Thrausmatos senghorensis, Watson & Chivers & Narayanaswamy & Lamont & Turnewitsch, 2014

Watson, Charlotte, Chivers, Adam J., Narayanaswamy, Bhavani E., Lamont, Peter & Turnewitsch, Robert, 2014, Chrysopetalidae (Annelida: Phyllodocida) from the Senghor Seamount, north-east Atlantic: taxa with deep-sea affinities and morphological adaptations, Memoirs of Museum Victoria 71, pp. 311-325 : 312-315

Thrausmatos senghorensis sp. nov. Watson, 2014

Zoobank LSID. http://zoobank.org/ urn:lsid:zoobank.org:act:

B2A32582-0DCC-43C5-840C-6DBC9B1935D4

Figures 2A–D .

Material examined. Holotype: NE Atlantic, Cape Verde Archipelago, Senghor Seamount, East transect, 17º09.66'N 21º53.12'W, some dead coral, 1656.5 m, Core #01, coll. DZMB, Oct 2009, SMF 22963 .

Paratypes: same details as holotype, Core 517 #08, coll. SAMS, 1, NMS.Z.2013.160.01; 1, NTM W25388 .

Description. Based on holotype, an anterior end of 15 segments, length 2.5 mm, width 1.35 mm. Prostomium with subulate median and two lateral antennae; two palps with ovoid bases, subulate distal halves with broad, rounded tips; ovoid caruncle; eye pigment absent. Segment I achaetose with 2 pairs of long cirri; segment II with 2 pairs of long cirri, notochaetal fascicle; segment III biramous with dorsal and ventral cirri, noto- and neurochaetae. Prostomium, caruncle, all ceratophores darker coloured, appear glandular; body epidermis dense with small, rounded structures, probably bacteria. Elongate pharynx with pharyngeal papillae and posterior muscular bulb, extends to segments 8–9 (figs. 2A, B).

Pale golden palaeal notochaetae insert in fans that cover the dorsum. Mid-body notochaetal fascicle with 2–3 short, pointed lateral palae with 5–6 ribs. Main palaea number 10–12 with 16–17 ribs and a couple of lightly raised ribs; medial main a little shorter with same number of ribs; widely spaced horizontal striae. Larval-type main palaea distally with broad ‘shoulders’; adult-type main palaea more slender with rounded ‘shoulders’; apices prominent (fig. 2C). Very thin, short dorsal acicula; slender dorsal cirri shorter or same length as fan. Neurochaetae number about 30; with long blades and bifid tips. Specialised superiormost fascicle with 2 short falcigers with large basal serrations, inserts supra to overlying long, robust ventral acicula (see asterisk indicating position, fig. 2C; detail, fig. 2D). Mid-superior, middle and inferior neurochaetal groups with very finely serrate falcigerous blades with tiny bifid tips; long, slender ventral cirri (fig. 2C).

Remarks. Elaboration of body shape and posterior end is not possible as all type material is fragmented. The two paratype specimens are both composed of anterior ends of seven segments and display no deviations in chaetal morphology from the holotype.

Thrausmatos species are deep-sea dwellers found only at depths> 1000 m. Thrausmatos dieteri Watson, 2001 was originally described from hydrothermal vents and seeps from Fiji and New Guinea, SW Pacific. Thrausmatos is a new record for the Atlantic and T. senghorensis sp. nov. is the first record from a nominal non-chemosynthetic habitat.

Thrausmatos senghorensis individuals are smaller bodied than those of T. dieteri and differ in: the more rounded shape of the main palaea and their lack of numerous heavy raised ribs; lesser number of lateral palaea (3 vs. 5–6); shorter dorsal cirri; short falcigers rather than long spinigers of the specialised neurochaetal superior fascicle (fig. 2D); and absence of pronounced ventral pads. It is very difficult to discern gametes with the opacity of the thick epidermis, which is covered in multiple rounded structures resembling bacteria (fig. 2C). This was also observed in T. dieteri ( Watson, 2001) .

Thrausmatos senghorensis is found at the Senghor Seamount in depths of between 1000 and 3000 m, where ferromanganese crusts are formed at the interface of waters of the oxygen minimum zone and deeper waters ( Wang et al., 2011). Although there is no indication of vents or seeps in the area (Chivers, unpublished data), a megacore sample from mid-slope depths on the East transect revealed numerous barnacle plates, suggesting a former vent community that had collapsed ( Christiansen et al., 2010). It is possible that the presence of T. senghorensis at Senghor Seamount indicates past or as yet undetected hydrothermal activity.

The specialised neurochaetal fascicle appears to be a permanent structure in both small and large individuals of Thrausmatos species. These compound chaetae insert in a superior position overlying the ventral acicula of the neurochaetal fascicle. They are much shorter than the transient, long fascicle observed in gametogenic swimming individuals of Arichlidon and Dysponetus . Larval stages of Thrausmatos species are not yet documented.

Distribution and habitat. Thrausmatos senghorensis is found at Senghor Seamount, NE Atlantic, at ~ 1600 m, among bare volcanic rock and patches of predominantly fine sand and shell fragments.

Etymology. The species name, senghorensis , is named after Senghor Seamount.

Figures

Figure 2. A–D: Thrausmatos senghorensis sp. nov., Senghor Seamount, SMF 22963. A, Anterior end, dorsal view, slide preparation; B, anterior end, dorsal view; C, mid-body parapodium, slide preparation; D, detail of superior-most neurochaeta (asterisked in fig. 2C). Scalebars: A-C, 100 µm; D, 10µm.