A new species of Synagoga (Crustacea, Thecostraca, Ascothoracida) parasitic in an antipatharian from Green Island, Taiwan, with notes on its morphology

Abstract A new ascothoracidan species has been discovered off Taiwan in the north part of the west Pacific at SCUBA depths. Twelve specimens including both sexes of the new species, described herein as Synagoga arabesquesp. nov., were collected from colonies of the antipatharian Myriopathes cf. japonica Brook, 1889. Three previously described species of Synagoga, morphologically the least specialized ascothoracidan genus, have been found as ectoparasites of antipatharians and an alcyonarian, whereas all other records of this genus have been based on specimens collected from the marine plankton. This is the second study of a new form of Synagoga to be based on more than a few mature specimens of a single sex or on a single juvenile. Furthermore, it is the second in which SEM has been used to document the fine-scale external morphology. The position of terminal pores in the anterior pairs of the lattice organs is different in Synagoga arabesquesp. nov. than those in S. grygieri Kolbasov & Newman, 2018 and S. millipalus Grygier & Ohtsuka, 1995. Species of Synagoga are small, host-specific predators or ectoparasites of antipatharians. This genus exhibits a major Tethyan reliction pattern.


Materials and methods
The ascothoracidans belonging to the new species Synagoga arabesque sp. nov. were collected alive from the two colonies of the antipatharian Myriopathes cf. japonica Brook, 1889. The colonies were first photographed and then collected alive in situ into sealed plastic bags (to prevent the escape of parasites) by GAK using SCUBA at depth of 35 m (Fig. 1A), at Green Island (Lyudao), Taiwan. Host specimens were transported in a portable ice box filled with seawater to the Green Island Marine Research Station, Biodiversity Research Center, Academia Sinica within 2 hrs of collection and subsequently maintained in an aquarium at 23-25 °C. Each colony was examined for crustacean Figure 1. Collection and natural coloration of living specimens of Synagoga arabesque sp. nov. A Collection of living specimens of Synagoga from black coral Myriopathes sp. B mature female with outstretched antennules, oral cone and abdomen, lateral view, left side C young female, lateral view, right side D male with outstretched antennules, oral cone, thoracopods and abdomen, lateral view, left side. Abbreviations: a1 -antennule, ab -abdomen, em -embryos, fr -furcal rami, gd -gut diverticulum, oc -oral cone, ov -ovary, thp1-6 -thoracopods I-VI. Scale bars: in µm. parasites using stereomicroscope. The seawater from the sealed plastic bags was filtered through a sieve and the sample was also examined under the stereomicroscope. The ascothoracidans thereby discovered were fixed one-two days later in 100% alcohol, formalin, and glutaraldehyde, after digital photography using a Lumix (Panasonic) GH4 camera equipped with a Leica DG Macro-Elmarit 45 mm f2.8 lens and the same camera body affixed to an Olympus SZ61 dissecting microscope. Two females (holotype and paratype) and two males (paratypes) were dissected and mounted in glycerol on glass slides. They were examined and illustrated using a WILD Heerbrugg M20-35369 light microscope. Line drawings were also made using oil immersion, Nomarsky differential interference contrast, and a drawing tube on an Olympus BX 51 microscope. For SEM, three non-type females and two non-type males were post-fixed in 2% OsO4 for 2 h, dehydrated in acetone and critically-point dried with CO 2 . Dried specimens were sputter-coated with platinum-palladium and examined on a JEOL JSM-6380LA scanning electron microscope operating at voltages of 15-20 kV at the University of Moscow. Resulting photographs were touched up using CorelDraw X3 Graphics Suite.

Diagnosis.
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.
Etymology. 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.
Relation to host and behavior. 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.
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. 1B-D).
Female (8)(9)(10)(11)(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 direc-tions; 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).
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).
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 twosegmented 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).
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, ventroposterior and posterior sides of mantle bearing setae with short setules (Fig. 13C, E).
Body of male resembling that of female (Figs 5A, 13B): head bearing similar Wshaped 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).
Main diagnostic characters of species of the genus Synagoga (modified from Kolbasov and Newman 2018). The finding of S. normani on alcyonarian Dendronephthya is questioned, because all other congeners attributed to hosts were found on antipatharians.

S. mira
Norman,   Comparison. 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 1983aGrygier , 1990aKolbasov and Newman 2018). The new species differs from S. mira (Norman 1888;Grygier Figure 16. Synagoga arabesque sp. nov., male. Mouth parts (SEM) A labrum, posterio-lateral view, anterior margin left B lateral surface of labrum C distal part of oral cone with exposed tips of mouth parts D tips of mandibles E tips of maxillae. Abbreviations: a1 -antennules, lb -labrum, md -mandible, ml -medial languette, mx1 -maxillule, mx2 -maxilla. Scale bars: in µm.

S. normani
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.

Discussion
Morphology of both sexes including external ultrastructure, as well as sexuality, host specificity, and biogeography of the genus Synagoga have been recently discussed in detail (Kolbasov and Newman 2018). In the present study we are providing new data on the structure of the lattice organs and anterior sensory pits of carapace, host specificity and biogeography of Synagoga. Developed anterior sensory pits (Figs 1A, 5A, B, 8E, F) are found on the inner side of valves in adult stages of both sexes of genera Synagoga and Sessiligoga (Grygier 1990b;Grygier and Ohtsuka1995;Kolbasov and Newman 2018;unpublished data). They are considered as possibly homologous to the pair of large anterio-ventral pores found externally on the ventral faces of the carapace valves of both sexes of both species of Waginella, Waginella sandersi (Newman 1974) and Waginella metacrinicola (Okada 1926), as well as two undescribed species of this genus (Newman 1974;Grygier 1990c;Itô and Grygier 1990;unpublished data). A chemosensory function was putatively proposed for these structures (Kolbasov and Newman 2018). Small pores and conspicuous volcano-shaped papillae observed on the surface of the canal of these pits in S. arabesque sp. nov. (Fig. 8E, F) may also be evidence in favor of chemosensory function.
In adults of both sexes of S. grygieri and Synagoga arabesque sp. nov. and the male of S. millipalus, all five pairs of lattice organs are situated co-linearly along the hinge line of the carapace valves, i.e., parallel to the hinge. A fully co-linear arrangement of the lattice organs has been considered plesiomorphic for ascothoracidans and also for all thecostracans (Jensen et al. 1994;Høeg and Kolbasov 2002;Celis et al. 2008;Kolbasov and Newman 2018). Apart from both S. grygieri and S. millipalus having the anterior terminal pore in lo1 and posterior terminal pore in lo2, the new species has the posterior terminal pore in lo1 and the anterior terminal pore in lo2. Thus, only posterior pairs of lattice organs (lo3, lo4, lo5) share the same position of terminal pores in the studied species of the genera Synagoga and Sessilogoga (Grygier and Ohtsuka 1995;Kolbasov and Newman 2018;herein;unpublished data). Species of both Synagoga and Sessilogoga share anterior terminal pores in lo3 and posterior terminal pores in lo4 and lo5. This is opposite to the condition in most thecostracans, which have a posterior terminal pore in lo3 (e.g., Jensen et al. 1994;Kolbasov et al. 1999;Høeg et al. 2004;Celis et al. 2008), and thus represents a potential synapomorphy of these two genera (unpublished data). The different position of terminal pores of the lattice organs even within congeners (terminal pores of anterior lattice organs in Synagoga) shown here for the first time might be evidence that the configuration of lattice organs in ascothoracidans is not constant, at least in adult stages.
Four of the seven described species of Synagoga are attributed to particular hosts ( Table 2) and three of them (S. mira, S. grygieri, and Synagoga arabesque sp. nov) were found on antipatharians. This may be evidence of the host specificity of Synagoga as exclusive ectoparasites or small predators of black corals. Therefore, we consider the attribution of S. normani to the alcyonarian host Dendronephthya as a possible misinterpretation. Grygier (1983a) described a single isolated female of S. normani 'collected by P. Hutchence from alcyonacean coral, Dendronephthya sp.' in Mombasa harbor and forwarded to him. We suspect that this record Dendronephthya may be of a non-specific substrate rather than an actual specific host for this species.
Synagoga arabesque sp. nov. is the second species of the genus found in the north part of the west Pacific after S. millipallus. Despite this fact, the new species resembles S. grygieri recently described from the Atlantic Ocean, Macaronesia (Kolbasov and Newman 2018; Table 2 herein) in many details. This may indicate that both Synagoga arabesque sp. nov. and S. grygieri evolved from a common ancestor and that the genus Synagoga exhibits the major Tethyan reliction pattern that is also characteristic of some ascothoracidans and barnacles (Newman and Ross 1971;Newman and Tomlinson 1974;Foster 1981;Kolbasov 2009;Kolbasov et al. 2015). Currently, studies of diversity of Ascothoracida are still based mainly on morphological approaches, future directions can involve molecular techniques to examine cryptic diversity and population genetics of Ascothoracida (see approaches in Chai et al. 2017;Chang et al. 2017;Ma et al. 2019;Jung et al. 2018)