New species of Melinna (Melinnidae, Annelida) from the Australian abyss with comments on M. albicincta, M. cristata and M. elisabethae

A new species of Melinna (family Melinnidae) is described from the abyssal depths off the east coast of Australia. All material was collected during the RV Investigator voyage “Sampling the abyss” (IN2017_V03) in May–June 2017 from Bass Strait, Tasmania/Victoria using a beam trawl at 4197–4133 m depth. The new species Melinna hamulus sp. nov. differs from all other species of Melinna by having around 45 abdominal segments, dorsal membrane with 13–17 projections, four pairs of branchiae completely free and occurring in depths of over 4000 m. Phylogenetic relationships between our new species and other species within the family Melinnidae were assessed using the nuclear 18S and the mitochondrial 16S and cytochrome oxidase subunit I (COI) gene fragments. The results revealed that Melinna hamulus sp. nov. was genetically distinct from all other species of Melinnidae. This is the first species of Melinna to be described from Australian waters. We provide a new standard for description of species of Melinna and provide further detail on three existing species: Melinna albicincta Mackie & Pleijel, 1995, Melinna elisabethae McIntosh, 1914 and the type species of the genus Melinna cristata (Sars, 1851) using morphological characters illustrated with light and scanning electron microscopy photographs. We provide a table with the main diagnostic characters of all described species of Melinna together with type localities and depths.