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Eight living classes (lineages) of molluscs have been recognised, primarily based on clad¹ (phylogenetic) analysis of morphological characters (Haszprunar et al. 2008). Aplacopora incorporates two classes, Solenogastres and Caudofoveata; these are worm‐shaped, deep‐water animals lacking a shell. Polyplacophora, often referred to as chitons, inhabit hard substrates on rocky shores and are characterised by eight dorsal shell plates. Aplacophora and Polyplacophora are grouped in the clade² Aculifera, which is regarded as monophyletic (i.e. all taxa in this group share a common ancestor) (Sigwart & Sutton 2007). The remaining five classes are grouped in the clade Conchifera, which is also regarded as a monophyletic group. Monoplacophora live in deep waters and are small and limpet‐like, with a single cap‐like shell. The class Bivalvia includes laterally compressed animals enclosed in two shell valves, such as clams mussels, oysters and scallops. Scaphopoda, commonly known as tusk shells, live in marine mud and sediments. The class Gastropoda is the largest and most diverse, containing spirally coiled snails, flat‐shelled limpets, shell‐less sea slugs and terrestrial snails and slugs. Octopus, squid and cuttlefish are in the class Cephalopoda and represent the largest, most organised and specialised of all the molluscs. The Monoplacophora are generally accepted as the earliest extant offshoot of the Conchifera (Haszprunar 2008).

Morphological disparity among the lineages has given rise to numerous conflicting phylogenetic hypotheses. Molecular investigations using nuclear ribosomal gene sequences (18S and 28S) offered little resolution (Ponder & Lindberg 2008). More recent studies, using phylogenomic‐scale molecular data sets (Kocot et al. 2011; Smith et al. 2011; Stöger et al. 2013), have significantly advanced understanding of molluscan phylogeny by providing well‐supported tree topologies and generally congruent results (Telford & Budd 2011; Kocot 2013; Schrödl & Stöger 2014). Probably the most important achievement of these studies is the establishment of the Aculifera and Conchifera groupings.

Several major evolutionary hypotheses and sister relationships have been proposed for the eight living molluscan classes, which are illustrated in Figure 1.1 (Sigwart & Lindberg 2015). Sigwart & Lindberg (2015) assembled a data set of 42 unique published trees describing molluscan interrelationships, which included at least five out of the eight classes. They found that almost 60% of trees based on morphological data (N = 27) were similar, while only 45% of those based on molecular data (N = 15) were similar; the distances separating morphological and molecular trees indicated that they were similar in almost 30% of pairs and different in 20%. It is interesting that there are no studies in which both molecular and morphological data have been simultaneously analysed for all eight classes of Mollusca (Sigwart & Lindberg 2015). Regarding some of the hypotheses illustrated in Figure 1.1, the authors found that support for Cyrtostoma or Diasoma was relatively weak, while the Serialia concept was deserving of due consideration (see also Stöger et al. 2013). They found no consensus support for the topology of the morphological Testaria concept. Integration of new molecular techniques with morphological and developmental data using multiple type species from all eight molluscan classes will no doubt continue to deepen our understanding of molluscan phylogeny and evolution (Kocot 2013; Sigwart & Lindberg 2015).

Figure 1.1 Schematic topology of the major evolutionary hypotheses and sister relationships proposed for the eight living classes in Mollusca: Aculifera (Solenogastres + Caudofoveata + Polyplacophora), Aplacophora (Caudofoveata + Solenogastres), Conchifera (Monoplacophora + Bivalvia + Scaphopoda + Gastropoda + Cephalopoda), Cyrtosoma (Gastropoda + Cephalopoda; historically also including Monoplacophora), Diasoma (Bivalvia + Scaphopoda), Serialia (Polyplacophora + Monoplacophora), and Testaria (Conchifera + Polyplacophora). Text in bold indicate the seven different hypotheses.

Source: From Sigwart & Lindberg (2015). Reproduced with permission from Oxford University Press.