On the Origin of Decapoda

Molecular methods are playing an increasingly important role in reconstructing phylogenetic relationships. Regardless of what source of DNA is used, the simple idea behind it is that the genetic distance (distinctness of DNA sequences) between any two taxa should be proportional to the time of their separation. Genetic markers with different degrees of variability appear appropriate for different taxonomic levels. The mitochondrial ribosomal RNA genes 12S and 16S have proven to be useful at the interspecific up to the interfamilial level in brachyuran crabs. Recent criticism has questioned the credibility of phylogenies based solely on mitochondrial DNA (mtDNA) as well as the specific value of commonly used mitochondrial markers such as 16S or Coxl. In this study, we present a molecular phylogeny of cancroid and portunoid crabs based on 1200 basepairs of mtDNA, which partly confirms and partly contradicts current morphology-based taxonomy. In order to test the reliability of mtDNA, we constructed a second phylogeny based on a nuclear gene corresponding to the histone H3. This phylogeny absolutely confirmed our initial results. Based on this independent evidence, we argue that mitochondrial DNA should still be considered a tool with high resolution power in decapod molecular phylogenies up to the interfamilial level. In view of the relatively unstable taxonomic classification of the two studied superfamilies, which are in the process of being revised (three new systems over the past three years), we propose a new taxonomy for the Cancroidea and Portunoidea that is based on significant evidence from two molecular markers and in part finds further support in larval morphology.