Archerd Shell Collection > Shell Classes > Bivalve Family Index p. 2

Bivalve Family Index, page 2

Overview, continued from page 1 

Most bivalves are dioecious; i.e., males are separate from females. A few bivalve species are, however, hermaphroditic; e.g., the fresh water families, Unionidae and Sphaeriidae, and the marine families in the Anomalodesmata subclass. Several others, notably the common oyster, Ostrea edulis, begin life excreting male gametes, later shift to female gametes, and may, later still, revert again to excreting male gametes.

Epifaunal families, i.e., those living with their shell attached to other shells or substrates, have a mantle that is partly or largely fused along the bivalve edges, and the molluscs do not generally form siphons. Oysters and jingles, for example, become attached permanently to rocks or wood, either using a bundle of hardened protein threads (the "byssus") or like oysters, by directly cementing themselves to rock or shell.

Mantle fusion seems to protect against excessive sediment accumulation within the body cavity. That which accumulates through the incurrent opening of the mantle may be expelled efficiently through the excretory opening; otherwise, excessive sediment deposits would clog the gill, thus impairing oxygen and carbon dioxide exchange with the water. Unlike the epifauna, bivalves that move about can readily flush their body cavity of sediments, and many move freely through the sand or mud substrate by thrusting a muscular foot into the sand and pulling themselves forward.

The Six Subclasses (Bivalvia)

The bivalves are classified mainly by shell structural features, in the following subclasses.

Paleotaxodonta ("ancient row teeth") and Cryptodonta ("concealed teeth"). These primitive subclasses are believed to be descended from the most ancient lineages of molluscs. The shells have a mother-of-pearl ("nacre") iridescent layer. Paleotaxodonts have two adductor muscles and a toothed hinge, whereas the Cryptodonts generally have a single adductor muscle and a hinge either without teeth or with vestigial teeth. None have siphons. 

Pteriomorpha ("wing-shaped"). This subclass includes mussels, scallops, oysters, and the bittersweet clam. The shells are solid, show great variation, and all are nacreous. Shells may be hinged with or without teeth. Adductor muscles are

variable in number and comparative size. None of these bivalves have siphons.

Paleoheterodonta ("ancient and variously toothed"). This subclass consists of two orders, Unionoida and Trigonioida. The Unionidae family is oyster-like and accounts for the majority of freshwater bivalves. Their shell is nacreous and the hinge teeth are variable. Two adductor muscles are present; there are no siphons, and these molluscs give rise to live births by internal incubation of the fertile egg. The Trigoniidae are a scarce family with a markedly triangular hard shell. Except for a few species found in southern Australia, most are fossils.

Heterodonta ("variously toothed"). This subclass holds 50% of all bivalves. Its most important order, Veneroida, includes clams and cockles, all of which have non-nacreous, toothed shells. Adductor muscles are usually paired, and all have siphons or at least partially fused mantles to direct the flow of water. Reproduction in these clams can be prodigious and provides a significant base to the foodchain of higher animal life. The second order, Myoida, includes the soft-shelled clams and the piddocks, which differ significantly from the Veneroida. The Myoida are deep burrowers in rock or substrata and include the destructive borer species in dock wood timbers, Teredo. In the Myoida, shells are without teeth and often have gaping openings on both the anterior and posterior ends, which may be covered by a chitinous plate (see Family: Pholadidae). These and other special characteristics, lead some authorities to place the Myoida order in the subclass below.

Anomalodesmata ("anomalously ligamented"). This subclass shares many features in common with the Pholadidae mentioned directly above. Siphons are present, and in several rather remarkable families the siphons are used for carnivorous predation. Unusual variation in the number and arrangement of adductor muscles is characteristic in both the Pholadidae and the several families comprising the Anomalodesmata. Similarly, auxiliary shell structures may be present, as well. The hinge does not have teeth, and the shell is usually thin and easily breakable. Unlike the Pholadidae, however, the shells do exhibit a nacreous layer, and the molluscs have both a distinct ovary and a distinct testis, opening on either side of the body. In most hermaphroditic bivalves, like these, a sex change usually occurs at a later point in their life cycle. Rather than than both reproductive organs becoming functional at the same time, only one sex organ is active (for details, see Morton, 1979).

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Archerd Shell Collection > Shell Classes > Bivalve Family Index p. 2