Stenotanais arenasi, Larsen, Kim, 2011

Larsen, Kim, 2011, The tanaidacean assemblage from the Central Pacific Manganese Nodule Province. II. The genera Stenotanais and Typhlotanais (Crustacea), Zootaxa 3088, pp. 39-53 : 40-43

Stenotanais arenasi n.sp.

( Fig. 1 – 2 )

Material examined. Holotype, male, dissected, (MNHN-Ta1033). St. CAROT-B, no 22, 06/06-2004. 14˚03.10’N, 130˚07.74’W. 4954 meters. No sediment date available.

Diagnosis. Male. Antennule article 4 only marginally longer than article 3, with fusion line. Cheliped carpal shield absent. Uropod basal article shorter than pleotelson and endopod; exopod biarticulated, shorter than endopod article 1, with one basally thick and flat seta. Female. Unknown.

Etymology. The species is named after Dr. Francisco Arenas who organises and leads the fieldwork collection expeditions at CIIMAR.

Description. Adult male, body length. 3.4 mm.

Body (fig. 1A) elongate, 13 times as long as wide. Carapace elongate, as long as pereonite 2, not wider than pereonites. Pereon with pereonites all longer than wide. Pereonite 1 somewhat shorter than pereonite 2. Pereonite 2 and 4 subequal, longer than other pereonites. Pereonite 3 as long as pereonite 5. Pereonite 6 shortest. Pleon short (17% of total body length). All pleonites subequal, with pleopods. Pleotelson with blunt apex, longer than last two pleonites combined.

Antennule (fig. 1B) shorter than carapace. Articles 1 and 2 twice as wide as article 4. Article 1 not much longer than article 2, with one simple and four setulose outer distal setae. Article 2 longer than articles 3 and 4 combined, almost square, with one robust outer and one small inner distal setae. Article 3 narrower than articles 1 and 2, marginally shorter than article 4, with one robust inner distal seta and outer row of setules. Article 4 about 0.25 times as long as article 1, with proximal fusion line (possibly an artefact), with six setae of which five are robust and one distal aesthetasc.

Antenna (fig. 1C) 0.85 times as long as antennule. Article 1 fused to cephalothorax, wider than following articles, naked. Article 2 with one dorsodistal seta arising from a wide dorsal projection. Article 3 about as long as article 2, with one dorsodistal seta arising from a narrower dorsal projection. Article 4 longer than other articles, with two simple and two small setulated distal setae. Article 5 longer than article 3, with one simple distal seta. Article 6 smallest, decreasing in width at midlenght, with one robust seta at midlenght and three robust distal setae.

Mouthparts. Labrum (fig. 1D) rounded, with distal setules. Mandibular molar tapering, with terminal spines. Left mandible (fig. 1E) incisor broad with three denticles; lacinia mobilis shaped like a broad spine. Right mandible (fig. 1F) incisor narrower than on left mandible with outer process. Labium (fig. 1G) outer corners with small, sparsely setulose process. Maxillule (fig. 1H) endite inner margin with small spines, with ten apical spiniform setae of which at least three are serrated; palp with two apical setae. Maxilla (fig. 1I) ovoid and featureless, but large relative to maxilliped. Maxilliped (fig. 1J) basis fused medially, with one seta at palp insertion; endites almost as wide as basis, divided into three lobe-like structures, outer corners with spiniform process. Palp article 1 without setae but with many scales; article 2 with one outer and three simple inner setae and scattered scales; article 3 with two larger bipinnate and three smaller simple inner setae; article 4 with four larger bipinnate and one smaller simple inner setae and one simple outer seta. Epignath (fig. 1K) naked.

Cheliped (fig. 2A) basis with slender proximal part, attached to cephalothorax by elongated sclerite wrapped beneath the cephalothorax, naked. Merus prominent (half as long as carpus) with one ventral seta. Carpus marginally longer than propodus including fixed finger, without ventral carpal shield, with two ventral setae and one small dorsodistal seta. Propodus robust with two smaller simple and on larger bipinnate inner setae, with one robust (as thick as a pereopodal unguis) outer seta at dactylus insertion. Fixed finger with two robust (as thick as a peropodal unguis) ventral setae and three robust (as thick as a pereopodal unguis) setae on inner margin, inner margin also with relatively weak processes. Dactylus naked.

Pereopod 1 (fig. 2B) Basis wider than those of pereopods 4–6, naked. Ischium with one ventral seta. Merus triangular, with one long (almost as long as carpus) spiniform seta. Carpus as long as merus, with two long distal spiniform setae and distal scales. Propodus with one spiniform ventral seta and one robust dorsal seta and dorsodistal spine, with distal scales. Dactylus and unguis of almost equal length, combined shorter than propodus, and not fused.

Pereopod 2 (fig. 2C) broken. Carpus with two short and one long (longer than half of propodus) spiniform distal setae. Propodus with ventral subdistal spiniform seta but without dorsal seta, with distal and ventral setules/ scales.

Pereopod 3 (fig. 2D) as pereopod 1 except: merus with one additional simple distal seta. Carpus with additional distal spiniform seta. Propodus with one additional simple distal seta.

Pereopod 4 (fig. 2E) basis naked. Ischium with two setae. Merus with two spiniform distal setae. Carpus with two short and one long (longer than half of propodus) spiniform distal setae, with dorsodistal bone-shaped seta. Propodus with three spiniform distal setae. Dactylus and unguis combined longer than propodus and not fused. Dactylus with row sof ventral spines.

Pereopod 5 (fig. 2F) as pereopod 4.

Pereopod 6 (fig. 2G) as pereopod 5 except: basis with one setulated dorsoproximal seta. Ischium naked. Propodus with only two spiniform ventrodistal setae and three robust dorsodistal setae.

Pleopods (fig. 1L) basal article naked. Endopod with 11 inner plumose setae, large gap between most proximal and other setae. Exopod with five plumose inner setae and one outer seta, with gap between most proximal and basal article.

Uropods (fig. 1M) as long as antennule. Basal article longer than exopod but much shorter than pleotelson, naked. Endopod slender, with two articles of subequal length; article 1 with two distal setae; article 2 with one subdistal simple, four simple distal, and one setulated distal setae. Exopod slender, with two articles of subequal length; article 1 with one simple distal seta; article 2 with one simple and one specialized distal setae with flat and wide basis.

Remarks. This species agrees well with most diagnostic characters of Stenotanais : the long pereonites 1–5, the wide basis of the anterior pereopods, possession of thick uropodal setae, structures of maxilliped endite distal margin, and the thick propodal setae of the cheliped. However, the lack of a carpal shield on the chelipeds is in sharp contrast to other species of Stenotanais . Also the mandibular molars, although tapering, are wider than seen in other species and the antennule article 4 shorter than in other species. These conflicting characters suggest that this species is early derived from Stenotanais or may not belong to this genus at all, and genus affiliation should be considered preliminary.

The wide antennule articles 1 and 2 leads me to believe this specimen is a male, although no penes could be observed. The thick antennule article is a character usually associated with males in the Tanaidomorpha ( Lang 1968, Larsen 2002, Larsen & Heard 2004). Also the subdivision (fusion line) of the 4th article, albeit incomplete and possibly an artifact, is indicative of the male gender ( Larsen & Shimomura 2007, 2009).

Figures

FIGURE 1. Stenotanais arenasi, male. Holotype. A, lateral view, scale bar 1 mm. B, antennule. C, antenna. D, labrum. E, left mandible. F, right mandible. G, labium. H, maxillule. I, maxilla. J, maxilliped. K, epignath. L, pleopod. L, uropod. Scale bars 0.1 mm.

FIGURE 2. Stenotanais arenasi, male. Holotype. A, cheliped. B, pereopod 1. C, pereopod 2. D, pereopod 3. E, Pereopod 4. F, pereopod 5. G, pereopod 6. Scale bar 0.1 mm.