Anobothrus gracilis can be up to 47 mm long. It can be found along the entire Norwegian coast and Svalbard at depths between 30 and 300 meters.

Type locality

Koster Islands, Bohuslän, Sweden, 230 m.

Key characters

Anobothrus gracilis. Anterior end. br: branchiae, pa: paleae.

Anobothrus gracilis. 11th thoracic chaetiger. cb: ciliated dorsal ridge, nrp: neuropodium, ntp: notopodium.

Measurements: Up to 47 mm long.

Number of thoracic chaetigers, excluding paleae: 15

Number of thoracic uncinigers: 12

Number of abdominal uncinigers: 13

Prostomium: Trilobed, two groups of eyespots.

Mouth tentacles: Smooth.

Branchiae: Four pairs of branchiae in two groups without any median gap; three branchiae in each group in transverse row, fourth branchia behind the innermost of the three.

Paleae: Long with filiform tips, 12–14 chaetae on each side.

Special features: First pair of notopodia very small and easy to overlook; Notopodia of 11th thoracic chaetigers slightly elevated and joined by ciliated ridge across dorsum.

Pygidium: Without cirri.

Tube: Thin layer of secretion incrusted with mud.

Distribution

Entire Norwegian coast and Svalbard; 30–300 m.

Remarks

Holthe (1986a) reports A. gracilis down to 3000 m. This depth is questionable, at least in Norwegian waters.

Similar species

Anobothrus gracilis is often misidentified as Ampharete finmarchica due to very small first pair of notopodia. The worm appears to have 14 instead of 15 thoracic notopodia. Both species have 13 abdominal uncinigers. Ampharete finmarchica has paleae with curved tips while A. gracilis has paleae with filiform tips. 

Sources

Holthe T (1986a). Polychaeta Terebellomorpha. Marine Invertebrates of Scandinavia 7: 1-192.

Malmgren AJ (1866). Nordiska Hafs-Annulater. [part three of three]. Öfversigt af Königlich Vetenskapsakademiens förhandlingar, Stockholm 22(5): 355-410, plates XVIII-XXIX. https://biodiversitylibrary.org/page/32339631

Personal observations, Alvestad T.

Anobothrus gracilis. Posterior end. nrp: neuropodium, p: pygidial papillae.

Ampharete gracilis. Thoracic neuropodium with uncini.