Florida Sea Grant College Program • http://www.flseagrant.org • SGEF–155 March 2009 rev.

CAN WE STOP “KILLER ALGAE” FROM INVADING FLORIDA?

by Charles Jacoby1 and Linda Walters2

What is “killer algae”? 

The Mediterranean strain of Caulerpa taxifolia earned 

the name “killer algae” because of its devastating 

effects on the Mediterranean coast. The story of this 

algae or seaweed represents one of the best-documented 

cases of accidental introduction, subsequent 

establishment and eventual invasion by a marine 

organism taken beyond its natural range (a non-native 

or nonindigenous marine organism). 

The green algae, Caulerpa taxifolia, normally grows in 

warm tropical oceans. Aquaria often collect and breed 

this tropical strain for displays. In this case, Caulerpa

taxifolia was bred at the Stuttgart Aquarium in 

Germany. Specimens were sent to the Monaco 

Oceanographic Museum, and as far as we can tell, some 

plants escaped or were released in 1984. When first 

discovered, Caulerpa taxifolia covered one square 

meter (10.8 square feet) of coastline. People ignored 

this accidental introduction and establishment because 

everyone felt confident this “tropical” algae would not 

survive winter temperatures. Big mistake! 

Following successive breeding, Caulerpa taxifolia had 

gained a tolerance of colder temperatures. By the end of 

2000, the algae covered over 131 square kilometers 

(50.6 square miles) of coastline in Croatia, France, Italy, 

Monaco, Spain and Tunisia. It continues to spread, and 

it has become invasive by causing ecological and 

economic harm. The Mediterranean strain of Caulerpa 

taxifolia overgrows native plants and animals. Loss of 

key species and blankets of algae have caused poor 

fishing and reduced tourism in many coastal 

communities. 

In 2000, the Mediterranean strain of Caulerpa taxifolia

appeared at two sites in southern California and three 

sites near Sydney, Australia. All introductions probably 

involved algae released by people. Rather than delay 

and risk an invasion, governments in California and 

New South Wales started eradicating “killer algae” 

before it spread. Millions of dollars have been spent on 

treating the algae with chemicals based on chlorine in 

California and copper sulfate in New South Wales. In 

addition, the New South Wales government sought to 

limit accidental spread of pieces of algae by banning 

swimming, boating and windsurfing in infested 

waterways and by asking people to keep their dogs out 

of these waters. Monitoring will continue for five years 

to determine if eradication is successful. 

What does Caulerpa taxifolia look like? 

All species of Caulerpa have upright fronds (blades) 

that are spaced out along a robust, creeping runner 

(rhizome). Fine, hair-like rhizoids branch off the 

rhizome. The rhizome and rhizoids grow underneath 

sand grains or adhere to rocks. The Mediterranean and 

tropical strains of Caulerpa taxifolia such as those 

founding Florida have featherlike, grass-green fronds 

with flattened branchlets that are 0.6–1.0 millimeters 

wide (0.02–0.04 inches). When mature, the branchlets 

are separated by about 1 millimeter (0.04 inches), and 

they are pinched at the tips and at the base where they 

attach to the midrib. 

1 UF/IFAS Program for Fisheries and Aquatic Sciences, University of 

Florida, Box 110600, Gainesville, FL 32611-0600; phone (352) 

392-96117, ext. 272; cajacoby@ufl.edu

2 Department of Biology, University of Central Florida, Orlando, FL, 

32186; phone (407) 823-2148; ljwalters@pegasus.cc.ufc.edu

Above: Caulerpa 

taxifolia showing 

fronds, rhizome and 

rhizoids (Linda 

Walters)

At left: Caulerpa 

taxifolia showing 

branchlets (Linda 

Walters)
Although similar in basic form, the two strains of 

Caulerpa taxifolia do differ. In south Florida, fronds of 

the native strain reach 3–15 centimeters in length (1.2–

5.9 inches). Small clumps grow in sheltered or moderately wave-exposed areas down to 15 meters deep (45 

feet). Fronds of the invasive strain reach 85 centimeters 

in length (nearly 3 feet). This strain can create huge 

meadows with over 8000 fronds per square meter (740 

fronds per square foot) down to 100 meters (328 feet). 

Unlike the south Florida strain, “killer algae” can survive in water as cold as 11o Centigrade (52o Fahrenheit). 

Several other species of Caulerpa with similar forms 

grow in Florida. Caulerpa ashmeadii has cylindrical 

rather than flattened branchlets. Caulerpa sertularioides

has needle-shaped branchlets that are only 0.5 millimeter wide (0.02 inches) and do not taper at the midrib. 

The most similar species, Caulerpa mexicana, has flattened fronds that average 2–25 centimeters (0.08–0.16 

inches) in height and tightly packed or overlapping 

branchlets that are 2–4 millimeters wide (0.08–0.16 

inches) and pointed at the tips but not at the base. 

Why is “killer algae” so invasive? 

Rapid spread is one key trait of a successful invader. 

Caulerpa taxifolia spreads most easily by fragmentation. Fragments as small as 10 millimeters in length (0.4 

inches) can rapidly produce a new rhizome, attach, and 

grow. The Mediterranean strain survives fragmentation 

better than the native strain in Florida. 

A fragment (2 centimeters or .8 inches long) of Caulerpa 

taxifolia regenerating a rhizoid (Linda Walters)

Will “killer algae” invade Florida? 

Florida has the right conditions for an introduction of 

the Mediterranean strain of Caulerpa taxifolia. It has an 

extensive coastline, a climate similar to previously 

invaded sites, and a large number of people with aquaria 

that could contain “killer algae.” Will an introduction 

turn into an invasion that causes ecological and economic harm? Hopefully not! Lessons from earlier invasions should help to significantly reduce the risk. 

Knowing that the Mediterranean strain can survive and 

spread in a wide range of temperatures, eradication must 

begin quickly. A rapid response plan will need to be to 

implemented if “killer algae” appear. Residents and 

visitors can help by being on the lookout for “killer 

algae” and reporting any sightings. 

How can I help? 

You can help prevent an invasion of “killer algae” just 

by following the law. In response to introductions in 

California and the potential for rapid expansion via 

fragmentation, the Mediterranean strain of Caulerpa

taxifolia was added to the U.S. Federal List of Noxious 

Weeds in 1999 and to the Plant Protection Act in 2000. 

Thus, importing the Mediterranean strain of Caulerpa

taxifolia or transporting it across state lines are federal 

offenses. Ordering Caulerpa taxifolia via the Internet 

would also be an offense if it turns out to be the Mediterranean strain. 

You can also help by: 

1) Never dumping unwanted aquarium plants or 

animals into our coastal waters. If you have unwanted algae, place it in a sealed freezer bag, freeze 

it for 24 hours, and dispose of it properly so that it 

will wind up in a landfill. 

2) Reporting sightings of either the native or 

Mediterranean strain of Caulerpa taxifolia. If you 

see Caulerpa taxifolia in an aquarium, contact Linda 

Walters so she can test its DNA to see if it is the invasive strain. If you see the algae in the wild, please 

collect 3–5 fronds connected by a rhizome and photograph them or preserve them in rubbing alcohol. 

Send the photograph or preserved specimen, along 

with the date, time and exact location of the sighting 

to Linda Walters at the address given on the front of 

this sheet. 

3) Learning more about Caulerpa taxifolia and other 

invasive species. The National Sea Grant College 

Program, Florida Sea Grant and other sources are 

developing additional fact sheets and complementary 

lesson plans on the Mediterranean strain of Caulerpa

taxifolia, other invasive species and the impacts of 

marine invasive species in Florida. If you would like 

copies of these materials when they become available, contact Linda Walters or Chuck Jacoby at the 

address given on the front of this sheet.