Mesotanais birdi, Larsen, Kim & Shimomura, Michitaka, 2007

Larsen, Kim & Shimomura, Michitaka, 2007, Tanaidacea (Crustacea: Peracarida) from Japan. II. Tanaidomorpha from the East China Sea, the West Pacific Ocean and the Nansei Islands, Zootaxa 1464, pp. 1-43 : 9-13

Mesotanais birdi n. sp.

( Figs 4 and 5 )

Material examined. Holotype, non-ovigerous female ( KMNH IvR 700.168), Station KG-2, 34°58.192’– 34°58.398’N, 140°05.188’– 140°05.115’E, 200– 169 m, muddy sand, Off Boso Peninsula, 17 November 2003. Paratypes, 1 non-ovigerous female (dissected) ( KMNH IvR 700.169), same data; 1 non-ovigerous female ( KMNH IvR 700.170), same data.

Diagnosis. Female. Antennule shorter than carapace, with long (almost as long as antennule) setae. Pereopod 5 with long robust setulated basal seta. Uropodal exopod shorter than first endopod article.

Etymology. This species is named in honor of Dr. G. Bird, the world’s leading authority on tanaidomorphan crustaceans.

Description (body of holotype, appendages of dissected paratype).

FEMALE.

Body ( Fig. 4 A). Body length 2.2 mm. Nine times as long as broad.

Cephalothorax. Shorter than combined length of pereonites 1 and 2. Eye-lobes present, no visual pigmentation.

Pereonites. Pereonites 1 and 6 wider than long. Pereonites 2–5 longer than wide.

Pleon. Very short (only 15% of total body length). Pleonites all with pleopods. Last pleonite larger than others.

Pleotelson. Longer than combined length of last two pleonites.

Antennule ( Fig. 4 B). Shorter than cephalothorax. With three articles; article 1 longer than rest of antennule combined, with several small proximal setae of which at least one is setulated, with one setulated and one simple medial setae, with two simple and one setulated distal setae; article 2 one-third as long as of article 1, with two simple and one setulated distal setae; article 3 about half length of article 1, with seven simple distal setae of varying length (some almost as long as antennule).

Antenna ( Fig. 4 C). Three-quarters length of antennule. With five articles; article 1 longer than article 4, with one spiniform dorsal setae; article 2 shorter than article 1, with one dorsodistal spine; article 3 longer than other articles, with one simple medial seta three long simple and 2 short setulated distal; article 4 slightly longer than half of article 3, with two distal seta; article 5 minute, with five setae.

Mouthparts. Labrum ( Fig. 4 D) smoothly curved, setose. Mandibles ( Fig. 4 E–G) molar broad and twisted relative to incisor, longer than incisor. Left mandible ( Fig. 4 E) lacinia mobilis larger than incisor, with several distal denticles on distal margin; incisor with tapering apex with denticles. Right mandible ( Fig. 4 F, G) incisor with bifurcate apex and serration on dorsal margin. Labium ( Fig. 4 H) consisting of two pairs of setose lobes. Maxillule ( Fig. 4 I) with nine spiniform terminal setae, several distal setules on both margins of endite shaft; palp ( Fig. 4 J) shorter than endite and with two distal setae. Maxilla ( Fig. 4 K) elongated and featureless. Maxilliped ( Fig. 4 L) endites narrower than basis, with three flat, short setae and one simple seta. Basis with two long seta at palp insertion. Palp article 1 naked with pointed process on outer margin; article 2 with four inner setae; article 3 with five–six inner setae; article 4 with five–six setae. Epignath not recovered.

Cheliped ( Fig. 4 M). 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 two setae at dactylus insertion. Fixed finger with two ventral setae and three setae on inner margin and with pointed denticles on cutting edge. Dactylus as long as fixed finger, with two dorsal setae.

Pereopod 1 ( Fig. 5 A). Almost twice as long as pereopods 2–6. Coxa with one seta.

Basis longer than three following articles combined, with one simple and one setulate seta. Ischium naked. Merus as long as carpus, with one dorsosubdistal seta. Carpus half as long as propodus, with three simple distal setae. Propodus longer than half of basis, with three simple subdistal setae and small dorsal spine. Dactylus and unguis combined longer than propodus and not fused. Unguis marginally shorter than dactylus.

Pereopod 2 ( Fig. 5 B). As pereopod 1 except: somewhat shorter; basis with one simple seta; ischium with one ventral seta; merus widening distally, with two distal setae; carpus three-quarters length of propodus, with one simple and one minute distal spiniform setae; propodus shorter than merus and carpus combined, with two simple and one minute spiniform subdistal setae, apparent without dorsal spine; dactylus and unguis shorter than propodus, not fused; dactylus naked.

Pereopod 3 ( Fig. 5 C). As pereopod 2 except: merus naked; propodus with two subdistal setae and dorsal spine.

Pereopod 4 ( Fig. 5 D). Without coxa. Basis marginally thicker than on pereopods 1–3, naked. Ischium naked. Merus as long as carpus, with one small spiniform ventral seta. Carpus with one simple and two small spiniform distal setae. Propodus shorter than combined length of merus and carpus, with one small ventral spiniform seta and three dorsodistal rigid setae. Dactylus and unguis fused to a slender claw, shorter than propodus.

Pereopod 5 ( Fig. 5 E). As pereopod 4 except: basis with one large circumplumose ventral seta; carpus with one simple seta.

Pereopod 6 ( Fig. 5 F). As pereopod 5 except: basis with two small setulated setae. Ischium with one seta; merus with two small spiniform setae; carpus with one simple and two small spiniform; propodus with two small ventral spiniform setae and four stout dorsodistal setae

Pleopods ( Fig. 5 G). Well developed. Basal article with one plumose seta. Exopod with nine outer and one inner plumose setae. Endopod with 12 outer plumose setae, gap between proximal seta and other setae.

Uropods ( Fig. 5 H). Biramous, more than half as long as pleon. Basal article naked. Exopod biarticulated, shorter than first endopod article; article 1 with one distal seta, article 2 with two distal setae. Endopod with four articles, with 0–4 distal setae.

Remarks. This new species can be separated from all other species of Mesotanais by the short antennule, by the strange long setulated basal seta of pereopod 5, and by the uropodal exopod shorter than first endopod article. In other respects Mesotanais birdi is identical to M. longisetosus Sieg & Heard, 1988 and even shares the long antennular setae character.

The dorsal spiniform seta on antenna article 2 seems to have fused into a real spine and no articulation could be observed.

This species has a four-articulated uropodal endopod, like that of M. styxis Larsen, BłaŻewicz-Paszkowycz & Cunha, 2006 but this character is likely ontogentically dependant ( Masunari 1983; Bird & Bamber 2000).

The antenna is diagnosed with six articles, but the new species described above has only five. This is a reoccurring conflict within many tanaidomorphan taxa, and probably reflects an inherent bias from dissection. It appears as though the antenna is attached to the head via a small pedestal (as in amphipods); this may be interpreted as the first article although it is fused to the head. After dissection it is often no longer possible to decide if there is an articulation or not.

Figures

FIGURE 4. Mesotanais birdi n. sp. A, holotype, female, lateral view, scale bar = 0.5 mm; B, antennule; C, antenna; D, labrum; E, left mandible; F, right mandible, incisor; G, same, molar; H. labium; I, maxillule; J, same, palp; K, maxilla; L, maxilliped; M, cheliped. Scale bars = 0.2 mm.

FIGURE 5. Mesotanais birdi n. sp. A, pereopod 1; B, pereopod 2; C, pereopod 3; D, pereopod 4; E, pereopod 5; F, pereopod 6; G, pleopod; H, uropod. Scale bar = 0.2 mm.