Figure 2. Known U.S. distribution of 

Melanoides tuberculatus as of April 2006; 

the non-native snail may have expanded 

its range from the areas shown. Map 

courtesy of A.J. Benson, USGS; adapted 

from 

http://nas.er.usgs.gov/queries/SpeciesList.

asp?Group=Mollusks

Biscayne 

National 

Park

1 mm

1 mm

1 mm

Why should you be concerned?

First, human health issues. Nobody 

wants to be home to liver or lung flukes. 

In Asia, where Melanoides tuberculatus

is native, the snails are part of the 

complex life cycle of several species of 

parasitic trematode worms, including 

liver flukes—Clonorchis sinensis, 

Opisthorchis spp., and Haplorchis

spp.—and a lung fluke—Paragonimus 

westermani. The life cycle of these 

trematode parasites (fig. 3) involves 

both vertebrate and invertebrate hosts; 

for example, humans, fish, snails, and 

crustaceans (such as crabs, crayfish, and 

shrimp). The cycle continues when a 

person (or alternate host such as a bird 

or raccoon) eats raw or undercooked fish 

or crab that has been infected.

Red-Rimmed Melania (Melanoides tuberculatus)—

A Snail in Biscayne National Park, Florida—

Harmful Invader or Just a Nuisance?

Potentially harmful to humans and 

other animals, the red-rimmed melania snail

(Melanoides tuberculatus; family Thiaridae) 

was discovered in Biscayne National Park,

Florida, in 2003 by U.S. Geological Survey 

(USGS) researchers (figs. 1, 2; box on p. 2). 

The discovery raised concerns for park 

managers because this aquatic non-native 

snail is present in significant numbers in 

areas frequently used by park visitors and 

poses a risk of exposure. Researchers are 

addressing questions such as: Is this species

a danger to human health? How widespread is it within the park? What factors 

control the distribution of the species? Is 

its presence a threat to native animals?

What We Know about

Melanoides tuberculatus

How did Melanoides tuberculatus get here?

Melanoides tuberculatus is native 

to tropical and subtropical regions of 

Africa and Asia (Clench, 1969). Specifics 

of how and when M. tuberculatus got 

to the United States are not known, but 

Murray (1971) and Roessler and others 

(1977) believed it was through the aquarium trade. The species was first reported 

in Arizona in the 1950s (Murray, 1971; 

Dundee, 1974) and in south Florida in 

1971 (Russo, 1973). By 1977, it had 

reached areas adjacent to Biscayne 

Bay (Roessler and others, 1977). The 

known distribution of the species in the 

United States is shown in figure 2.

Photographs by G. Lynn Wingard, USGS

Miracidiae stage of 

 the trematode enters

 Melanoides tuberculatus

 and larvae develop

Metacercariae stage of the 

 trematode develops in

 crustacean or fish

Adult 

worms

Proper food preparation

stops cycle here

for humans

Adult worms are

 ingested and 

 infect mammals

 or birds

Free-swimming

 life stage—Miracidium

Free-swimming life

 stage—Cercariae

Eggs expelled in feces

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2008–3006

May 2008 Printed on recycled paper

Figure 2. Known U.S. distribution of 

Melanoides tuberculatus as of April 2006; 

the non-native snail may have expanded 

its range from the areas shown. Map 

courtesy of A.J. Benson, USGS; adapted 

from http://nas.er.usgs.gov/queries/

SpeciesList.asp?Group=Mollusks.

Figure 1. Juvenile and adult Melanoides 

tuberculatus. This non-native species shows 

many variations and can be easily confused 

with native snails. mm, millimeter.

Figure 3. Generalized life cycle of trematodes that infect Melanoides tuberculatus and other 

aquatic snails. Hosts differ at each stage, depending on the species of trematode. Trematodes 

are parasitic worms (also called “flukes”) that can cause illness in humans and other animals. 

Proper food preparation can prevent humans from being infected.
 

Dried samples are then sorted under a microscope (4 and 5) to 

remove the Melanoides tuberculatus debris (animals not alive 

when sample was collected). M. tuberculatus specimens are 

lined up on specimen trays, arranged by size, and counted (6). 

Some live M. tuberculatus

are selected for DNA 

analysis (7) by polymerase 

chain reactions (PCR), or 

they are tested for their 

salinity tolerance in 

experimental tanks (8).

Samples are collected (1) in the field in 

Biscayne National Park (fig. 4) by using a 

petite ponar device while being supervised by 

local wildlife (brown pelican). Samples are 

sieved immediately in specially designed 

buckets (2). The samples are placed in plastic 

bags with some of the water from the site to 

keep the snails alive. Back in our lab at the 

U.S. Geological Survey, after all live 

specimens have been removed from samples, 

the samples are washed through sieves to 

remove any mud (3), then dried.

How do we collect our samples and what happens to them?

1 2

6

3

4 5

7 8

What are

you up to?

1 cm

In the 1970s, some researchers 

(Murray, 1971; Roessler and others, 

1977) thought the risk to human health 

from these parasites was minimal in the 

United States because sewage treatment 

and food preparation methods would 

prevent the spread of infection. The 

advent of more ethnic and diversified 

methods of food preparation in recent 

decades (Simonne and others, 2004), 

however, increases the chances of live 

parasite ingestion. Infection by the lung 

fluke (Paragonimus) has been documented in people in the United States 

(Stoll, 1947; Mariano and others, 1986; 

DeFrain and Hooker, 2002). Parasite 

infections can last for years (Stauffer and 

others, 2004), increasing the chances of 

the host passing infectious materials on 

and starting the life cycle again.

Second, animal health issues.

Trematode flukes affect waterfowl, fish, 

and other animals (including humans). 

Melanoides tuberculatus is a known 

host for several of these parasites and 

a potential host for others. Centrocestus 

formosanus is a trematode parasite that 

burrows into the gills of fish and then 

enters mammals or birds that consume 

the fish. Centrocestus causes losses of 

over $3 million annually to ornamental 

fish producers and has been found on 

fish in the wild in Texas, Florida, and 

Utah (Mitchell, 2005). Haplorchis spp., 

another parasite present in M. tuberculatus, infects the muscle tissue of fish.

Other parasites are not yet known to 

include Melanoides tuberculatus in their 

life cycles but are cause for concern. The 

trematode parasite Philophthalmus megalurus affects the eyes of birds, including 

waterfowl, exposed to M. tuberculatus

while feeding in the shallow waters 

of Biscayne Bay. These parasites can 

potentially adapt to new animal hosts. 

Penner and Fried (1963) found a marine 

species of Philophthalmus along the 

Gulf Coast of Florida to Key West that 

utilizes another snail—Batillaria minima

(West Indian false cerith)—as its host. 

Because B. minima and M. tuberculatus co-exist in Biscayne Bay, there is a 

potential for the parasite to expand from 

one host species to an alternate, previously unaffected, host population.

Third, potential displacement of 

native species. A new, prolific species 

that can out-compete native species 

for food sources could cause serious 

ecological damage (Murray, 1971; 

Roessler and others, 1977; Mitchell, 

2005). Melanoides tuberculatus has 

been found in Florida in densities of 

10,000 per square meter at St. Johns River 

(Thompson, 2004) and 23,000 per square 

meter near Coral Gables (Roessler and 

others, 1977). M. tuberculatus (and other 

members of the family Thiaridae) can 

reproduce asexually; thus, a single snail 

is all it takes to populate a new area, and 

the reproductive rates are extremely high. 

In addition, M. tuberculatus snails do not 

lay eggs, but brood their young internally; 

this reproductive strategy may also give 

them an advantage over native species.

2
North Canal

FLA City Canal

Princeton 

 Canal

Goulds Canal

Military Canal

Cutler Drain Canal

Mowry Canal

Black Creek Canal

Homestead Air Force Base

Black Point

Convoy Point

Landfill

Water Treatment 

Facility

Turkey Point

Florida Turnpike

Biscayne National Park Boundary

Alive

Melanoides tuberculatus

 detected in field surveys

EXPLANATION

Shell debris

80°25'W 80°20'W 80°15'W

25°35'N

25°30'N

25°25'N

Biscayne 

National Park

Area of

enlarged

map

Transect at Black Point follows

channel out from shore

Alabama

Gulf of Mexico

Atlantic

Ocean

Georgia

Florida

3

Figure 4. Satellite image showing location of sites in and near Biscayne National Park, Florida, where Melanoides tuberculatus

snails (live and debris) were found during field surveys conducted by the USGS between October 2004 and July 2007. Inset at 

upper left shows part of transect at Black Point. Inset maps at right show location of larger view. Base image is a mosaic of 

Landsat orthorectified data obtained from the USGS EROS Data Center. Photograph of Black Point by James B. Murray, USGS.
22,261

1,536

50,333

3,473

6,304

435 2,551

176 696

48

59,826

4,128

464

32

87

6

27,913

1,926 19,667

1,357

5,739

396 3,826

264

DISTANCE FROM SHORE AT BLACK POINT

0

1,000

2,000

3,000

4,000

5,000

NUMBER OF INDIVIDUALS COUNTED

October 2004

Actual count December 2006

TR1 TR2 TR3 TR4 TR5 TR6

Estimate/m2

How do conditions in south Florida increase 

the potential for Melanoides tuberculatus

to affect human and animal health?

The potential danger to human health

and animal health is increased by the 

presence of Melanoides tuberculatus in 

south Florida because conditions here 

favor the parasitic life cycle shown in 

figure 3. First, the 

potential snail, fish, 

and crustacean hosts 

are present in large numbers. Second, many activities 

bring people and animals into 

contact with the hosts. Fish and 

shellfish are popular food sources 

in the region and are sometimes 

eaten raw or undercooked. 

From 1990 to 2000, Florida 

had the highest incidence in 

the Nation of illness outbreaks 

caused by ethnic food (Simonne

and others, 2004). Water birds 

(photos at right) and small mammals, such as raccoons, frequent 

the shoreline to feed on crabs, fish, 

and invertebrates. Fishing, swimming, and other water sports bring 

people into contact with the water. 

Third, exposure to potentially

infected, untreated human waste

is a concern because MiamiDade County is underlain by 

porous limestone, and about 27 percent 

of households used septic tanks in 

2000 (data from http://www.doh.state.

fl.us/Environment/ostds/statistics/

ostdsstatistics.htm and http://www.

miamidade.gov/planzone/Library/Census/

MiamiDadeCountyFacts-2005.pdf). 

In addition to having all the components of the trematode life cycle in place, 

south Florida has a climate similar to that 

of southeast Asia, where M. tuberculatus is

native. These factors increase the likelihood 

of the spread of M. tuberculatus and the displacement of native south Florida species.

What We’ve Learned in This Study 

What is the distribution of Melanoides 

tuberculatus within Biscayne National Park?

Both living Melanoides tuberculatus

snails and shell debris have been found 

near canal mouths and in the nearshore areas in Biscayne National Park 

(figs. 4, 5). The highest concentrations 

of live and dead shells were near Black 

Point. Field surveys in 2004 and 2006 

showed that M. tuberculatus is becoming

salinity tolerance illustrates the hardiness of the species (fig. 6). None of the 

original adult specimens died as salinity was gradually increased during the 

6-week-long experiment, and juveniles 

appeared during the second week and 

survived, even though one tank reached 

extreme conditions (45 ppt). Although 

experimental results such as these may 

not translate directly to the natural environment, they indicate that the population 

of M. tuberculatus in Biscayne Bay is no 

longer restricted to freshwater.

Other environmental factors such as 

temperature, oxygen levels, and substrate 

need further investigation. In Biscayne 

Bay, temperature is not likely to be a 

limiting factor because water temperatures are similar to those in the snail’s 

native habitat. Mitchell (2005) reported 

that M. tuberculatus is resistant to desiccation, and Neck (1985) found that the 

species is resistant to low oxygen levels.

increasingly abundant at Black Point 

(fig. 5). The estimated number of M. tuberculatus snails per square meter approaches 

60,000 on the Black Point transect at site 

TR4, which is about 500 meters offshore.

In order to determine how many 

introductions of Melanoides tuberculatus

have taken place in Biscayne Bay, we 

are analyzing mitochondrial DNA from 

selected locations. Analysis of samples 

from Black Creek Canal and site TR2 

on the Black Point transect show that 

the two populations are indistinguishable. This finding suggests that the 

population at Black Point is the result of 

a single introduction of one clonal type of 

M. tuberculatus. The numbers shown in 

figure 5 illustrate how rapidly this species 

is spreading from a single introduction and indicate the level of care 

necessary to prevent the release 

of non-native species.

What factors control

 the distribution 

of Melanoides 

tuberculatus?

In its native 

habitat, Melanoides 

tuberculatus is 

considered primarily a freshwater

species; however, 

in south Florida, it is adapting to estuarine salinity conditions (Russo, 1973; 

Roessler and others, 1977). In Biscayne 

Bay, we have collected live M. tuberculatus snails at just under normal marine 

salinity (up to 33 parts per thousand (ppt) 

of dissolved salts). Experiments to determine what environmental conditions 

might deter the spread of M. tuberculatus

are being conducted. One experiment on 

Bird photographs by G. Lynn Wingard, USGS

4

Figure 5. Raw count of Melanoides tuberculatus, live and dead, collected in 2004 and 2006 

along a transect from Black Point (fig. 4). Black numbers are actual counts from three bottom 

grab samples collected (by using a petite ponar device) at each site. Red numbers are 

estimates of the numbers of individuals per square meter (m2) based on the raw count.

Great blue heron

Double-crested cormorant
How does Melanoides tuberculatus

affect native animal populations?

The numbers of Melanoides tuberculatus snails found at some of the sites 

around Black Point (fig. 5) are far in 

excess of the numbers of any of the native 

mollusk species present. Although we 

have not directly tested the effect on the 

native mollusks and other invertebrates 

in Biscayne National Park, we assume 

that there is competition for food (the 

microalgae on which M. tuberculatus

and other invertebrate organisms feed). 

Additional experiments and fieldwork are 

needed to determine the precise effects on 

the native invertebrates within Biscayne 

Bay and on the fish, birds, and mammals 

that feed upon these organisms.

What are the risks from Melanoides 

tuberculatus in Biscayne National Park?

There is good news and bad news. 

The good news is that, to date, the 

Melanoides tuberculatus population 

in Biscayne National Park has not 

revealed evidence of harmful parasites. 

The bad news is that if parasites appear 

in a small portion of the population, both 

parasites and host snails could spread rapidly. As mentioned above, all components 

of the trematode life cycle are in place in 

the park, and other snails known to carry 

parasites co-occur with M. tuberculatus

populations. Continued monitoring of 

the snail population in the park should be 

done to detect any parasites that appear.

More good news is that all Melanoides tuberculatus snails found within 

Biscayne National Park to date are 

concentrated near canal mouths and in 

the nearshore areas between canals. 

The more open regions of Biscayne Bay 

appear to be free of this non-native snail. 

The area around Black Point has an especially high concentration, yet this population appears to have begun from a single 

introduction. If it is possible to get rid of 

M. tuberculatus at Black Point, there is 

less likelihood of it being reintroduced 

than if it had entered the environment 

repeatedly. Additional genetic testing 

should be done on other populations to 

determine how many introductions have 

occurred. M. tuberculatus is variable 

and hard to distinguish from some native 

snails; careful notes and measurements 

(figs. 7, 8) are needed and so are studies 

to examine the diversity of the populations within Biscayne National Park.

The expansion of M. tuberculatus from freshwater into the brackish 

estuaries of south Florida increases 

the potential number of species that 

can be involved in the transmission of 

any parasites that may exist to include 

other snails, blue crabs, spiny lobsters, 

shrimp, stone crabs, and other saltwater 

crustaceans and fish. It also indicates the 

species’ ability to adapt and survive and 

implies that the distribution of M. tuberculatus will likely expand in the future 

unless actions are taken to control the 

further spread of the species.

Is Melanoides tuberculatus

an invasive species?

According to Federal law (Executive Order 13112 of Feb. 3, 1999), an 

alien species is any species capable of 

propagating in a particular ecosystem 

that is not native to that ecosystem. 

An invasive species is an alien species 

5

Figure 7. An example of the type of notes and sketches made by scientists when 

studying and measuring distinguishing characteristics of snails. Melanoides 

tuberculatus is variable and difficult to distinguish from some native species.

Figure 6. Salinity during a 6-week experiment to 

test the tolerance of Melanoides tuberculatus

for increasing salinity. Starting salinity in all 

three tanks was less than 5 parts per thousand 

(ppt) dissolved salts. Salinities in tanks B and 

C were gradually increased while tank A was 

maintained at approximately 5 ppt as a control. 

All 30 adults placed in each tank survived, and 

juveniles appeared in tanks B and C, confirming 

that the freshwater snail M. tuberculatus is 

adapting to estuarine salinities. SALINITY IN TANKS (ppt)

Number of adults was constant

 at 30 individuals in each tank

1/4/07

1/17/07

2/5/07

2/20/07

Juveniles

appear

0

10

20

30

40

50

Freshwater

Marine 

Hypersaline

Tank A

Tank B

Tank C
1 cm

By G. Lynn Wingard, James B. Murray, 

W. Bane Schill, and Emily C. Phillips

For further information, please contact:

G. Lynn Wingard

U.S. Geological Survey

926A National Center

Reston, VA 20192

Telephone: 703–648–5352

E-mail: lwingard@usgs.gov

Web sites: http://sofia.usgs.gov

http://sofia.usgs.gov/exchange/flaecohist/

(data for this project)

“whose introduction does or is likely to 

cause economic or environmental harm 

or harm to human health” (http://www.

invasivespeciesinfo.gov/laws/execorder.

shtml#sec1). Melanoides tuberculatus is 

officially listed as an alien species or nonnative species by most sources, not as an 

invasive species. What do you think?

What should be done?

Regardless of its official status as 

an alien or invasive species, Melanoides 

tuberculatus is a species of concern for 

Biscayne National Park and elsewhere. 

Researchers should monitor its distribution, frequently check for the presence of 

parasites, and measure the effects on the 

native animals of the park. If parasites 

become a problem, future restrictions 

may need to be placed on fishing and 

consumption of species from the park 

and on recreational use of the nearshore 

waters. If flushing of freshwater through 

canal systems increases (a possible consequence of south Florida restoration) and if 

the global climate warms in this century as 

predicted by the Intergovernmental Panel 

on Climate Change (IPCC, 2007), then 

M. tuberculatus will be likely to spread 

within Biscayne National Park and beyond. 

References Cited

Clench, W.J., 1969, Melanoides tuberculata (Muller) in Florida: Nautilus, v. 83, 

no. 2, p. 72.

DeFrain, Michael, and Hooker, Robert, 

2002, North American paragonimiasis; 

Case report of a severe clinical infection: 

Chest, v. 121, p. 1368–1372.

Dundee, D.S., 1974, Catalog of introduced 

mollusks of eastern North America 

(north of Mexico): Sterkiana, no. 55, 

p. 1–37.

Intergovernmental Panel on Climate Change 

(IPCC), 2007, Climate change 2007; The 

physical science basis; Contribution of 

Working Group I to the Fourth assessment 

report of the IPCC: New York, Cambridge 

University Press, 996 p., 1 CD–ROM. 

(Also available online at http://www.ipcc.

ch/ipccreports/ar4-wg1.htm.)

Mariano, E.G., Borja, S.R., and Vruno, M.J.,

1986, A human infection with Paragonimus kellicotti (lung fluke) in the United 

States: American Journal of Clinical 

Pathology, v. 86, p. 685–687.

Mitchell, A.J., 2005, Centrocestiasis; 

A serious gill trematode problem in 

cultured and wild fishes [abs.]: World 

Aquaculture Society Book of Abstracts, 

2005, p. 283.

Murray, H.D., 1971, The introduction and 

spread of thiarids in the United States: 

The Biologist, v. 53, no. 3, p. 133–135.

Neck, R.W., 1985, Melanoides tuberculata 

in extreme southern Texas: Texas Conchologist, v. 21, no. 4, p. 150–152.

Penner, L.R., and Fried, Bernard, 1963, 

Philophthalmus hegeneri sp. n., an 

ocular trematode from birds: Journal of 

Parasitology, v. 49, no. 6, p. 974–977.

Roessler, M.A., Beardsley, G.L., and Tabb, 

D.C., 1977, New records of the introduced snail, Melanoides tuberculata

(Mollusca: Thiaridae) in south Florida: 

Florida Scientist, v. 40, no. 1, p. 87–94.

Russo, T.N., 1973, Discovery of the 

gastropod snail Melanoides (Thiara) 

tuberculata (Müller) in Florida: Florida 

Scientist, v. 36, no. 2–4, p. 212–213.

Simonne, A.H., Nille, A., Evans, K., 

and Marshall, M.R., Jr., 2004, Ethnic 

food safety trends in the United States 

based on CDC foodborne illness data: 

Food Protection Trends, v. 24, no. 8, 

p. 590–604.

Stauffer, W.M., Sellman, J.S., and Walker, 

P.F., 2004, Biliary liver flukes (Opisthorchiasis and Clonorchiasis) in immigrants 

in the United States; Often subtle and 

diagnosed years after arrival: Journal of Travel Medicine, v. 11, no. 3, 

p. 157–159.

Stoll, N.R., 1947, This wormy world: 

Journal of Parasitology, v. 33, p. 1–18; 

reprinted in 1999 in v. 85, no. 3, 

p. 392–396.

Thompson, F.G., 2004, An identification 

manual for the freshwater snails of Florida (last edited March 5, 2004): Gainesville, Fla., Florida Museum of Natural 

History, available online at http://www.

flmnh.ufl.edu/natsci/malacology/fl-snail/

snails1.htm. 

Acknowledgments

This project was funded by the 

U.S. Geological Survey Geology 

Venture Capital Fund for 2007. The 

work was done in cooperation with 

Biscayne National Park. We thank 

Dr. Fred Thompson, Florida Museum of 

Natural History, for verifying our initial 

identification of Melanoides tuberculatus. 

Ruth Ortiz and Carlos Budet, USGS, 

prepared samples for analysis; high school 

volunteers Christopher Wingard and 

Tara Colley assisted with this project.

6

Figure 8. Adult specimens of Melanoides tuberculatus that have been sorted from a sample 

(see box on p. 2) and prepared for counting. Comparing all of the shells from each sample 

allows us to see the variation within a population. cm, centimeter.