Arichlidon reyssi ( Katzmann, Laubier & Ramos, 1974 )

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 : 317-319

Arichlidon reyssi ( Katzmann, Laubier & Ramos, 1974)

Figures 4A, B .

Bhawania reyssi Katzmann, Laubier & Ramos, 1974: 313–317 , Fig. 1A–G . Type locality: Adriatic Sea.

Paleanotus heteroseta Rullier, 1964: 142–3 . Cape Verde Islands.

Chrysopetalum debile Cazaux, 1968: 536–541 . Arcachon, France (larvae).

Arichlidon reyssi Watson Russell, 1998: 159–176 , Figs 4C , 6G, H . Adriatic, Mediterranean, Cape Verde Islands.

Arichlidon reyssi Watson Russell, 2000: 465–477 , Fig. 1A–D . Eastern Mediterranean

Material examined: NE Atlantic, Cape Verde Archipelago, Senghor Seamount, East summit, 17º12.30'N 21º53.12'W, shelly sand, 133.6 m, Core 1510 #08, coll. SAMS, 14, NMS.Z.2013.160.02; 17º10.62'N 21º56.83'W, 103.1 m, coarse sediment, Core 1531 #11, coll . SAMS, 3, NMS.Z.2013.160.03; 17º12.29'N 21º57.69'W, 132.4 m, Core #01, coll . DZMB, 4, NMS.Z.2013.160.04; 17º10.62'N 21º56.84'W, 103.1 m, Core #01, coll . DZMB, 3, NMS.Z.2013.160.05; 17º10.62'N 21º56.82'W, 102.7 m, Core #01, coll . DZMB, 1, NMS.Z.2013.160.06; East summit, 17º12.30'N 21º57.70'W, 133.6 m, shelly sand, Core 1510 # 12, coll . SAMS, 2, NMS.Z.2013.160.07; East summit, 17º12.30'N 21º57.70'W, shelly sand, 133 m, Core1509 #02, coll . SAMS, 2, NMS.Z.2013.160.08; 17º09.66'N 21º53.12'W, dead coral, 1656.5 m, Core 1517 #08, coll . SAMS, 2, NMS.Z.2013.160.09; East summit, 17º12.30'N 21º57.70'W, shelly sand, 133.6m,Core1510 #10,coll . SAMS,3, NMS.Z.2013.160.01; 17º10.62'N 21º56.82'W, Core #04, 102.7 m, coll . DZMB, 2, SMF 22965 ; 17º1210.62'N 21º56.84'W, Core #05, 103.1 m, coll. DZMB, 6, SMF 22966 ; 17º12.29'N 21º57.69'W, Core #07, 132.4 m, coll. DZMB, 17, SMF 22967 ; 17º10.62'N 21º56.84'W, Core #08, 103.1 m, coll. DZMB, 2, SMF 22968 ; 17º10.62'N 21º56.82'W, Core # 10, 102.4 m, coll. DZMB, 8, SMF 22969 ; 17º12.29'N 21º57.69'W, Core 864 #02, 132.4 m, coll. DZMB, 10, NTM W 025386 ; East summit, 17º12.30'N 21º57.70'W, Core 1509 #01, shell sand, 133 m, coll. SAMS, 3, NTM W25387 .

Description. Largest individual measuring 50 segments, length 5.0 mm and width 1.1 mm. Body relatively short, broad, with silver to pale-golden palaeal fans, often with brownish scale bands, covering dorsum. Prostomium with two pairs of violetblack eyes often fused, forming rectangular block visible beneath palaea of anterior segments (fig. 4A). Segment I with two pairs of dorsal and ventral tentacular cirri; segment II with palaeal notochaetae, dorsal cirri, neurochaetae, ventral cirri absent. Lateral palaea fascicle intergrades smoothly with main palaea fascicle; distinctive group of asymmetrical ornate median palaea interlock middorsal line forming smooth convex ridge. From segment VI median group palaea number 3–5. Long lateral-most median palae appears first at segment VI and continues down body as tallest palae in entire fan (fig. 4B). Dorsal surface of notochaetal palae with tubercules and raised serrate ribs. Neurochaetae comprising superior group of spinigers; mid group with strongly dentate falcigers; inferior group falcigers with short, broad, curved articles with smooth to minutely serrate margin and blunt tip.

Remarks. One character, not reported on previously, and observed in 67 individuals of Arichlidon reyssi at Senghor Seamount and in material of all other re-examined Arichlidon species, is a distinctive paired structure at a level near the top of the pharynx. It is composed of two small, brownish, triangular structures sitting opposite each other, either side of the pair of stylets (fig. 4A). When dissected they spill out densely packed, tiny, golden-brown globules; function currently unknown. They are reminiscent of the oil globules common to many chrysopetalid species that occur as larger, singular globules inside parapodia (Watson, 2012).

Epitokous swimming neurochaetae, described from both benthic and planktonic adults in all nominal Arichlidon species ( Watson Russell, 1998, 2000), were not seen in any Arichlidon reyssi individuals in the present study.

Adult specimens of Arichlidon reyssi from the Cape Verde Archipelago (Maio, Brava and Boavista Islands) in sponge, shell and sediment samples, depth 20–425 m, were included in the description of the new genus Arichlidon and a redescription of A. reyssi from the Adriatic and Mediterranean Seas and NE Atlantic ( Watson Russell, 1998). Arichlidon reyssi specimens observed in this present study from Senghor Seamount morphologically agree with the former Cape Verde material examined in all characters of body shape, size, colouration, notochaetal and neurochaetal characters, including numbers of palaeal ribs and chaetal types.

Previously, Arichlidon reyssi have been collected in moderately large numbers (e.g. 82 individuals from one station) and over large depth ranges (10–4000 m) in the Eastern Mediterranean ( Watson Russell, 1998). At Senghor Seamount, A. reyssi ranges from the summit at 102 m to mid-slope depths of over 1000 m. In both cases, no discernible morphological differences were found between individuals at different depths.

Arichlidon is one of a number of chrysopetalid taxa that possess primarily cryptic species with a very conservative morphology. Watson Russell (2000) described a new species, Arichlidon gathofi from the western Atlantic, and compared it with A. reyssi on the basis of one character in particular. In A. reyssi , the long lateral-most median palae, with a higher number of ribs, is taller than the main fan (fig. 4 B); in A. gathofi , the lateral-most median palae, with a slightly lesser number of ribs, is the same height or shorter than the main fan (fig. 6E). This singular median-palae is evident in mid-body segments in juvenile and adult material examined and dissected from both species ( Watson Russell, 2000: 476). In order to identify chrysopetalid larvae to species, it is essential to study chaetal patterns throughout the entire body. In the interests of distinguishing Atlantic Arichlidon larvae to species, and to elaborate on the sequence of changes in the morphology of planktonic to benthic individuals, larvae of A. reyssi and A. gathofi are described below.

Distribution and habitat. Benthic adults of Arichlidon reyssi are found from the Mediterranean, NE Atlantic coast, and the islands and seamount of the Cape Verde Archipelago. Among the Senghor Seamount chrysopetalid fauna, A. reyssi comprises the largest number of individuals, which predominantly dwell in coarse sediments at the summit at ~ 100 m, among the largest polychaete biomass recorded. There is also one record from mid-slope at 1651 m.

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 4. A–B: Arichlidon reyssi, adult, Senghor Seamount, NMS.Z.2013.160.09, slide preparations. A, Anterior end; B, mid-body notopodium from anterior end. Scalebars: A, 100 µm; B, 50µm.

Figure 6. A–F: Arichlidon gathofi, 7-segmented larva, Carolina, West Atlantic, USNM 186017. A, Entire larva, dorsal view; B, ventral view of A; C, notopodium segment IV; D, notopodium segment VI (figs A, C, after Watson Russell, 1987: Figs 28.4, 6: as ‘new genus 1’). E, A. gathofi, adult, mid-body notopodium, detail median fascicle; F, mid-body neuropodium with epitokous swimming neurochaetae (figs E, F after Watson Russell, 2000: Figs 1D, 5A). Scalebars: A, 200µm; B, 350µm; C-E, 40 µm; F, 100µm.