Florida’s Introduced Nonindigenous and Invasive Mollusks (Clams and Snails): Final Installment of Our 3-part Series on Biological Invasions in Florida

This article discusses the species of introduced mollusks (bivalves and gastropods, better known as clams and snails) of Florida’s terrestrial and aquatic habitats along with a general discussion of the possible effects of biological invasions on native wildlife and habitats.  The first part of this three-part series was on introduced fishes in the state, and the second was on introduced amphibians and reptiles.  This discussion on introduced mollusks of Florida will wrap up our series!

As of this writing, at least 31 species of nonindigenous mollusks representing 17 families have been introduced to Florida (Exhibit 3).  Of these, about 68% have established breeding populations in one or more counties.  There are at least 5 species of introduced clams and 26 species of introduced snails, including terrestrial, freshwater, and marine species.  Of these mollusks, about 68% have established breeding populations in one or more counties.  Examples of established invasive species include the Asian green mussel (Perna viridis) (Exhibit 1) and the giant East African snail (Achatina fulica) (Exhibit 2).  

Exhibit 1.  Asian Green Mussels (Perna viridis) Coat the Lower Surface of a Buoy in Tampa Bay, Florida.  The Species Is Introduced and Invasive throughout Florida’s Coastline.  The Barnacles in the Photo Are Most Likely Striped Barnacles (Balanus amphitrite)—Another Invasive Species in Florida.  Source: UF Fisheries & Aquatic Sciences (http://sfrc.ufl.edu/greenmussel/).

Some well-known negative effects of introduced snails are large-scale consumption and decimation of native vegetation and out-competing native species through direct competition for limited resources and through predation on their eggs and young. 

The giant East African snail (Achatina fulica) is a member of a family that contains the largest land snails in the world (Abbott 1989).  The species was first introduced to Florida in 1966 when a young boy brought three live snails from Hawaii (where it is also introduced) to Miami as pets.  Upon discovery of the smuggled snails, the boy’s grandmother released the snails into her garden.  Over the next several years the snails multiplied and spread to neighboring lands.  Florida state agricultural authorities were eventually alerted to the establishment of this destructive species and the species was eradicated by 1972 to the tune of $300,000 (Abbott 1989) to more than $1 million (FDACS 2011).  Between 1966 and 1972, the three specimens brought to Florida by the boy had multiplied to over 18,000 snails.  One specimen of the Miami colony reportedly measured a whopping seven inches in shell length (Abbott 1989)! 

In September 2011, the giant East African snail was found to have been reestablished in Miami after the Florida Department of Agriculture and Consumer Services responded to a call from a Miami homeowner.  Within 6 months, over 40,000 snails were collected in Miami by state and federal authorities (USDA 2012).  Although authorities are working hard to remove all the individuals of this species from Florida, the eradication will prove very difficult and the likelihood of complete eradication currently appears low.

The giant East African snail is introduced and invasive in several other parts of the world, including Hawaii and other islands in the Pacific, the Philippines, Madagascar, and parts of Asia.  The species is known to consume some 500 species of plants in both agricultural and natural areas.  Because the snail requires large amounts of calcium to grow and strengthen its great shell, the species causes damage to plaster and stucco while consuming these products for their calcium content.  It is a known carrier of a parasitic nematode that is capable of spreading meningitis in humans (FDACS 2011). 

The parasitic nematode known as rat lungworm (Angiostrongylus cantonensis) is carried by the giant East African snail and has an interesting life cycle.  The larvae are ingested by the snail (the intermediate host of the worm) when feeding on rat feces (don’t ask).  The larvae grow and approach maturity inside the snail.  It takes the consumption of an infected snail by a rat (the definitive host) for the parasitic nematode to complete its life cycle by reaching maturity and producing eggs inside the rat.  The mature nematode eggs hatch into larvae while still within the rat and are expelled with the rat’s feces.  People can become infected by eating undercooked or raw (who eats raw snails?) infected snails.  People may also become infected by eating raw produce such as lettuce that contains a small snail or slug.  An infected person cannot transmit the disease to other people.  Infection of rat lungworm in humans is rare in the continental United States, but at least one case was recorded in 1993 in New Orleans where a boy ingested a raw snail (apparently on a dare) and became infected with rat lungworm.  The parasitic nematode is host-specific and humans are not its intended host, so the parasite typically dies inside an infected person, even without treatment.  However, the symptoms range from headache, muscle aches, stiff neck, skin irritation, fever, nausea, and vomiting until the parasite dies (CDC 2010). 

In 2012, a captive orangutan (Pongo sp.) housed in Miami was found to have been infected with the rat lungworm.  The animal had a history of eating snails.  Researchers from the University of Florida collected snails and rat feces from around the area where the orangutan was housed and examined the samples for evidence of the parasitic nematode.  Several of the snails and all of the rat feces tested positive for rat lungworm (UF 2015).  The species of snails found to have been infected included the introduced species Asian trampsnail (Bradybaena similaris), garden zachrysia (Zachrysia provisoria), and the striate drop (Alcadia striata) (J. Slapcinsky, Florida Museum of Natural History, Gainesville, FL, pers. comm. 03/03/015).

Although reducing the effects of invasive nonindigenous species (such as those listed in Exhibit 2) is an important part of restoration and management efforts in natural areas of Florida and elsewhere, introduced mollusks are typically a lower priority than other organisms, such as invasive plants or fishes, except when they are known carriers of disease or damage agricultural crops or other property.  Nonetheless, invasive organisms of all kinds can cause significant stress to native ecosystems and biological invasion is widely viewed as a major cause of the reduction in native plant and animal diversity (Elton 1958, Wilcove et al. 1998).  Invasive species are known to affect most natural areas of the United States (Villazon 2009) and worldwide (Sala et al. 2000).

It should go without saying that the intentional introduction of any nonindigenous species, whether it be a plant or animal and regardless of size or assumed innocuousness, should never be attempted.  The reasons are many and the costs can be severe in terms of biological effects, human health, and economic impacts.  Nonindigenous species introduced to new areas have the capacity to explode in numbers and outcompete native species for limited resources such as food, water, and shelter.  Native species are at a competitive disadvantage because they have not had time to evolve defense mechanisms that would otherwise allow them to successfully compete against the introduced species.  The introduced species can have a competitive edge where it is introduced outside its native range partly because these species lack the predators they would have in their native range.  This idea was coined fairly recently by scientists with the term ‘predator release’.  The competition between native and nonindigenous species can result in the extinction of native species, spread of diseases and parasites, and displacement of whole communities, and may even cause physical changes to the environment. 

Exhibit 2.  Nonindigenous Mollusks Recorded in Florida

Sources: Florida Museum of Natural History (http://www.flmnh.ufl.edu/scripts/dbs/malacol_pub.asp), USGS Nonindigenous Aquatic Species online database (http://nas.er.usgs.gov/queries/SpeciesList.aspx?Group=Mollusks&Sortby=1&state=FL), Seitz (2014), J.C.Seitz unpublished data.

Sources:

Abbott, R.T.  1989. Compendium of Landsnails.  A Color Guide to More than 2,000 of the World’s Terrestrial Shells.  American Malacologists, Inc., Melbourne, FL.

Centers for Disease Control (CDC).  2010.  Parasites – Angiostrongyliasis (Also Known as Angiostrongylus Infection) [online resource].  Accessed 03/24/15 at http://www.cdc.gov/parasites/angiostrongylus/‌gen_info/faqs.html#whatangiostrongylus#whatangiostrongylus.

Elton, C.S.  1958.  The Ecology of Invasions by Animals and Plants. Methuen and Co., Ltd., Strand, London.

Florida Department of Agriculture and Consumer Services (FDACS).  2011.  Florida Department of Agriculture and Consumer Services Identified Giant African Land Snails in Miami-Dade County [online resource].  Accessed 03/24/15 at http://www.freshfromflorida.com/News-Events/Press-Releases/2011-Press-Releases/Florida-Department-of-Agriculture-and-Consumer-Services-Identifies-Giant-African-Land-Snails-in-Miami-Dade-County.

Florida Museum of Natural History (FLMNH).  2015.  Invertebrate Zoology Master Database [online resource].  Accessed 03/23/15 at http://www.flmnh.ufl.edu/scripts/dbs/malacol_pub.asp.

Sala, O.E. F.S. Chapin, J.J. Armesto, E. Berlow, J. Bloomfield, R. Dirzo, E. Huber-Sanwald, L.F. Huenneke, R.B. Jackson, A. Kinzig, R. Leemans, D.M. Lodge, H.A. Mooney, M. Oesterheld, N.L. Poff, M.T. Sykes, B.H. Walker, M. Walker, and D.H. Wall.  2000.  Global biodiversity scenarios for the year 2100. Science 287:1770–1774.

Seitz, J.C.  2014.  Assessing Stream-mediated Seed and Shoot Dispersal of Invasive Plants in an Urban Riparian Wetland [thesis].  University of Florida, Gainesville, FL.

University of Florida (UF).  2015.  UF Researchers: Rare Parasite Colonizing Snails in South Florida [online resource].  Accessed 03/24/15 at http://news.ufl.edu/archive/2015/02/uf-researchers-rare-parasite-colonizing-snails-in-south-florida.html#prettyPhoto.

U.S. Department of Agriculture (USDA).  2012.  Escargot? More like Escar-No! [online resource].  Accessed 03/24/15 at http://blogs.usda.gov/2012/04/19/escargot-more-like-escar-no/.

U.S. Geological Survey (USGS).  2015.  NAS – Nonindigenous Aquatic Species [online resource].  Accessed 03/23/15 at http://nas.er.usgs.gov/queries/SpeciesList.aspx?Group=Mollusks&Sortby=1&state=FL.

Villazon, K.A.  2009.  Methods to Restore Native Plant Communities after Invasive Species Removal: Marl Prairie Ponds and an Abandoned Phosphate Mine in Florida.  MS thesis, University of Florida, Gainesville, FL.

Wilcove, D.S., D. Rothstein, J. Dubow, A. Phillips, and E. Losos.  1998.  Quantifying threats to imperiled species in the United States. Bioscience 48:607–615.

Wilson, L.D. and L. Porras.  1983.  The Ecological Impact of Man on the South Florida Herpetofauna.  The University of Kansas Museum of Natural History Special Publication No. 9, University of Kansas, Lawrence, KS.