Synagoga mira Norman, 1888

Gongguan harbor, Green Island (Ludao), ca. 33 km off the southeastern coast of Taiwan, 22°41.438'N, 121°29.678'E, 35 m depth, 08 and 09 September 2017.

Twelve specimens of the new species, Synagoga arabesque sp. nov. (five males and seven females), were collected from two colonies of the antipatharian Myriopathes cf. japonica. Slides of the holotype female Mg 1243, and three paratypes (female, Mg 1244 and two males, Mg 1245) are deposited in the Zoological Museum of Moscow State University in Moscow, Russian Federation. The remaining two undissected paratypes (female and male) are deposited in alcohol in the Biodiversity Research Museum, Biodiversity Research Center, Academia Sinica, Taipei, Taiwan (ASIZCR000412). The other four SEM specimens and two undissected specimens in alcohol have been retained by the first author for further study and comparison with other synagogids.

Diagnoses for both adult females and males are provided for the new species, and a full list of interspecific differences is given in Table 2.

Females: carapace oval, slightly elongated in posterio-dorsal direction, up to 2.3 mm long and 2.0 mm high, with projecting posterio-dorsal tip. Massive setae (spines) of fourth antennular segment with row of dense, conspicuous denticles along anterior edge and rare, tiny denticles on posterior edge; fifth segment with 6–9 large setae; concave margin of antennular claw serrate in middle part. Exopod of second segment of thoracopod I with seven setae. Telson spines ca. 1/3 of blade length of furcal ramus; inner surface of furcal ramus with eight setae. Gut diverticulum red-orange, W-shaped, with numerous branches; dorsal, ventral, anterior and posterior branches terminate with light orange, wide areas at the edge of carapace.

Males: carapace ellipsoidal, up to 1.5 mm long and 0.9 mm high, with slightly projecting posterio-dorsal tip. Massive setae (spines) of fourth antennular segment differing slightly in length, with anterior and posterior rows of small denticles; fifth segment with 4–6 large setae; other characters of antennules similar to those in female. Exopod of second segment of thoracopod I with eight setae. Telson spines ca. 1/3 of blade length of furcal ramus; inner surface of furcal ramus with six setae. Gut diverticulum red-orange, W-shaped, with short anterior, posterior, and two ventral branches; branches terminate with light orange wide areas at edge of carapace.

From French arabesque borrowed from Italian arabesco - foliate ornament, used in the Islamic world, referring to the complex ornament of gut diverticula in carapace valves. The name arabesque has no appropriate equivalent in Latin and is used in this context as an arbitrary combination of letters (sensu ICZN Article 11.3) to avoid using the word in the vernacular.

Animals were seen freely swimming from one branch of the antipatharian colony to another and represent small predators rather than ectoparasites. All live specimens of Synagoga were collected after washing the colonies. Animals were quite motile and moved in a Petri dish by jumping. To accomplish these jumping movements, they bent and unbent their developed abdomen with furca, while thoracopod beating was used for slow swimming.

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. 1B–D).

Female (Figs 1–4, 8–12): Carapace oval, up to 2.3 mm long and 2.0 mm high, bivalved (Figs 1B, C, 2A–C, 8A, B), valves joined and hinged along dorsal margin (Fig. 17A). Dorsal and posterior margins of valves feebly convex, meeting at slightly produced posterio-dorsal angle; anterior and ventral margins rounded (Figs 1B, C, 2A–C, 8B).Exterior of carapace smooth, lacking setae but covered with small pores (Figs 8A, 17A, B, 18A–D). Right and left gut diverticula (Figs 1B, C, 2A–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. 1B, C). Inner surface of carapace valves with cuticular lining or mantle (Fig. 8B–F). Small, narrow pit on inner surface of anterior part of each valve (Figs 2A, C, 8E). Anterior pit of carapace infundibuliform, with wide entrance and long, narrowed internal part (Figs 2D, 8E); cuticle of pit wrinkled, with circular folds, small pores and volcano-shaped papillae (Fig. 8E, F). Body situated within mantle cavity (Figs 2B, 8A); oval brood chamber for embryos in posterior portion of each valve (Fig. 2B). 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. 8C); anterior, ventro-posterior and posterior sides of mantle bearing long setae with short setules, these being absent ventro-anteriorly (Fig. 8B–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. 2B). Frontal filament complex (Fig. 8D) 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 2B, 9A, B), each with pair of biramous natatory thoracopods described in detail below. Dorsal sides of segments (II–VI) covered with thin setae (Fig. 9A, B). Posterio-ventral angles of sixth thoracic segment formed as small triangular projections or epaulets, their surface covered by rounded plaques (Fig. 9B).

Abdomen U-shaped, five-segmented, including telson (Figs 2B, 8A, 9B). First segment with vestigial penis on ventral side (Figs 2E, 9B, C), an unpaired process – 140–190 μm long, its distal part bearing ctenoid scales (Fig. 9C). 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 2B, 9B, D) approximately 190 μm long with row of nine or ten sharp denticles along their dorsal margins. Furcal rami unsegmented (Figs 2B, F, 9B), 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 2F, 9B, E, G); proximal half of ventral margin armed with large, sharp denticles and ctenoid scales (Figs 2F, 9D, F, H). Inner subdorsal margin of each ramus with row of eight long natatory setae with long setules (Figs 2F, 9F, H); row of dense ctenoid scales along inner side of dorsal margin (Fig. 9F–H).

Extendable, prehensile antennules subchelate, folded into W-shape, consisting of six segments with complex of intrinsic and extrinsic flexor and extensor muscles (Figs 2B, 3A, B, 10A). First segment rectangular, narrowing somewhat distally, without setae. Second segment irregularly rectangular, with dense, thin omniserrate setae along postaxial/ventral margin (Figs 3A, B, 10A, C). Third segment equilaterally triangular, narrowing toward ventral margin; preaxial/dorsal margin curved, densely covered by thin omniserrate setae (Figs 3A, B, 10A). Fourth segment rectangular, trapezoid (appearing triangular in folded antennules, Fig. 3A, 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 3A, B, 10A, 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 3A, B, 10A). Sixth segment longer than fifth segment and armed with sensory and grasping structures (Figs 3A, B, 10A, D, E, G–I). Short proximal sensory process on lower margin at base of sixth segment (Figs 3A, B, 10A, 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 3A, B, 10A, D, E); concave margin of claw serrate, with sharp tiny denticles in middle part (Fig. 10D, E); three small setae at base of claw, two lateral on inner and outer surfaces and one on anterior dorsal margin (Figs 3A, B, 10D, H). Relaxed claw sheathed by grooved claw guard (Figs 3A, B, 10D, E, H, I), latter approximately 110 μm long, with wide flange on inner side, thin, membranous, apical ctenoid hood (Fig. 10I) and four small terminal setae including two longer and one tiny subapical (Fig. 10E) and 1 tiny apical seta (Fig. 10I). Cuticle on sides of antennular segments bearing dense small ctenoid scales (Fig. 10B).

Oral cone prominent, approximately 600–650 μm long; distal end often protruding outside carapace (Figs 1B, D, 2A–C, 11A); formed by cone-shaped labrum surrounding piercing mouth parts (Fig. 11A). Posterior margins of labrum free, unfused (Fig. 11A). Tuft of long, thin simple setae in middle of anterior face of labrum; dense, small ctenoid scales on external cuticle (Fig. 11A). Mandibles in form of lanceolate stylets, approximately 350 μm long (Figs 3D, 11B); 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. 11D); 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 3D, 11B). Maxillules consisting of a wide basal half and narrow distal half (Figs 3E, 11B, 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. 11B, C, E); tip with thin, curved setiform projections (probably setae, Fig. 11F). Maxillae (Figs 3F, 11C) thin, fused at bases, with row of thin, needle-shaped denticles along inner cutting edge at distal end (Fig. 11G); tips not distinctly bifid, not harpoon-shaped, with apical projection and adjacent tiny process (probably seta, Fig. 11G). 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. 3C).

All thoracopods natatory and biramous (Figs 4, 12). Seminal receptacles found in lateral proximal parts of coxae of thoracopods II–V (Fig. 4B–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. 4A) 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 4B–E, 12A–E). Coxae of thoracopods II and III (Figs 4B, C, 12B, 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. 4D, 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 4F, 12F) 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 1D, 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 5A, B, 13A). Dorsal margin almost straight; anterior, ventral and posterior margins rounded. Exterior of carapace smooth, lacking setae but covered with small pores (Figs 13A, 17E, F, 18E–H). Conspicuous deep pit with curved lumen opening on inner surface of anterior part of each valve (Fig. 5A, B). Gut diverticulum of simplified W-shape in comparison to female (Figs 1D, 5A, 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. 5A). Cuticular armament of mantle is similar to that of female (Fig. 13C–E). Edge of mantle forming thin marginal fold adjacent to margin of carapace and consisting of dense, tiny cuticular projections (Figs 13E, 15A). 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. 13C–E); anterior, ventro-posterior and posterior sides of mantle bearing setae with short setules (Fig. 13C, 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 5C, 12B, 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 5A, 13B): head bearing similar W-shaped antennules and well-developed oral cone; trunk consisting of 6 thoracic and 5 abdominal segments (Figs 5A, 13B, 14B); telson spines of same proportions and morphology (Figs 5F, 14E). Furcal rami resembling these of female in many details (Fig. 14E–H) but differ in having fewer long natatory setae on inner subdorsal margin (six instead of eight, Fig. 5F). Unlike in females, epaulets of sixth thoracic segment more strongly developed (Fig. 14A).

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 5D, E, 14B). Penis complex, approximately 600 μm long, ~ 4 times longer than supporting segment, and consisting of three parts: basal, medial and distal (Figs 5D, 14B, 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. 5D) covered by thin layer of epicuticle. Distal part consisting of two rami originating from medial part and narrowing toward tips (Figs 5D, 14B, C). Cuticular setiform projections 10–20 μm long with apical pore (not setae) present along anterior margin of each ramus (Figs 5D, 14B, C). Tip of each ramus terminating in pair of these projections (Fig. 14C).

Antennules of male resembling those of female (Figs 6A, 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. 15A). Fifth segment with 4–6 rather than 6–9 setae on anterior margin (Figs 6A, B, 15B). Sensory and grasping structures of sixth segment of same morphology as in females, but ctenoid scales denser in lateral surfaces of segment (Figs 6B, 15C, D).

Oral cone and mouth parts similar to those of female (Figs 6C–G, 16), consisting of labrum (Figs 6G, 16A–C) enclosing an unpaired medial languette (Fig. 6D) and paired mouth parts, mandibles (Figs 6E, 16C, D), maxillules (Figs 6F, 16C) and maxillae (Figs 6G, 16C, E); tips of maxillules bifid, not harpoon-shaped, apical projection and adjacent process slightly larger than in females (Figs 6G, 16E). 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. 7A), coxae of thoracopods II and III and bases of thoracopods II–IV have fewer setae along inner margins (Fig. 7B–D).

(Figs 17, 18): both sexes with five pairs of trough-like slits along hinge line of carapace (lattice organs: lo1-5, Figs 17, 18), situated co-linearly in two groups: anterior pairs 1–2 and posterior pairs 3–5. Those of both female and male are of similar morphology and arrangement and are therefore described together.

Lattice organs straight, each trough containing one short, modified seta (so-called crest) with terminal pore at free distal end (Figs 17D, 18B), terminal pore maybe hidden by debris, shrinkage or trough. Normally, each trough has oblique and rounded ends; distal part of crest lies at rounded end (Figs 17C, D, 18B–D). Cuticle of crests smooth, not perforated by small pores. Anterior lattice organs situated just posterior to point of divergence of carapace valves (Fig. 17A, E). Lo1 15 μm long in female (Fig. 17B, C) and 10–11 μm long in male (Fig. 17F, G), with posterior terminal pore, located 5–6 μm from hinge line (Fig. 17B, F). Lo2 100 μm behind first pair in female (Fig. 17B) and 80 μm behind in male (Fig. 17F), 16 μm long in female (Fig. 17D) and 10 μm long in male (Fig. 17H), with anterior terminal pore, located 9–10 μm from hinge line (Fig. 17B, D). Posterior lattice organs situated somewhat anterior to point of divergence of carapace valves, near their apices (Figs 17A, E, 18A, E), 530–550 μm behind anterior organs in mature female (Fig. 17A), 370–380 μm behind in male (Fig. 17E). Lo3 14–15 μm long in female (Fig. 18B) and 12 μm long in male (Fig. 18F), with anterior terminal pore, located 5–6 μm from hinge line. Lo4 25–28 μm behind lo3 in female and 40–45 μm behind in male (Fig. 18A, E), 17–18 μm long in female (Fig. 18C) and 13–14 μm long in male (Fig. 18G), with posterior terminal pore, located 7–8 μm from hinge line (Fig. 18A). Lo5 45–50 μm behind lo4 in female and 40 μm behind in male (Fig. 18A, E), 17 μm long in female (Fig. 18D) and 14 μm long in male (Fig. 18H), with posterior terminal pore, located 10–15 μm from hinge line.

Having both sexes of S. arabesque available makes it possible to compare this species with all other described species of Synagoga. The main characters used for comparison are given in Table 2. Owing to a lack of detailed description, no meaningful comparison with the juvenile “McKenzie’s larva” from the eastern Indian Ocean (cf. Grygier 1988) can be made. Only one species, S. millipalus represented by a single male, found in the Pacific Ocean off Okinawa, Japan. It differs in having fewer setae on the fifth antennular segment (three instead of four-six) and on the inner side of the furcal ramus (three instead of six), and also relatively longer telson spines (Grygier and Ohtsuka 1995). Only a single species, S. normani (based on a female), is known from the western Indian Ocean (Grygier 1983a). It has fewer setae on the fifth antennular segment (five instead of 6–9) and on the inner side of the furcal ramus (five or six instead of eight), and more setae on the second exopodal segment of thoracopod I (nine instead of seven). Four species inhabit the Atlantic and adjacent seas, these are S. mira, S. bisetosa, S. paucisetosa and S. grygieri (Norman 1888; Grygier 1983a, 1990a; Kolbasov and Newman 2018). The new species differs from S. mira (Norman 1888; Grygier 1983a) by having smooth, unscalloped edges of the gut diverticula, fewer setae on the fifth antennular segment (4–9 instead of 15), the second exopodal segment of thoracopod I (seven(eight) instead of 18) and the inner side of the furcal ramus (eight(six) instead of 14). It can be distinguished from S. bisetosa (Grygier 1990a) by having fewer setae on the fifth antennular segment (four-nine instead of ten), the second exopodal segment of thoracopod I (seven(eight) instead of ten) and the inner side of the furcal ramus (eight(six) instead of 13). The new species differs from S. paucisetosa (Grygier 1990a) in having more setae on the fifth antennular segment (four-nine instead of three) and the inner side of the furcal ramus (eight(six) instead of three); it also has relatively shorter telson spines. Finally, it can be distinguished from S. grygieri (Kolbasov and Newman 2018) by fewer setae on the fifth antennular segment of males (four to six instead of eight) and more setae on the inner side of the furcal ramus of females (eight instead of six); it also has relatively shorter fifth antennular segment and telson spines.