Urstylis thiotyntlus, Riehl & Wilson & Malyutina, 2014

Riehl, Torben, Wilson, George D. F. & Malyutina, Marina V., 2014, Urstylidae - a new family of abyssal isopods (Crustacea: Asellota) and its phylogenetic implications, Zoological Journal of the Linnean Society 170 (2), pp. 245-296 : 277-285

URSTYLIS THIOTYNTLUS GEN. ET SP. NOV.

FIGURES 22–25

Zoobank registration urn:lsid:zoobank.org:act:E3150B05-CAC6-4C03-B0AB-71FB6DB862FD

Diagnosis

Body subcylindrical; anterior pereonites medioventrally keeled; all sternites with projecting spines; spines directed posteriorly; without dorsal setae, posterolateral margins of all pereonites with prominent, robust spine-like seta; pereonites 1–4 tightly packed, pereonite 4 wider than pereonite 5, posterolateral margin with prominent, spine-like seta and simple setae; pereonite 6 shorter than pereonite 5; pereonite 7 posterolateral margins not projecting posteriorly. Pleotelson rectangular, length 2.0 width, waist weakly pronounced; paired dorsal sensory organ absent. Pereopod I projecting laterally and dorsally, ischium dorsal setose lobe longer than merus dorsal lobe, with three enlarged setae; pereopods V–VI ischium with seta mid-dorsally, carpus mid-dorsally with no seta. Female pleopod II distal setae apically sensillate.

Etymology

This name, derived from the Greek words theiodes meaning sulphur-like and tyntlos mud, refers to the sulphide-rich sediments around the hydrothermal

280 T. RIEHL ET AL.

mounds of the Galapagos mid-ocean ridge system. It is a masculine noun in apposition.

Type fixation

Adult female holotype, 1.6 mm, USNM 1208016 , designated here .

Type material examined

USNM 1208016 : adult female holotype . USNM 1208017 : manca stage 1 paratype .

Type locality

Galapagos Hydrothermal Mounds region , R / V Gillis st. 301 (‘away from mounds’ – see Grassle et al., 1985), 0°35.0′N, 86°05.7′W, 2730 m, box core (one of 25 subcores) .

Type material – remarks

Holotype female missing antennal flagellum and uropods; several pereopods broken at basis; pleopod II

(operculum) removed and mounted on slide. Manca stage 1 specimen missing antennae and uropods.

Description of female

Body ( Fig. 22 ) subcylindrical, length 1.6 mm, 4.0 width, tergite surfaces hirsute, setation of lateral tergal margins present, with long setae along lateral margins of pereonites; posterior pereonites and pleotelson with dorsal robust setae. Ventral spines acute, keel-like, directed posteriorly. Pereonite 1 spine small. Pereonite 2 spine small, placed midway on midline. Pereonites 3 and 4 spines small, closer to posterior segment border. Pereonite 5 spine absent. Pereonite 6 spine prominent, triangular in lateral view, closer to posterior segment border. Pereonite 7 spine prominent. Imbricate ornamentation absent on all pereonites. Cephalothorax length 0.61 width, 0.14 body length; frons in dorsal view convex, smooth, frontal furrow present, convex anterior margin adjacent to clypeus, not projecting. Posterolateral setae present. Posterolateral margins angular, blunt.

Pereonite 1 length 0.35 width, 0.08 body length, anterior margin straight. Posterolateral setae asensillate, simple. Pereonite 2 length 0.33 width, 0.08 body length. Posterolateral setae robust. Pereonite 3 length 0.42 width, 0.10 body length; posterolateral setae asensillate, robust, flexibly articulated. Pereonite 4 width 1.04 pereonite 5 width, length 0.63 width; lateral margins curved, in dorsal view lateral margins narrowing posteriorly to coxae, with distinct indentation posterior to coxa. Posterolateral margins tapering. Posterolateral setae sensillate, robust, flexibly articulated.

Pereonites 5–7 posterior tergite margin with two sensillate, robust, flexibly articulated setae; setae extending beyond posterolateral margin. Posterolateral margins not produced posteriorly. Tergite posterolateral setae sensillate, robust. Pereonite 5 length 0.75 width, 0.80 pereonite 4 length. Pereonite 6 length 0.59 width, 0.94 pereonite 5 length. Pereonite 7 length 0.56 width.

Pleotelson ( Fig. 22A, B ) length 0.26 body length, 2.0 width, narrower than pereonite 7; paired dorsal sensory organ absent. Posterior margin apex length 0.11 pleotelson length. Posterior apex setae absent. Pleopodal cavity width 0.85 pleotelson width.

Labrum anterior margin in dorsal view concave. Concavity on left side, margin sinusoid, no distal cuticular spinules.

Antennula ( Fig. 22E ) length 0.51 head width, length 0.46 antenna length, width 0.69 antenna width; relative length ratios of articles 1.0, 0.75, 0.34, 0.28, 0.18, 0.34; L/W ratios of articles 1.8, 2.3, 1.4, 1.2, 1.1, 2.2. Article 1 distinctly longer than wide, dorsally flattened, ventrally semicircular, longest and widest. Articles 2–4 distinctly longer than wide. Article 3 with one asensillate seta. Article 4 with one asensillate large, distally curled seta. Article 5 length subequal to width. Terminal article with two simple tubular aesthetascs.

Antenna ( Fig. 22F ) length 0.25 body length. Basis angular with dorsolateral projection, shorter than coxa, rudimentary scale present. Ischium longer than coxa. Merus longer than coxa, basis, and ischium combined, distally with two asensillate setae, articulating distolaterally on ischium, antennal proximodistal axis with distinctly sharp bend. Carpus longer than merus, distally with one asensillate seta. Flagellum with nine articles.

Mandibles ( Figs 22C, D , 23A ): left mandible incisor process simplified, mono- or bidentate rounded, blunt; right mandible incisor process multidentate with dorsal and ventral subdistal teeth that partly enclose lacinia.

Maxilliped ( Fig. 23A ) with three coupling hooks, article 2 wider than articles 1 and 3, article 1 shorter than article 3, article 4 distomedial extension present; epipod length 2.8 width, 0.87 basis length.

Anterior pereopodal coxae ring-shaped, coxal setation present. Pereopod I ( Fig. 24A ) positioned laterally and dorsally; length 0.28 body length; article L/W ratios 1.96, 0.83, 0.90, 1.64, 2.43, 4.27; relative article length ratios 1.00, 0.62, 0.41, 0.55, 0.35, 0.30; ischium dorsal margin with lobe projecting much greater than basal width of segment, with three enlarged setae: one simple seta, two distally biserrate. Merus dorsal margin with two setae: one simple, one distally bidenticulate; ventral margin with three setae: two distally curled, one robust subdistally sensillate. Carpus dorsally with two distally biserrate setae. Propodus with one long distodorsal seta and one short ventral seta, articular plate on propodus absent; dactylus distally with two sensillae, dorsal claw length slightly shorter than dactylus.

Pereopod II ( Fig. 24B ) longer than pereopod I, length 0.37 body length; article L/W ratios 4.0, 2.3, 1.3, 2.6, 3.3, 4.0; relative article length ratios 1.0, 0.50, 0.31 0.46, 0.30, 0.26. Ischium dorsally with 1 distally curled simple seta. Merus dorsally with two setae, ventrally with one seta; setae distally curled simple. Carpus dorsally with one broom seta, ventrally with three setae: one distally curled, simple and two robust, subdistally sensillate. Dactylus distally with one sensilla, dorsal claw length similar to dactylus length.

Pereopod III ( Fig. 25A ) length 0.39 body length; article L/W ratios 3.4, 2.0, 1.8, 2.5, 3.0, 4.0; relative article length ratios 1.00, 0.48, 0.44, 0.50, 0.36, 0.34. Ischium with no seta proximodorsally, dorsal lobe flat, rounded; proximally with one seta. Merus dorsally with two simple, distally curled setae, at dorsodistal margin, ventrally with two simple, distally curled setae, along ventral margin. Carpus dorsally with two simple setae, ventrally with three setae: two simple marginally, one sensillate robust distally. Dactylus with one sensilla.

Pereopod IV ( Fig. 25B ) length 0.48 body length, more robust and longer than pereopod III (∼43% longer); article L/W ratios 4.7, 2.9, 1.6, 3.0, 3.7, 5.9; relative article length ratios 1.0, 0.63, 0.36, 0.48, 0.32, 0.30. Pereopod V ( Fig. 25C ) length 0.39 body length; article L/W ratios 5.8, 2.9, 3.4, 5.5, 7.3, 6.3; relative article length ratios 1.0, 0.61, 0.51, 0.65, 0.65, 0.41. Ischium mid-dorsally with one small simple seta, midventrally with two simple distally curled setae. Merus distoventrally with one simple distally curled seta. Carpus distodorsally with two setae: one simple, one penicillate; midventrally with one seta; distoventrally with two elongate, robust, sensillate setae.

Pereopod VI ( Fig. 25D ) length 0.48 body length; article L/W ratios 4.7, 3.8, 4.5, 9.1, 8.5, 2.9; relative article length ratios 1.0, 0.73, 0.75, 1.12, 0.93, 0.52. Ischium dorsally with one seta, midventrally with two distally curled simple setae. Merus distodorsally with one distally curled simple seta; midventrally with one distally curled simple seta; distoventrally with one simple seta. Distodorsally with two simple setae; midventrally with two thin robust sensillate setae; distoventrally with two thin robust sensillate setae.

Pereopod VII basis ventral margin with row of three elongate setae; setae shorter than basis width.

Operculum ( Fig. 23C ) length 1.4 width, 0.82 pleotelson dorsal length; apical width 0.50 operculum width; distally tapering. With lateral fringe consisting of eight bifurcate distally sensillate setae, with con- tinuous transition to apical row of 17 setae; apical setae asetulate, distally sensillate, extending to anal opening.

Pleopod III ( Fig. 23B ) length 2.3 width; protopod length 2.4 width, 0.61 pleopod III length. Uropod broken.

Remarks

Urstylis thiotyntlus gen. et sp. nov. differs from the other two species in the genus by its laterally positioned pereopod I. This limb is further modified by being more robust and having distinctive dorsal projections on the ischium, merus, and carpus, all of which bear large distally denticulate setae. The attitude of the first pereopod is reminiscent of that seen in macrostylids and the Desmosomatidae although, in these taxa, the relevant limbs are pereopods II–III. Given that this limb position is common amongst desmosomatid and macrostylid species known to be fossorial ( Hessler & Strömberg, 1989), we infer that this species may also be fossorial. Differences between pereopods II–IV and V–VI are more pronounced and

Figures

Figure 22. Urstylis thiotyntlus gen. et sp. nov., holotype female USNM 1208016. A, dorsal habitus. B, ventral habitus. C, D, head anterior and left lateral view, respectively. E, antennula, lateral view. F, antenna, medial view. Scale bars = 0.5 mm (A, B); 0.2 mm (C, D); 0.1 mm (E, F).

Figure 23. Urstylis thiotyntlus gen. et sp. nov., holotype female USNM 1208016. A, maxilliped, ventral view, with enlargement of palp articles 2–3. B, pleotelson, ventral view, operculum removed to show pleopod III. C, operculum ventral view. Scale bars = 0.2 mm (A); 0.2 mm (B, C).

Figure 24. Urstylis thiotyntlus gen. et sp. nov., holotype female USNM 1208016. A, pereopod I, lateral view. B, pereopod II, lateral view, with enlargement of dactylus and claws (arrow). Scale bar = 0.2 mm.

Figure 25. Urstylis thiotyntlus gen. et sp. nov., holotype female USNM 1208016. A–D, pereopods III–VI, left side, lateral view. A, C, showing enlargement of dactylus. D, pereopod VI dactylus, right side, lateral view. Scale bar = 0.2 mm (A–D).