Portaratrum birdi, Chim & Tong, 2020

Chim, C. K. & Tong, Samantha J. W., 2020, Two new species of paratanaoid tanaidaceans of the family incertae sedis (Crustacea: Peracarida) from polymetallic nodule fields in the eastern Clarion-Clipperton Fracture Zone, Zootaxa 4758 (3), pp. 461-485 : 473-479

Portaratrum birdi n. sp.

( Figs 6–11 , Table 3)

Material examined. Holotype, non-ovigerous female, dissected and mounted on 32 slides ( ZRC 2020.0162 ), 3.4 mm, Clarion-Clipperton Fracture Zone ( CCFZ), OMS exploratory area, station S08 (12°11.404’N, 117°22.281’W), 4172 m, box corer (deployment # BC24), layer 2–5 cm, sieve mesh size 300 µm, ABYSSLINE II ( AB02 ) cruise, 14 March 2015; paratype, non-ovigerous female ( ZRC 2020.0163 ), 2.2 mm, layer 0–2 cm, other collection data as holotype; paratype, non-ovigerous female ( ZRC 2020.0164 ), 2.1 mm, station S11 (12°00.559’N, 117°22.819’W), 4148 m, box corer (deployment # BC25), layer 0–2 cm, sieve mesh size 300 µm, 14 March 2015.

Diagnosis. Pleotelson not narrower at join with pleonite-5. Labium with spiniform setae distally. Maxilliped endite with two setae distally; palp article-2 outer margin with long seta. Chela fixed finger with one large tooth distally. Pereopods merus and carpus with long spiniform setae (“bayonet spines” in Bird, 2014) that often extend beyond half the length of the succeeding article. Pereopods 1–3 carpus with simple seta dorsodistally; propodus without spinules ventrally; unguis tip rounded. Pereopods 4–6 ischium with two setae; dactylus with spinules ventrally. Pleopod basal article subovate; exopod biarticulate. Uropod exopod biarticulate, article-2 cap-like with long seta extending beyond endopod.

Description. Non-ovigerous female holotype.

Body ( Fig. 6A ) elongate, subcylindrical, about 8.5 times as long as wide, and fragile. Cephalothorax about 17% of body length, suboval in dorsal view, and with one minute seta posterior to antennule.

Pereon about 57% of body length; pereonites 1 and 6 shortest, wider than long; pereonites 2 to 5 of subequal length, almost as long as wide; pereonite-1 bearing one minute seta at each dorsoanterior corner.

Pleon about 26% of body length; all five pleonites of subequal length, each 0.3 times as long as wide, slightly rounded and with one simple seta laterally; pleonite-5 with a large ventrally-directed spur ( Figs 6C, 6D , 11A ); pleo- telson ( Figs 6A, C ) about 31% of pleon length, subpentagonal, not narrower at join with pleonite-5, with six simple setae dorsodistally, posterior end pointed but blunt, bearing four minute setae.

Antennule ( Fig. 6E ) with four articles, about 0.8 times as long as cephalothorax; article-1 longest, about 0.5 times as long as total length of antennule, with some setules proximally, five plumose and one simple setae subdistally; article-2 about 0.5 times as long as article-1, with four plumose and two simple setae distally; article-3 shortest, about 0.6 times as long as article-2, with one plumose and two simple setae distally; article-4 slightly longer than article-3, with one complex aesthetasc and six simple setae.

Antenna ( Fig. 6F ) with six articles, about 0.8 times as long as antennule; article-1 damaged during dissection and not examined; article-2 with one simple seta distally; article-3 about 0.8 times as long as article-2, with one simple seta distally; article-4 longest, about 0.7 times as long as total length of antenna, with fusion line (faint un- der light microscopy; Fig. 10 for CLSM image of paratype female ZRC 2020.0164), with one long plumose seta at mid-length and three plumose and three simple setae distally; article-5 about 0.4 times as long as article-4, with one simple seta distally; article-6 shortest, about 0.2 times as long as article-5, with seven simple setae.

Labrum ( Figs 7A ,A`) hood-shaped, densely covered by setules. Left mandible ( Figs 7B , 11B ) molar acuminate, with numerous fine setae distally; incisor bicuspid; lacinia mobilis large and conical. Right mandible ( Fig. 7 B`) damaged during dissection. Labium ( Figs 4C , 11C ) with one pair of lobes, each with some setules proximally and one minute spiniform seta and many setules distally. Epignath not examined. Maxillule ( Fig. 7D ) endite with eight spiniform setae and numerous setules distally and numerous simple setae along entire length; palp with two long terminal setae. Maxilla ( Fig. 7E ) subtriangular. Maxilliped ( Fig. 7F ) bases not fused distally, each with one simple seta distally; endite with one lobe, one rounded tubercle and two simple setae distally; palp article-1 naked; article-2 with two simple and one setulose setae on inner margin and one setulose seta on outer margin; article-3 with two short and two long simple setae on inner margin; article-4 with one simple seta on outer margin and five simple setae distally.

Cheliped ( Fig. 8A ) basis attached to a massive sclerite, slightly wider than long, with small posterior lobe and one dorsal seta; merus ventral margin as long as carpus margin, with one simple seta; carpus about 1.3 times as long as wide, with one simple seta dorso-proximally, one simple seta dorsodistally and two simple setae ventrally; propodus palm dorsal and ventral margins sub-parallel, with two simple setae and about four rows of microtrichia on inner margin near dactylus insertion ( Fig. 11D ), fixed finger with one large tooth on cutting edge distally, three simple setae on outer margin near cutting edge and two simple setae ventrally; dactylus with one simple seta on inner margin.

Pereopod-1 ( Fig. 8B ) coxa with one simple seta; basis slender, about 6.0 times as long as wide, naked; ischium short, less than 0.1 times as long as basis, with one simple seta ventrodistally; merus about 0.3 times as long as basis, with one long spiniform seta distally; carpus slightly longer than merus, with microtrichia along ventral margin and one short simple and two long spiniform setae distally; propodus about 0.5 times as long as basis, with microtrichia along ventral margin ( Fig. 11E ), one small spiniform seta dorsodistally and one long spiniform seta ventrodistally; dactylus and unguis naked, slender and slightly curved ventrally, combined length slightly longer than propodus; unguis about 1.3 times as long as dactylus, with rounded tip ( Fig. 11F ).

Pereopod-2 ( Fig. 8C ) similar to pereopod-1, except carpus with three long spiniform setae.

Pereopod-3 ( Fig. 8D ) similar to pereopod-2, except propodus with some setules along distal margin.

Pereopod-4 ( Fig. 8E ) coxa with one simple seta; basis slender, about 4.0 times as long as wide, with two plumose setae at mid-length where it is also widest; ischium short, less than 0.1 times as long as basis, with two simple setae ventrodistally; merus about 0.3 times as long as basis, with two long spiniform setae distally; carpus about 1.3 times as long as merus, with microtrichia along entire length and one short simple and three long spiniform setae distally; propodus about 0.4 times as long as basis, with microtrichia along entire length, one plumose seta dorsomedially, one small and one long spiniform setae dorsodistally, two long spiniform setae ventrodistally and many setules along entire distal margin; dactylus and unguis slender and almost straight, combined length about 1.4 times as long as propodus; dactylus with spinules along ventral margin ( Fig. 11G ); unguis about 0.8 times as long as dactylus.

Pereopod-5 ( Fig. 8F ) similar to pereopod-4, except basis with three plumose setae and propodus without dorsal plumose seta.

Pereopod-6 ( Fig. 8G ) similar to pereopod-5, except basis naked; propodus with three long spiniform setae dorsodistally, dactylus with one minute seta distally and unguis about 0.4 times as long as dactylus.

Pleopods ( Fig. 9A ) 1–5 similar; basal article subovate, about 2.0 times as long as wide, naked; endopod slender, about 5.5 times as long as wide, with one plumose seta subdistally and four very long feather setae distally; exopod slightly longer and wider than endopod, biarticulate, article-1 very short (not obvious under light microscopy; Fig. 10 for CLSM image of paratype female ZRC 2020.0164), article-2 bearing eight feather setae distally with a conspicuous gap between the two most exterior setae.

Uropod ( Fig. 9B ) basal article about 1.8 times as long as wide, naked; endopod biarticulate; article-1 about 0.8 times as long as basal article, with two plumose setae distally; article-2 about 1.2 times as long as article-1, with one simple seta medially and one plumose and five simple setae distally; exopod biarticulate, article-1 about 0.5 times as long as basal article, with one simple seta distally; article-2 about 0.1 times as long as article-1, with one very long simple seta distally.

Female paratypes ZRC 2020.0163 ( Fig. 6B ) and ZRC 2020.0164 ( Fig. 10 ): Similar to female holotype .

Etymology. This species is named in gratitude of Dr. Graham J. Bird for his prompt replies to our questions on tanaidacean taxonomy. Dr. Bird is a leading tanaidacean expert and his extensive work includes Portaratrum and related taxa.

Type locality. Station S08 of OMS exploratory area at the Clarion Clipperton Fracture Zone, 4172 m.

Distribution and habitat. Two stations (S08 and S11) of OMS at CCFZ, 4148–4172m; 0–5 cm layer of soft sediment with shear strength of 0.625 –2.250 kPa and surface nodule coverage of 0.0–23.8%.

Remarks. Portaratrum birdi n. sp. is the fourth species of the genus. It conforms to the genus diagnosis modified by Bird (2014), except that its pleotelson is not narrower at join with pleonite-5. The current species is extremely similar to P. holdichi Bird, 2014 , especially in having (1) one long seta on maxilliped palp article-2 outer margin; (2) one large tooth on chela fixed finger cutting edge; (3) two setae on pereopods 4–6 ischium; (4) biarticulate uropod exopod with cap-like article-2 ( Table 3). The major characters distinguishing P. birdi from P. holdichi include the (1) labium with spiniform setae; (2) pereopods merus and carpus with spiniform setae mostly extend- ing beyond half the length of succeeding article; (3) pereopods 1–3 with dorsodistal simple seta on carpus instead of propodus; (4) pereopod-4 propodus with one plumose seta dorsally; (5) pleopod exopod with two articles. The other differences between these two species involve the presence of minute setae and microtrichia, which can be easily overlooked under light microscopy. Despite their morphological similarities, P. holdichi was described from the ‘Northern Atlantic Boreal’ lower bathyal province, which is far removed from the ‘Equatorial Pacific’ abyssal province where P. birdi was collected ( Watling et al., 2013; Fig. 12 ).

Amongst other members of the genus Portaratrum , P. fascinatus was first described from the neighbouring abyssal province ‘ Chile, Peru and Guatemala Basins’. Interestingly, these two species are also the most different from each other, most notably P. fascinatus in having (1) antenna article-2 wider than long and bearing one spine and several setules; (2) antenna article-3 naked; (3) one long seta on maxilliped palp article-1 outer margin; (4) pereopods merus and carpus with most spiniform setae less than half the length of the succeeding article; (5) one seta on pereopods 4–6 ischium; (6) several small teeth on chela fixed finger cutting edge; (7) indistinct pereopod coxa; (8) pereopod-1 carpus bearing one spiniform seta; (9) pereopods 4–6 dactylus without spinules ventrally; (10) pereopod-6 propodus with only two dorsodistal setae; (11) uniarticulate uropod exopod.

The unusual cap-like article-2 of the uropod exopod is also present in L. biarticulata , and this characteristic was singled out by Bird (2014) as evidence for the close relationship between the genera Leptognathioides and Portaratrum . Furthermore, P. birdi shares some characters with L. biarticulata that are absent in congeneric species in Portaratrum , and these include (1) pleotelson not narrower at join with pleonite-5; (2) maxilliped endite with two setae distally; (3) pereopod-4 propodus with one plumose seta dorsally; (4) pleopod exopod with two articles. It is probable that Portaratrum is very closely related to Leptognathioides and most likely belong to the same family, but it can be easily distinguished by the large ventrally-directed spur on pleonite-5 and the small cheliped basis posterior process.

Figures

FIGURE 4. Unispinosus eopacificus n. gen., n. sp. Holotype female (A–F, H) and allotype male (G). (A–F) pereopods 1–6; (G) pleopod-1; (H) uropods, ventral view. Scale bar: 100 µm.

FIGURE 6. Portaratrum birdi n. sp. OMS exploratory area, Clarion-Clipperton Fracture Zone. (A, C–F) Holotype female (ZRC 2020.0162) and (B) paratype female (ZRC 2020.0163): habitus, (A) dorsal and (B) lateral views; (C) pleonite-5, pleotelson and uropod, lateral; (D) ventral spur on pleonite-5; (E) antennule; (F) antenna. Scale bars: 1 mm for (A, B); 100 µm for (C–F).

FIGURE 7. Portaratrum birdi n. sp. Holotype female. Labrum, (A) dorsal and (A`) lateral views; (B) left and (B`) right mandibles; (C) labium; (D) maxillule; (E) maxilla; (F) maxillipeds. Scale bar: 100 µm.

FIGURE 8. Portaratrum birdi n. sp. Holotype female. (A) Left cheliped, with setae and microtrichia on inner margin (denoted by asterisks *); (B–G) pereopods 1–6. Scale bars: 100 µm.

FIGURE 9. Portaratrum birdi n. sp. Holotype female. (A) pleopod-1; (B) uropod. Scale bar: 100 µm.

FIGURE 10. Portaratrum birdi n. sp. Paratype female (ZRC 2020.0164). Confocal laser scanning microscopy images of habitus ventral with insets highlighting article-1 of pleopod exopods and fusion line of antennae. Scale bars: 200 µm.

FIGURE 11. Portaratrum birdi n. sp. Holotype female. (A) pleonite-5 ventral spur; (B) left mandible; (C) labium spiniform setae; (D) chela inner margin with two simple setae and four rows of microtrichia; (E) pereopod-1 propodus microtrichia; (F) pereopod-1 unguis tip; (G) pereopod-4 dactylus spinulation. Scale bars: 50 µm.

FIGURE 12. Type localities of Portaratrum species. (A) P. afer; (B) P. fascinatus; (C) P. holdichi; (D) P. birdi n. sp.