Mollusks (A. J. Benson)

There have been at least 22 species of mollusks introduced into the southeast region including bivalves (clams, mussels) and gastropods (snails, nudibranchs) (Table 3). Of these 22 species, one-half are freshwater inhabitants and the other half marine. Many of these introduced species are established and have spread to surrounding waters while others have failed to establish populations for one reason or another. Florida leads the other states in the region with 20 species of nonindigenous mollusks.  The probability of species becoming established in Florida over the states could be attributed to its climate, abundance of water, and many ports of entry.  A majority of Florida's nonindigenous mollusks would not be expected to survive elsewhere in the Southeast because of habitat requirements such as temperature for example.

Although for many species the pathway of the introduction cannot be precisely documented, it can be assumed for most species based on life history traits or known anthropogenic movements (Table 4). Most of the marine species are introduced by either ship hull fouling or ballast water dumping. The freshwater species, particularly the snails, have been introduced through the aquarium trade where they have been sold in retail pet stores for years.  No other vectors have been documented to show a large importation of freshwater snails to this country.  Therefore, the assumption has been made that the majority of freshwater snails were initially introduced by the aquarium trade and the subsequent release of unwanted specimens.  Other human and natural vectors have since taken over to distribute them further. Of all the introduced mollusks, freshwater snails may be the greatest threat to biodiversity because of their ability to denude vegetation, displace native snails, and act as intermediate hosts for trematode parasites that can infect humans.

The following section gives a brief description of some selected organisms.

Bivalves

Corbicula fluminea (Müller, 1774) Asian clam

Since the introduction of Corbicula fluminea to the United States in 1938 in the Pacific Northwest, this species has spread into many of the major waterways across the country. The Asian clam has become established in all of the states in Region 4 including most recently, Puerto Rico. Corbicula are so widespread in Alabama, Arkansas, Florida, Georgia, Kentucky, and Mississippi that specimens can be found in most counties.  In the other states, the distribution is more limited such as in Louisiana (Pearl, Atchafalaya, Mississippi, and upper Red drainages), North Carolina (Cape Fear, Catawba, Chowan, Eden, Little, Meherrin, Neuse, Roanoke, Rocky, Tar, Uhwarrie and Waccamaw rivers and Richardsons Creek), South Carolina (Savannah, Cooper, Santee, Pee Dee, Little Pee Dee, Edisto, Waccamaw, and Salkahatchie rivers; the Intracoastal waterway; and several industrial facilities in Aiken and Pickens counties), and Tennessee (Tennessee drainage). Possible pathways of introduction include the bilge waters of pleasure boats, fishing bait, or aquarium hobbyists releases, waterfowl and dredging activity.

Impacts in man-made system include the restriction of water flow in power plants, water treatment facilities, and irrigation systems. In natural systems, high densities of Corbicula most likely stress native benthic communities through competition for food and space.  Corbicula is a prey species for some fish and waterfowl, but not significant enough to control populations.

Perna viridis (Linnaeus, 1758) green mussel (MARINE)

The green mussel is a native of tropical marine waters of the Indo-Pacific region surrounding the continent of Asia. It was discovered inside water intake tunnels by power plant workers in the Tampa, Florida area in 1999.   This mussel can grow to about six inches, making it a potentially serious biofouler in made-made systems as well as natural systems.  Its range in Florida extends from Tampa Bay southward to Charlotte Harbor. The green mussel was probably a ballast water introduction from the Caribbean region where green mussels arrived there in the early 1990s.

Gastropods

Pomacea bridgesi (Reeve, 1856) spiketop applesnail

Another native of South America, the spiketop applesnail was introduced into southern Florida where is also established probably as the result of unwanted aquarium animals. The significant impact of this introduction is that is may be displacing the native Pomacea paludosa, a primary food of the rare Everglades kite (Hale 1964).

Pomacea canaliculata (Lamarck, 1822) channeled applesnail

The channeled applesnail is native to South America and was introduced to Florida again as the probable result of unwanted aquarium animals.

Pomacea paludosa (Say, 1829) Florida applesnail

This species of applesnail is native to Florida and Cuba.  It has been reported from single localities in both Georgia and Alabama.

Marisa cornuarietis (Linnaeus, 1758) giant rams-horn snail

A native of South and Central Americas, this freshwater snail has been introduced into many canals in southern Florida where it is established.  It is a probable aquarium release.  Impacts include the ability to completely denude vegetated areas and negatively interact with native snails through competition and direct predation of eggs and young (Hunt, 1958). It is not a host for trematodes, which can later infect humans or animals.

Melanoides tuberculatus (Müller, 1774) red-rim melania

The red-rim melania is native to subtropical and tropical regions in Africa, Asia, and Australia. It is now distributed nearly statewide in Florida possibly due to aquarium releases.  Densities reached 10,000/m² in the St. Johns River in 1976 (Thompson 1984). Populations were also discovered in New Orleans (Dundee and Paine, 1977).  This species of snail is capable of replacing native species and can be a vector for human parasites.

Tarebia granifera (Lamarck, 1758) quilted melania

The quilted melania is native from India to eastern Asia, Japan, and Hawaii. It is established in Florida waters near Tampa and Miami most likely from discarded aquarium animals.   Abbott (1950) reported densities of 400/m² in one spring.  This species can serve as an intermediate host for trematode parasites that can eventually infect humans.

Cipangopaludina chinensis (Reeve, 1863) Chinese mysterysnail

The Chinese mysterysnail is native in much of eastern Asia.  It has been introduced into several locations in Florida, North Carolina, and South Carolina.  Again, this was probably another aquarium introduction. This species could be a vector for some human infections.

Literature Cited

Abbot, R.T. 1950. Snail invaders. Natural History 59(2):80-85.

Dundee, D.S., and A. Paine. 1977. Ecology of the snail Melanoides tuberculata (Müller), intermediate host of the human liver fluke (Opisthorchis sinensis) in New Orleans, Louisiana. The Nautilus 91(1):17-20.

Hale, M. C. 1964. The ecology and distribution of the introduced snail, Marisa cornuarietis (Ampullariidae) in South Florida. M.S. thesis, University of Miami, Coral Gables. 115 pp.

Hunt, B.P. 1958. Introduction of Marisa into Florida. The Nautilus 54(2):53-55.

Thompson, F.G. 1984. The freshwater snails of Florida. A manual for identification. The University of Florida Press. Gainesville. 93 pp.

Table 3.  Mollusks introduced into states with U.S. Fish
and Wildlife Service Region 4. - click to enlarge

Table 4.   Year first documented, origin, pathway of introduction,
and status of mollusks introduced into states in Region 4. - click to enlarge

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