Terebellides Group C (sensu Nygren et al. 2018)

Figs 3, 4, 9, 10, 11, 12, 13

Description.

The morphological features of the examined species in Group C in this paper (clade 24) are itemised below. Some of these are also shared by Groups A, B, and D as defined in Nygren et al. (2018) (see Remarks below). Clade 25 will be studied elsewhere; formal description of clade 22 will wait until more material is available.

Body appearance. Incomplete individuals ranging from 10.0-17.0 mm in length. Body tapering posteriorly with segments increasingly shorter and crowded towards pygidium. Prostomium compact; large tentacular membrane surrounding mouth (Fig. 3D), with typical buccal tentacles with expanded tips (Fig. 3D). SG 1 as an expanded structure below tentacular membrane in a lower lip (Fig. 3D).

Branchiae. Branchiae arising as single structure from SG 3, with a single stalked mid-dorsal stem, lobes not fused (Fig. 4C). Dorsal lobes ending posteriorly in short terminal papilla (Fig. 3D) and ventral lobes ones ending in long filaments. Anterior projection of dorsal lobes (fifth lobe) present. Posterior end of dorsal lobes reaching TC 4. Ciliary rows of cilia and ciliary tufts in inner face of branchial lamellae not observed. Ciliary papillae absent in branchial lamellae margin.

Thorax. Eighteen pairs of notopodia (SG 3-20) (Fig. 3D), those of TC 1 approximately as long as subsequent ones (Fig. 4C). Lateral lappets and dorsal projections of notopodia in anterior thoracic chaetigers with different degree of development depending on size and preservation conditions, but both more conspicuous on TC 1-5. All notochaetae as simple capillaries. Size of notochaetae of TC 1 similar to subsequent ones. Neuropodia as sessile pinnules from TC 6 to body end, with uncini in single rows, from TC 7 throughout. Neuropodia on TC 6, provided with several sharply bent, acute-tipped, geniculate chaetae (Fig. 13B) with minute teeth forming a capitium only visible with SEM (Fig. 13B). From TC 7, neuropodia with one row of uncini per torus (Fig. 13C), with long shafted denticulate hooks, with large main fang (rostrum) longer than upper crest of teeth (capitium), rostrum/capitium length ratio of ~ 2:1, capitium composed by several teeth above main fang of decreasing length (Fig. 13D).

Abdomen and pygidium. Approximately half as long as thorax and progressively thinner (Fig. 3D). Neuropodia ranging from 18-20 chaetigers and forming erect pinnules with several uncini per torus, number depending on specimen size. Uncini provided with several teeth above rostrum surmounted by a capitium composed of several teeth of decreasing length (Fig. 13E, F). Pygidium blunt, as funnel-like depression.

Colour pattern. Colour in preserved specimens whitish (Fig. 3D). MG staining pattern characterised by compact green colourant in SG 1-4, then turning into striped pattern in SG 5-14 and fading in following segments (Fig. 12).

Remarks.

Among the above-mentioned characters, branchial features might serve to distinguish most of Group C species from those of Groups A, B and D. Those include branchial lobes size, presence of filaments in ventral ones and lobes which are not fused. Other taxa such as Terebellides mira Schüller & Hutchings, 2013 and T. rigel Schüller & Hutchings, 2013 also bear branchiae with similar shape (Parapar et al. 2016a).