Macrostylis antennamagna, Riehl, Torben & Brandt, Angelika, 2010

Riehl, Torben & Brandt, Angelika, 2010, Descriptions of two new species in the genus Macrostylis Sars, 1864 (Isopoda, Asellota, Macrostylidae) from the Weddell Sea (Southern Ocean), with a synonymisation of the genus Desmostylis Brandt, 1992 with Macrostylis, ZooKeys 57, pp. 9-49 : 23-36

Macrostylis antennamagna sp. n. Figs 918

Material examined.

Holotype. Adult male, 3.4 mm, ZMH (K-42168), Southern Ocean, North-western Weddell-Enderby Abyssal Plain, south of the South Orkney Islands, station 110-8 (ANDEEP III), 64°59.20'S; 043°002.05'W, 4698 m depth. Paratypes. 1 preparatory female, 4.7 mm, ZMH (K-42169) and 1 adult male ZMH (K-42169) from type locality; 1 male, 3.1 mm, ZMH (K-42171); 1 male fixed for SEM, 2.8 mm, ZMH (K-42172) Southern Ocean, northern Weddell Sea abyssal plain, south of the Endurance ridge, station 138-6 (ANDEEP II), 64°1.67'S; 39°7.68'W, 4760 m depth. For further material examined for comparison see Table 1.

Diagnosis.

Cephalothorax semicircular, antenna 2 long, basal 5 articles pronounced, flagellum reaching to the posterior end of pereonite 3 when bent backwards, little shorter in female; mandible flat, dorsoventrally constricted proximally to laciniae; pars incisiva without pronounced or sharp teeth but bump-like structures; dominant setae on pereopods of bifurcate type; setation pattern on male pleopod 1 mostly not symmetrical, distolateral lobes of pleopod 1 projecting less distally than medial lobes, bent laterally (~90°) and ventrally, medial lobes rounded, each with 7 setae.

Description holotype male.

Body (Figs 9, 10) elongate, 5.3 times longer than wide; maximal body width in pereonites 2 and 3, maximal pereonite width 1.5 times maximal width of pleotelson; pereonites 1-3 about the same width, from pereonite 3 towards pleotelson slightly but gradually narrowing, pereonite 7 0.8 times as wide as pereonite 3. Surface of body (Fig. 10) bearing comb-like structures which can be worn off in exposed areas, e.g. posterolateral protrusions, cephalothorax or dorsal surface of pereonite 3; numerous small simple setae and setules on general cuticle; pereonites 3-7 unsymmetrical with respect to number and length of 1-2 posterolateral setae. Cephalothorax almost semicircular, length 0.7 times maximal width posteriorly to antenna 2 articulation and less than 0.2 times total body length; width 0.9 times pereonite 1 width; transverse ridge on frons not recognized. Fossosome 0.2 times total body length; as long as wide, laterally slightly convex; median legth: 1:1:1.3; frontal spine in sternite of pereonite 1, dorsoventral constrictions close to the segment borders of pereonites 2-3. Posterior pereonites anteriorly strongly constricted; anterior margins overlapped by preceding tergite; relative medial lengths of pereonites 4-7: 1:1.2:1.4:1.2; laterally concave. Posterolateral parts of tergites slightly tapered backwards; pereonites 5-7 with ventral spines directed posteriorly. Pleotelson 0.2 times body length; about 1.7 times longer than wide, laterally concave, narrowing towards a constriction anterior to uropodal articulations; caudal apex concave surrounded by setules; compared to rest of body, most prominent sculpturation of cuticle, shingle-like appearance; cuticle slightly translucent, statocysts visible; slot-like apertures in dorsal cuticle; breathing cavity (Fig. 15) maximal opening width 0.7 times maximal pleotelson width, narrowest width of longitudinal excavation 0.2 times maximal pleotelson width.

Antenna 1 (Figs 10, 11) with 5 articles, small, barely reaching halfway to posterior margin of cephalothorax when directed backwards; length 2.6 times article 1 width; all articles about as long as wide, gradually decreasing in size towards distally; fifth article 0.1 times total length; at least 4 aesthetascs on each of articles 4 and 5, some broken off; broom setae around distal margin of article 2; some distally fringed sensillae on distal margins of articles 1-3; with 1 seta distally on article 5, next to aesthetascs.

Antenna 2 (Figs 10, 11) relatively long and broad, peduncle reaching to the posterior end of pereonite 2 and flagellum reaching to the posterior end of pereonite 3; each of articles 1-3 1.4 times longer than wide; article 4 longest, length 0.3 times total length, as wide as articles 1-3; article 5 0.8 times article 4 length, about 0.4 times article 4 width; flagellum of 7 articles; all 5 basal articles with 1-several distal broom setae (not shown in illustration) and some simple setae; tubular setae with large apical pores distally on flagellar articles 1-4 ans 7, articles 1-3 with 3 setae each, articles 4 and 7 with 2; article 7 with long simple distal setae, 3 times longer than article.

Mandible (Fig 11) flat, dorsoventrally constricted proximally to lacinia; pars incisiva without teeth, but bump-like structures, left mandible with 1 terminal, 1 ventral and 2 dorsal cusps, right mandible with a centered lobe-like cusp; left lacinia mobilis longer and more robust than right lacinia mobilis, with 4 strong blunt teeth, 1.5 times longer than following spines of spine row; right lacinia mobilis twice as long as following spines of spine row, with 6 teeth, more acute than in left lacinia mobilis, dorsoventrally arranged, with ventral teeth projecting most distally; spine row of about 8 spines with multiple cusps, partially serrated; Maxilla 1 and 2: see description of female paratype.

Maxilliped (Fig. 11) epipod oblong-subtriangular, distally narrowing to multiple small tips, distolaterally concave, without any setae or setules, reaching distal end of palp article 2; endite with 2 coupling hooks, densely covered with fine but rather long simple setae, medially broadening, more proximally and dorsally forming lobe-like protrusion with 4 setae on lateral blunt apex, together constituting a sheath in which the epipod rests; lateral margins of basis, and palp articles 1-2 with row of small setae, basis and epipod subequal in length; palp article 2 3 times longer than article 1, articles 2-4 distomedially and distolaterally with medially scaled and distally pappose (fringed) sensillae, article 5 with the same setae terminally and subterminally.

Anterior pereopods (Fig. 12) length ratios: 1:1.1:1.1, all articles covered with tiny setules of varying density of coverage.

Pereopod 1 relative length ratios: 1:0.5:0.4:0.3:0.4:0.2, basis 3.8 times longer than wide, with 4 broom setae and 1 short simple seta on dorsal side, 3 short simple setae and 1 broom seta ventrally,; ischium 2.3 times longer than wide, with 2 setae distodorsally and 1 small seta distoventrally; merus almost 1.4 times longer than wide, with distodorsal extension bearing 4 unequally long setae; carpus 2.4 times longer than wide, distodorsally with 1 bifurcate seta and 1 broom seta, 3 setae ventrally; propodus 3.5 times longer than wide, dorsally with 2 short setae, ventral side with 3 sensillae, terminally expanded to a subtriangular lobe; dactylus 4.6 times longer than wide, 0.7 times the length of carpus, with 7 sensillae of different lengths.

Pereopod 2 relative length ratios: 1:0.6:0.3:0.5:0.2:0.2, basis with 3 broom setae dorsally and row of short setae ventrally, 4.2 times longer than wide; ischium 3.1 times longer than wide, with 1 small seta bent backwards proximodorsally, row of 4 long simple setae distodorsally and 1 small seta distoventrally; merus 1.5 times longer than wide with distodorsal extension bearing row of 4 long simple setae; carpus 2.6 times longer than wide, distodorsally with row of 4 long setae, most distal one bifurcate, with 4 unequal simple setae ventrally; propodus 3 times longer than wide, dorsally with 2 simple setae, with 1 broom seta dorsally, terminally expanded to a subtriangular lobe; dactylus 5.2 times longer than wide, subterminally with 3 sensillae.

Pereopod 3, relative length ratios: 1:0.6:0.8:0.7:0.3:0.3, basis 2.9 times longer than wide with at least 3 broom setae, row of short setae on ventral side; ischium 1.8 times longer than wide, dorsal extension strongly expanded, sub-triangular, with 7 robust setae, apical 2 setae bifurcate, about as long as maximum ischium width, proximal 2 and distal 3 setae simple, but slightly increasing towards apex, proximodorsally 1 short seta, bent and directed towards proximal hollow of ischium articulation; merus twice as long as wide with weak distoventral extension and strong distodorsal extension bearing 6 bifurcate setae of similar length, 1.4 times maximal merus width, furcation not observed in most proximal seta of row, 4 simple setae ventrally; carpus 2.8 times longer than wide, distodorsally with 5 bifurcate setae and 1 broom seta, with 4 simple setae ventrally; propodus of 0.4 times carpus length, 3.6 times longer than wide, distoventrally with 1 simple and 1 broom seta, distodorsally with sensilla; dactylus 5.5 times longer than wide, 1.1 times carpus length, with 4 sensillae of different lengths arranged in 2 pairs on opposite sides.

Posterior pereopods (Fig. 13) length ratios: 1:1.7:2.3:1.9.

Pereopod 4 0.6 times pereopod 2 length, relative length ratios: 1:0.4:0.3:0.3:0.2:0.1, basis 3.9 times longer than wide, with at least 4 broom setae; ischium twice as long as wide, with 2 short and 2 long setae; merus about 1.8 times longer than wide, row of 3 setae of same length as merus, most distal seta bifurcate, with 1 simple seta distally on opposite side of the article; carpus 3 times longer than wide, 1.3 times longer than merus, setation similar to merus but most proximal seta of row with bifurcation, and 1 very long bifurcate seta, almost reaching distal tip of propodus; with 1 very long bifurcate seta reaching the tip of the most distally reaching seta of propodus, projecting beyond all setae of dactylus; propodus distally projecting into a subtriangular lobe, 2.3 times longer than wide, distally with 1 long simple seta and 1 robust and acute seta; dactylus very small, twice as long as wide, about 0.4 times propodus length, with 1 terminal bifurcate claw of 1.3 times dactylus length and 1 subterminal claw of 0.5 times dactylus length, with 1 thin terminal seta of intermediate length.

Pereopod 5 as long as pereopod 2, relative length ratios: 1:0.6:0.4:0.6:0.5:0.2, basis 4.1 times longer than wide with at least 6 broom setae dorsally, 2 rows with 5 small, acute and distally directed setae respectively, arranged dorsally and ventrally, with 1 large simple seta distoventrally; ischium 2.3 times longer than wide, with 2 setae dorsally, 2 groups of 3 closely together articulating setae, 1 group medioventrally and 1 group distoventrally; merus 1.8 times longer than wide, distodorsally extending with 1 simple and 1 very long bifurcate seta of 1.1 times merus length, with 3 setae ventrally; carpus 2.5 times longer than wide, ventrally with 3 setae and distoventrally with 3 setae, the strongest of which is bifurcate; propodus 3 times longer than wide, distodorsally projecting into an acute subtriangular lobe, with 2 simple setae distodorsally and 2 on the opposite side, 3 setae ventrally, articulating closely together; dactylus small, 3.7 times longer than wide, of half propodus length, 2 claws, terminal claw of 0.4 times dactylus length, subterminal claw of 0.3 times the length of dactylus, with 1 thin subterminal seta ventrally.

Pereopod 6 1.3 times pereopod 2 length, relative length ratios: 1:0.6:0.4:1:0.7:0.2, basis 3.9 times longer than wide, setation as in pereopod 5 except no setae dorsally; ischium 2.4 times longer than wide, with 5 setae dorsally, 9 setae in 3 groups ventrally; merus about 2.3 times longer than wide, distodorsally extending with 4 unequal setae, the longest little longer than the article and bifurcate, 2 groups of setae ventrally; carpus elongated, as long as basis, 7.7 times longer than wide, dorsally with 1 seta and 1 broom seta, with 1 simple and 1 bifurcate seta distodorsally, ventrally with 3 robust bifurcate setae and distoventrally with a group of 3 bifurcate setae of varying lengths; propodus slender, 6.8 times longer than wide, distodorsally projecting into an acute subtriangular lobe, 2 bifurcate setae ventrally and 2 distoventrally, the longest one 1.5 times longer than dactylus; dactylus 5 times longer than wide, 0.3 times propodus length, with a terminal claw of 1.4 times dactylus length and a subterminal claw less than half as long.

Pereopod 7 1.1 times longer than pereopod 2, relative length ratios and setation similar to pereopod 6 except basis without broom setae, but dorsally with row of more than 25 simple setae of twice the length of basis width and a ventral row of at least 5 simple setae.

Pleopods 1-4 (Fig. 14) relative length ratios: 1:0.9:0.6:0.4. Operculum of 0.7 times pleotelson length, opening width of distoventral excavation about 0.2 pleotelson width. Pleopod 1 elongate, proximal width about 0.4 times length, width at distolateral lobes 0.3 times length, laterally convex with minimal width about 0.1 times length, setation pattern mostly not symmetrical, distolateral lobes not projecting as wide distally as medial lobes, bent laterally and ventrally, medial lobes rounded, with 7 simple setae of different lengths and strengths arranged symmetrically. Pleopod 2 elongate, proximal width 0.3 times length, maximal width (including stylet) 0.4 times length, stylet almost 0.5 times total length of protopod, laterally with row of at least 11 pappose setae, and 9 long distal pappose setae, medially some simple setae. Pleopod 3 protopod of rhomboid shape, 0.4 times total length, endopod not considerably narrowing, with 3 plumose setae and 2 pairs of 2 short setae distally, exopod 0.9 times total endopod length, distally narrowing, with 1 seta subterminally, row of numerous long simple setae surrounding the exopod laterally incl. distal apex and getting shorter medially. Pleopod 4 protopod and exopod broken off and missing; with several very small setae on apex margin and 1 pronounced seta subterminally.

Uropod (Figs 10, 14) of two articles, 0.8 times pleotelson length, protopod 7.9 times longer than wide, endopod small and slender, less than 0.2 times the length of protopod.

Description paratype female.

Body (Fig. 15) elongate, 4.5 times longer than wide; maximal body width in pereonite 4, 1.6 times wider than pleotelson; pereonites 2-4 about the same width, 1.1 times pereonite 1 width, small ventral spine posteriorly on pereonite 3. Cephalothorax (Fig. 16) almost semicircular with maximal width at posterolateral corners, length 0.8 times width; slightly longer than 0.1 times total body length; transverse ridge on frons not very prominent, of curved shape. Anterior pereonal division (Fig. 16) 0.2 times total body length; 1.1 times longer than wide. Posterior pereonites (Fig. 16) relative medial segment lengths: 1:1.4:1.1:1.2; relative segment widths of pereonites 4-7: 1:0.9:0.8:0.8, pereonite 7 0.8 the width of pereonite 1. Pleotelson (Fig. 16) 0.2 times body length; about 1.7 times longer than wide; caudal apex slightly concave; breathing cavity maximal opening width 0.7 times pleotelson width, longitudinal excavation 0.2 times maximal pleotelson width.

Antenna 1 (Fig. 16) of 5 articles, 3.4 times longer than article 1 width; articles 1 and 5 as long as wide, fifth article 0.1 times total length, articles 2-4 2 times longer than wide, gradually decreasing in size; 1 aesthetasc on article 5, 0.6 times the length of antenna 1.

Antenna 2 (Fig. 16) relatively slender compared to male antenna 2; articles 1-3 as long as wide; article 4 half as wide as articles 1-3; article 5 0.6 times article 4 length and of same width.

Maxilla 1 (Fig. 16) inner endite shorter and more slender than outer endite, terminally narrowing, around distal apex, dorsally and along lateral margin with numerous long and some very small simple setae; outer endite 1.4 times longer than inner one, narrowing distolaterally, with at least 11 simple setae on lateral margin, with 4 combs of 2-4 simple setae on medial margin, numerous simple setae of different lengths distomedially and 12 robust distal setae, some two-sided denticulate on distal margin.

Maxilla 2 (Fig. 16) inner endite broadest, outer endite of intermediate width, medial endite longest, outer endite 0.8 times middle-endite length, inner endite 0.9 times middle-endite length; margin of inner endite with 12 long simple basomedial setae of more than half the length of endite, numerous small simple setae ventrally and laterally, distal margin with 8 robust setae: 5 simple and 3 denticulate; medial endite with 1 lateral row of simple setae, distally with 4 long simple setae; outer endite with small simple setae along both margins, distally with 4 stiff simple setae of different lengths, the longest of which is of more than half the length of the outer endite. Measurements of article lengths ratios and length-width ratios of articles are listed in Table 3.

Anterior pereopods (Fig. 17) increasing in length, relative length ratios, omitting basis: 1:1.2:1.3, with basis longest article. Shapes and setation as in holotype. Pereopod 2 basis damaged. Except: pereopod 3 relative length ratios and setation different from holotype: basis relatively elongated; ischium only 2.2 times longer than wide, dorsal expansion less prominent than in holotype; propodus 0.5 times the length of carpus; dactylus long and slender, 0.9 times carpus length; lengths and robustness of setae as well as the number of setae per row increased in merus and carpus.

Posterior pereopods (Fig. 18). Pereopod 4 shortest, length ratios: 1:1.6:2.0; pereopod 7 missing. Pereopod 4 0.7 times pereopod 1 length; dactylus 0.6 times propodus length. Pereopod 5 1.1 times longer than pereopod 1; all articles elongated compared to holotype, especially carpus and propodus. Pereopod 6 1.4 times longer than pereopod 1; with more setae in carpus and propodus.

Operculum (Figs 9, 18) length/width ratio 1.6, densely covered with small setules, with 4 setae alternating along medial line of fusion, numerous setae ventrolaterally along proximal margin, distally with 15 pappose setae. Pleopod 3 and 4 as in male (Fig. 14). Uropods broken off and missing in analysed specimens.

Sexual dimorphism.

The male specimen is more slender than the female, its pleotelson is less narrowing posteriorly. The antenna 1 is relatively smaller but wider than in female and with more aesthetascs. The antenna 2 is little larger in the male, almost 0.4 times the length of the total body, 0.3 in female. Pereopods are dimorphic by means of article length ratios while pereopod length in relation to body length and setation are identical in both sexes.

Etymology.

“Antennamagna” is derived from the Latin word for "big antenna" as the second antennae are conspicuously large in both sexes.

Remarks.

Macrostylis antennamagna sp. n. can be delimited from all other species by the large antenna 2, the three-lobed pars incisiva, the roundish appearance of the cephalothorax without a transverse ridge on frons and the shape of male pleopod 1. Macrostylis antennamagna sp. n. is most similar to Macrostylis urceolata Mezhov, 1989 which is the only known species of this genus with comparably prominent antenna 2. Both species also share the general appearance in dorsal view and have high similarities in following characters: mandibles, male pleopod 1 and ventral spines. Macrostylis antennamagna sp. n. can be distinguished from Macrostylis urceolata by the transverse ridge on frons lacking, antenna 1 being stouter, lacinia mobilis being smaller, the male pleopod 1 being wider and stouter, the smaller relative length of the pleotelson, and the “bifurcate” caudal tip of the pleotelson. Macrostylis gerdesi (Brandt, 2002), Macrostylis carinifera Mezhov, 1988, Macrostylis grandis Birstein, 1970, Macrostylis hirsuticaudis Menzies, 1962, Macrostylis longiremis (Meinert, 1890), Macrostylis minuscularia Mezhov, 2003, Macrostylis sarsi Brandt, 1992, Macrostylis sensitiva Birstein, 1970, Macrostylis ovata Birstein, 1970, and Macrostylis vinogradovae Mezhov, 1992 have a comparably long antenna 2. However, these species show distinct characters clearly delimitating them from Macrostylis antennamagna sp. n. of which only the most obvious are listed below: antenna 1 of Macrostylis gerdesi has nine articles, Macrostylis carinifera bears a posterior ventral spine on pereonite 4 and a much more stout pleopod 2, Macrostylis grandis and Macrostylis ovata have a much wider habitus in dorsal view, Macrostylis hirsuticaudis has an uniquely shaped pleotelson with an almost straight posterior end, the habitus of Macrostylis longiremis is constantly narrowing towards posteriorly and the cephalothorax bears spines in posterolateral corners, Macrostylis minuscularia has a comparably long but thinner antenna 2 and antenna 1 differs in article length ratios and size, Macrostylis sarsi shows a stouter habitus and more slender antenna 2, the latter is also true for Macrostylis sensitiva and Macrostylis vinogradovae .

Antenna 2 can be found even bigger in Macrostylis spinifera Sars, 1864, Macrostylis polaris Malyutina & Kussakin, 1996 and Macrostylis porrecta Mezhov, 1988. However, in Macrostylis spinifera shape and size ratios of male and female pleopods 1 and 2 differ from Macrostylis antennamagna sp. n. and in the other two species antenna 2 has more slender peducular articles and antenna 1 is bigger compared to Macrostylis antennamagna sp. n.

The slender habitus with almost parallel sides in dorsal view shown by Macrostylis antennamagna sp. n. is common in Macrostylidae . The “bifurcated” caudal end is also present in Macrostylis bifurcatus Menzies, 1962, although much stronger developed there. The pleopods 5 have not been found in both sexes. It is unclear, though, whether these have been broken off during dissection or if they are generally reduced in this species. The analysis of five specimens from two stations has revealed little individual variation. Variations in setal counts are probably allometric (compare Hessler 1970 for Desmosomatidae ).