Aphelochaeta falklandica Paterson & Neal, 2020

Neal, Lenka, Paterson, Gordon L. J., Blockley, David, Scott, Ben, Sherlock, Emma, Huque, Cate & Glover, Adrian G., 2020, Biodiversity data and new species descriptions of polychaetes from offshore waters of the Falkland Islands, an area undergoing hydrocarbon exploration, ZooKeys 938, pp. 1-86 : 1

Aphelochaeta falklandica Paterson & Neal sp. nov. Figures 23 , 24

Materials.

Holotype: Sample 17MFA, 449 m, -49.2679625, -59.0563564, coll. 15/04/2012, ind. 1, NHM.2018.21708. Paratypes: Sample 31MFA, 442 m, -49.3230311, -59.1674461, coll. 15/04/2012, ind. 1, NHM.2018.21728. Sample 44MFB, 429 m, -49.3585975, -59.1117606, coll. 14/04/2012, ind. 1, NHM.2018.21744. Sample 6MFC, 458 m, -49.2320192, -59.0316156, coll. 25/04/2012, ind. 2, NHM.2018.12714-12715. Sample 66MFA, 445 m, -49.258322, -59.1058241, coll. 18/03/2012, ind. 1, NHM 2018.25373. Sample T10MFB, 702 m, -53.059303, -58.026135, coll. 02/02/2009, ind. 1, NHM.2018.25370.

Description.

Holotype NHM.2018.21708, anterior fragment 14.1mm in length with 127 chaetigers. Body widening gradually in thorax then narrowing at junction with abdomen; dorsally flattened; ventrally a shallow groove runs down body from mid to posterior thorax. Thoracic chaetigers much wider than long, crowded; in abdomen chaetigers, as wide as long or longer, before becoming crowded in the pygidial segments.

Prostomium wide, conical with rounded or ‘stumpy’ point. Peristomium as wide as long, with domed dorsal crest; three annulations, first two obvious in lateral and ventral view, interrupted by dorsal crest, third complete dorsally and extending distally into first chaetiger. Dorsal tentacles arising between peristomium and first chaetiger, sometimes appearing to arise in the first chaetiger due to the peristomium extending back into the first chaetiger; dorsal to and on the same level or slightly anterior to first branchiae; second branchial pair close to first pair (Figs 23e ; 24e ).

Chaetae all simple capillaries, thoracic notochaetae of two basic lengths-relatively short capillaries ca. four chaetigers in length or equivalent to half the chaetiger width, and longer chaetae of at least ten chaetigers length; long chaetae arising from posterior thoracic region occurring in subsequent chaetigers into abdominal region; 6-10 chaetae per fascicle sometime appearing as if in two groups. Thoracic neurochaetae short capillaries, similar in length as short capillaries in the notopodia; 5-6 chaetae per fascicle. Abdominal notochaetae longer than those of neuropodia, smaller numbers of chaetae in abdominal fascicles than in thorax.

Pygidium with last 18 segments expanded, anal opening with lobes, ventralmost lobe largest, blunt and extended (Figs 23f ; 24c ).

Methyl Green stain pattern.

Band ventral under the prostomium extending into the mouth, narrow bands along lateral and ventral edges of thoracic chaetigers (Fig. 23 ).

Remarks.

Aphelochaeta falklandica sp. nov. is a common species in the Sealion samples. It closely resembles A. cf. longisetosa ( Hartmann-Schröder 1965), differing in being more dorso-ventrally flattened with an obvious widening of the body in the thoracic region, with an obvious junction between the thoracic and abdominal chaetigers and in the blunt shape of the ventral anal lobe (see Figs 23f ; 24c ). In mixed samples thisspecies can often be distinguished by a full gut obvious in posterior thoracic and anterior abdominal chaetigers and, in fresh material, by the bases of branchiae in the posterior thorax showing as bright red dots. The domed dorsal crest is also characteristic.

Aphelochaeta falklandica sp. nov. closely resembles A. palmeri Blake, 2018 and A. spectabilis Blake, 2018 both recently described from Antarctic and South American waters; A. falklandica sp. nov. has a similar body shape to these species but appears to differ in the following ways: the definition of the annulation differs slightly in that with A. spectabilis the junctions of the annulations are only weakly defined whereas in A. falklandica sp. nov. the junctions of the annulations are distinct although can be less easy to see dorsally if the dorsal crest is well developed. The dorsal tentacles of A. spectabilis arise on the posterior part of the peristomium or in the junction between the peristomium and first chaetiger, with the first branchia arising on the first chaetiger. In A. falklandica sp. nov. the dorsal tentacle also arises in the junction between the first chaetiger and peristomium but the first branchia arises ventral to the dorsal tentacle and in the junction between the first chaetiger and the peristomium. Aphelochaeta falklandica sp. nov. has a ventral groove running from the mid to posterior thoracic region of the body. Aphelochaeta spectabilis does not have a ventral groove. In A. spectabilis thedevelopment and shape of the posterior chaetigers and pygidial region is fusiform and well developed with a simple pygidium with a rounded lobe, whereas in A. falklandica sp. nov. this region is not as fusiform and the pygidium has an extended blunt ventral lobe. The staining pattern for A. spectabilis differs from A. falklandica , being more developed occurring dorsally on the peristomium, there is a distinct patch posterior of the mouth and the first chaetiger. These patches are missing from A. falklandica sp. nov., but this species has patches on the lateral parts of the peristomium, there is a small band ventrally on the prostomium.

The shape of the prostomium in A. palmeri is pear-shaped, slightly indented at its junction with the peristomium; the prostomium in A. falklandica sp. nov. is conical and with a blunt rounded tip. The dorsal crest in A. falklandica is not always well developed and may extend only as far as the second annulation whereas the dorsal crest in A. palmeri is usually well developed extending to the end of the peristomium. Aphelochaeta palmeri has a well-developed posterior region with a fusiform shape and a blunt lobed pygidium. In A. falklandica sp. nov. this region is not so well developed and expanded. The staining pattern differs in that there are distinct bands ventrally in the prostomium and a large patch laterally on each side of the peristomium in A. falklandica sp. nov. which are absent in A. palmeri .

Aphelochaeta falklandica sp. nov. can be distinguished from the A. malefica Elias & Rivero, 2009 by the arrangement of the dorsal tentacles, third annulation of the peristomium and MGSP. The shape of the body suggests similarities with A. williamsae Blake, 1996 from California but A. falklandica sp. nov. differs in MGSP which have banding extending laterally while in A. williamsae they do not. Aphelochaeta falklandica sp. nov. differs from the other Californian species in that the dorsal thorax is flattened and ventrally more rounded, whereas most of Blake’s (1996) species are dorsal-ventrally rounded or have a rounded (hump-backed) dorsal thoracic shape. Table 3 gives the comparison of Aphelochaeta species that might be found in the Falkland Islands region.

Etymology.

This species is named after the Falkland Islands where it was discovered.

Distribution.

Recorded from the NW Falkland Islands at depths of 400-500 m

Figures

Figure 23. Aphelochaeta falklandica sp. nov. (holotype NHM. 2018.21708), Methyl Green Staining patterns a complete specimen b ventral view of thoracic chaetigers showing methyl green stained bands across the thorax c dorsal view of head and thoracic chaetigers d close up of head region e dorsal view of anterior of specimen stained with Shirla stain showing dorsal tentacles f detail of pygidial region showing anal lobes. Scale bars: 1 mm (a).

Figure 24. Comparison of Aphelochaeta falklandica sp. nov. and A. cf. longisetosa (Hartmann-Schroeder, 1965). Aphelochaeta cf. longisetosa a drawing of pygidium showing pointed ventral anal lobe b drawing of anterior showing rounded peristomium. Aphelochaeta falklandica sp. nov. c drawing of pygidium showing rounded ventral anal lobe d drawing of anterior showing domed dorsal peristomium and widened flat thoracic chaetigers e detail drawing of prostomium and peristomial area showing arrangement of dorsal tentacles and 1 st branchiae.