Nothria otsuchiensis Imajima, 1986 complex

Figs 1, 24, Tables 2, 3, S 1

Nothria otsuchiensis Imajima, 1986: 108, fig. 8.— Imajima 1999: 46, fig 26.— Budaeva & Paxton 2013: 1492, figs 11–17.

Material examined (21). AM W.49014, IN 2015_C01_110 (1). AM W.49037, IN 2015_C01_110 (1). AM W.49038, IN 2015_C01_114 (1). AM W.49015, IN 2015_C01_117 (1). AM W.49025, IN 2015_C02_196 (1). AM W.49026, IN 2015_C02_330 (1). AM W.49939, IN 2017_ V 03_128 (1). AM W.51635, IN 2018_ V 06_184 (1). AM W.51636, IN 2018_ V 06_184 (1). AM W.51637, IN 2018_ V 06_184 (1). AM W.51638, IN 2018_ V 06_184 (1). AM W.53847, IN 2018_ V 06_184 (10).

Diagnosis. Prostomium anteriorly rounded to subtriangular. Small anterior and large posterior eyes present or absent. Palpo- and antennophores with 2–3 rings, median antennae extending to chaetiger 4–5, lateral to 3–4. Branchiae with short to long filaments, from chaetiger 8–9, most often 9. First 2 chaetigers enlarged, parapodia directed anteriorly, often surpassing peristomium. Last postchaetal lobe on chaetiger 15. Robust bidentate simple and slender pseudocompound hooks on chaetiger 1; slender bidentate simple and pseudocompound hooks on chaetiger 2; bidentate pseudocompound to compound hooks on chaetiger 3; pectinate chaetae with about 20 teeth and limbate chaetae from chaetiger 3; subacicular hooks from chaetiger 10–15.

Remarks. All of our specimens are anterior fragments measuring 1.9 to 3 mm in width. Nothria otsuchiensis was described from Otsuchi Bay, in 48–79 m depth based on the holotype and 27 paratypes. The complete holotype measured 18 mm in length for 49 chaetigers and 3 mm in width including parapodia, the morphological characteristics are here presented in Table 3. In a monograph of the Onuphidae from Japan, Imajima (1999) reported numerous additional records of the species from Otsuchi Bay to Kagoshima Bay, in 26–1070 m depths. In spite of the huge increase of material examined and range of distribution and depth, the diagnostic features were almost identical. The only widening of parameters were the start of branchiae from chaetiger 9–10 rather than 10, and subacicular hooks from chaetiger 10–12 rather than 10 in the original description. In a study of ontogenetic variation of diagnostic characters Budaeva & Paxton (2013) examined a large number of N. otsuchiensis from off eastern Australia in 24–2900 m and a single specimen from New Caledonia in 440–450 m depth. Again, the morphological characteristics were basically the same, showed only a slightly wider range in the length of the antennae, last postchaetal lobes and start of subacicular hooks from the original description (Table 3). As far as diagnostic characters could be evaluated, in spite of the large geographical and depth ranges all specimens examined appeared to match the description of N. otsuchiensis .

In the present study we examined material from three research cruises to the Great Australian Bight and eastern Australia in 400–1761 m depth with similar results (Table 3). Several specimens were sequenced as discussed in the “Molecular Results” section. We obtained several clades, indicating that we are dealing with a species complex that can only be resolved with a deeper molecular investigation of a larger number of specimens .

Distribution. Eastern Australia and Great Australian Bight, in 400–1761 m depth (Fig. 24).