STYRACONYXIDAE Kristensen & Renaud-Mornant, 1983

Fontoura, Paulo, Bartels, Paul J., Jørgensen, Aslak, Kristensen, Reinhardt Møbjerg & Hansen, Jesper Guldberg, 2017, A dichotomous key to the genera of the Marine Heterotardigrades (Tardigrada), Zootaxa 4294 (1), pp. 1-45 : 35-37

1.8 STYRACONYXIDAE Kristensen & Renaud-Mornant, 1983 (emended by Fujimoto et al. 2016)

Considered as subfamily Styraconyxinae assigned to the family Halechiniscidae by Kristensen & Renaud-Mornant (1983). Elevated to family level by Fujimoto et al. (2016) as previously suggested by Jørgensen et al. (2010).

Arthrotardigrada without sclerotized cuticular plates. Cephalic appendages usually consist of unpaired median cirrus, three pairs of cirri and two pairs of clavae. Buccal clavae may be indistinguishable or third pair of clavae may be present ( Angursa Pollock, 1979 ). Unmodified papillate sensory organs on leg IV. Adult legs terminate in four digits, usually with multipointed claws, rarely single-pointed ( Lepoarctus Kristensen & Renaud-Mornant, 1983 ; Pleocola Cantacuzène, 1951 ). Peduncles present in all digits, in external digits only with heart-shaped pads on internal digits or absent. Rosette-like female gonopore and paired seminal receptacles.

Selected literature for further information: Jørgensen et al. (2014). See Kristensen & Renaud-Mornant (1983) and Kristensen & Higgins (1984a) for keys to this family.

1.8.1 Angursa Pollock, 1979 (emended by Bussau, 1992; Figs 8 I, 12C)

Styraconyxidae with four digits. External digits supported by peduncles, internal digits with proximal pads. Claws with two divergent points (hooks), primary hook stronger than secondary hook. Claws retractable into membranous sheath. Dorsal cuticular plates absent. Complete set of cephalic cirri present, sometimes median cirrus may be reduced or absent. Configuration of primary clavae and lateral cirri variable, arising from distinct lateral extension or directly from head. Secondary clavae occasionally present, occurring as indistinct dome-shaped areas. Sometimes tertiary clavae with appearance similar to those of secondary clavae present, situated ventrally to the secondary clavae. Spines present on legs I, II, III or only on legs I. Usually clavoid papillae, mostly with spine, on legs IV. Body smooth and slender.

Type species: Angursa bicuspis Pollock, 1979

Selected literature for further information: See Renaud-Mornant (1981b); Bussau (1992) and Villora-Moreno (1998) for keys to this genus.

1.8.2 Bathyechiniscus Steiner, 1926 (emended by Pollock 1983; Fig. 8 B, no drawing of claws)

Cylindrical Styraconyxidae with four fully developed toes, each bearing claws with three or more vertically arrayed exposed points, on medial digits at least. Well-developed cephalic cirri present, including median cirrus. Single pair of clavae immediately adjacent to lateral cirri with one or both of these structures arising from a distinct lateral extension of the head, i.e., from a pedestalate base. Spines present on legs I–III and papilla or papilla/spine combination on legs IV. Lateral folds of cuticle or alae absent.

Type species: Bathyechiniscus tetronyx Steiner, 1926

Remark: Bathyechiniscus was described by Steiner (1926) as having claws with four points. However, his description was based on only one specimen, and his drawings are rough sketches. Steiner’s Fig. 3 shows four small points, but in his Fig. 5 the fourth point could be interpreted as the posteriormost tip of a crescent-shaped claw ( Pollock 1983). The type material for Bathyechiniscus no longer exists, no paper after Steiner ever mentions studying it and no other records of Bathyechiniscus have ever been made. As we cannot determine whether Bathyechiniscus actually has four points or only three, we have not included a drawing for Bathyechiniscus in our illustration of feet ( Figs 10–14 ).

1.8.3 Lepoarctus Kristensen & Renaud-Mornant, 1983 (translated from French; Figs 8 D, 12F)

Styraconyxidae with enlarged bell-shaped epicuticle around the body excluding the cephalic region. Complete set of cephalic appendages with cylindrical scapus. Primary clavae retractable into a membranous epicuticular sheath. Secondary clavae indistinct. Cirrus E located in a funnel-shaped epicuticular structure. Spines on legs I; reduced on legs II and vestigial on legs III. Papillae on legs IV with pointed tip. Buccal tube long, stylets and stylet supports very thin, placoids thin. Simple claws retractable into membranous sheath.

Type species: Lepoarctus coniferus (Renaud-Mornant, 1975)

Selected literature for further information: Kristensen & Renaud-Mornant (1983) transfer Pleocola coniferus Renaud-Mornant, 1975 to Lepoarctus coniferus ( Renaud-Mornant, 1975a) .

1.8.4 Paratanarctus D’Addabbo Gallo, Grimaldi de Zio, Morone De Lucia & Troccoli, 1992 ( Figs 3 C, 9A, 11I)

Styraconyxidae with a complete set of cephalic sense organs. Cephalic cirri short and thin; primary clavae tubular. Sense organs of the fourth leg coxal. Cirrus E articulated with an elementary accordion-plated portion. Lance-shaped tarsus with pretarsus. Basal pads on all digits. All claws with external dorsal calcars. Peduncles not present.

Type species: Paratanarctus kristenseni D’Addabbo Gallo, Grimaldi de Zio, Morone De Lucia & Troccoli, 1992

Selected literature for further information: D’Addabbo Gallo et al. (1992).

1.8.5 Pleocola Cantacuzène, 1951 (translated from French, following Renaud-Mornant 1979a; Figs 8 C, 12E)

Styraconyxidae with strongly convex conical head; Cirri A and clavae implanted in a posterior plan relative to median cirrus. Simple claws.

Remark: The original description does not exactly give a diagnosis of the genus: A subsequent incomplete diagnosis has been given by Renaud-Mornant (1979a). Based on the reexamination of Pleocola specimens by Kristensen & Renaud-Mornant (1983) a body smooth and slender; the presence of a complete set of cephalic sense organs; primary clavae short and ovoid; secondary clavae indistinct; spines on legs I, II, III; papillae on legs IV, and cirrus E present are some missing characters characterizing the genus.

Type species: Pleocola limnoriae Cantacuzène, 1951 . Commensal on the isopod Limnoria lignorum ( Rathke, 1799) .

Selected literature for further information: Kristensen & Renaud-Mornant, 1983.

1.8.6 Raiarctus Renaud-Mornant, 1981 (emended by Jørgensen et al. 2014; Figs 8 G, 12G)

Styraconyxidae with the dorso-lateral epicuticular coat supported by long thin pillars 5–6 µm regularly arranged in close parallel lines. With 3-pointed claws42, with or without heart shaped pads on the internal digits. With or without peduncles on every tarsus. Minor primary clava with a cone or tube-like shape; the short, secondary clava either reduced or minor. Body dorso-ventrally flattened. With lateral tube-like openings to the seminal receptacles. Halechiniscid male gonopore (oval papilla with a crescent-shaped opening).

Type species: Raiarctus colurus Renaud-Mornant, 1981

Selected literature for further information: see Renaud-Mornant (1981a), D’Addabbo Gallo et al. (1986) and Jørgensen et al. (2014).

1.8.7 Rhomboarctus Renaud-Mornant, 1984 (emended by Hansen et al. 2003a; Figs 8 H, 12D)

Styraconyxidae with convex frontal edge; indistinct or lenticular secondary clavae; dorsal cuticle punctated; cuticular fins supported by medium sized pillars, extending on both sides of the body between primary clavae and the fourth pair of legs. More or less evident caudal appendage with cuticular fan with pillars. Peduncles present only on the external digits, proximal pads on the internal ones; all the claws with very thin or vestigial secondary hooks.

Type species: Rhomboarctus thomassini Renaud-Mornant, 1984

Remark: Claw bases can appear like a third point.

1.8.8 Styraconyx Thulin, 1942 (emended by Kristensen & Higgins 1984a; Figs 8 A, 12A)

Styraconyxidae with four digits. External digits always supported by peduncles; internal digits with proximal pads, peduncles present or absent. Claws with three exposed points (hooks43), may be retracted within claw sheath. Dorsal cuticular plates absent although grid-like pattern of 18–23 folds sometimes present on epicuticle. Complete 42. In the terminology of this key “three points” = primary, secondary and accessory points. 43. In the terminology of this key “three points or hooks” = a primary point and 2 secondary points or an accessory point, primary point and secondary point.

set of cephalic cirri always present; configuration of primary clavae and lateral cirri variable, arising from distinct lateral extension or directly from head; secondary clavae occurring as indistinct flat sacs or dome-shaped papillae. Stylet supports, stylets and placoids present. Spines present on legs I–III, clavoid papillae, each with spine, on leg IV. Paired seminal receptacles with ducts opening posterior to the female gonopore present between legs III and IV.

Type species: Styraconyx haploceros Thulin, 1942

Selected literature for further information: See Kristensen & Higgins (1984a) and D’Addabbo Gallo et al. (1989) for keys to this genus. See also Chang & Rho (1998) and Bartels et al. (2015) for more recently described species.

1.8.9 Tetrakentron Cuénot, 1892 (emended by van der Land 1975; Figs 8 F, 12H)

Styraconyxidae with the body dorsoventrally flattened. Cephalic appendages (including cirrus medianus) very short; only the cephalic papillae are relatively large. Eyes lacking. Mouth opening on buccal cone and anus subterminal. Legs short, with short tibia and digits. Claws compound (with primary and secondary hooks) and with peduncles; those of internal digits with large accessory spine44 on secondary hook; no spurs. Marine, littoral ectoparasitic on burrowing holothurians.

Type species: Tetrakentron synaptae Cuénot, 1892

Selected literature for further information: Kristensen (1980).

1.8.10 Tholoarctus Kristensen & Renaud-Mornant, 1983 (emended by Jørgensen et al. 2014; Figs 8 E, 12B)

Styraconyxidae with barrell-shaped outer epicuticle surrounding the trunk, the body itself is thin and elongated. Complete set of cephalic appendages. Cone-shaped primary clavae and secondary clavae enlarged surrounding both the internal and external cephalic cirri. Cirri A and E partially surrounded by funnel-shaped epicuticular folds. Leg sense organs always present on the first and fourth pair of legs, though much smaller sense organs may be present on the second and third legs. Flagellum on the sense organ of the first leg always very long. Sense organ on the fourth leg partially enclosed in funnel-shaped epicuticular folds. Females have claws with a main hook, a basal accessory hook and a terminal accessory spur45. Males and larvae migth have simple claws in the external digits. Long buccal tube with two lateral projections at ca. the mid-point. Long and thin stylet supports articulate with the furcae. Placoids consist of a non-calcified cuticular lining, but three small calcified apodemes occur in the anterior part of the pharyngeal bulb. Mouth cone terminal and protruding. Globular coelomocytes seem to be attached to the midgut. Sexual dimorphism in body size and claws.

Type species: Tholoarctus natans Kristensen & Renaud-Mornant, 1983

Figures

FIGURE 3. Foot and claw morphology. A. Generalised leg. B. Chimaeric foot showing various claw configurations. External claw at left of image has a single point (sometimes called a “ simple claw “), the next internal claw has two points, and the final two claws have three points. C. Paratanarctus foot, redrawn from D’Addabbo Gallo et al. (1992). D. Mutaparadoxipus foot, redrawn from Gross et al. (2014). E. Batillipes foot, redrawn from Kristensen & Mackness (2000). F. Megastygarctides foot, redrawn from Hansen & Kristensen (2006). ap = accessory point, bm = basal membrane, cl = claw, co = coxa, cr = calcar, cs = claw sheath, db = disc brace, dc = disc, de = distal stalk enlargement, di = digit, ed = external digit, fe = femur, id = internal digit, pa = adhesive pad, pe = peduncle, pp = primary point, ppa = proximal adhesive pad, pta = pretarsus, sp = secondary point. ta = tarsus, ti = tibia, to = toe.

FIGURE 5. Habitus drawings of marine heterotardigrades. A Renaudarctus, redrawn from Hansen et al. (2012). B Megastygarctides, redrawn from McKirdy et al. (1976). C Faroestygarctus, redrawn from Hansen et al. (2012). D Stygarctus, redrawn from McKirdy et al. (1976). E Mesostygarctus, redrawn from Hansen et al. (2012). F Pseudostygarctus, redrawn from McKirdy et al. (1976). G Prostygarctus, original drawing. H Parastygarctus, redrawn from McKirdy et al. (1976). I Neostygarctus, Fujimoto & Miyazaki (2013).

FIGURE 8. Habitus drawings of marine heterotardigrades. A Styraconyx, redrawn from Marcus (1936). B Bathyechiniscus, redrawn from Steiner (1926). C Pleocola, redrawn from Pollock (1976). D Lepoarctus, redrawn from Renaud-Mornant (1975 a). E Tholoarctus, redrawn from D’Addabbo Gallo et al. (1992). F Tetrakentron, redrawn from Kristensen (1980). G Raiarctus, redrawn from Jørgensen et al. (2014). H Rhomboarctus, redrawn from Hansen et al. (2003 a). I Angursa, redrawn from Renaud-Mornant (1988) and Bussau (1992).

FIGURE 10. Drawings of feet of marine heterotardigrades. A Coronarctus (heteronych claws), redrawn from Renaud-Mornant (1983). B Coronarctus (isonych claws), redrawn from Hansen (2007). C Trogloarctus, original drawing. D Megastygarctides, redrawn from Hansen & Kristensen (2006). E Pseudostygarctus, Mesostygarctus, Faroestygarctus redrawn from Renaud- Mornant (1983). We have indicated the four-claw condition, but there is variation in this group. Faroestygarctus and Pseudostygarctus galloae have only two claws, Pseudostygarctus triungulatus has three, while all the other species in these genera have four claws. Faroestygarctus has dorsal spurs on claws. F Stygarctus, Parastygarctus, redrawn from Renaud- Mornant (1982 a). G Neoarctus, redrawn from de Zio Grimaldi et al. (1992). H Echiniscoides, Isoechiniscoides redrawn from Renaud-Mornant (1982 a). I Anisonyches, redrawn from Renaud-Mornant (1982 a).

FIGURE 11. Drawings of feet of marine heterotardigrades. A Neostygarctus, redrawn from Fujimoto & Miyazaki (2013). B Prostygarctus, original drawing. C Renaudarctus, redrawn from Kristensen & Higgins (1984 b). D Actinarctus, Tanarctus, Zioella, redrawn from Renaud-Mornant (1982 a). E Dipodarctus leg I, redrawn from Jørgensen et al. (2014). F Dipodarctus leg IV, redrawn from Jørgensen et al. (2014). G Florarctus, Wingstrandarctus, redrawn from Renaud-Mornant (1989 a). H Ligiarctus, redrawn from Renaud-Mornant (1982 b). I Paratanarctus, redrawn from D’Addabbo Gallo et al. (1992).

FIGURE 12. Drawings of feet of marine heterotardigrades. A Styraconyx, redrawn from Kristensen & Higgins (1984 a). B Tholoarctus female, redrawn from Kristensen & Renaud-Mornant (1983). C Angursa, redrawn from Bussau (1992). D Rhomboarctus, redrawn from Hansen et al. (2003 a). E Pleocola, redrawn from Renaud-Mornant (1984). F Lepoarctus, redrawn from Kristensen & Renaud-Mornant (1983). G Raiarctus, redrawn from Jørgensen et al. (2014). H Tetrakentron, redrawn from Renaud-Mornant (1982 a). I Archechiniscus, redrawn from Renaud-Mornant (1982 a).

FIGURE 13. Drawings of feet of marine heterotardigrades. A Clavarctus, redrawn from Renaud-Mornant (1983). B Parmursa, redrawn from Renaud-Mornant (1984). C Euclavarctus, Exoclavarctus, Moebjergarctus, Proclavarctus, redrawn from Renaud- Mornant (1983). D Halechiniscus, redrawn from Renaud-Mornant (1982 a). E Quisarctus, redrawn from Fujimoto (2015 b). F Chrysoarctus, redrawn from Renaud-Mornant (1984).

FIGURE 14. Drawings of feet of marine heterotardigrades. A Paradoxipus, redrawn from Kristensen & Higgins (1989). B Mutaparadoxipus leg I, redrawn from Gross et al. (2014). C Mutaparadoxipus leg IV, redrawn from Gross et al. (2014). D Opydorscus leg I, redrawn from Renaud-Mornant (1989 b). E Opydorscus leg II (legs III and IV identical), redrawn from Renaud-Mornant (1989 b). F Orzeliscus, redrawn from Kristensen & Higgins (1989). G Batillipes, redrawn from Kristensen & Mackness (2000).