Phalacrostemma, Marenzeller, 1895

Zhang, Jinghuai, Hutchings, Pat, Burghardt, Ingo & Kupriyanova, Elena, 2020, Two new species of Sabellariidae (Annelida, Polychaeta) from the abyss of eastern Australia, Zootaxa 4821 (3), pp. 487-510 : 502

Phalacrostemma sp.

Fig. 10

Material examined. St. 128, 23º37.872´– 23º 39.54´S 154º39.582´– 154º38.628´E, 1761–1770 m, beam trawl, 13 th June 2017, AM W.50676, 1 spec. (tissue sample taken) .

Description. Body incomplete, reddish-brown preserved in alcohol, gradually tapering to posterior end ( Fig 10A ). Body length 4.0 mm from paleae to parathoracic segments, 0.9 mm maximum in body width.

Opercular lobes completely fused with distal end perpendicular to longitudinal axis ( Fig 10B ). Operculum with 12 pairs of broken outer paleae; outer paleae golden as simple spines with pointed tips and compact thecae with straight, not expanded margins ( Fig. 10A , E–G).

One pair of inner paleae present, amber, with smooth surface and tapering tips.

Eight pairs of robust and tapering opercular papillae, peripheral to outer paleae ( Fig. 10 B–C), not extending to tip of outer paleae.

Two pairs of flattened nuchal hooks, with poorly developed limbation on concave side ( Fig. 10D ). A medial organ present at dorsal junction of opercular lobes. Eyes not observed. Tentacular filaments absent ( Fig. 10 B–C). A pair of buccal flaps present ( Fig. 10B ). A pair of short robust palps, not extending to operculum ( Fig. 10B ).

Thoracic segment 1 (chaetiger 1) with pair of long and tapering neuropodial cirri and capillary chaetae ( Fig. 10 B–C). Thoracic segment 2 (chaetiger 2) with one pair of broad triangular lateral lobes between branchia and neuropodia ( Fig. 10C ). Eight pairs of dorsal tapering branchiae present from thoracic segment 2, largest ones on parathoracic segments, reducing posteriorly in size from abdominal segment 8.

Segments 3–6 (parathoracic) with two types of notochaetae arranged in transverse rows; about 4–6 robust nonretractile lanceolate chaetae interspersed with some fine capillaries ( Fig. 10A, H ). Segments 3–6 with two types of neurochaetae arranged transversely, about four fine lanceolate chaetae interspersed with several fine short capillaries. Notopodial lanceolate chaetae much stouter than those in neuropodia ( Fig. 10A ).

Abdominal notopodia as erect expanded tori, with number of uncini within torus decreasing posteriorly. Neuropodia with long fine capillaries. Cauda lost ( Fig. 10A ). Tube unknown.

Remarks. This single specimen from 1761–1770 m is different from the specimens of P. timoharai n. sp. It has 12 pairs of outer paleae (but many are broken), a pair of inner paleae, two pairs of nuchal hooks and non-retractile lanceolate notopodial chaetae on segments 3–6 ( Fig. 10F & Table 3). While the specimen clearly represents a new species as also confirmed by molecular results ( Fig. 11 ), it is too damaged to be described formally as a new species. We have included it here to highlight that an additional species of this genus is present in the Coral Sea.

Figures

FIGURE 10. Photos of Phalacrostemma sp. (AM W.50676). A. Anterior end, lateral view. B. Close-up view of buccal cavity with palps, ventral view. C. Close-up view of anterior end, lateral view. D. Nuchal hooks. E–G. Close-up view of distal (E), middle (F) and proximal (G) parts of outer paleae. H. Notochaetae from segment 6. Abbreviations: bf, buccal flaps; cac, capillary chaeta; lac, lanceolate chaetae; nec, neuropodial cirri; nh, nuchal hooks; op, outer paleae; pa, palp; s, segment.

FIGURE 11. Bayesian majority rule consensus phylogram of the family Sabellariidae based on the concatenated dataset of COI, 16S, 18S and 28S sequences. Outgroups: Marenzellaria viridis and Lepidonotus squamatus. Numbers next to branches are posterior probabilities obtained from the Bayesian analysis.