A recent molecular phylogenetic study on Glaucus, a genus of neustonic aeolid nudibranchs, revealed undescribed cryptic diversity. Glaucus atlanticus is sister to the traditional species Glaucus marginatus, which is a complex of four genetically distinct cryptic species (Informal clade ‘Marginatus’). The present paper revises the systematics of Glaucus and provides formal descriptions for three new species in the informal clade ‘Marginatus’ substantiated by species delimitation analyses. Molecular and morphological evidence confirms that the type species of Glaucus, Glaucus atlanticus, has a cosmopolitan subtropical distribution and is characterised by having a uniseriate ceratal arrangement, a penial spine and a longitudinal, medial silver stripe on the sole of the foot. Examination of type material indicates that the name G. marginatus should be retained for the most widespread of these species, found in the Indian and Pacific Oceans. This species is characterised by molecular diagnostic characters as well as the presence of a bursa copulatrix. Glaucus marginatus is sister to the undescribed species Glaucus bennettae, sp. nov., which is found in the South Pacific Ocean and lacks a bursa copulatrix. The other two undescribed species, Glaucus thompsoni, sp. nov. and Glaucus mcfarlanei, sp. nov. are only known from the North Pacific Ocean, and are characterised by molecular diagnostic characters as well as possessing and lacking a bursa copulatrix, respectively. Because sister species of Glaucus differ in their reproductive anatomy, we hypothesise that mating behaviour has played a role in cladogenesis in this group.

ZooBank Publication code:  http://zoobank.org/References/E352E264-A440-4AF1-8565-B57B7EEE25BC