Dysponetus caecus ( Langerhans, 1880 )

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 : 315-317

Dysponetus caecus ( Langerhans, 1880)

Figures 3A, B .

Chrysopetalum caecum Langerhans, 1880: 278–279 , NE Atlantic, Madeira Island.— Laubier, 1964: 125–138, Mediterranean, 32 m.

Dysponetus caecus Dahlgren and Pleijel, 1995: 159–173 , NE Atlantic, Mediterranean, intertidal to 85 m. — Bőggemann, 2009: 283– 296, East Atlantic , Angola Basin, to 5494 m .

Material examined. Dysponetus caecus NE Atlantic , Cape Verde Archipelago , Senghor Seamount , 17º21.82'N 21º57.93'W, North transect, Core 1511 #11, 3241 m, coll. SAMS, Oct 2009, SMF 22964 .

Description. Anterior fragment with 13 segments, 3.4 mm long, 1.6 mm wide. Streamlined body, with tapered anterior end. Transparent to silvery notochaetal spines in long fascicles covering dorsum; neurochaetae extend out beyond notochaetae. Prostomium rounded to quadrate, with glandular, ovoid, unpigmented patches on the prostomium, lateral antennae broken, medial papillae (median antenna?) present; 2 ventrolateral palps with broad bases, subulate tips, moderate length. Elongate, single lobe present on posterior margin of mouth; elongate pharynx to segment 7–9 with pair of slender, red-brown stylets (fig. 3A).

Anterior segment I very contracted, with 2 pairs of cirri, dorsal tentacular cirri broken, ventral tentacular cirri present. Segment II biramous with notochaetae and dorsal cirri, neurochaetae, no ventral cirri; notopodia of segment III with notochaetae and dorsal cirri, neuropodia with subulate ventral cirri.

Notochaetal spines long, especially mid-body; with 2 rows of long spinelets. Notopodia with elongate dorsal ceratophores; cirrostyles mostly broken. Shorter dorsal cirri on anterior segments become longer after segment 5. Compound neurochaetae with slender shafts with bifid tip at joint and long, slender, finely serrate blades, minute blade tips unidentate to bifid. Very long-shafted, specialised swimming neurochaetae, numbering 4–6 insert in superior-most position (fig. 3B).

Remarks. In the absence of extant type material of Chrysopetalum caecum ( Langerhans, 1880) from Madeira Island, NE Atlantic, Dahlgren and Pleijel (1995) designated a neotype from southern France, Mediterranean. The authors redescribed the species and placed it within the genus Dysponetus . More recently Böggemann (2009) described Dysponetus caecus from abyssal depths off Angola, West Africa, South-east (SE) Atlantic.

Dysponetus caecus Senghor Seamount and Madeira Island specimens of Langerhans (1880: Fig. 9C) have moderate length palps. Palps are lost in Böggemann’s specimens of abyssal material from Angola (2009: Figs 20A, B). All Mediterranean material described by Laubier (1964: Fig. 1A ) and Dahlgren and Pleijel (1995: Fig. 3A ) have longer palps. The arrangement of segments of the anterior end, based primarily on Mediterranean material, and agreed on by Laubier (1964) and Dahlgren and Pleijel (1995), are as follows: segment 1 with 2 pairs of cirri; segment 2 uniramous with notochaetae and dorsal and ventral cirri; segment 3 biramous with dorsal and ventral cirri and chaetigerous lobes. Segment 1 of Senghor material agrees with the above but segment 2 is biramous with chaetigerous lobes and dorsal cirri but no ventral cirri. There appears no sign that ventral cirri were broken off from neuropodia 2, although cirri are fragile and easily lost in dysponetids. More entire material would be needed for confirmation.

A marked increase in notochaetal length has not been observed before in Dysponetus (CW, pers. obs.). These longer notochaetae appear in D. caecus from Senghor Seamount in segments 9–13, the same segments that possess epitokous neurochaetae (fig. 3A). Slender, non-epitokous neurochaetal blades of D. caecus appear spinigerous under the light microscope. Only on highest magnification do the tips of neurochaetae appear unidentate or bifid within the same individual (also observed by Dahlgren and Pleijel (1995)). Neuropodia are very slender with a compressed, dense neurochaetal fascicle. Simple neurochaetae, described in D. caecus ( Dahlgren and Pleijel, 1995) , were not discerned.

Epitokous swimming neurochaetae, similar to those described in planktonic adults of Arichlidon species ( Watson Russell, 1998, 2000), have been observed in Dysponetus gracilis Hartman, 1965 from deep waters of the NE Atlantic by Aguirrezabalaga et al. (1999) and in gametogenic undescribed species of Dysponetus and Pseudodysponetus ( Böggemann, 2009) from southern Australia (CW, unpubl. obs.). These extended, very long-shafted and bladed, compound chaetae insert in a superior position within the neurochaetal fascicle and are recorded for the first time in the male D. caecus from Senghor Seamount (fig. 3B).

Very little is known of the larval stages of Dysponetus species. The only two instances recorded are of benthic larvae of Dysponetus pygmaeus ( Watson Russell, 1987) and planktonic larvae of Dysponetus cf. pygmaeus (Yokouchi, Fig. in litt.).

Dysponetus caecus can be separated from its congeners based on a few combinations of characters. However, it is clear that within D. caecus there are a number of morphological and ecological disparities between Mediterranean and NE Atlantic forms, e.g. palp length and anterior segment formulae; and large depth differences reported between regions e.g. intertidal in Mediterranean to ~ 5000 m off Angola. Morphological revision and genetic analysis of fresh material would help to resolve whether NE Atlantic and Mediterranean Dysponetus caecus , as presently understood, is a single species or a complex of cryptic species.

Habitat and distribution. At Senghor Seamount Dysponetus caecus occurs among the least-biomass and fine clay-like sediments recorded at the base in ~ 3000 m depths ( Chivers et al., 2013). The nominal distribution of D. caecus is currently from 52ºN to 19ºS in the East Atlantic, including the Mediterranean. Dysponetus caecus has been collected from hard and soft substrates, from 1 m to depths of over 5000 m ( Dahlgren and Pleijel, 1995; Böggemann, 2009).

Figures

Figure 1. A, Map of Senghor Seamount, located in the Cape Verde Archipelago, NE Atlantic. Data extracted from Smith and Sandwell (1997); dataset created by A. Dale (SAMS). B, Senghor Seamount with the location of transects. Data and map created by Thor Hansteen and Alexander Schmidt, GEOMAR. (A, B, reproduced from Chivers et al., 2013.)

Figure 3. A–B: Dysponetus caecus, Senghor Seamount, SMF 22964. A, Anterior end, dorsal view, slide preparation; B, neuropodium XII with superior swimming neurochaetae. Scalebars: A-B, 100µm.