Phylogeny of Rhizocephala (Cirripedia) based on the discovery of a new species of Thylacoplethus Coutière, 1902 (Cirripedia: Rhizocephala) parasitizing squat lobsters (Decapoda: Anomura: Munidopsidae)

Two specimens of the munidopsid squat lobster Munidopsis arietina Alcock & Anderson, 1894, collected off New South Wales, Australia (> 2,500 m depth), were parasitized by a thompsoniid rhizocephalan. This is the first record of Thompsoniidae Høeg & Rybakov, 1992, a family of parasitic barnacles characterized by extreme morphological simplification and extensive character loss, infesting a species of Munidopsidae. A combination of molecular evidence and morphological data obtained by both micro-computed tomography and light microscopy indicates that the parasites represent a new species of Thylacoplethus Coutière, 1902. This species is described herein with an assessment of its phylogenetic placement within Rhizocephala. Mitochondrial (COI, 16S) and nuclear (18S) markers reveal instability in the monophyly of both Thylacoplethus and Thompsonia Kossmann, 1872, as well as unresolved relationships among several other rhizocephalan genera and species in various families.