Terebellides Group D (sensu Nygren et al. 2018)

Figs 2, 3, 4, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18

Description.

The morphological features shared by all examined species in Group D in this paper (clades 2 and 3) are itemized below. Some of these are also shared by Groups A, B, and C as defined in Nygren et al. (2018) (see Remarks below). Clade 15 will be studied elsewhere.

Body appearance. Complete individuals ranging from 5.0-34.0 mm in length. Body tapering posteriorly with segments increasingly shorter and crowded towards pygidium. Prostomium compact; large tentacular membrane surrounding mouth (Figs 2D-F, 3E, F), with typical buccal tentacles with expanded tips (Figs 2E, F, 3E, F). SG 1 as an expanded structure below tentacular membrane in a lower lip (Figs 2D, E, 3E, F).

Branchiae. Branchiae arising as single structure from SG 3, with a single stalked mid-dorsal stem (Figs 2D, E, 3E, F), one pair of dorsal (upper) partially fused lobes (Figs 2D, E, 3E, F), and a pair of shorter ventral (lower) lobes (Fig. 3E, F) obscured or not by dorsal ones (Figs 2D-F, 3E, F). Dorsal lobes ending posteriorly in short terminal papilla (Fig. 3E, F) and ventral lobes ending in long filaments. Anterior projection of dorsal lobes (fifth lobe) present (Fig. 2D-F). Posterior end of dorsal lobes reaching TC 4-5 (Figs 2D-F, 3E, F). Branchial lamellae provided with several parallel rows of cilia and ciliary tufts present in inner face (Figs 14B, C, 16B, C, 17B). Ciliary papillae absent on the margin of branchial lamellae.

Thorax. Eighteen pairs of notopodia (SG 3-20) (Fig. 2D, E), those of TC 1 approximately as long as subsequent ones (Fig. 2D, E). 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 (Figs 2D-F, 3E, F). White ventral colouration present on TC 1-4 (Figs 2D, 3E) or only on TC 4 (Figs 2E, F, 3F). All notochaetae as simple capillaries (Fig. 15A). Size of notochaetae of TC 1 similar to subsequent ones. Neuropodia as sessile pinnules from TC 6 to body end, with uncini in single or double rows, from TC 7 throughout. Neuropodia on TC 6, provided with several sharply bent, acute-tipped, geniculate chaetae (Figs 15B, 18A) with minute teeth forming a capitium only visible with SEM (Fig. 18A, B). From TC 7, neuropodia with one row of uncini per torus (Figs 15C, 18C), 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 (Figs 15D, 18D).

Abdomen and pygidium. Approximately half as long as thorax and progressively thinner (Fig. 2D, E). Neuropodia ranging from 18-44 chaetigers and forming erect pinnules (Figs 15E, 18E) 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 (Figs 15F, 18F). Pygidium blunt, as funnel-like depression.

Colour pattern. Colour in preserved specimens whitish or pale brown (Fig. 3E, F). MG staining pattern characterised by compact green colourant in SG 1-5 and SG 7-13, SG 6 white and SG 14 striped (Fig. 12).

Remarks.

Among the aforementioned characters, the white ventral colouration in anterior thoracic chaetigers may be a useful character to distinguish Group D species from those of Groups A-C. Other taxa described or reported worldwide showing this colouration pattern are Terebellides distincta Williams, 1984 and T. ceneresi Lavesque, Hutchings, Daffe, Nygren & Londoño-Mesa, 2019.