Freshwater Molluscan Shells
Lampsilis cardium Rafinesque, 1820, in a very shallow creek in southwest
Missouri. The gray mantle flaps have black and white "eyespots" and
undulate back and forth rhythmically. Click Here for animated sketch.
The Unionidae, of worldwide distribution, are most diverse in eastern and central North America, and secondarily in China and Southeast Asia. These include North America's most abundant, interesting, and economically valuable shells. Because of their long association with activities of leisure, livelihood and trade, many have acquired colorful common names.
Historically, "rushes" on pearls from freshwater mussels were important in settling of parts of Texas (Howells, 1996, available at http://www.conchologistsofamerica.org/articles/y1996/9606_howell.asp). Preston (1915) mentions legends "which may or may not be true" that the lust for freshwater pearls is one of the reasons Caesar invaded the British Isles. He was probably referring to passages in Gibbon's classic Decline and Fall of the Roman Empire (1776-'88). Mussels were used as food by native peoples, and in the late nineteenth and early twentieth centuries, were widely harvested to make buttons for clothing. With the advent of plastics in the 1940s and '50s, this industry came to an end, the last plant closing in the mid-1960s (Parmalee, 1998). Soon afterwards, small spheres fasioned from mussel shell came to be used as nuclei for cultured pearls grown in marine pearl oysters (Pinctada) in Japan. It is now estimated that 95% of the world's round cultured pearls contain nuclei from American freshwater mussels. More recently, some freshwater mussels have been used for pearl production themselves using modifications of the Japanese techniques.
Reproduction in the Unionacean freshwater clams is unique. In the Family Unionidae, most species are dioecious (having separate sexes in separate individuals) and some are strongly sexually dimorphic. Eggs are fertilized by water-borne sperm while being stored in the gills of the female animal, modified as pouches or marsupia, for this purpose. There they develop into tiny larva known as glochidia. These are obligate parasites on fish, and each species of mussel requires certain fish species in order to develop. A variety of ingenious schemes have evolved to improve the glochidium's chances of attaching to a host fish. In some mussels, as shown above, a portion of the mantle is modified to appear like a minnow or darter, while others use insect-shaped clumps of the glochidia themselves, attached to gelatinous "fishing line." The glochidium usually attaches in the mouth or gills, or on the skin, where it becomes covered over in a small cyst. The parasitic stage lasts from one to six weeks, depending on water temperature and other factors, after which it drops off to begin life as a free-living filter feeder.
The classification of the Family Unionidae is, according to all authors, still under review. Even identification at the species level can be difficult due to considerable variation between individuals and populations regionally. There also appears to be ecophenotypic variability, where an individual develops differently as a function of where it lives, for example, "large-river forms" in some species. Most classification schemes are based on soft-part anatomy, particularly the nature and location of the marsupium in the female animal. In eastern North America there are roughly 300 species grouped into 43 genera, which are in turn grouped into six tribes in two subfamilies. The following classification is from Banarescu (1990), and Burch (1975) for North America. North American species numbers (full species only) are from Turgeon, et al. (1988).
Amblema 2 Elliptoideus 1 Fusconaia 13 Plectomerus 1 Quadrula 18 Quincuncina 3 Tritogonia 1
Worldwide, other "lineages" in the Subfamily Ambleminae include:
Tribe Gonideini: western North America, Gonidea, 1 species
Tribe Pseudodontinae: east Asia
Tribe Leguminaiini: three genera, Microcondylaea, Pseudontopsis, Leguminaia; western Asia, Italy
Tribes Arcidopsini, Oxynaiini, Rectidentini: southeast Asia
Tribe Parreysiini: two genera, Burma, India
Tribe Lamprotulini: seven genera, eastern Asia
Tribe Coelaturini: nine genera, Africa and southeast Asia (given family status by some authorities)
Subfamily Unioninae (North American)
Cyclonaias 1 Elliptio 31 Hemistena 1 Lexingtonia 2 Plethobasis 3 Pleurobema 32 Uniomerus 5
Alasmidonta 11 Anodonta (includes new
13 Anodontoides 2 Arcidens 1 Arkansia 1 Lasmigona 6 Simpsonaias 1 Strophitus 3
The Anodontinae (Subfamily status for Anodontini used by Banarescu in his worldwide synthesis) and the Genus Anodonta have wide distribution in the Northern Hemisphere. 11 genera have been listed for east and southeast Asia, and two in western Asia.
Twenty North American genera further grouped into five subtribes by Burch. Names in parenthesis are older synonyms in common use;
Actinonaias 2 Ellipsaria 1 Epioblasma (Dysnomia, Plagiola) 20 Glebula 1 Lampsilis 28 Lemiox 1 Ligumia 3 Leptodea 3 Medionidus 7 Obovaria 6 Potamilus (Proptera) 6 Toxolasma (Carunculina) 8 Truncilla 4 Venustaconcha 2 Villosa 18 -- Ptychobranchus 5 -- Cyprogenia 2 Obliquaria 1 -- Dromus 1 -- Cyrtonaias 1
Subfamilies having distributions wholly outside North America include:
Acuticostinae: east and southeast Asia, eight genera.
Lamellidentina: one genus in India and Afghanistan.
Psilunioninae: one genus in southern Europe, Northern Africa.
Unioninae s.s. (not equivalent to the North American Unioninae, above) six genera in Europe, east Asia and Africa, including Unio itself.