Molecular phylogenetics and the evolution of labral spines among eastern Pacific ocenebrine gastropods

Labral spines are sharp projections of the apertural lip found in some marine gastropods that are used to penetrate hard-shelled prey. The majority of gastropod genera that contain labral spine-bearing species are found in the subfamily Ocenebrinae (Gastropoda: Muricidae). To reconstruct the evolutionary history of labral spine-bearing and labral spine-lacking gastropods in the eastern Pacific (EP) Ocean, partial sequences of two mitochondrial genes (cytochrome oxidase I and 12S rRNA) were obtained from representative taxa. Despite high nucleotide bias, a variety of phylogenetic reconstruction methods produced the same tree topology. The traditional taxonomic view that all "Nucella-like" spine-bearing taxa in the EP belong to a monophyletic "Acanthina" is rejected due to nonmonophyly of this group. The more recently recognized "Acanthinucella" is also not monophyletic, and we therefore propose the new genus Mexacanthina for two Mexican species formerly assigned to Acanthinucella. The genus Ocinebrina, which first appears in the middle Eocene, is not a stem EP ocenebrine lineage and may also not be a monophyletic clade. Tracing the evolutionary history of labral spines among extant lineages indicates that the absence of a labral spine is ancestral for all EP ocenebrines. Ancestral conditions could not be resolved unambiguously for all nodes of the phylogeny based on extant taxa. However, by jointly considering both molecular phylogenetic relationships and the phylogenetic affinities of several extinct taxa, all remaining character state transformation can be inferred unambiguously. Based on this analysis, a labral spine likely evolved independently in at least four lineages of EP ocenebrines. Although homoplasy appears to characterize labral spine evolution among ocenebrine gastropods, the structural position of a labral spine was evolutionarily altered in one lineage, indicating that different types of labral spines do not necessarily reflect convergent evolution.