What Are Invasive Species? Invasive species are exotic plants or animals that take up residence in a new environment. Some invasive species are relatively harmless, but most are not. The spread of invasive species is a complex international problem that has existed since the age of exploration. Scientists have long been aware of the problems that land-based exotic plants and animals can cause. You are probably familiar with the gypsy moth, or you've heard of the African killer bee - both invasive species! We are just beginning to learn more about aquatic invaders. The best-known freshwater invader in the United States is the zebra mussel. The mussel was released into the Great Lakes by ships in the 1980s. It has since spread throughout the region and has recently entered lakes and rivers in New England. The zebra mussel breeds prolifically, encrusting power plant and industrial water intakes, causing more than $150 million in damages and threatening the survival of native species. The spread of saltwater invasive species into our coastal waters has accelerated in recent decades due to expanded international shipping, the growth of aquaculture, the baitfish industry, the aquarium trade, and even international Internet purchases. But there are some simple things that we can do to prevent these different pathways from allowing invaders to spread into our local waters....

Case study: Caulerpa

In the early 1980s, curators of public aquariums throughout Europe began to cultivate an alga imported from the Caribbean for use in tanks displaying tropical fish. This bright green, feathery alga called caulerpa (Caulerpa taxifolia) provided a great habitat for tropical fish in the artificial aquarium environments.

In 1984, a small patch (only about one square meter) of (Caulerpa taxifolia) was noted on a submerged flat under the windows of the Oceanographic Museum of Monaco on the Mediterranean Sea, likely discharged with wastewater from an aquarium display. Scientists soon noted the remarkable tendency of this alga to aggressively crowd out native plant and animal species.

Photo Courtesy of Meinesz

Its ability to spread by extending long stems, or rhizomes (which are toxic to most plants and fish) makes it a fierce competitor for space. The alga forms large, dense mats, uninhabitable by almost all other species. The rhizomes also cause caulerpa to be easily entangled in and transported with boat anchors and fishing gear. By 1990, caulerpa had spread as far as 120 miles from its original point of introduction. In 1997, caulerpa covered a total area of 17-square miles, and by the spring of 2001, caulerpa (now commonly referred to as the "Killer Algae") had invaded the coastlines of six Mediterranean countries.

The adaptable and aggressive characteristics of Caulerpa taxifolia and other invasive species are causing great concern not only in the Mediterranean, but also among scientists and natural resource managers across the globe, including those working here in Massachusetts. Many organisms that thrive in artificial or stressful environments, both plants and animals, are in high demand for use in home aquariums, water gardens, aquaculture operations, research facilities, or as bait for fresh or saltwater fishing. As a result, many of these species are transported for sale around the world. In fact, though it is now illegal to possess caulerpa in the United States, a brief search of the Internet will reveal a number of companies willing to deliver the "Killer Algae" right to your doorstep.

Photo Courtesy of Grosholz

In the summer of 2000, populations of caulerpa were discovered in two Southern California estuaries, both likely getting their start as a result of releases by aquarium hobbyists. The cost of a one-year Caulerpa taxifolia eradication effort in a southern California lagoon totaled over $1.1 million, and state agency officials are seeking additional funds for continued control of the persistent algae - click here to learn more about caulerpa in California.

The very characteristics that make species like caulerpa so desirable for use in artificial environments also make them a great threat to the natural diversity of aquatic systems. Species transported beyond their native range and introduced to natural systems often have no competitors, allowing them to grow rampant and displace many or all native species. As a result, these invasive species are considered to be second only to human development in causing declines in the total number of species in the United States and worldwide. The explosive growth of invasive species also causes major conflicts with human uses of aquatic resources. These species often completely clog waterways and water intakes, making them unusable for recreational, municipal, or industrial purposes.

If you decide to include live plants in your aquarium, it is best to select native species. There are several common native species sold at pet stores that are beautiful additions to an aquarium. These include Coontail ( Ceratophyllum ), Water Celery ( Vallisneria americana ) and Common Waterweed ( Elodea ). Click here for more information on creating an invasive-free aquarium.

Adapted from "Biological Invaders: Threats from Aquatic Invasive Species and What You Can Do to Keep Them Out of Our Waters," written for Coastlines by Jay Baker.