Description

Description. Living specimens of both sexes semitransparent, light colored, but with bright red-orange gut diverticula; rounded embryos brooded inside female mantle cavity visible through carapace (Figs 1, 2). Abdomen and antennules often extending out of carapace during movements (Fig. 1 B-D). Female (Figs 1 - 4, 8 - 12): Carapace oval, up to 2.3 mm long and 2.0 mm high, bivalved (Figs 1 B, C, 2 A-C, 8 A, B), valves joined and hinged along dorsal margin (Fig. 17 A). Dorsal and posterior margins of valves feebly convex, meeting at slightly produced posterio-dorsal angle; anterior and ventral margins rounded (Figs 1 B, C, 2 A-C, 8 B). Exterior of carapace smooth, lacking setae but covered with small pores (Figs 8 A, 17 A, B, 18 A-D). Right and left gut diverticula (Figs 1 B, C, 2 A-C) lying within respective carapace valve, resembling letter " W "; short main branch descending toward ventral margin and bifurcating, with anterior branch shorter than posterior and numerous simple and bifid small branches extending from them in various directions; dorsal, ventral, anterior and posterior small branches terminated with light orange, wide areas at edge of carapace (Fig. 1 B, C). Inner surface of carapace valves with cuticular lining or mantle (Fig. 8 B-F). Small, narrow pit on inner surface of anterior part of each valve (Figs 2 A, C, 8 E). Anterior pit of carapace infundibuliform, with wide entrance and long, narrowed internal part (Figs 2 D, 8 E); cuticle of pit wrinkled, with circular folds, small pores and volcano-shaped papillae (Fig. 8 E, F). Body situated within mantle cavity (Figs 2 B, 8 A); oval brood chamber for embryos in posterior portion of each valve (Fig. 2 B). Cuticular armament of mantle similar to that in S. grygieri (see Kolbasov and Newman 2018). Main cuticular structures of mantle arrayed along its margin: anterior and ventral sides with submarginal underlying folder consisting of dense row of cuticular projections forming fringe or palisade (Fig. 8 C); anterior, ventro-posterior and posterior sides of mantle bearing long setae with short setules, these being absent ventro-anteriorly (Fig. 8 B-D). Body proper consisting of unsegmented head and segmented thorax and abdomen. Head bearing W-shaped prehensile antennules followed by large ventral oral cone formed of mouth parts surrounded by labrum (Fig. 2 B). Frontal filament complex (Fig. 8 D) originating on mantle rather than body proper, ~ 380 μm long and trifid, with anterior ramus longest (ca. 350 μm) and densely covered by long, setiform protrusions; ampuliform, short basal ramus (ca. 70 μm) with smooth cuticle; and small, thin posterior ramus (ca. 40 μm). Thorax consisting of six segments (Figs 2 B, 9 A, B), each with pair of biramous natatory thoracopods described in detail below. Dorsal sides of segments (II-VI) covered with thin setae (Fig. 9 A, B). Posterio-ventral angles of sixth thoracic segment formed as small triangular projections or epaulets, their surface covered by rounded plaques (Fig. 9 B). Abdomen U-shaped, five-segmented, including telson (Figs 2 B, 8 A, 9 B). First segment with vestigial penis on ventral side (Figs 2 E, 9 B, C), an unpaired process - 140 - 190 μm long, its distal part bearing ctenoid scales (Fig. 9 C). Second segment trapezoid, bigger than either third or fourth. Last body segment (telson) cylindrical, ca. 300 μm long, its posterio-ventral margin bearing fringe of ctenoid scales and pair of conspicuous telson spines (Figs 2 B, 9 B, D) approximately 190 μm long with row of nine or ten sharp denticles along their dorsal margins. Furcal rami unsegmented (Figs 2 B, F, 9 B), approximately 410 - 560 μm long, thus approximately 2.5 - 2.9 times longer than telson spines; ventral margin with one medial, one subdistal and two distal setae, rarely with long setules (Figs 2 F, 9 B, E, G); proximal half of ventral margin armed with large, sharp denticles and ctenoid scales (Figs 2 F, 9 D, F, H). Inner subdorsal margin of each ramus with row of eight long natatory setae with long setules (Figs 2 F, 9 F, H); row of dense ctenoid scales along inner side of dorsal margin (Fig. 9 F-H). Extendable, prehensile antennules subchelate, folded into W-shape, consisting of six segments with complex of intrinsic and extrinsic flexor and extensor muscles (Figs 2 B, 3 A, B, 10 A). First segment rectangular, narrowing somewhat distally, without setae. Second segment irregularly rectangular, with dense, thin omniserrate setae along postaxial / ventral margin (Figs 3 A, B, 10 A, C). Third segment equilaterally triangular, narrowing toward ventral margin; preaxial / dorsal margin curved, densely covered by thin omniserrate setae (Figs 3 A, B, 10 A). Fourth segment rectangular, trapezoid (appearing triangular in folded antennules, Fig. 3 A, B), very narrow, narrowing towards dorsal margin, with two massive and denticulate setae (' spines') armed with row of dense, conspicuous denticles along anterior edge and, rarely, tiny denticles on posterior edge; these two spines sitting on dorsal projection apex bearing ctenoid scales with sharp denticles (Figs 3 A, B, 10 A, F). The two spines form a fork to accept movable claw. Fifth segment conical, forming a palm against which sixth segment can fold in order to grasp host tissue, with 6 - 9 strong, simple setae along upper margin (Figs 3 A, B, 10 A). Sixth segment longer than fifth segment and armed with sensory and grasping structures (Figs 3 A, B, 10 A, D, E, G-I). Short proximal sensory process on lower margin at base of sixth segment (Figs 3 A, B, 10 A, G), with 3 terminal setae, middle one setulated and longest, and 1 thick, blunt sub-basal seta (at least this seta probably an aesthetasc). Curved claw on distal end of sixth segment apparently with muscles attached (Figs 3 A, B, 10 A, D, E); concave margin of claw serrate, with sharp tiny denticles in middle part (Fig. 10 D, E); three small setae at base of claw, two lateral on inner and outer surfaces and one on anterior dorsal margin (Figs 3 A, B, 10 D, H). Relaxed claw sheathed by grooved claw guard (Figs 3 A, B, 10 D, E, H, I), latter approximately 110 μm long, with wide flange on inner side, thin, membranous, apical ctenoid hood (Fig. 10 I) and four small terminal setae including two longer and one tiny subapical (Fig. 10 E) and 1 tiny apical seta (Fig. 10 I). Cuticle on sides of antennular segments bearing dense small ctenoid scales (Fig. 10 B). Oral cone prominent, approximately 600 - 650 μm long; distal end often protruding outside carapace (Figs 1 B, D, 2 A-C, 11 A); formed by cone-shaped labrum surrounding piercing mouth parts (Fig. 11 A). Posterior margins of labrum free, unfused (Fig. 11 A). Tuft of long, thin simple setae in middle of anterior face of labrum; dense, small ctenoid scales on external cuticle (Fig. 11 A). Mandibles in form of lanceolate stylets, approximately 350 μm long (Figs 3 D, 11 B); cutting edge of each bearing approximately 80 - 90 sharp, complex teeth with four tips (quadrifid), length of teeth increasing towards middle part of blade, with row of small setae paralleling them (Fig. 11 D); neck of mandible lacking denticles or teeth but bearing small simple setae; distal part with row of 16 - 20 curved teeth on posterior margin (Figs 3 D, 11 B). Maxillules consisting of a wide basal half and narrow distal half (Figs 3 E, 11 B, C); cutting edge bearing numerous denticles with serrate margin and cuticular setiform projections, these denticles being massive in proximal part and thin and elongate in middle and distal parts (Fig. 11 B, C, E); tip with thin, curved setiform projections (probably setae, Fig. 11 F). Maxillae (Figs 3 F, 11 C) thin, fused at bases, with row of thin, needle-shaped denticles along inner cutting edge at distal end (Fig. 11 G); tips not distinctly bifid, not harpoon-shaped, with apical projection and adjacent tiny process (probably seta, Fig. 11 G). Unpaired process or medial languette (fused paragnaths?) originating from between paired mouth parts, with sharp tip and two rows of denticles on anterior margin (Fig. 3 C). All thoracopods natatory and biramous (Figs 4, 12). Seminal receptacles found in lateral proximal parts of coxae of thoracopods II-V (Fig. 4 B-E), consisting of ampuliform sacs with proximal parts converging but external opening (s) not observed; thoracopods II with four seminal receptacles, thoracopods III and IV each with three and thoracopod V with one. Thoracopodal setation summarized in Table 1. First thoracopod (Fig. 4 A) slightly separated from others, with elongate protopod comprised of coxa and basis and two-segmented exopod and endopod; margins of basis with tufts of short setae; segments of exopod with ctenoid scales and small denticles, inner margin of basal segment lined with dense thin, small setae; seven long, plumose setae situated at distal end of second segment; basal segment of endopod bearing three long, plumose setae, margins being lined with dense thin, small setae; distal segment with three terminal plumose setae. Thoracopods II-V with three-segmented endopods and two-segmented exopods (Figs 4 B-E, 12 A-E). Coxae of thoracopods II and III (Figs 4 B, C, 12 B, C) with large, distal seta in position " 1 " (see Table 1 for further explanation) and row of plumose setae along inner edge (position " 9 "); these setae absent on coxae of thoracopods IV and V (Fig. 4 D, E). Number of setae on rami of thoracopods II and III much more numerous than on thoracopods IV and V. Protopod of thoracopod VI (Figs 4 F, 12 F) narrow; coxa and basis without setae; both rami two-segmented with long, plumose terminal setae on distal segments; two tufts of thin, small setae on basal segment of endopod and distal segment of exopod. Surface of all thoracopods bearing conspicuous ctenoid scales (Fig. 12). Male (Figs 1 D, 5 - 7, 13 - 16): Carapace bivalved, ellipsoidal, up to 1.5 mm long and 0.9 mm high, with slightly produced posterio-dorsal tip (Figs 5 A, B, 13 A). Dorsal margin almost straight; anterior, ventral and posterior margins rounded. Exterior of carapace smooth, lacking setae but covered with small pores (Figs 13 A, 17 E, F, 18 E-H). Conspicuous deep pit with curved lumen opening on inner surface of anterior part of each valve (Fig. 5 A, B). Gut diverticulum of simplified W-shape in comparison to female (Figs 1 D, 5 A, B), with 4 short lateral branches extending from anterior, posterior and ventral parts and terminated with light orange, widened areas at edge of carapace. Lobed testis within each carapace valve along lower part of gut diverticulum (Fig. 5 A). Cuticular armament of mantle is similar to that of female (Fig. 13 C-E). Edge of mantle forming thin marginal fold adjacent to margin of carapace and consisting of dense, tiny cuticular projections (Figs 13 E, 15 A). Anterior, ventral and posterior sides with submarginal underlying folder consisting of dense row of cuticular projections forming fringe or palisade, these projections longer in posterior side (Fig. 13 C-E); anterior, ventro-posterior and posterior sides of mantle bearing setae with short setules (Fig. 13 C, E). Frontal filament long, trifid, more complex or less reduced than in female, with well-developed anterior and posterior rami covered by long, setiform cuticular projections (Figs 5 C, 12 B, F). Anterior ramus thicker and shorter than posterior, approximately 200 - 250 μm long; medial (basal) ramus short (50 - 80 μm), ampulliform, with smooth cuticle; posterior ramus longest, approximately 470 μm. Body of male resembling that of female (Figs 5 A, 13 B): head bearing similar W-shaped antennules and well-developed oral cone; trunk consisting of 6 thoracic and 5 abdominal segments (Figs 5 A, 13 B, 14 B); telson spines of same proportions and morphology (Figs 5 F, 14 E). Furcal rami resembling these of female in many details (Fig. 14 E-H) but differ in having fewer long natatory setae on inner subdorsal margin (six instead of eight, Fig. 5 F). Unlike in females, epaulets of sixth thoracic segment more strongly developed (Fig. 14 A). Condition of penis considerably different between male and female, tergite of penis-bearing first abdominal segment with conspicuous pair of long (approximately 100 μm), posteriorly directed pleural processes with four sharp terminal extensions that are absent in females (Figs 5 D, E, 14 B). Penis complex, approximately 600 μm long, ~ 4 times longer than supporting segment, and consisting of three parts: basal, medial and distal (Figs 5 D, 14 B, C). Basal shaft cylindrical, approximately 160 μm long. Medial part swollen, ~ 136 μm long, with unpaired thin process ~ 110 μm long extending from anterior side, tip of process (Fig. 5 D) covered by thin layer of epicuticle. Distal part consisting of two rami originating from medial part and narrowing toward tips (Figs 5 D, 14 B, C). Cuticular setiform projections 10 - 20 μm long with apical pore (not setae) present along anterior margin of each ramus (Figs 5 D, 14 B, C). Tip of each ramus terminating in pair of these projections (Fig. 14 C). Antennules of male resembling those of female (Figs 6 A, B, 15) but relatively thinner and longer with respect to body size. Second and third segments with dense, thin setae in same positions as in female. Two massive spines of fourth segment armed with row of conspicuous denticles along both anterior and posterior edges (Fig. 15 A). Fifth segment with 4 - 6 rather than 6 - 9 setae on anterior margin (Figs 6 A, B, 15 B). Sensory and grasping structures of sixth segment of same morphology as in females, but ctenoid scales denser in lateral surfaces of segment (Figs 6 B, 15 C, D). Oral cone and mouth parts similar to those of female (Figs 6 C-G, 16), consisting of labrum (Figs 6 G, 16 A-C) enclosing an unpaired medial languette (Fig. 6 D) and paired mouth parts, mandibles (Figs 6 E, 16 C, D), maxillules (Figs 6 F, 16 C) and maxillae (Figs 6 G, 16 C, E); tips of maxillules bifid, not harpoon-shaped, apical projection and adjacent process slightly larger than in females (Figs 6 G, 16 E). Thoracopodal setation of male (Table 1, Fig. 7) similar to that of female (Fig. 4) but showing some differences (only thoracopod VI have same setation); distal segment of exopod of thoracopod I with eight rather than seven setae (Fig. 7 A), coxae of thoracopods II and III and bases of thoracopods II-IV have fewer setae along inner margins (Fig. 7 B-D).

Common Names

Taxonomic Hierarchy

Kingdom: Animalia

Phylum: Arthropoda

Class: Maxillopoda

Order: Laurida

Family: Synagogidae

Genus: Synagoga

Species: Synagoga arabesque Kolbasov, Petrunina, Ho & Chan, 2019