The smile of Amphiporus nelsoni Sanchez, 1973 (Nemertea: Hoplonemertea: Monostilifera: Amphiporidae) leads to a redescription and a change in family

A common intertidal hoplonemertean species, Amphiporus nelsoni Sanchez, 1973, from Chile is re-described based on the investigation of material from the type locality and one other locality in Chile. The species is transferred to the genus Prosorhochmus Keferstein, 1862 (Prosorhochmidae) based on the presence of a dorsal epidermal fold, i.e., "prosorhochmid smile" on the bilobed head, truncated stylet basis, well-developed frontal organ with characteristic epithelial specialization, structure of the nephridial system and other characters of internal anatomy. Placement of this species into Prosorhochmus expands the geographical distribution of the genus, previously known only from the North Atlantic and Mediterranean, to the South Pacific. Interestingly, Prosorhochmus nelsoni (Sanchez, 1973) is characterized by separate sexes and oviparity, unlike all the other members of the genus, which combine hermaphroditism with ovoviviparity. We compare P. nelsoni to the other species of Prosorhochmus and reassess some of the morphological characters used in the systematics of the genus.