George.

Yes, Gladys.
There's a man at the door with a moustache.
Tell him I've already got one. . . All right, all right.  What's he want then?
He says do we want a documentary on mollusks.
Mollusks!
Yes.
What's he mean, mollusks?
[screaming] MOLLUSKS!!  GASTROPODS!  LAMELLIBRANCHS!  CEPHALOPODS!
Oh mollusks. I thought you said bacon. . . All right, all right.  What's he charge then?
It's free.

Monty Python
************


Freshwater Molluscan Shells
by
Martin Kohl


[pelecypoda (bivalves, clams)] [gastropoda (snails)] [measurement scales] [links] [references] [site map]
Machine translations (Babelfish) available for
Chinese
, Deutch, Espanol, Francais, Italiano, Korean , Portugues. Human Translation: Belorussian.


Introduction

These pages constitute an attempt to fill a void in popular materials covering worldwide molluscan shells.  An abundance of literature, from small field guides to gorgeously produced coffee-table books, is available for marine shells.  For land snails there is Abbott's 1982 Compendium of Landshells.  With few exceptions these resources have excluded freshwater aquatic mollusks.  Much of the existing literature is technical, covering a limited taxonomic or geographic range.  The material presented here is organized in a manner typical of Abbott -- zoologically down to the family level, then (where I have sufficient material) regionally.  I would like to acknowledge the assistance of Paul Parmalee of the University of Tennessee, and Alan Gettleman for assistance in identification, Karl-Otto Nagel of the Senckenberg Naturmuseum, Frankfurt for specimens and help with European species and systematics, Stephanie Clark, Marc Keppens, and others for providing specimens from worldwide locations, Thomas Eichhorst for images and information on the family Neritidae, and Lynn Scheu and Mike Emery for helpful comments and suggestions.

Classification

Freshwater mollusks, being thoroughly polyphyletic, cannot be classified independently of the rest of the great invertebrate phylum mollusca.  Below is an evolutionary tree diagram for the mollusca modified from Brusca and Brusca (1990), p 762.  Arrangements for the gastropods and pelecypods, and lower taxonomic rankings with significant representation in fresh waters, are reflected in the linked pages.


to Freshwater pelecypoda (bivalves, clams, mussels)
to Freshwater gastropoda (snails, slugs)

In addition to standard taxonomic classification, other schemes have been devised that consider ecological or biogeographical factors, the degree of evolution that a taxon has undergone since its ancestral invasion of fresh water, and how salt-tolerant an otherwise freshwater group may be.  The following zoogeographic classification is abstracted from Banarescu (1990).  He adapts an earlier system used for freshwater fishes, and generally based on salt tolerance, to mollusks.  The groupings include:

  • Primary (dispersal confined to freshwater continental routes)
  • Secondary (freshwater fish with a range that would require some exposure to seawater)
  • Vicarious (distribution by continental routes, but closely related to marine species)
  • Peripheral (more recent marine ancestors, colonization from the marine "periphery")

"Primary" and "secondary" are merged into "continental" where applied to freshwater prosobranch snails.  Freshwater pulmonate snails, which did not migrate from salt water, are not included in the following.  

Primary bivalves (clams). Superfamily Unionacea (pearly freshwater mussels, all families), Pisidiidae and Sphaeriidae (fingernail and pill clams).

Secondary bivalves (clams). Dreissenidae (zebra mussels and false mussels), Corbiculidae (basket clams).

"Continental" gastropods (snails). Superfamily Viviparoidea ("mystery snails", the large "apple snails"), Pleuroceridae ("hornshells," largely North American), Syrnolopsidae (endemic to Lake Tanganyika), and Pomatiopsidae, Bithyniidae, Baicaliidae, Benedictiidae, Pyrgulidae, and the large and probably polyphyletic Hydrobiidae (all small to tiny snails).

Peripheral gastropods (snails). Neritina and other freshwater members of Neritidae, members of the primarily marine Stenothyridae, Iravadiidae, Tornidae, Assimineidae, Hydrococcidae, Littorinidae, Truncatellidae; and some of the HydrobiidaeMelanopsidae (Asian and New Zealand snails).   Thiaridae (parthenogenic "hornshells", tropical) are also included.  There are freshwater members of other marine families including Buccinidae, Marginellidae, Turbonillidae, Trochidae, Umbonidae, Muricidae, Nassariidae, Epitoniidae, and from the Systellommatophorans (an order conataining certain tropical slugs, but not most common garden slugs and snails), two freshwater Onchidiidae.

Peripheral bivalves. Freshwater members of the families Arcidae, Mytilidae, Lyonsiidae, Donacidae, Cardiidae, including many geologically recent extinctions, and Mactridae. Members of many other families may live in brackish waters.

Other malacologists prefer to categorize using the fossil record, or degree of kinship with a marine ancestor.  One such system includes:

  • Paleolimnic (higher taxa that have no marine ancestors)
  • Mesolimnic (higher taxa with distant marine ancestors)
  • Neolimnic (more recent marine ancestors)

Zoogeography

Freshwater mollusks are interesting from the standpoint of zoogeography.  First, a common notion that these animals are present in the creeks and rivers because "this area was once covered by the sea" needs to be dispelled.  Continents have moved, and seas have gone, come, and gone again since the ancestors of "primary" freshwater mollusks first crawled up the rivers. Molluscan evolution spans a good chunk of geologic time, and their distribution bears witness to a number of causes--some yet unknown.  Drainage divides represent substantial barriers to these creatures. The more isolated two rivers are from each other, or the more important the drainage divide between them is, the fewer species and genera their molluscan faunas have in common.  Besides saltwater seas, large trunk rivers also form substantial barriers to the species of headwaters and small tributaries. For example, although creeks in the Missouri Ozarks share many things in common with those in East Tennessee, including being tributary to the Mississippi River, there are few unionid mussels and no pleurocerid snail species in common in the two regions.

Another general observation is the lack of the strong arctic-to-tropics gradient of increasing diversity, size, color, etc. This is in contrast to most other living things, notably marine mollusks, where a huge diversity of large showy species live in the tropics, while arctic species are few, and often drab and chalky.  There are no freshwater equivalents to the conchs, cowries, and giant clams of marine tropical waters.  With the exception of three tropical snail families Neritidae, Thiaridae and Ampullariidae, most freshwater families achieve their greatest size and diversity in temperate, or even cold parts of the world. 

Several authors have observed similarities between the unionacean pearly mussels of Australia and South America, noting that they have more in common with each than they have with species from the northern hemisphere.  Prior to the development of plate tectonic theory in the 1960s, various explanations were proposed, including land bridges, foundered beneath the waves long ago, or ancestral migrations to both areas from the north, with the fauna subsequently dying out in their native (norther hemnisphere) lands.  Had researchers written just a few years later, plate tectonics theory would have provided a much better explanation, with southern freshwater mussels survivors of the breakup of Gondwana.

In eastern North America, the Pleuroceridae (among the gastropods) and the Unionidae tend to have endemic species with restricted ranges. Species composition will change from one river or creek to another. Where ranges overlap, as in the case with three Elimia snails surveyed in one area, they seem to be present semi-continuously, with a single widespread species, and one of the other two. Only rarely were all three species present (personal observation).  The Hydrobiidae seem to be discontinuous, with concentrations of a single species locally, and wide areas where none are present.

Pilsbry and Becquaert (1927), describing the zoogeography of African freshwater mollusks, observe that many species and genera are widespread in the Congo Basin and its vicinity. A major exception to this is Lake Tanganyika, with its abundance of strange endemic snails, some of which closely resemble marine shells. They classify this lacustrine fauna into three groups. Their group 1 includes nonendemic species, and species belonging to genera found in surrounding waters. Most such mollusks live in shallow lagoons near the lakeshore. Group 2 includes strictly endemic genera, though they are still obviously related to the surrounding fauna. These generally live in the shallower waters of the lake. Finally, group 3 includes the "thalasoid" [Greek, Thalassa, sea] snails, a strange and extremely varied assemblage of forms. These are so different from freshwater snails elsewhere in Africa that "their relationship can only be traced through a careful study of their soft parts and anatomy, and is in some cases still open to question." (Pilsbry and Becquaert, 1927). Group 3 species are adapted to diverse habitats including the wave-washed and algae covered rocks of the shore, the shallower waters of the lake where they coexist with snails and mussels of group 2, and deeper waters to 200 meters down, below which there is insufficient oxygen. Pilsbry and Becquaert suggest that the mollusk fauna of Lake Tanganyika resulted from a few generalized freshwater types finding their way into the lake and evolving in isolation over time.

Geomorphologic aspects such as stream capture or piracy may explain the zoogeography of some species. For instance, many pearly Elliptio mussels live in Atlantic Coastal Plain rivers of the southeastern United States-- some endemic, others wide-ranging-- while only two species exist in the continental interior, west of the Great Eastern Divide.  Of possible explanations, two are likely.  One involves an origin in the American interior (the heartland of unionid diversity generally), with escape and subsequent adaptive radiation in the many parallel but separate river systems of the Atlantic Coastal Plain.  The second involves a long period of speciation along the Atlantic coast (the center of Elliptio's diversity), with the eventual escape of two species back across the Divide to the continental interior.

Importance to man

The importance of freshwater mollusks to humans and the environment is greater than their general absence in collections and popular guides would lead one to believe, although with increasing environmental awareness, and threats of extinction, this lately has started to change.  Some negative aspects, where they are a source of problems themselves, will be considered first:

Parasite vectors:  While landsnails and slugs may play minor roles in the transmission of certain tapeworms and nematodes, the importance of freshwater mollusks as intermediate hosts for a great variety of human and animal parasitic flatworms cannot be ignored.  Prosobranch gastropod taxa known to be involved in parasite life cycles include Pila, snails belonging to the Hydrobiidae and related families, Thiaridae including Thiara, Pleuroceridae including Elimia (Juga), and Semisulcospira, and also several marine gastropods in the related family Potamididae.  Pulmonate gastropod hosts include the Planorbidae, Bulinidae (now included within the Planorbidae), and the Lymnaeidae (Chandler and Clark, 1961).

Invasive species:  The invasion of North America by the zebra mussel has had catastrophic consequences in some places.  Their colonization of Unionid mussel habitat and the shells of living native mussels has been lethal to the native inhabitants.  In the Illinois River, a mass die-off of zebra mussels in the summer of 1994 resulted in the death of large numbers of fish and native Unionid mussels.  Populations of zebra mussels had exploded to densities of over 60,000 per square meter, a concentration of biomass that could not be sustained by available oxygen during summer low-water.  Mortalities then precipitated a snowballing effect.

Previous displacement of native American species by the Asiatic clam Corbicula proceeded more slowly.  Both Corbicula and more so, the zebra mussel, have the capability of fouling up water intake systems for power plants and municipal purification plants. 

Passive indicators of environmental degradation:  The news they tell us is bad.  Besides suffering from natural calamities (both drought, and floods when augmented by anthropogenic erosion and siltation), a number of species in North America have become endangered, or been driven to extinction due to the fourfold attack of pollution, siltation due to agricultural runoff, river impoundment, and invasive exotic species.  The Unionidae were among the first victims of widespread pollution.  For example, in 1922 it was observed that the mussel fauna in the Big Vermillion River had been completely destroyed for a distance of 14 miles below the sewage outfall of Urbana, Illinois (Parmalee, 1967).

Siltation from agricultural runoff and other disturbances harms freshwater species directly through smothering them, covering them with silt, and altering the habitat by covering what was formerly clean sand, gravel, and algae-covered stones with anywhere from a thin layer to many meters of silt. 

Dam construction changes habitat radically, both above and below the dam.  Populations in the free-flowing reaches above the impoundment become isolated.  In the reservoir itself, deep quiet waters no longer favor the abundance and variety of species originally present.  Below the dam, released water may be clear and clean, but with radically different chemical and physical properties.  This is typical of deeper reservoirs, where cold winter temperatures and low oxygen contents are retained in the bottom waters and released through the turbines year-round.

Shell:  Freshwater shells have always been present in the seashell trade, though often not so labeled for the end user.  Their most important use economically was for the manufacture of pearl buttons before the application of plastic in the 1940s and '50s.  Presently, mussel shells are exploited for use as pearl blanks in the Japanese cultured pearl industry.

Foodstuff:  Piles of mussel shells and the larger freshwater snail shells are found along the banks and in the soil near, and sometimes well away from, many of the larger rivers throughout North America. Extinct (recently, due to anthropogenic causes) or regionally extirpated species are often encountered.  This suggests that aboriginal cultures used freshwater mussels and snails as a source of protein.  In Africa, river oysters (Etheria) were harvested and dried in the Belgian Congo, now Zaire, later to be made into a stew.  Neolithic tribes living along the Nile had many uses for the large Spathiopsis shells, and they form the bulk of the Neolithic kitchen middens there.  They have also been recovered from many Egyptian graves, their meaning and significance suggested by the existence of a beautifully carved diorite representation found in an Egyptian necropolis (Van Damme, 1984).

Parmalee (1998) indicates that a variety of methods may have been used in their preparation in North America, but that white men generally find them tough and completely unpalatable.  Shakespeare alludes to this in The Tempest (Act 1, scene 2, lines 460-465 (Riverside edition)):

I'll manacle thy neck and feet together;
Sea water shalt thou drink; thy food shall be
The fresh-brook mussels, wither'd roots, and husks
Wherein the acorn cradled
.

It should be noted that primitive people often use elaborate methods to prepare and process otherwise toxic or unpalatable foodstuffs. 

Gastropods have helped to feed many cultures. Tribal elders in Zaire ate local Pila snails ritualistically at certain times of the year, and the leftover shells were worn as charms to protect against drowning, or as headdresses assuring fertility (Van Damme, 1984). In the Amazon basin, large apple snails like Pomacea canaliculata and P. bridgesii are raised, and their mild-tasting muscular portions are eaten as the dish "churo." Pila snails are eaten raw by the Ilocano people of the Philippines, but may be a source of parasitic infection (Chandler and Clark, 1961). The Chinese Mysterysnail (Cipangopaludina)--collected and offered for sale in San Francisco's and Boston's traditional Chinese markets-- can also harbor parasites.

Finally, in classical times, Pliny the Elder indicates the flesh of "river snails" [fluviatilium coclearum] was used raw or boiled, as an antidote for scorpion venom. It could either be eaten, or applied directly to the wound.

The Images

Most of the images are direct scans of the shells. They were scanned at resolutions of 600, 300, 150, 75 and 50 pixels per inch.  This results in magnifications close to 8, 4, 2, 1, and 2/3 respectively when viewed on an 11 inch (28 cm) wide computer monitor.  [Click here for scales] In most cases this information is included at the end of the filename, accessible by right clicking the image, and going down the pop-up menu to "properties."  The "upside down" perspective of the flat-bed scanner introduces a distortion somewhat different from other illustrations and photographs.  It is most apparent in short-spired snail shells resting on their apertures, and causes spires to appear foreshortened.

Comments, errors, additions, suggestions to
Mkohl1@aol.com


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