Invasive Species Specialist Group, 2006). A substantial amount of the $120 billion figure is also spent on attempts to eradicate or control the spread of introduced species (Pimentel, 2005).

Case Studies

One situation that can help an invasive species spread is habitat degradation. When an ecosystem has been damaged, either by human or natural causes, it creates the opportunity for a species to colonize the area that was damaged. An invasive species that is well-adapted to live in that damaged area can gain a foothold in an otherwise difficult area to invade, and can then spread quickly due to lack of natural predators and diseases that would normally control its population growth. As human activities alter and degrade a larger part of the Earth’s surface, they create additional opportunities for invasive species to gain a foothold and spread, particularly in many coastal areas, which are facing numerous manmade threats. A few classic examples of invasive species demonstrate how devastating they can be to ecosystems and economies.

Zebra Mussels

The zebra mussel, Dreissena polymorpha (Pallas, 1771), is a freshwater mussel native to the freshwater lakes of the Caspian Sea region in Asia. Outside its native range, however, it is causing widespread ecological and economic damage in a growing number of countries. It was first detected in the United States in Lake St. Clair near Detroit, MI in 1988, probably carried in via ballast water from a transatlantic cargo ship.

Photo 3: The Zebra mussel, Dreissena polymorpha. Photo courtesy of the U.S. Geological Survey Archives.

Zebra mussels spread by producing planktonic larvae that can drift in the water for miles before finding an appropriate hard surface to settle on and begin growing. These surfaces are typically rock, piers, boats, anchors and water intake pipes. The larvae even settle on other clams and hardshelled animals, typically killing them by suffocation or out-competing them for food. Zebra mussels even settle on other zebra mussels, producing colonies up to one foot thick numbering tens to hundreds of thousands per square meter (ZMIS, 2007).

In fewer than 10 years, from 1988-1998, zebra mussels spread to all five great lakes; the Mississippi, Ohio, Tennessee and Hudson river basins; and numerous freshwater lakes in over 24 states and provinces in the United States and Canada. It is estimated that these vast colonies of zebra mussels will cause over $5 billion in economic damage over the next 10 years in the USA alone, and will continue to severely alter the lake and river ecosystems they colonize (ZMIS, 2007). Their spread has led to population declines of many native species, and could lead to extinctions or extirpations. Once a body of water is invaded, no reliable method has been found to eradicate the mussels, and the best hope to stop their spread is to prevent them from entering new bodies of water with public education efforts and enforceable restrictions on the movement of boats that have recently visited mussel-colonized waters.


Switching to invasive species on land, the Kudzu plant (Pueraria montana) is an ivy-like vine native to Southeastern Asia. It was intentionally introduced to the United States in the late 1800s for use as a livestock feed and ornamental plant. In the early half of the 20th century it gained popularity throughout the South as a groundcover to reduce soil erosion. Government agencies even paid farmers to plant it. Eventually, though, people realized that kudzu was a little too good at covering things. It began covering houses, farmland and wooded areas (Van Driesche, 2002). It soon became commonly known as “the plant that ate the South.”

Photo 4: Kudzu overgrowing and killing trees. Photo courtesy of James H. Miller, USDA Forest Service.

As it overgrows other plants, it shades them from the sun and crushes them under its weight, eventually killing them. Where it is established, it can cover acres of land, growing as much as a foot per day! Few other living plants are visible in these areas and, other than insects, animals are also rare (Van Driesche, 2002).

Kudzu is an extremely difficult plant to control due to its quick, dense growth and its resistance to herbicides. Many parts of the roots and stem can form new plants, which makes mowing and manual removal nearly impossible. Careless disposal of the plant can also spread it (Van Driesche, 2002).

Caulerpa taxifolia

Kudzu’s ability to spread from small fragments and overgrow objects is mirrored by the invasive marine algae Caulerpa taxifolia. Unlike kudzu, however, C. taxifolia was not a planned introduction, but an accidentally introduced invasive species with roots in the aquarium industry. It is believed that an aquarium-altered species was created in the Stuttgart Aquarium in Germany, and subsequently spread into the wild from the Oceanographic Museum in Monaco. This was recently confirmed through genetic testing. The aquarium-altered species can thrive in much colder water and can grow up to 10 times as long as the natural strain of C. taxifolia.

Kudzu’s spread on land is limited by the spreading of plant fragments and seeds by humans, animals or the wind. The only method of spreading the aquarium strain of C. taxifolia is the plant's fragmentation (Žuljević & Antolić, 2000) and subsequent transport by currents and ships. This allows C. taxifolia to spread over long distances quickly, and increases the difficulties associated with eradication efforts because even small fragments carried by boats or currents can establish new stands of algae.

Photo 5: Caulerpa taxifolia monospecific habitat in the Mediterranean Sea. Photo courtesy of Ante Žuljević.

The invasion that started near Monaco has spread to over 30,000 acres of seafloor in Spain, France, Italy, Croatia and Tunisia. The same aquarium strain of Caulerpa taxifolia was found in the United States in California in 2000 near San Diego, and was recently found in southern Australia. It is assumed that an aquarium owner emptying water into either the bay or a storm drain introduced it into the wild in these locations. Caulerpa taxifolia invasions have negatively impacted tourism, commercial and recreational fishing, and recreational activities such as SCUBA diving. Its impacts on ecosystems are still being studied, but the invasive algae C. taxifolia is becoming known for creating dense monocultures that are degrading and monopolizing ecosystems wherever it can establish a population.

Fungia and Tubastraea

Recently, Pacific coral species that most hobbyists are familiar with have started to appear in the Caribbean. According to Bush et al (2004), the plate coral Fungia scutaria was introduced to a Jamaican reef by a researcher during the 1960s. The reef near the research station was used as a holding area for the corals prior to his experiments. At that time this practice wasn’t of great concern. Unfortunately, the researcher died suddenly and the corals weren’t removed. For the past 35 years divers in the area have been periodically finding and removing small groups of adult Fungia, with the most recent discovery of two in 2003. While they aren’t invasive yet, it’s a scary thought that despite multiple attempts to eradicate them, they’ve managed to persist for so long. They’ve even survived two major hurricanes, several bleaching events and an almost total decline of the natural reef in the area.

Another invading coral causing much more concern is the sun coral from the genus Tubastraea. A few species have made their way into the Caribbean, probably introduced by boats or by using oil rigs as “stepping stones.” Though it probably arrived during the 1940’s, T. coccinea has recently accelerated its spread around the Caribbean and Gulf of Mexico, including the Flower Garden Banks and Florida (Fenner and Banks, 2004). Another species, Tubastraea taguensis, has also been recently found in Brazil. Both species are stinging and overgrowing corals that are found nowhere else (Creed, 2006).

Photo 6: Tubastraea sp. reproduction is exciting in home aquariums, but a growing danger in the Caribbean and Gulf of Mexico. Photo courtesy of Andrew Berry.

Implications for Hobbyists

Too often, upon realizing that a specimen is poorly suited to their home aquarium, a well-meaning but misguided hobbyist decides that it should be released back into the wild. Although this may seem to some people like a noble thing to do, hopefully the previous examples have shown what a potential disaster this can create. With sexually reproducing organisms such as fish, the potential danger may seem small because a single release is unlikely to become established. However, even in the case of fish, once an organism has been kept with species from different parts of the world, it has picked up exotic protozoans, viruses, bacteria, and other pest species and potential pathogens. For that reason, even local species should never be re-released once they’ve shared a tank or water with species from different areas.

Many times when people are advised not to release their pets, someone chimes in stating that he, or someone he knows, has been releasing organisms for years without a problem, implying that the risk has been overblown. However, an aquarist is unable to track released animals, and it is nearly impossible to know if the release has caused harm or not. It can take decades to realize the impact of a new invasive species given the vastness of lakes, rivers and oceans and the limited funds for monitoring. Unfortunately, by the time the species is finally noticed it is typically widespread and very difficult to eradicate. Prevention is the easiest and cheapest way to prevent invasive species.

It takes only one successful introduction of a species to create a huge economic or environmental disaster, and although many people have released organisms without apparent harm, the success of some invasive species shows the potential for damage. Furthermore, the danger of a species becoming established varies largely with where a species is introduced and the species in question. An apparently harmless introduction in South Carolina is likely to have different results in Florida. Beyond the environmental danger of introductions, in many places it’s also illegal to intentionally release plants or animals into the wild.

The truth is that the risk of introducing a species that will become invasive this way is small. Pimentel, et al (2001) estimated that only 20-30% of introduced species become invasive. Most of those, though, were microbes or invertebrates accidentally introduced along with intentional introductions. The risks are not just of the released organism, that is a solitary macroorganism, but of the countless species that are unnoticed or microscopic. The epizootic loss of Diadema is a prime example of the potentially catastrophic effects of a pathogen.

In one of his many articles on the subject, David Lodge (1993) compiled a list of some factors that contribute to the danger of a species becoming invasive. Among those characteristics are high reproductive or dispersal potential, adaptability, mutability, short generation times, broad geographic range and opportunism. Many of these characteristics are the very traits that make an organism desirable to the aquarium hobby, especially at a time when there is increased interest in captive breeding. The hobby's popularity also adds another element of risk in that these species are being shipped to all corners of the globe. Because of the increased likelihood of the species we keep becoming invasive, hobbyists should take special precautions, listed below, to ensure that their specimens never see the wild after they are collected.

Measures Hobbyists Can Take to Prevent Invasive Species

  1. Never release any contents from an aquarium into a local waterway. This includes fish, animals and plants, as well as water and any substrate or cleaning materials. Any unwanted pets should be returned to a responsible pet store, traded to another hobbyist or frozen for at least 24 hours and then disposed of in a sanitary landfill.
  1. Used aquarium water should be properly treated with chlorine bleach and disposed of in a wastewater system that leads to a wastewater treatment plant, to reduce the likelihood of accidental introduction. Aquarium water should never be poured into storm water drains or local waterways.
  1. Hobbyists can reply to questions in discussion forums about whether releasing an animal into the wild is okay with a simple statement that there are no good reasons to do this and that it is not a responsible thing to do because it could hurt the local environment, and then post a link to this article. Even old threads should be updated to prevent lurkers from thinking that an old practice is still acceptable today.
  1. It is important to educate hobbyists, local fish store owners and school children of this danger. An excellent brochure and video titled, “Don’t Release a Pest” could be placed in local schools or fish stores, or handed out at aquarium club meetings. It is available at:


Hopefully, this article has educated readers about the danger that invasive species from aquarium inhabitants pose to the Earth’s oceans and rivers. While this is a global problem, hobbyists can take measures to help prevent accidental introductions of invasive species to their waterways. As the danger to the world's coral reefs, oceans and waterways increases, it is imperative that hobbyists take every step they can to prevent additional invasive species from becoming established. Anyone can be a hero in this battle.


We would like to thank Eric Borneman for his help with locating references on Caribbean Sea and Gulf of Mexico invasive species, Ante Žuljević for his permission to use his C. taxifolia photograph and Roman Lustrik for finding and getting permission to use the photograph. We would also like to thank Andrew Berry for his Tubastraea sp. photograph.

If you have any questions about this article, please visit Mike's author forum or Brian's author forum on Reef Central.


Bush, S. L., Precht, W. F., Woodley, J. D. and Bruno, J. F. 2004. Indo-Pacific mushroom corals found on Jamaican reefs. Coral Reefs, 23(2), 234.

Calado, R. & Chapman, P.M. 2006. Aquarium species: Deadly invaders. Marine Pollution Bulletin, 52, 599-601.

Creed, J.C. 2006. Two invasive alien azooxanthellate corals, Tubastraea coccinea and Tubastraea tagusensis, dominate the native zooxanthellate Mussismilia hispida in Brazil. Coral Reefs, 25(3), 350.

Department of Defense. 2006. FY 2007 Department of Defense Budget.

Fenner, D. & Banks, K. 2004. Orange Cup Coral Tubastraea coccinea invades Florida and the Flower Garden Banks, Northwestern Gulf of Mexico. Coral Reefs, 23(4), 505-507.

Invasive Species Specialist Group. 2006. Global Invasive Species Database.

Lodge, D.M. 1993. Species invasions and deletions: community effects and responses to climate and habitat change. Biotic Interactions and Global Change. P.M. Karieva, J.G. Kingsolver and R.B. Huey, eds. Sunderland, MA. Sinauer. 367-387.

NASA. 2006. National Aeronautics and Space Administration President’s FY 2007 Budget Request.

Padilla, D.K. & Williams, S.L. 2004. Beyond ballast water: aquarium and ornamental trades as sources of invasive species in aquatic ecosystems. Front. Ecol. Environ, 2, 131-138.

Phinney, J.T. et al. 2001. Using remote sensing to reassess the mass mortality of Diadema antillarum 1983-1984. Conservation Biology. 15(4), 885-891.

Pimentel, D. et al. 2001. Economic and environmental threats of alien plant, animal and microbe invasions. Agriculture, Ecosystems and Environment. 84(1), 1-20.

Pimentel, D., R. Zuniga & D. Morrison. 2005. Update on the environmental and economic costs associated with alien-invasive species in the United States. Ecological Economics, 52(3), 273-288.

United States Geological Survey. 2007. United States Geological Survey Photographic Library.

USA. 1999. Executive Order 13112 of February 3, 1999: Invasive Species. Federal Register, 64(25), 6183-6186.

Van Driesche, R., et al. 2002. Biological Control of Invasive Plants in the Eastern United States.USDA Forest Service Publication FHTET-2002-04. 413.

ZMIS. 2007. Zebra Mussel Information System. U.S. Army Corps of Engineers.

Žuljević, A. & Antolić, B. 2000. Synchronous release of male gametes of Caulerpa taxifolia (Caulerpales, Chlorophyta) in the Mediterranean Sea. Phycologia, 39(2), 157-159.

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Wanted: Hobbyist Heroes to Stop An Alien Invasion by Mike Greenemeier and Brian Plankis -