Freshwater Mussels are Bivalved Mollusks
Freshwater mussels of the order Unionoida are classified as part of the Bivalvia, which is itself grouped in the Mollusca. The other molluscan groups are introduced on the Mollusca page. There are certain important characteristics that mollusk species share: a shell and mantle, a foot, ctenidia, a radula, etc. These characters are modified in important and interesting ways among the bivlaves. In addition certain bivalve groups have evolved some uniquely bivalvian traits, like a byssal thread to fix themselves to rocks and things, and labial palps for feeding. This page is meant to be a brief introduction. More in-depth synopses can be found in Boss (1982) and Brusca & Brusca (1990). For a detailed (and rather technical) discussion of bivalve characters, we recommend Giribet & Wheeler (2002).
 Bivalved Shell.— Bivalve species share certain characteristics that bind them together in the same molluscan group. Like most mollusks, bivalves have a shell. But the shell consists of two distinct halves or valves — a left one and a right one. (They have two valves, hence the nomen BI-VALVIA.) Like other mollusks, the bivalve shell is composed of calcium carbonate and organic compounds layed down by the mantle. The mantle is also divided into two halves called lobes. The two rigid valves are connected dorsally by an elastic ligament that is secreted by a narrow isthmus of mantle between the two lateral lobes. Also connecting the two valves are the adductor muscles; usually two, although one of them may be reduced or absent. When the adductors contract, the valves close, and when the bivalve relaxes the shell opens. The scars of the adductor muscle attachments are often evident on the inner surface of the shell.
Burrowing Foot.— Among bivalve species there are two basic styles of foot: a flattened creeping sole and a wedge-shaped foot for burrowing. (A third foot-plan, found among bivalves that attach themselves to rocks, etc., is to reduced the foot all together!) The creeping sole is like the foot you have seen on a snail or chiton, sliming its way up the side of an aquarium or over a rock; among bivalves, that type of foot is found among protobranchs (see below). The typical bivalve foot is hatchet-shaped— rather than being ventrally flattened, it is laterally compressed. Burrowing bivalves are typically epifaunal or infaunal, and their foot is used to move through the sediment. Deeply buried species need long snorkle-like siphons formed from their mantle to keep up contact with the open water above them.
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Enlarged Ctenidia.— Besides retaining a primitive type of foot morphology, the Protobranchia also have atypical bivalve ctenidia (keep reading to learn more about protobranchs). The stereotypical "higher" bivalve has ctenidia modified for filter-feeding. That is, the current that the mollusks use to draw water over their "gills" for the exchange of respiratory gases serves double duty by supplying food as well. Water passes through the seive-like ctenidia, and food particles, which are trapped in mucus, are passed forward toward the mouth to be sorted and ingested. The structure of the ctenidia is a character of some importance in the classification of the Bivalvia, as discussed below.
Other Bivalve Characteristics.— Besides modifications to the shell, foot and ctendidia, certain other characteristics are useful for diagnosing bivalves. One of these is the lack of a radula. In fact, many of the features associated with molluscan heads are missing from bivalves: for example, eyes and tentacles! While many of these head-parts are gone, bivalves do retain a mouth, and they have elaborated their lips into structures referred to as labial palps. Labial palps are used for sorting food particles pasted to the mouth from the ctenidia.
Another trait found in many bivalves is a byssus. The byssus is composed of threads made of protein that are secreted by a special gland in the foot. The byssus helps the bivalve attach itself to rocks and other firm substrates rather than burrow into soft sediment. Some groups, like oysters, actually grow their shell right to the surface on which they have settled and cement themselves into place.
A characteristic that many marine bivalves share with snails is a planktonic larval stage known as a veliger.
Bivalves can be divided into five subgroups.
Just as the Mollusca can be divided into 7 classes (or more, according to some authorities), the Bivalvia can also be subdivided to recognize major groups within the class. We will refer to these groups as subclasses.
 Protobranchia — aka Paleotaxodonta, Cryptodonta. These bivalves are generally small and infaunal, using their labial palps to sort food from the surrounding sediment (i.e., detritophages). Many malacologists consider protobranchs to be the most primitive bivalves because they share some characteristics with other molluscan classes, like a creeping sole on their foot, and because they lack certain traits of more advanced bivalves like a veliger and modifications of their ctenidia for filter feeding. Protobranchs are strictly marine.
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 Pteriomorpha — Pteriomorphs are often found attached to the substrate, either by a byssus or by cementing — things like true mussels and oysters. But some, like scallops, are capable of swimming using jets of water to escape predators! Rock oysters that produce pearls belong to this group. Pteriomorphs generally have veliger larvae and their ctenidia are composed of folded filaments built for filter feeding. They are mainly marine.
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 Heterodonta. Heterodonts are species belonging to the diverse group of bivalves often referred to as "clams." On the menu, the two biggies are Mya arenaria (aka soft-shell clams, belly clams, steamers) and Mercenaria mercenaria (hard-shell clams, quahogs, cherry stones); other heterodonts are also eaten in the United States, and even more around World. Adult heterodonts generally lack a byssus, tending to be active burrowers or nestlers. But, as with the Pteriomorpha, the larvae, at least among marine forms, are generally veligers and the ctenidia are constructed for filter feeding.
Heterodonts have made some rather conspicuous invasions into freshwater water. The Corbiculidae and Sphaeriidae are two families of bivalves that are conspicuous in freshwater environments around the world, and they have had to face some of the same adaptive challenges as freshwater mussels (see Life History). Dreissena polymorpha, also known as the zebra mussel, is a relative new-comer to the freshwater schene. |
 Anomalodesmata — includes the septibranchs. Some of these bivalves are truly weird! Anomalodesmatan bivalves are generally infaunal filter-feeders with well-developed siphons, but some have evolved along bizarre lines. They are all marine and usually infaunal, although some become attached or form tubes. The ctenidia are either the typical filter-feeding bivalve variety or they are modified into muscular septa. These septibranchs use their ctenidia for a carnivorous lifestyle, sucking hapless small prey into their mantle cavity to be devoured! Hermaphrodism is common and the veliger stage, if present, is short.
While it is quite clear that Anomalodesmata is a group of bivalves distinct from the others, it has recently been shown by Giribet & Wheeler (2002) to be part of the heterodont clade. |
  Palaeoheterodonta — These are our bread-n-butter. The Palaeoheterodonta is comprised of two orders. One, Trigonioida, is very very small: just the genus Neotrigonia with its 5-6 strictly marine species found in Australian waters. The other order, Unionoida, is the diverse group of freshwater mussels to which the MUSSELp web site is dedicated; roughly 840 species distributed all over the world.
While Neotrigonia retains mainly of the characters of the Pteriomorpha, it shares certain characteristics with freshwater mussels that call of its placement in the same subclass. While many of these morphological characteristics have been dubbed dubious in a phylogenetic context, recent molecular evidence strongly supports the traditional Palaeoheterodonta.
Freshwater mussels also possess a suite of unique characters that set them apart from all other bivalves, namely their restriction to freshwater, parental care and (especially) their parasitic larvae. The order Unionoida has also received a lot of attention because of their imperiled conservation status. |
References:
- Boss, K.J. 1982. Mollusca. pp. 945-1166. [In] S.P. Parker (ed.). Synopsis and Classification of Living Organisms. Volume 1. McGraw-Hill Book Company, New York.
- Brusca, R.C. & G.J. Brusca. 1990. Invertebrates. Sinauer Associates, Inc. Sunderland, Massachusetts.
- Giribet, G. & W. Wheeler. 2002. On bivalve phylogeny: a high-level analysis of the Bivalvia (Mollusca) based on combined morphology and DNA sequence data. Invertebrate Biology 121: 271-324.
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