Phylogenetic Signal in Shell Morphology of the Chemosymbiotic Lucinidae (Bivalvia)

Lucinidae are the most specious family of extant chemosymbiotic bivalves and occupy a wide range of habitats worldwide. All extant lucinids examined to date house chemosynthetic endosymbionts within their gill tissues. Fossil evidence suggests a Silurian origin for the family, with chemosymbiotic associations dating back to at least the Late Jurassic. Previous systematics work indicates that shell characters contain limited phylogenetic signal, and when they do carry phylogenetic signal, it is typically at more derived, genus-level positions. Instead of using categorical states for shell characters, which are ubiquitous in morphological phylogenetics, this study uses landmark-based geometric morphometrics as a quantitative approach to capture morphological variation in shell shape. To test for phylogenetic signal in morphology using the multivariate version of the K statistic (Kmult), we analyzed 627 specimens from eight lucinid species. Bayesian phylogenetic analyses were performed on a subset of 62 specimens using four sequenced genes: the nuclear ribosomal genes 18S rRNA and 28S rRNA, and the mitochondrial genes cytochrome b (cyt b) and cytochrome oxidase I (COI). This study presents the first COI phylogenetic tree for lucinids. Shell shape exhibited a strong phylogenetic signal (Kmult = 0.98), that was visualized through phylomorphospace analysis (PA), phylogenetically aligned component analysis (PACA), and phylogenetic principal component analysis (Phy-PCA). Although phylogenetic signal was present throughout the entire landmark configuration, thin-Plate Spines (TPS) of mean shell shape illustrated that the inhalant channel shape had the greatest variation among species. Therefore, the phylogenetic signal could not be contributed to any single shell feature or shape.