LUCINIDAE TO OTHER BIVALVE GROUPS

Taylor, John D. & Glover, Emily A., 2006, Lucinidae (Bivalvia) - the most diverse group of chemosymbiotic molluscs, Zoological Journal of the Linnean Society 148 (3), pp. 421-438 : 426

RELATIONSHIP OF LUCINIDAE TO OTHER BIVALVE GROUPS

McAlester (1966) suggested that Babinka and the lucinoids were derived from monoplacophorans independently from the rest of the bivalves and Pojeta (1978) considered them a sufficiently distinct group to warrant separation at subclass level (Lucinata). However, these concepts were countered by Boss (1969, 1970) who pointed out the many morphological characters of lucinids that are shared with heterodont bivalves. Subsequent morphological and molecular analyses have confirmed the position of the Lucinidae amongst the Heterodonta ( Healy, 1995; Steiner & Hammer, 2000; Giribet & Wheeler, 2002; Giribet & Distel, 2004; Williams, Taylor & Glover, 2004).

Some workers have proposed, on the basis of morphology, a phylogenetic relationship between the Crassatelloidea and Lucinoidea (in old concept) ( Allen, 1958; Boss, 1969; Scarlato & Starobogatov, 1978; Johnston, 1993; Morton, 1996). Molecular analyses of species of Astartidae and Carditidae ( Giribet & Wheeler, 2002; Giribet & Distel, 2004), indicated that they group together in a monophyletic clade that forms a sister group to the remaining heterodont bivalves, but with no close relationship to the Lucinidae . Inclusion of a crassatellid species, Eucrassatella donacina (Lamarck, 1818) , in the molecular analysis ( Taylor, Glover & Williams, 2005) shows ( Fig. 2 ) that a combined monophyletic clade of Astartidae , Crassatellidae and Carditidae forms a basal sister group to all other heterodont bivalves including the Anomalodesmata. The monophyly of this clade is corroborated by morphological characters, including those of sperm ( Healy, 1995) and presence of extracellular haemoglobin of high molecular weight in all three families. This result supports the idea, based on morphological studies, that the Crassatelloidea and Carditoidea are the most primitive of the living heterodont bivalves ( Yonge, 1969). Suggestions of a relationship between the Crassatellidae and any of the families previously included in the Lucinoidea ( Allen, 1958; Boss, 1969; Johnston, 1993; Morton, 1996) are not supported.

A molecular phylogeny of the Anomalodesmata ( Dreyer, Steiner & Harper, 2003) showed them rooting, in parsimony analysis, as a monophyletic clade amongst basal heterodonts between the Carditoidea/ Crassatelloidea clade and the rest of the heterodonts but in a maximum likelihood analysis they formed a sister group to the Lucinidae . Nevertheless, we are at present unable to identify a sister group to the Lucinidae and, in our molecular analyses, the clade falls into a polytomy with several other major groups of heterodont bivalves ( Fig. 2 ).

Figures

Figure 2. Molecular phylogeny of heterodont and palaeoheterodont bivalves produced by Bayesian analysis of partial sequences from the 18SrRNA gene. Branches with posterior probabilities <85% have been collapsed. Nodal support is posterior probability/bootstrap (Neighbour-joining using Maximum Likelihood distance, 10 000 reps). Details of taxa and methods in Taylor et al., 2005.