Azygokeras columbiae Koeller & Littlepage, 1976

Bradford-Grieve, Janet M. & Boxshall, Geoffrey A., 2019, Re-description of Azygokeras columbiae Koeller & Littlepage, 1976 (Calanoida: Aetideidae) and musculature of the male grasping antennule, Zootaxa 4565 (3), pp. 361-372 : 362-370

Azygokeras columbiae Koeller & Littlepage, 1976

( Figs. 1–6 )

Description: Female: 2.6 mm. Generally, as in the original description with some details corrected. Rostral margin not extended, without rostral filaments ( Fig. 1A ), pedigerous somites 4 and 5 separate, posterior borders of genital double-somite and urosomites 2 and 3 lined with dorsal and ventral spinules ( Fig. 1B ). Caudal rami without seta I, seta II small, inserted on outer distal corner, terminal setae III-VI largest and seta VII small, inserted on ventral surface.

Antennule with joints between functional telescopic segments separated by ring of arthrodial membrane that allows for limited flexion in any direction. Nearly all setae pseudoannulate and segments II-IV, X-XI and XXVII- XXVIII fused ( Fig. 1 C-F). Setation as follows: I—1 s; II, III, IV , V, VI—2 s; VII—2 s, 1a; VIII, IX, X— 2 s; XI—2 s, 1a; XII, XIII—2s; XIV—2s, 1a; XV—2s; XVI—2s, 1a; XVII, XVIII, XIX, XX—2 s; XXI—2 s, 1a; XXII, XXIII— 1 s; XXIV—1+1s (plumose); XXV, XXVI—1 +1s; XXVII—1+1s; XXVIII—2s, 1a. Setae on at least segments V, IX, XX, XXIII, XXV, XXVI, XXVII and possibly XVI, XXII, and XXIV are particularly long although many are broken. Musculature composed of large bundle of muscles running ventrally (muscle 3) and pair of smaller bundles (muscles 2 and 4) originating dorsally as well as muscle 1, originating in prosome, extending along posterior surface with an undiscernible insertion point ( Table 1). Ventral bundle of muscles (3) originates on segment I and extends along ventral side of antennule to insert on antero-proximal corner of segment XXVII; posteriormost section of muscle bundle 3 extends shorter distance, inserting on proximal rim of, at least, segment V. Posterodorsally-inserted muscle (2), originating on segment I, extends along posterior border and appears to insert on proximal rim of segment XXVII. Anterodorsally-inserting muscle (4), originating on segment I, extends distally to proximal rim of segment XXII.

Antenna exopod slightly longer than endopod; coxa with 1 plumose seta and comb of long setules, basis with 2 setae. Endopod segment 1 with 2 setae and segment 2 with 9 (1 vestigial) + 7 (1 short) setae, outer distal border lined with small spinules, position of border between segments 2 and 3 marked by row of very small spinules on one surface. Exopod ( Fig. 1G ) with 7 articulated segments: I separate, II-IV fused, V, VI, VII, VIII separate and IX- X fused. Exopod segment I without seta, segments II-VIII with 1 seta each, segment IX without seta, segment X with 3 terminal setae.

Mandible ( Fig. 2A, B ) gnathobase with transverse rows of spinules ventrally at base of blade; basis with 3 well-developed setae, endopod segment 1 with 2 setae, segment 2 with 10 terminal setae, 1 of them much short than others; exopod segments 1–5 with 1, 1, 1, 1, 2 setae, respectively.

Maxillule praecoxal arthrite with 9 terminal stout spines and 1 posterior and 1 anterior surface seta; coxal endite with 4 setae; basal endites 1 and 2 with 4 and 5 setae, respectively, basal endite 2 with posterior surface crescent of conspicuous spinules; elongate basis and endopod segment apparently separated, endopod segments 1 and 2 fused, endopod segment 3 separate, endopod with 4, 4, and 6 setae, respectively; exopod small with 11 setae, coxal epipodite with 9 setae, 2 proximalmost vestigial.

Maxilla ( Fig. 2C, D ) praecoxal endites 1 and 2 with 3 setae each, medial seta on endite 2 short; coxal endites 1 and 2 with 3 setae each, medial seta short; basal endite longest with 3 setae, medial seta short and distal seta more robustly developed than others with longitudinal row of very fine spinules; endopod 4-segmented, segment I with small endite, segments 1–4 with 2 (1 very short), 1, 1, 3 setae respectively. Praecoxal endite 1 with surface spinules arranged approximately longitudinally.

Maxilliped ( Fig. 2E, F ) syncoxa first to fourth endites with 0, 2, 3, and 3 setae respectively; basis with 3 setae and patch of small proximal spinules; endopod segments 1–6 with: 2, 4, 4, 3, 3+1, 4 setae, respectively, longest setae extending to proximal border of basis.

Legs: Legs 1–4 typical for Clausocalanoidea. Leg 1 with 3-segmented exopod and 1-segmented endopod; setation as follows: Coxa 0-0. Basis I-1. Exopod I-0; I-1; I-I-3. Endopod 0,2,3; coxa and basis with inner and outer borders lined with setules, outer border spine of basis vestigial ( Fig. 1H ). Leg 2 with 3-segmented exopod and 2- segmented endopod (arthrodial membrane between segments 2 and 3 has failed to develop); setation as follows: Coxa 0-1. Basis 0-0. Exopod I-1; I-1; III-I-4. Endopod 0-1; 1,2,2. Legs 3 and 4 with 3-segmented exopod and endopod; setation as follows: Coxa 0-1. Basis 0-0. Exopod I-1; I-1; III-I-4. Endopod 0-1; 0-1; 1,2,2. Coxae of legs 2 and 3 with lateral spinules and inner setules and basis of both legs with posterodistal spinules. Posterior surfaces of endopod segments of legs 2–4 decorated with small spinules (e.g. Fig. 2G ). Leg 4 coxa without lateral spinules or posterodistal spinules on basis ( Fig. 2G ). Leg 5 absent.

Male: 2.1 mm. Generally, as in the original description with some details corrected.

Rostral margin not extended, without rostral filaments, pedigerous somites 4 and 5 apparently fused, posterior borders of urosomites 2–4 lined with dorsal and ventral spinules. Caudal rami without seta I, seta II small, inserted on outer distal corner, terminal setae III-VI largest and seta VII small, inserted on ventral surface.

Left antennule ( Fig. 3A ) with nearly all setae pseudoannulate and segments II-IV, X-XII, XIV-XV, at least, fused and appear to have limited flexion in any direction as in the female antennule. Setation as follows: I—1 s, 1a; II—2 s, 1a; III—2 s, 2a; IV—2 s, 1a; V, VI, VII, VIII, IX, X, XI—2 s, 2a; XII—1 a; XIII, XIV, XV, XVII, XVIII, XIX, XX—2 s, 1a each, remainder of antennule broken off. Segments become gradually narrower towards distal part of left antennule. Musculature appears to be like that of female up to segment XX including presence of muscle 1 but its course is difficult to determine. The only exception is that anterodorsally originating muscle (5) on segment I attaches to proximal border of segment VI, altering course to run dorsally and insert on proximal rim of segment X .

Right antennule ( Fig. 3 B–D) has several specialized distal features that allow greater movement between segments in several planes. Arthrodial membranes between segments XXI and XXII, XXIII and XXIV and XXV and XXVI allow greater movement than in proximal segments. Segment XXII-XXIII has proximal border that allows dorsal-ward flexion around 2 pivot points. Oblique proximal border of segment XXIV allows mainly ventral-ward flexion and slightly anteriorly, and segments XXV and XXVI flex anteriorly. Bulky muscles that bring about this movement in distal segments explain greater width of distal segments on right compared with homologous segments of female antennule. Knee joint absent and joints around segments XIV-XVI without specialized musculature. Nearly all setae pseudoannulate and segments II-IV, X-XII, XIV-XV, XXII-XXIII and XXVII-XXVIII fused. Setation as follows: I—1 s, 1a; II—2 s, 1a; III—2 s, 2a; IV—2 s, 1a; V, VI, VII, VIII, IX, X, XI—2 s, 2a; XII—1 a; XIII, XIV, XV, XVI, XVII, XVIII, XIX, XX, XXI—2s, 1a; XXII—1s; XXIII—1s, 1a; XXIV—1+1s, 1a; XXV—1+1s, 1a; XXVI—1+1s; XXVII—1+1s; XXVIII—2s, 1a. Setae on segments II, III, V and XX are very long although some are broken. Midlength seta on segments XX, XXI, XXII-XXIII and anterodistal seta on segment XXIV swollen proximally compared with related, more proximal setae, with proximal cuticle thickened and fine, whip-like distal part. Muscle 1 present, extending along posterior surface and inserting on proximodorsal rim of segment X. Ventral bundle of muscles (4) originates broadly on segment I and continues along ventral surface, apparently through boundary between segments XXI and fused XXII-XXIII, to insert on ventroproximal wall of segment XXII-XXIII ( Fig. 6A ; Table 1). Anterodorsally inserting muscle (5) on segment I attaches to proximal border of segment VI, alters course to run dorsally to attach to proximodorsal rim of segment X. Segment I with posterodorsally-originating muscle (2) apparently extending distally along posterior border of antennule ( Fig. 6B ). Muscle 2 has an intermediate attachment on posteroproximal wall of segment XXI, then continues into segment XXII-XXIII where half of the muscle bundle attaches on posterodistal wall of segment XXII. On segments XIX-XXVI, short, anteriorly situated muscles originate from ventral surface of each segment, level with midlength seta or more proximally, inserting at proximodorsal corner of the following segment ( Fig. 6A ) (muscles 6.1–6.3). On segment XXII-XXIII, muscle 6.4 extends from its origin on ventroposterior wall, obliquely across ventral face of segment and inserts on posteroproximal corner of segment XXIV and similarly muscles 6.5 and 6.6 insert on posteroproximal corners of segments XXIV and XXV, respectively, and continue to posteroproximal borders of segment XXV and XXVI, respectively. On segment XXVI muscle 6.7 originates proximodorsally and extends to the proximal rim of segment XXVII. Muscles illustrated in figures 6A and 6B increase in bulk from proximal to distal and appear to work antagonistically to straighten antennule.

Antenna coxa with 1 plumose seta and comb of long spinules; basis with 2 setae; endopod segment 1 without pair of setae but with outer distal patch of spinules, segment 2 with 7 + 4 setae and outer distal patch of spinules; exopod segments II-IV without setae, segments V-VIII with 1 seta each segment X with 3 terminal setae but may be damaged.

Mandible ( Fig. 4A ) gnathobase atrophied, basis with 1 seta, endopod segment 1 with 2 reduced setae, endopod segment 2 with 10 setae 1 of which very small, exopod with 1, 1, 1, 1, 2 setae.

Maxillule and maxilla very reduced.

Maxilliped ( Fig. 4B ) syncoxa with endites 1–4 with 0, 0, 2, 3 vestigial setae; basis with 1 well-developed seta and 2 vestigial setae; endopod segments 1–6 with 2, 4, 4, 3, 3+1, 4 setae, respectively.

Legs 1–4 as in female, leg 2 ( Fig. 4C ) endopod 2-segmented (arthrodial membrane between ancestral segments 2 and 3 not developed), coxa, basis and endopod with well-developed posterior surface spinules.

Leg 5 ( Fig. 4D ) with rami slender without swollen basis, exopods 3-segmented, endopods 1-segmented, misshapen on right, left endopod with partial suture which may represent vestige of segmentation; left leg longer relative to right leg than in the original description, right exopod terminal spine does not extend as far as distal border of left exopod segment 1.

Remarks. The examined paratype specimens of Azygokeras columbiae conform to the general habit as in the original description but several differences in the setation of the female antenna, maxillule, maxilla, maxilliped and leg 1 and the male mandible, maxilliped and fifth leg are corrected here. The antennules of both sexes are extensively described because these details were not given in the original description.

Azygokeras columbiae is typical of the family Aetideidae within the Clausocalanoidea in having: leg 1 endopod 1-segmented, leg 2 endopod 2-segmented, legs 3–4 outer border of endopod segment 3 with 1 seta, and legs 2–4 exopod segment 3 inner border with 4 setae and a maximum of 3 setae on the maxilla praecoxal endite 1, 2 setae on female antennular segments XV and XXI and the female leg 5 usually absent (e.g. Andronov 1974; Markhaseva 1996; Boxshall & Halsey 2004: 57; Markhaseva & Schulz 2006:13; Markhaseva & Renz 2015:1041).

Amongst the Aetideidae , Azygokeras is most closely related to Farrania , Parabradyidius and Pseudotharybis in having 1 seta on antennule segment I. Azygokeras is also related to a broader group of benthopelagic genera, although not united by any synapomorphy ( Bradford-Grieve & Boxshall 2019). This broader group of benthopelagic genera have very long pseudoannulate setae on many or some antennular segments, typified by that on segment XXIII which extends beyond the distal border of segment XXVIII. The males of this extended group of genera, where known, have slender, usually biramous fifth legs and females may or may not have rudimentary fifth legs ( Bradford-Grieve & Boxshall 2019, and references therein).

Figures

FIGURE 1. Azygokeras columbiae female: A, anterior head, lateral view; B, posterior prosome and urosome, lateral view; C, antennule, segments I–XVI; D, antennule, segments XVII-XXIII; E, antennule, segments XXIV–XXVIII; F, antennule, segments XXVII–XXVIII; G, antenna exopod; H, leg 1, basis and segment 1 of exopod and endopod, outer seta of basis indicated with arrow. Scale bars represent 0.1 mm.

FIGURE 2. Azygokeras columbiae female: A, mandibular palp; B, mandibular gnathobase; C, maxilla; D, endopod of maxilla; E, basis and endopod of maxilliped; F, syncoxa of maxilliped; G, leg 4, posterior surface, remainder of exopod missing. Scale bars represent 0.1 mm.

FIGURE 3. Azygokeras columbiae male: A, left antennule, segments I–XX; B, right antennule, segments I–XV; C, right antennule, segments XVI–XXVIII; D, right antennule, segments XXVII–XXVIII. Scale bars represent 0.1 mm.

FIGURE 4. Azygokeras columbiae male: A, mandible; B, maxilliped; C, leg 2, posterior surface, remainder of exopod missing; D, leg 5, posterior surface, note right endopod bent. Scale bars represent 0.1 mm.

FIGURE 5. Azygokeras columbiae female antennule musculature: A, dorsal muscles 2 and 4; B, ventral muscle 3. See Table 1 for equivalent nomenclature of Boxshall (1985).

FIGURE 6. Azygokeras columbiae male right antennule distal musculature: A, muscles 4 and 6; B, muscle 2. See Table 1 for comparison of female and male right antennular muscles.