A revision of the genus Petta Malmgren, 1866 (Annelida: Pectinariidae), with two new species from deep waters of southeastern Australia, and comments on phylogeny of the family.

Petta Malmgren, 1866 is a small and poorly known genus of the annelid family Pectinariidae Quatrefages, 1866. Prior to this study, the genus comprised four species P. pusilla Malmgren, 1866 (type locality Gullmarsfjord, west coast of Sweden), P. assimilis McIntosh, 1885 (type locality between Prince Edward and Kerguelen Island, southern Indian Ocean), P. pellucida (Ehlers, 1887) (type locality Santarem Channel between Cay Sal Bank and Bahamas, Caribbean Sea) and P. tenuis Caullery, 1944 (type locality Sulu, Philippines, tropical Pacific Ocean), the two last ones were known only from the original description. We revised the genus by re-examining the types and providing updated illustrated re-descriptions of its species, except for P. assimilis of which the type material has been lost. Commonly used morphological characters of the genus are expanded to also include new ones such as the presence of pair of lateral ear-shaped lobes adjacent to dorsal base of cephalic veil, pair of ventral lappets on segment 1, pair of dorso-lateral pads on segment 5, large basal hump on branchiae, and a rounded anterior peg with a blunt tip and a longitudinal row of two major teeth on uncini. The type species P. pusilla is recognised as having four lappets on the anterior margins of cephalic veil and a large lower lip posterior to buccal cavity. Two species P. investigatoris n. sp. and P. williamsonae n. sp. are described from deep water off the coast of southeastern Australia and represent the first records of this genus in Australian waters. A phylogenetic position of one new species was assessed in the framework of a phylogeny based on a fragment of the mitochondrial cytochrome oxidase I gene (mtCOI). An updated taxonomic key to Pectinariidae genera and all species of Petta is given.