Ampharete undecima can be up to 5 mm long. It can be found in the Norwegian Sea and the Barents Sea at depths between 300 and 1800 meters.
Norwegian Sea, 72.367oN 14.895oE, 770 m.
Measurements: Up to 5 mm long.
Number of thoracic chaetigers, excluding paleae: 14
Number of thoracic uncinigers: 12
Number of abdominal uncinigers: 11
Prostomium: Divided into three lobes; eyespots absent.
Mouth tentacles: Papillose.
Branchiae: Four pairs of branchiae; three pairs of branchiae arranged in anterior transverse row without median gap, fourth pair of branchiae located slightly posterior to anterior row, between 2nd outermost and innermost branchiae of anterior row; bases of branchiae in anterior row completely fused, forming characteristic and well-marked edge above head in frontal view.
Paleae: Long with filiform tips; 9–12 chaetae on each side.
Special features: Anterior tip of prostomium with particularly strong colour when stained with methyl blue.
Pygidium: With two short lateral cirri and several small rounded papillae.
Tube: Loose mud with shell fragments
Norwegian Sea and Barents Sea; 300–1800 m (moderate depths in Barents Sea, deep in Norwegian Sea).
The small size of Ampharete undecima makes it difficult to see characters and the species can be confused with other small-sized species and juvenile specimens of larger ampharetids. In particular, Amythasides macroglossus and Anobothrus laubieri as well as juveniles of other Ampharete can be difficult to differentiate from this species. Ampharete undecima has four pairs of branchiae while Amythasides macroglossus and Anobothrus laubieri have three pairs. Ampharete undecima has 11 abdominal uncinigers while other Ampharete have 12 or more abdominal uncinigers.
Alvestad T, Kongsrud JA, Kongshavn K (2014). Ampharete undecima, a new deep-sea ampharetid (Annelida, Polychaeta) from the Norwegian Sea. Memoirs of Museum Victoria 71: 11-19. http://doi.org/10.24199/j.mmv.2014.71.02
Personal observations, Alvestad T.