Mesotanais styxis, Larsen & Błażewicz-Paszkowycz & Cunha, 2006

Larsen, Kim, Błażewicz-Paszkowycz, Magdalena & Cunha, Marina R., 2006, Tanaidacean (Crustacea: Peracarida) fauna from chemically reduced habitats-the lucky strike hydrothermal vent system, mid-atlantic ridge, Zootaxa 1187, pp. 1-36 : 10-15

Mesotanais styxis n. sp. ( Figs. 3 , 4 )

Material examined

Holotype: non­ovigerous female ( ZMUC CRU­4910 ), south of the vent field, 37°16.484’ N, 32º15.799’ W, 1750 m, RV Prof. Logachev, TV­assisted grab, TTR­10, station AT­280­GR, August 2000 . Paratypes:1 non­ovigerous female ( ZMUC CRU­4911 ), same data. 1 non­ovigerous female (dissected), same data. Other material: 2 non­ovigerous females, 1 manca III, inside the vent field, 37º17.289’ N, 32º16.522’ W, 1718 m, RV Prof. Logachev, TV­assisted grab, TTR­12, station AT­428­GR, August 2002 . 1 non­ovigerous female, 2 (half) non­ovigerous females, 37º17.300’ N, 32º16.563’ W, 1709 m, RV Prof. Logachev, TV­assisted grab, TTR­12, station AT­436­GR, August 2002 .

Diagnosis Female. Antennule without long (almost as long as antennule) setae. Maxilliped basis with only one distal seta. Uropodal endopod with four articles.

Etymology

This species is named after the mythological subterranean River Styx separating the world of the living from the underworld, due to the collection sites at another such ‘river’ (the vent plume).

Description

Female. Body length 1.85 mm.

Body ( Fig. 3A, B ). Seven times as long as broad.

Cephalothorax. Shorter than combined length of Pereonite 1 and 2. Eyelobes present, no visual pigmentation.

Pereonites. Pereonite 1 and 6 wider than long. Pereonite 2 square. Pereonites 3–5 slightly longer than wide.

Pleon. Very short (only 15% of total body length). Pleonites marginally wider than pereonites, all with pleopods and with one robust lateral seta on each margin. Pleotelson longer than combined length of last two pleonites (in lateral view).

Antennule ( Fig. 3C ). Long (as long as combined length of cephalothorax and pereonite 1. With three articles; article 1 longer than rest of antennule combined, with three simple medial and five simple distal setae; article 2 one­third as long as of article 1, with two simple distal setae; article 3 about half length of article 1, with three long, two short simple distal setae and one aesthetasc.

Antenna ( Fig. 3D ). Three­quarters length of antennule. With six articles; article 1 not broader than following articles, naked; article 2 shorter than article 5, with one spiniform dorsodistal seta and one simple seta on each distal margin; article 3 shorter than article 2, with one spiniform dorsodistal seta; article 4 longer than other articles, with one simple and one pinnate medial seta and eight distal setae; article 5 slightly longer than half of article 4, with one distal seta; article 6 minute, distally with three long simple setae and two short setae.

Mouthparts. Labrum ( Fig. 3E ) setose and with flat apex. Mandibles ( Fig. 3 F–H) large and well developed; molar broad and twisted relative to incisor, longer than incisor. Left mandible ( Fig. 3F ) lacinia mobilis larger than incisor, with several distal denticles on upper margin; incisor with tapering apex without denticles. Right mandible ( Fig. 3G & H ) incisor with tapering apex and serration on both margins. Labium ( Fig. 3I ) consisting of two pairs of lobes, outer lobe setose on distal margin, inner lobes apparently naked. Maxillule ( Fig. 3J ) with nine spiniform terminal setae, several distal setules and row of setae proximally on inner margin of endite shaft; palp ( Fig. 3K ) shorter than endite and with two distal setae. Maxilla not recovered. Maxilliped ( Fig. 4A ) endites small and narrow, with three rounded, flat, short setae (middle one with irregular distal edge). Basis with only one long simple seta at palp insertion. Palp article 1 naked; article 2 with one outer seta, four short medial setae, and four long distal setae (of which at least one is serrated) on inner margin; article 3 with multiple inner setae of variable thickness, (of which several are serrated); article 4 with six distal inner and one outer setae. Epignath not recovered.

Cheliped ( Fig. 4B ). Basis shorter than carpus, naked, attached via anterior sclerite. Merus with one ventral seta. Carpus shorter than propodus including fixed finger, with three ventral and two dorsal setae. Propodus elongated and with several setae at dactylus insertion. Fixed finger with two ventral setae and three setae on inner margin and with conspicuous denticles on inner margin. Dactylus as long as fixed finger and naked.

Pereopod 1 ( Fig. 4C ). Almost twice as long as pereopods 3–6. Coxa with one seta. Basis longer than three following articles combined, naked. Ischium naked. Merus as long as carpus, rectangular and naked. Carpus less than half as long as propodus, with three simple distal setae. Propodus longer than half of basis, with three simple dorso­subdistal setae, one minute ventro­subdistal seta and without apparent dorsal spine. Dactylus and unguis combined longer than propodus and not fused. Dactylus with one simple dorsal seta. Unguis shorter than dactylus.

Pereopod 2 ( Fig. 4D ) Coxa with one seta. Basis longer than three following articles combined. Ischium with one ventral seta. Merus widening distally, with one ventral distal seta. Carpus three­quarters length of propodus, with three minute distal setae. Propodus shorter than merus and carpus combined, with one ventral and two simple subdistal setae and without apparent dorsal spine. Dactylus and unguis shorter than propodus, not fused. Dactylus naked. Unguis shorter than dactylus.

Pereopod 3 ( Fig. 4E ). As pereopod 2 except: coxa naked. Basis with one dorsoproximal seta. Merus with two simple distal setae. Carpus with two simple distal setae. Propodus with one subdistal seta on each margin.

Pereopod 4 ( Fig. 4F ). Without coxa. Basis marginally thicker than on pereopods 1–3, with one dorsoproximal pinnate seta. Ischium with one ventral seta. Merus as long as carpus, with two spiniform ventral setae. Carpus with three spiniform distal setae. Propodus shorter than combined length of merus and carpus, with dorsomedial pinnate seta, one ventral and three dorsal spiniform and one stout distal setae. Dactylus and unguis incompletely fused to a slender claw, shorter than propodus.

Pereopod 5 ( Fig. 4G ). As pereopod 4 except: ischium naked. Propodus with one dorsodistal stout seta, and one ventral and two dorsal spiniform distal setae.

Pereopod 6 ( Fig. 4H ). As pereopod 5 except: basis naked. Ischium with one seta. Carpus with two spiniform and one simple distal setae. Propodus with five spiniform and two stout dorsodistal setae

Pleopods ( Fig. 3L ). Well developed. Basal article with one plumose seta (setulation not illustrated). Exopod with seven outer and one inner plumose setae. Endopod with eleven outer plumose setae, gap between proximal seta and other setae.

Uropods ( Fig. 4I ). Biramous, more than half as long as pleon. Basal article naked. Exopod biarticulated, longer than first endopod article; article 1 with one distal seta, article 2 with two distal setae. Endopod with four articles, all with distal setae; most articles also with pinnate setae.

Remarks. This new species can be separated from all other species of Mesotanais by the presence of only one seta at the maxilliped bases. Furthermore, the new species differs from M. vadicola Sieg & Heard, 1989 by a slender, almost straight cheliped and from M. longisetosus Sieg & Heard, 1989 by the absence of long antennular setae. From M. elongatus Sieg & Bird, 1989 it differs by having three short flat maxillipedal endite setae and from M. dubius Dollfus by the simple shape of the right mandibular incisor, the lack of maxilliped basal simple setae, and by the two meral spiniform setae on pereopods 5 and 6.

The incomplete fusion of dactylus and unguis of the pereopods 4–6 is also seen in M. vadicola and M. longisetosus and, although not described, has been illustrated to vary within the individuals ( Sieg & Bird 1989: 177, fig. 8).

This species is also unique in the having a four­articulated uropodal endopod, but this character has shown to be dependent on ontogeny in many species ( Larsen 2005) which is also the case in the family Leptocheliidae ( Masunari 1983; Bird & Bamber 2000).

Family NOTOTANAIDAE Sieg, 1976

Figures

FIGURE 3. Mesotanais styxis n. sp. A, holotype, dorsal view; B, holotype, lateral view; C, antennule; D, antenna; E, labrum; F, left mandible; G, right mandible, incisor; H, right mandible, molar; I, labium; J, maxillule, endite; K, maxillule, palp; L, pleopod. Scale bars: A & B = 1 mm. others = 0.1 mm.

FIGURE 4. Mesotanais styxis n.sp. A, maxilliped; B, cheliped; C, pereopod 1; D, pereopod 2; E, pereopod 3; F, pereopod 4; G, pereopod 5; H, pereopod 6; I, uropod. Scale bars = 0.1 mm.