Systematic status of the caridean families Gnathophyllidae Dana and Hymenoceridae Ortmann(Crustacea:Decapoda):a further examination based on molecular and morphological data

The four palaemonoid(sub)families Anchistioididae,Gnathophyllidae,Hymenoceridae,and Pontoniinae are similar in morphology,and all live in marine habitats.Their systematic relationships are controversial.In this study,we used sequences from a mitochondrial ribosomal gene(16S rRNA) and three nuclear genes(H3,NaK,and enolase) to explore the phylogenetic relationships of these four taxa.Our tree based on 43 species belonging to 28 genera shows that Gnathophyllidae and Hymenoceridae are nested within Pontoniinae.This result is consistent with evidence from larval morphology.The defining characteristics of Gnathophyllidae and Hymenoceridae,a vestigial or missing mandibular incisor process and a broadened third maxilliped,can also be found in Pontoniinae;conversely,on the basis of published species descriptions,gnathophyllids and hymenocerids meet most of the defining characteristics of Pontoniinae.The peculiar form of the third maxilliped in gnathophyllids and hymenocerids might be the result of adaptive evolution,as these particular features are also present in pontoniines.According to our phylogenetic tree,Anchistioididae are more remote from Pontoniinae,which is consistent with the distinct morphological differences in the pleopods.The pontoniine genera analyzed(together with Gnathophyllidae and Hymenoceridae) are divided into two clades.The members of Clade I exhibit primordial characteristics similar to those of the Palaemoninae,and might be direct descendants of the ancestor of the Pontoniinae;members of Clade II are more specialized.