Terebellides nkossa

Parapar, Julio, Martin, Daniel & Moreira, Juan, 2020, On the diversity of Terebellides (Annelida, Trichobranchidae) in West Africa, seven new species and the redescription of T. africana Augener, 1918 stat. prom., Zootaxa 4771 (1), pp. 1-61: 38-43

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Terebellides nkossa

sp. nov.

Terebellides nkossa  sp. nov.

Figures 11CView FIGURE 11, 28AView FIGURE 28, 29–32View FIGURE 29View FIGURE 30View FIGURE 31View FIGURE 32, 37View FIGURE 37; Tables 1, 2 urn:lsid:zoobank.org:act:8C262B7A-F8EC-4ABC-A6AE-538652C71D1B

Material examined. Type material. Fifty specimens: holotype ( MNCN16.01View Materials /18594); forty-nine paratypes ( MNCN16.01View Materials /18592, 16.01/18593 and 16.01/18595 to 16.01/18610). Non-type material. Five dried-out specimens ( MNCN16.01View Materials /18611 and MNCN16.01View Materials /18612); one specimen (NHMD-231434) (Table 1)  .

Diagnosis. Body small sized (5–15 mm in length); semi-transparent body wall. Branchial anterior lobe absent; posterior lobes not fused; ventral ones thinner and shorter than dorsal ones, with long terminal filament, directly emerging from branchial stem. TC1 notopodia and notochaetae smaller than following ones. Thoracic neurochaetae with rostrum / capitium length about 1/1 and capitium with a first row of 4–7 medium-sized teeth followed by progressively shorter ones.

Description based on holotype

Measurements and general body features. Complete specimen, with coelomic cavity filled with oocytes, 12 mm long and 1.0 mm wide ( Fig. 28AView FIGURE 28, 29AView FIGURE 29); body tapering posteriorly with segments increasingly shorter and crowded towards pygidium. Prostomium compact; tentacular membrane surrounding mouth, with a low number of buccal tentacles ( Fig. 29AView FIGURE 29). SGI as an expanded structure below tentacular membrane (lower lip) ( Fig. 29AView FIGURE 29, 30AView FIGURE 30). Lateral lappets weakly developed on SGIII–VII ( CH 1–5).

Branchiae. Branchiae arising as single structure from SGIII, with a single stalked mid-dorsal branch having of one pair of dorsal (upper) lobes not fused along their lengths, and a pair of ventral (lower) lobes much shorter, neither fused together nor to dorsal ones ( Fig. 28AView FIGURE 28, 29AView FIGURE 29, 30View FIGURE 30 A−B, 32A). Anterior projection of dorsal lobes (lobe 5) not present. Pointed projection of posterior region of both upper and lower lobes much longer in ventral ones, becoming a long terminal filament ( Fig. 30View FIGURE 30 A−B, 32A). Both sides of branchial lamellae with several parallel bent rows of cilia and several ciliated tufts near outer edge ( Fig. 30View FIGURE 30 C−D).

Thorax. Eighteen pairs of notopodia (SGIII−XX); those in TC1 slightly smaller than subsequent ones ( Fig. 30AView FIGURE 30, E−F, 32A). Neuropodia as sessile pinnules, from TC6 (SGVIII) to pygidium, with uncini in single rows start- ing from TC7 (SGIX) throughout. First neuropodia (TC6) with 3–4 sharply bent, acute-tipped, geniculate chaetae with well-developed minute teeth forming a capitium ( Fig. 31AView FIGURE 31). From TC7, neuropodia with up to 7–9 uncini per torus in one row ( Fig. 31BView FIGURE 31), with long-shafted denticulate hooks, rostrum about same length as capitium, with 4−7 medium-sized teeth above main fang, surmounted by 4−6 slightly shorter teeth ( Fig. 31CView FIGURE 31, 32View FIGURE 32 C−D).

Abdomen. 29−30 pairs of neuropodia, as erect pinnules with about 20 uncini per torus ( Fig. 31View FIGURE 31 D−E) having three teeth above main fang surmounted by two teeth above one in middle, an upper crest of two teeth and a variable number of smaller teeth ( Fig. 31FView FIGURE 31).

Other features. Paired, mulberry-like nephridial papilla behind first thoracic notopodia and dorsal to each SGVI and VII ( CH 4 and CH 5) notopodium ( Fig. 30AView FIGURE 30, E−F, 32A−B). Pygidium blunt, as funnel-like depression.

Methyl green staining pattern. Anterior CH 1 to CH 4 solid; CH 5 to CH 11 striped; CH 12 and CH 13 only neuropodia slightly stained; pattern 1 of Schüller & Hutchings (2010) ( Fig. 37View FIGURE 37).

Variability. Most specimens of T. nkossa  sp. nov. are complete, with well-defined branchiae, ranging from 5 to 15 mm long, showing intra-coelomic oocytes from 9-mm long specimens ( Fig. 29DView FIGURE 29). The digestive tract main parts, especially the hind stomach, can be easily seen by transparency through the delicate body wall ( Fig. 29CView FIGURE 29). The only specimen from the Atlantide cruise (NHMD-231434), identified as T. stroemii  by Kirkegaard (1959), comes from shallow waters (108 m depth) off Sierra Leone, measures 7 mm in length and overall agrees with T. nkossa  sp. nov. description, except in having much shorter, comma-shaped branchiae, both features not present in our specimens of this species, and therefore it was labelled as T. af. nkossa  .

Type locality. Gabon; 43 m depth (Table 1)  .

Distribution and bathymetry. Gabon and Democratic Republic of Congo; 43−250 m depth ( Fig. 11CView FIGURE 11; Table 1).

Etymology. The species is named after the N’Kossa gas field located off Congo, where most specimens were collected.

Remarks. The diagnostic characters above mentioned clearly distinguishes T. nkossa  sp. nov. from all other species here described. However, it shares some branchial features with six previously described species: Terebellides mundora Hutchings & Peart, 2000  ( Australia), Terebellides atlantis Williams, 1984  (New England slope), Terebellides parapari Lavesque, Hutchings, Daffe, Nygren & Londoño-Mesa, 2019  , T. sepultura  , T. irinae  and T. shetlandica  .

Terebellides mundora  differs from T. nkossa  sp. nov. in having branchiae with posterior ventral lobes almost as long as dorsal ones and thoracic uncini with rostrum shorter than capitium, while the new species has ventral lobes much shorter and rostrum longer than capitium; T. sepultura  has a very large branchial stalk (short in T. nkossa  sp. nov.), and T. irinae  has very long thoracic notopodial chaetae (short in T. nkossa  sp. nov.).

The Atlantic species T. atlantis  , T. shetlandica  and T. parapari  closely resemble T. nkossa  sp. nov. in their small body size ( T. atlantis  : 8–18 mm; T. shetlandica  : 06– 19 mm; T. parapari  : 8–23 mm and T. nkossa  sp. nov.: 5–15 mm) and several other morphological features. Thus, even though the brief original description of T. atlantis ( Williams, 1984)  does not allow accurate comparisons, the Icelandic specimens described by Parapar et al. (2011) differ from T. nkossa  sp. nov. in the size and shape of dorsal branchial lobes vs. ventral ones (which are all highly deciduous in the former) and the shape of thoracic uncini, which show the capitium endowed with minute and numerous teeth (type 4 in Parapar et al., 2016c) (long and scarce in T. nkossa  sp. nov., type 3 in Parapar et al., 2016c). Terebellides shetlandica  and T. parapari  most closely resemble T. nkossa  sp. nov. in general branchial shape with ventral branchial lobes only fused at base with dorsal ones and provided with a terminal filament and thoracic uncini. Furthermore, all three species were found at a similar depth range: 100–160 m for T. shetlandica  , 120 m for T. parapari  and 43–250 m for T. nkossa  sp. nov. Nevertheless, T. nkossa  sp. nov. mostly differs from T. shetlandica  in the number of uncini per abdominal neuropodium (6–7 vs. ca. 20, respectively), and from T. parapari  in having smaller TC1 notopodial and notochaetae than following (much longer in T. parapari  ) and having ciliated papillae dorsal to thoracic notopodia and ciliature in branchial lobes (absent in T. parapari  ).

Other species with four-lobed branchiae are: Terebellides ehlersi McIntosh, 1885  ( Fiji Islands), Terebellides intoshi Caullery, 1944  ( Indonesia) and Terebellides lobatus Hartman & Fauchald, 1971  (New England). Nevertheless, all of them are much larger than T. nkossa  sp. nov. (30−40 mm vs. 5−15 mm) and T. intoshi  also bears two chaetigers with geniculate chaetae (cfr. Imajima & Williams, 1985), instead of one.