Major Subgroups of Bivalves

Bivalves are classified mainly by shell structural features and internal soft body parts. There are six subclasses.

• PALAEOTAXODONTA "ancient row teeth" and CRYPTODONTA "concealed teeth".
  These subclasses are known from the Ordovician to the present day. They are primitive and are thought to have descended from the most recent ancient lineages of molluscs. They are characterised by taxodont dentition, equivalved shells and protobranch gills. The shells have a mother-of-pearl iridescent layer. Palaeotaxodonts tend to have two adductor muscles, whereas cryptodonts have a single muscle. Neither has siphons. Most are detritus-feeding, infaunal marine animals, such as Nucula, and are most abundant in deeper-water environments.

• PTERIOMORPHA "wing shaped" (Ordovician to Recent).
  Mainly marine benthic organisms attached by byssal threads. This subclass includes mussels, scallops, oysters and certain clams. The shells are solid and tend to show large variation, and can be hinged with or without teeth. Adductor muscles are variable in size and number. These bivalves do not have siphons.

• PALAEOHETERODONTA "ancient and variously toothed" (Ordovician to Recent).
  This subclass consists of two orders, Unionoida and Trigonioida. The Unionoida are oyster-like and they account for the majority of freshwater bivalves. The shell is mother-of-pearl and the hinge teeth are variable. Two adductor muscles are present and there are no siphons. Interestingly, these molluscs give birth to live young via internal incubation of the fertilized 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 or HETEROCONCHS "variously toothed" (Ordovician to Recent).
  This subclass holds 50% of all bivalves. Its most important order is the Veneroida which includes clams and cockles, all of which have toothed shells, usually paired adductor muscles and siphons. The second order is Myoida, which includes soft-shelled clams and paddocks, which differ significantly from the Veneroida. The Myoida are deep burrowers in the rock or substrate, and include the genus that bores in dock wood timber, Teredo. Shells lack teeth and often have gaping openings at both the anterior and posterior ends, which may be covered by a chitinous plate.

• ANOMALODESMATA "anomalously ligamented" (Ordovician to Recent).
  These are predominantly suspension-feeding marine forms. Siphons are present and in several remarkable families they are used for carnivorous predation. The hinge line does not have teeth and the shell is unusually thin and easily breakable. These molluscs have both distinct testes and ovaries on either side of the body. In most hermaphroditic bivalves, like these, a sex change usually occurs at a later point in the life cycle.