Systematics and phylogeny of arcoid bivalves (Arcoida: Pteriomorphia: Bivalvia).

摘要

The Arcoida is a large group of mostly marine bivalves, with a global distribution. Familiar taxa in this group include the arks, bittersweets and dog cockles. Relationships among the higher-level taxa of the Arcoida are not well understood and the classification of this group has been the subject of debate and rearrangement. While many views exist as to the evolution of this group, none of them are based explicitly on a phylogenetic analysis. In this study, the phylogenetic relationship of the Arcoida is inferred from a systematic analysis based on both morphological and molecular data. This is the first analysis in which representatives of all seven nominal families are included. 141 morphological characters from the external shell and internal anatomy were coded for 131 taxa. The phylogenetic signal of both these character types was explored. Few nonhomoplastic synapomorphies for the group were recovered; shell tubules are confirmed as the sole non-homoplastic synapomorphy for the order. Shell characters failed to recover the majority of the higher taxonomic ranks that they were initially used to describe. Little coherent signal was received from the analysis of anatomy alone. Four molecular markers, the nuclear 18S rRNA, 28S rRNA, protein coding histone H3 and mitochondrial cytochrome c oxidase I, were also investigated using direct optimization as implemented in POY (Varon et al., 2008). These data were analyzed individually as well as simultaneously with the morphological data. A Sensitivity Analysis (Wheeler, 1995) of the molecular data was also performed---this explores the effects of parameter costs (i.e. indels and transition/transversion ratios) on the phylogenetic results. The results of these phylogenetic analyses do not reflect the current classification of the group. In this study, the majority of the higher taxonomic groups of Newell (1969) were not recovered, including the two superfamilies Arcoidea and Limopsoidea, as well as five of the families; only the monophyly of the Glycymerididae and Noetiidae is supported. A major taxonomic review of the order is necessary. This analysis is the largest and most comprehensive phylogenetic analysis of the Arcoida to date.