Terebellides Group A (sensu Nygren et al. 2018)
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https://dx.doi.org/10.3897/zookeys.992.55977 |
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lsid:zoobank.org:pub:0F038B5B-120E-4583-8E85-4092C9798566 |
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https://treatment.plazi.org/id/C08F6C59-B495-5836-AA9E-9EA4CACE44F1 |
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Terebellides Group A (sensu Nygren et al. 2018) |
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Terebellides Group A (sensu Nygren et al. 2018)
Description.
The morphological features shared by all studied species in Group A are itemized below. Some of these are also shared by Groups B, C and D as defined in Nygren et al. (2018) (see Remarks below).
Body appearance.
Complete individuals ranging from 10.0-50.0 mm in length. Body tapering posteriorly with segments increasingly shorter and crowded towards pygidium (Fig. 14A-C View Figure 14 ). Prostomium compact; large tentacular membrane surrounding mouth (Figs 5C View Figure 5 , 14B View Figure 14 ), with typical buccal tentacles with expanded tips (Figs 15A View Figure 15 , 20A View Figure 20 ). SGI as an expanded structure below tentacular membrane in a lower lip (Figs 14C View Figure 14 , 15A View Figure 15 , 22A View Figure 22 , 24A View Figure 24 ).
Branchiae. Branchiae arising as single structure from SGIII, with a single stalked mid-dorsal stem (Figs 5A View Figure 5 , 11C View Figure 11 , 15A View Figure 15 ), one pair of dorsal (upper) partially fused lobes (Figs 11B View Figure 11 , 15B View Figure 15 , 20A View Figure 20 ), and a pair of shorter ventral (lower) lobes (Fig. 5A, B View Figure 5 ) obscured or not by dorsal ones (Figs 5A, C View Figure 5 , 15A, B View Figure 15 ). Both dorsal and ventral branchial lobes ending each posteriorly in short terminal papilla (Fig. 20B View Figure 20 ). Anterior projection of dorsal lobes (fifth lobe) present but short (Fig. 5A, B View Figure 5 ) and usually obscured by tentacular membrane and buccal tentacles (Fig. 14A, C View Figure 14 ). Posterior dorsal lobes reaching TC4 (Figs 3 View Figure 3 , 4 View Figure 4 , 19 View Figure 19 ). Branchial lamellae provided with several parallel rows of cilia in inner face (Fig. 15C View Figure 15 ); ciliated papillae not present, ciliary tufts present, sometimes not clearly visible (Fig. 5B, D View Figure 5 ).
Thorax. Eighteen pairs of notopodia (SGIII-SGXX) (Fig. 14B, D View Figure 14 ), those of TC1 approximately as long as following ones (Figs 20A View Figure 20 , 22A View Figure 22 ) or slightly shorter (Fig. 15A View Figure 15 ). 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 TC2-4/5 (Figs 15A View Figure 15 , 22A View Figure 22 ). All notochaetae as simple capillaries (Figs 11F View Figure 11 , 15A View Figure 15 ). Neuropodia as sessile pinnules from TC5 or TC6 to body end, with uncini in single or double rows, from TC7 throughout. Neuropodia on TC5 or TC5 and TC6, provided with several sharply bent, acute-tipped, geniculate chaetae (Figs 16B View Figure 16 , 23A View Figure 23 ) with minute teeth forming an ill-defined capitium only visible with SEM (Figs 12B View Figure 12 , 25B View Figure 25 ). From TC7, neuropodia with one or several rows of uncini per torus (Figs 16C View Figure 16 , 23C View Figure 23 ), with long shafted denticulate hooks, with large main fang (rostrum) longer than upper crest of teeth (capitium), which is composed by several teeth above main fang of decreasing length (Figs 23D View Figure 23 , 25D, E View Figure 25 ).
Abdomen and pygidium. Approximately half as long as thorax and progressively thinner (Fig. 14B View Figure 14 ). Neuropodia ranging from 18-38 chaetigers and forming erect pinnules (Figs 6F View Figure 6 , 12F View Figure 12 ) with several uncini per torus, number depending of specimen size. Uncini provided with several teeth above rostrum surmounted by a capitium composed of several teeth of decreasing length (Figs 6G View Figure 6 , 16E View Figure 16 , 21F View Figure 21 ). Pygidium blunt, as funnel-like depression.
Colour pattern. Colour in preserved specimens pale brown (Fig. 3 View Figure 3 ). MG staining pattern 1 sensu Schüller and Hutchings (2010: 10, fig. 4) and characterised by compact green colouration in CH1-3, then turning into striped pattern in CH4-12 and fading in following segments.
Remarks.
Among the aforementioned characters, branchial features might serve to distinguish most of Group A species (except for A3 species) from those in Groups B-D. Those include branchial size, lobes size (i.e., whether dorsal and ventral are of similar size or differ), presence of terminal papilla/filament on posterior lobes, and presence of ciliary structures (rows, tufts or buttons) on lamellae. Other taxa described or reported worldwide bear similar branchiae including T. stroemii sensu Parapar et al. (2011) from Iceland and sensu Parapar et al. (2013) from the Adriatic Sea, T. kerguelensis McIntosh, 1885 and T. longicaudatus Hessle, 1917 from Antarctic latitudes ( Parapar and Moreira 2008a, 2008b), and T. kobei Hessle, 1917 from Japan ( Imajima and Williams 1985).
The other species groups as found in Nygren et al. (2018) were not studied in depth here and will be the aim of a subsequent study. However, Group B seems to be characterised by having a shorter body and free branchial lobes; these features are shared with T. atlantis Williams, 1984 and T. irinae Gagaev, 2009 as already suggested by Nygren et al. (2018). Members of Group C are apparently not defined by any unique shared morphological character but show the same geographic distribution as T. irinae . Finally, the three putative species in Group D were related to T. gracilis Malm, 1874 and T. williamsae Jirkov, 1989 by Nygren et al. (2018) even though the latter was proposed to be synonymised with the former by Parapar et al. (2011). These species seem characterised by having ventral white colouration in a number of anterior chaetigers and similar-sized branchial lobes; these characters are not shared with Group A.
Regarding Group A, six morphological characters have been considered to delineate subgroups and species (Table 1 View Table 1 ). Two characters can be determined with the aid of the STM: 1) general branchial shape, 2) number of thoracic chaetigers with geniculate chaetae; four characters require SEM examination: 3) presence of papillae on lamellae of dorsal branchial lobes, 4) presence of ciliated papillae dorsal to thoracic notopodia, 5) features of thoracic and 6) abdominal uncini shape dentition. Branchial typology (1) is defined according to Parapar et al. (2016c) and thoracic uncini (5) follows Parapar et al. (2020). Typology of abdominal uncini (6) is described here (see Discussion).
Furthermore, species will be also characterised according to geographic and bathymetric distribution according to available data.
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