Aphelochaeta cf. longisetosa ( Hartmann-Schroeder , 1965)

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 cf. longisetosa ( Hartmann-Schroeder, 1965) Figures 21 , 22 , 24

Materials.

Sample 16MFC, 459 m, -49.268443, -59.085888, coll. 25/04/2012, ind. 1, NHM.2018.21707; Sample 60MFA, 450 m, -49.254527, -59.049453, coll. 18/03/2012, ind. 1, NHM.2018.21754; Sample 20MFB, 451 m, -49.286853, -59.140553, coll. 23/04/2012, ind. 2, NHM.2018.12702-12703.

Description.

Specimen 16 MFC anterior fragment 16.3mm long with 79 chaetigers, specimen 60 MFA 11mm long with 67 chaetigers. Body widening gradually in thorax region, dorsally hump-backed, thoracic chaetigers crowded, narrow, wider than long, becoming longer and more flaccid in abdominal chaetigers; ventrally, groove present from mid thorax down the body (Fig. 21b ).

Prostomium conical, bluntly pointed. Peristomium wider than long, domed dorsal crest over first two annulations of peristomium, three annulations seen laterally and horizontally (Fig. 21a, c ). Dorsal tentacles separated, situated laterally on distal edge of peristomium and first chaetiger, dorsal and proximal to first branchiae (Fig. 21c ). Following branchiae situated dorsal to notochaetae separated by at least one branchial diameter from dorsalmost notochaetae.

Chaetae positioned laterally in thoracic segments. Thoracic notochaetae of two sizes, long capillaries approximately up to 10 chaetigers long, giving specimens a‘silky’ appearance, long chaetae similar in length perhaps slightly longer in distalmost thoracic chaetigers; thoracic neurochaetae slightly shorter; 7-10 notochaetae and 6-8 neurochaetae per fascicle. Abdominal notochaetae fewer in number than neurochaetae and less dense than in thoracic chaetigers; a few long chaetae appear from anterior thoracic chaetigers and occur down body into the abdominal chaetigers, shorter capillaries less than six chaetigers in length also present.

Pygidial region expanded, pygidium with lobes the ventralmost enlarged and pointed (Figs 21d ; 24a )

Methyl Green stain pattern.

Falkland Islands specimens with band or spot on ventral prostomial region extending into mouth, diffuse staining on sides of peristomium, distinct partial bands ventrally on thoracic chaetigers along edges of the chaetigers, becoming denser in mid and proximal thoracic chaetigers, some banding dorsally on thoracic chaetigers (Fig. 22 ).

Remarks.

Aphelochaeta cf. longisetosa ( Hartmann-Schröder, 1965) was identified from a number of samples during the contract, but only a few specimens have so far been registered in the Museum’s collection. The Falkland Islands specimens closely resemble Hartmann-Schröder’s holotype, particularly in possessing the long silky chaetae (see Table 3 ). The Falkland Islands specimens differ in the shape of the body; they show a distinct staining pattern which is absent in A. longisetosa ; however, this might be due to difference in the age of the specimens, and the abdominal notochaetae are fewer in number than neurochaetae and less dense than in thoracic chaetigers. Given these observations and that the Falkland Islands lie outside the previous range of A. longisetosa we are taking a cautious position and assigning the specimens to A. cf. longisetosa .

Aphelochaeta cf. longisetosa resembles A. falklandica sp. nov. having similar MGSP but differs in the shape of the body, A. cf. longisetosa has a near cylindrical body with only a slight widening in the proximal thoracic region and with no obvious junction between the thorax and abdomen, whereas in A. falklandica sp. nov. the thorax is flat with the body widening in the thoracic region and narrowing again at the junction between the thorax and abdomen; finally, the pygidium has an extended lobe with a pointed tip, whereas A. falklandica sp. nov. has a rounded ventral lobe. Such differences appear to be consistent and so it has been decided to consider them as separate species.

In terms of other known Aphelochaeta species in the region, A. cf. longisetosa resembles A. malefica Elias & Rivero, 2009, in the body shape as the species from the River Plate has a body more uniform in width. However, the two species differ in that the peristomium is much shorter in the Falkland Islands specimens, only as long as wide, whereas in A. malefica Elias & Rivero, 2009 the peristomium is longer than wide in the position of the dorsal tentacles and first branchiae and that the latter also has the peristomium extending back displacing the first chaetiger, while A. cf. longisetosa the junction is not displaced proximally. The MGSP for the two species is also different. The Falkland Islands species have only diffuse staining in the peristomium and prostomium while A. malefica has these areas darkly stained.

Distribution.

NW Falkland Islands at depths of 400-500 m.

Figures

Figure 21. Aphelochaeta cf. longisetosa Falkland Islands specimen a dorso-lateral view of anterior of specimen b ventral view of anterior c close-up showing arrangement of dorsal tentacles and first branchiae d pygidium.

Figure 22. Aphelochaeta cf. longisetosa Falkland Islands specimen - MGSP plate a ventrolateral view of anterior end b dorsal view of anterior end.

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.