Robustochelia pacifica, Larsen, Kim, 2011

Larsen, Kim, 2011, Tanaidaceans (Crustacea) from the Central Pacific Manganese Nodule Province. I. The genera Collettea, Robustochelia and Tumidochelia, ZooKeys 87, pp. 19-41 : 31-35

Robustochelia pacifica sp. n. Figs 810

Material examined.

Holotype manca II (MNHN-Ta1030) SUB-CStation CAROT-0 # 18, 05/06-2004, 14°02.6665'N, 130°05.9948'W, 5045 m.

Diagnosis.

Manca II. Carapace longer than the lengths of pereonites 1 and 2 combined. Pereonites 2-5 longer than wide. Pleotelson longer than combined length of three pleonites. Mandibular molar tapering. Maxillipedal endites with medial setae. Cheliped propodus and proximal part of fixed finger with calcified keel; fixed finger with clear tubercle distal from the keel; dactylus with dorsoproximal seta. Pereopods 1-3 propodus setae only reaching unguis insertion. Uropods as long as pleotelson.

Male. Unknown.

Etymology.

Named after the Pacific Ocean.

Description.

Manca II (body and appendages of holotype).

Body (Fig. 8A) length 1.7 mm. About ten times as long as wide (owing to the curved nature of the holotype, body morphometric calculations are based on the lateral view).

Cephalothorax more than 1.5 times as long as wide. Shorter than the lengths of pereonites 1 and 2 combined. Eye-lobes absent.

Pereon lateral margins almost straight. Pereonite 1 and 6 wider than long. Pereonites 2-5 longer than wide.

Pleon short (including pleotelson about 0.25 times total body length).

Pleotelson longer than the lengths of three free pleonites combined, apex blunt and covered by dorsal plate.

Antennule (Fig. 9A) 0.67 times as long as carapace, with four plus one minute terminal articles. Article 1 shorter than rest of antennule, with three setulose setae medially. Article 2 longer than article 3, with two setulated and one simple distal setae. Article 3 shorter than article 4, with one simple distal seta. Article 4 longer than article 2, with two long and one short simple distal setae, with one very wide distal aesthetasc. Article 5 minute, with two aesthestascs and one seta.

Antenna (Fig. 9B) with six articles. Almost as long as antennule. Article 1 wider than long, naked, not significantly wider than other articles. Article 2 longer than article 3, with one simple dorsal and ventral setae. Article 3 longer than article 5, with one simple dorsal seta. Article 4 longer than other articles, with three setulose and one simple distal setae. Article 5 as long as article 2, with two simple distal setae. Article 6 minute, with four distal setae.

Mouthparts: Labrum (Fig. 9C) finely setose, with flat apex. Mandibular molar tapering, ending in a few small spines. Left mandible (Fig. 9D) lacinia mobilis longer and broader than incisor; incisor blunt. Right mandible (Fig. 9E) incisor with medial depression. Labium (Fig. 9F) as wide as maxillipedal endites, outer corners with only a few setules. Maxillule (Fig. 9G) endite with 12 spiniform distal setae; palp with two terminal setae. Maxilla (Fig. 9H) elongated and featureless. Maxilliped (Fig. 9I) endites without denticles but with two distal setae, the medial one longest; palp article 1 with one outer seta, article 2 with three inner setae, article 3 with three setae on inner margin, article 4 with five setae. Epignath not recovered.

Cheliped (Fig. 10A, B) basis shorter than carpus, naked. Merus prominent with one simple ventral seta. Carpus appears twisted in relation to propodus, shorter than propodus including fixed finger, with two simple ventral setae and prominent distal process. Propodus massive, with one seta at dactylus insertion and ventral calcified keel. Fixed finger with keel only on proximal part, but with a clear tubercle distally from the keel, with one ventral seta and two on inner margin, and with one large distal process. Dactylus as long as fixed finger, with small medial process and small dorsoproximal seta.

Pereopod 1 (Fig. 10C) coxa with one simple seta. Basis longer than the three succeeding articles combined. Ischium with one ventral seta. Merus as long as carpus, widening distally, naked. Carpus shorter than 0.5 times propodus, with two spiniform and one simple distal setae. Propodus with spinnules and one distal seta on both margins and dorsal spine. Dactylus and unguis combined shorter than propodus, not fused, dactylus shorter than unguis.

Pereopod 2 (Fig. 10D) as pereopod 1 except: coxa apparently without seta. Merus with ventral seta. Carpus with simple ventral seta.

Pereopod 3 (Fig. 10E) as pereopod 1 except: basis with ventromedial setulose seta.

Pereopod 4 (Fig. 10F) with no visible coxa. Basis about as long as the three succeeding articles combined, naked. Ischium naked. Merus with one simple ventral seta. Carpus with two small spiniform and one simple distal setae. Propodus longer than carpus, with two spiniform ventral and one simple dorsal setae. Dactylus and unguis combined shorter than propodus, not fused, with spinnules on both margins. Unguis still longer than dactylus, but shorter than in pereopod 1-3.

Pereopod 5 (Fig. 10G) as pereopod 4 except merus with two spiniform setae and carpus with three.

Pereopod 6 absent in manca II.

Pleopods absent in manca II.

Uropod (Fig. 8B) longer than pleotelson. Basal article naked and about as long exopod. Endopod with weak traces of a medial pseudo-articulation, with two medial setulose setae, one subdistal seta and four distal setae. Exopod less than half of endopod length, with two distal setae of unequal length.

Remarks.

Bacause the studied specimen represent manca stageit is difficult to compare it with the other species of the genus. The carapace being longer than combined length of pereonites 1 and 2 and the weak armament of the pereopods (particularly the carpus) differentiate Robustochelia pacifica sp. n. from Robustochelia longa . As for Robustochelia angusticephala , this species is only incompletely described and nothing is known about the mouthparts so comparison with this species is difficult. However, the weaker fixed finger of Robustochelia pacifica and the shape of the cephalothorax indicates that these are not conspecific. Robustochelia pacifica most closely resemble Robustochelia robusta , but differs in the stout antennule (particularly article 4) and antenna, but also the more rectangular shape of the cephalothorax, the pereonites 2-5 being longer than wide, the large tubercle distally on the fixed finger, and the much longer pleotelson. Robustochelia pacifica differs from Robustochelia virilis by the pereonite 4 and 5 being longer than wide and by the pereopods 1-3 propodus setae not reaching longer than unguis insertion.

The species Robustochelia solida Larsen, 2005 was removed from the genus by Józwiak and Błażewicz-Paszkowycz 2007, based primary on the present of the present of a broad molar. This removal is undoubtly correct and thus clearly illustrate that the heavy cheliped is not a sufficient character for defining the genus Robustochelia .