On the identity of two Antarctic brooding nemerteans: redescription of Antarctonemertes valida (Bürger, 1893) and description of a new species in the genus Antarctonemertes Friedrich, 1955 (Nemertea, Hoplonemertea)

Abstract

Nemerteans (ribbon worms) constitute an abundant and occasionally conspicuous group of benthic invertebrates in the Southern Ocean. Although recent work has confirmed that this group is far more diverse than previously recognized, the Antarctic nemertean fauna remains poorly understood when compared to other geo- graphic regions. In most cases, the taxonomic information on the known nemertean fauna is incomplete for this region and/or has been inappropriately documented. As a conse- quence, many of the species described are considered species inquirendae. Among the nearly 50 species described so far for the Southern Ocean, two hoplonem- erteans are known to brood eggs in cocoons: Amphiporus incubator Joubin, 1914 and Amphiporus michaelseni Burger, 1895a. Here, we redescribe Antarctonemertes valida (Burger, 1893), a senior synonym of A. michaelseni, and describe a new congeneric species, Antarctonemertes riesgoae sp. nov. Both species show a similar reproductive strategy by brooding their cocoons, and similar external appearance, but clearly differ in other aspects of their morphology, such as the cephalic coloration pattern and the number of proboscidial nerves. We provide novel infor- mation about their life habitus, reproductive behaviour, internal anatomy, and their phylogenetic placement within hoplonemerteans using one nuclear (28S rRNA) and two mitochondrial (cytochrome c oxidase subunit I (COI) and 16S rRNA) markers. We also provide a parsimony haplo- type network using 16S rRNA, COI, and the internal transcribed spacer region 2 (ITS-2) showing a clear dis- tinction between individuals of both species. Our results stress the need of combining molecular and morphological information when dealing with closely related species of nemerteans.