Phylogeny of Thalassinidea (Crustacea, Decapoda) inferred from three rDNA sequences: Implications for morphological evolution and superfamily classification

The infraorder Thalassinidea is a group of cryptic marine burrowing decapods of which the higher taxonomy is often contentious. The present analysis attempts to reconstruct phylogenetic relationship among 12 of the 13 currently recognized families using partial nuclear 18S, 28S rDNA and mitochondrial 16S rDNA sequences. The infraorder is divided into two distinct clades, with the first clade consisting of Thalassinidae, Laomediidae, Axianassidae and Upogebiidae, and the second clade including Axiidae, Calocarididae, Eiconaxiidae, Callianassidae, Ctenochelidae, Micheleidae, Strahlaxiidae and Callianideidae. Within the first clade, the Upogebiidae is the basal family. The Axianassidae shows low affinity to other laomediid genera indicating that it is a valid family. The interfamilial relationships are less well resolved in the second clade. The Axiidae is paraphyletic with respect to Calocarididae and Eiconaxiidae. Thus, the status of these two latter families is not supported if the currently defined Axiidae is maintained. All three families appear to be basal in the thalassinidean clade. The Micheleidae is closely related to the Callianideidae and they form a sister group to the Strahlaxiidae. The monophyletic Callianassidae aligns with the Micheleidae + Callianideidae + Strahlaxiidae clade. The relationship among the Axiidae + Calocarididae + Eiconaxiidae clade, Callianassidae + Micheleidae + Callianideidae + Strahlaxiidae clade and the Ctenochelidae cannot be resolved which might be due to a rapid radiation of the three lineages. Our results do not support the generally used classification scheme of Thalassinidea and suggest that the infraorder might be divided into two superfamilies instead of three as suggested based on larval morphology, second pereiopod morphology in adults and gastric mill structure. The two superfamilies are Thalassinoidea (i.e. Thalassinidae, Laomediidae, Upogebiidae and Axianassidae) and Callianassoidea (i.e. Axioidea + Callianassoidea, as defined in Martin and Davis (2001) but excluding Laomediidae and Upogebiidae). It also appears that gill-cleaning adaptations are important in thalassinidean evolution while the presence of linea thalassinica is a result of parallel evolution.