Cephalotrichella echinicola, sp. nov. (Palaeonemertea : Cephalotrichellidae), a new nemertean associated with sea urchins from Nha Trang Bay (South China Sea)

About 50 nemertean species have been reported to live in symbiotic relationships with other invertebrates, but only two hoplonemertean species are associated with echinoderms (starfish). The palaeonemertean Cephalotrichella echinicola, sp. nov. is described from samples collected in Nha Trang Bay, Vietnam, South China Sea. The species is the first known nemertean associated with sea urchins (Metalia sternalis and M. spatagus), living on both the oral and the aboral surfaces of the host and freely moving among its spines. The internal morphology of the new species is described based on histological sections and confocal laser scanning microscopy with phalloidin and antibody labelling. Sequences of three nuclear gene markers (18S rRNA, 28S rRNA, and histone H3) and two mitochondrial gene markers (16S rRNA and COI) were compared with those of other palaeonemertean species, and a phylogenetic analysis suggested that C. echinicola is closest to the free-living Cephalotrichella alba Gibson & Sundberg, 1992. Both the morphological data and the phylogenetic analysis provide additional evidence for distinguishing the families Cephalotrichidae and Cephalotrichellidae and support the rejection of Cephalotrichella as a junior synonym of Cephalothrix. A new diagnosis of the genus Cephalotrichella is given. http://zoobank.org/urn:lsid:zoobank.org:pub:091B5D56-71B2-4F4C-9AD8-F666B4610DE2