Sessilogoga perfuga Kolbasov & Savchenko sp. nov.

urn:lsid:zoobank.org:act: 98671130-19AA-4F28-A184-EFBE7EAD5DA5

Figs 1–12

Type and non-type material. Type locality: 5°35’59.4”N 103°03’44.1”E in the vicinity of Bidong Island, Kuala Nerus District, Terengganu, Malaysia, South China Sea, from coral reefs at 25 to 29 m depths. Holotype female, slide preparation (no. Mh-1252) and 3 paratypes (female no. Mh-1254 and 2 males, no. Mh-1253), along with 5 more paratypes fixed in alcohol (2 females and 3 males; no. Mh-1261), deposited in Zoological Museum of Moscow State University, Moscow, Russian Federation. Seven paratypes (3 females, 4 males), on SEM stubs, 25 specimens (15 females, 10 males) fixed in 96 % EthOH unregistered retained by first author in White Sea Biological Station’s facilities at the Biological Faculty, Moscow State University). Eight specimens were deposited in the White Sea Biological station branch of Zoological Museum MSU with registration numbers: ZMMU XZ 6362, ZMMU XZ 6363, ZMMU XZ 6364, ZMMU XZ 6365, ZMMU XZ 6366, ZMMU XZ 6367, ZMMU XZ 6368, ZMMU XZ 6369, ZMMU XZ 6370 and later these were used for DNA extraction.

Etymology. A noun from Latin perfuga meaning refugee in memoriam of numerous Vietnamese refugees fleeing from the communist regime in Vietnam in 1975–2005. The name refers to the type locality, Bidong Island, which was used as a refugee camp with population at its peak reaching 40000 people, highlighting negative consequences of any war conflicts.

Diagnosis. In females, carapace ovoid, up to 2.3 mm long and 1.8 mm high, with distinct posteriodorsal enlargement; posterior part distinctly higher than anterior third. In males, carapace elliptical, up to 1.7 mm long, 1.2 mm high, without distinct posteriodorsal enlargement. In both sexes, diverticula of gut and gonads shaped as rounded W with short anterior and longer posterior branches in each valve, diverticula of testis with side branches. In antennules of both sexes, pair of ‘fork setae’ of fourth segment conspicuous, proximal seta 2–3 times larger than distal seta (especially in females); fifth segment with 4–5 setae. In males, thoracopods 2–4 with one outer distal coxal seta; in females, thoracopods 2–5 with one outer distal coxal seta. Thoracopods 2–5 of females each with 4–5 long, ampuliform seminal receptacles within coxa. Telsonic spines smooth; furcal rami elongated, 6–6.5 times longer than basal height, with small ventral spines in males.

Description

Female (Figs 1 A, B, 2, 3, 4, 8, 9, 10). Living specimens semitransparent, light-colored; ovary white; brooded embryos round and visible through carapace (Fig. 1A, B). Abdomen and antennules often extending beyond margins of carapace. Carapace bivalved, laterally compressed, up to 2.3 mm long and 1.8 mm high; rounded at both ends, with anterior third (free anterior ends of valves) less high than posterior part, and with no distinct posteriodorsal enlargement beyond posterior end of hinge line (Figs 1B, 2A). Dorsal hinge line slightly curved in mature specimens (Figs 1B, 2A). Exterior of carapace smooth, lacking setae and conspicuous pores (Fig. 8A). Right and left gut diverticula (Figs 1B, 2A) lying within respective carapace valves and resembling rounded letter “W”, with main branch descending toward ventral margin from point near adductor muscle and then bifurcating; anterior branch smooth (Fig. 2A) or with 1–2 tiny secondary branches (Fig. 1B) and shorter than posterior branch smooth and unbranched. Ovaries lying parallel and lateral to gut diverticula, with the same rounded-W configuration. Inner surface of carapace valves (so-called ‘mantle’) consisting of thin cuticle (Fig. 8A, C–E). Each half of mantle cavity continuous with oval brood chamber in posterior part of respective valve (Fig. 8A). Main cuticular structures of mantle arrayed along its margin: anterior part with sparse thin setae and short setae with small setules, tiny pores, and underlying row of ctenes (Fig. 8C, D); posterioventral and posterior parts bearing long setae with short setules (Fig. 8E); opening to brood chamber sparsely guarded by long, thin, simple setae (Fig. 8E), but brood chamber itself without setae.

Cephalon bearing a pair of W-shaped prehensile antennules flanking and longer than conspicuous,anterioventrally directed oral cone, latter formed of mouth parts surrounded by labrum (Figs 2B, 3A, 8B); for details, see below. Thorax consisting of 6 limb-bearing somites (Figs 2B, 8B), each with a pair of biramous natatory thoracopods described in detail below. Dorsal sides of somites V and VI slightly longer than those of somites I–IV and covered with short, fine setae (Figs 2B, 8B). No lateral epaulets present on sixth segment. Abdomen U-shaped, consisting of 5 segments, including telson (Figs 2B, 8B). First abdominal segment larger, with vestigial penis ventrally, representing an unpaired process about 160–170 μm long with sparse short setae and ctenate scales on its lateral and distal parts (Fig. 2F); second segment bigger than either third or fourth, all with ctenate scales on dorsal sided. Last body segment (telson) cylindrical, about 270–312 μm long (Figs 2B, 8F) with rows of ctenate scales along dorsal and distal margins (Fig. 8F–H); posterioventral margin bearing conspicuous pair of smooth telsonic spines about 90–103 μm long (Figs 2G, 8F, H). Furcal rami unsegmented, about 520–590 μm long, 5.75–6.0 times longer (excluding setae) than basal height; ventral margin with 1 seta at midlength, 1 subdistal seta, and 2 distal (terminal) setae (Figs 3G, H, 8I), longer terminal seta with ctenoid ornamentation along distal margin (Fig. 8L); ctenate scales along entire dorsal margin and proximal half of ventral margin (Fig. 8I–K), but ventral spines absent; 3 natatory subdorsal setae along inner face of ramus (Figs 3G, 8I).

Antennules extendable, prehensile, subchelate consisting of 6 segments (Figs 2B, C–E, 8B, 9A). The shape of segments clearly visible in outstretched antennules (Fig. 2C–E), but the manner of folding is better observed in SEM plate (Fig. 9A). First segment irregularly rectangular, narrowing somewhat distally, without setae. Second segment nearly parallelogram-shaped, also lacking setae, distal part of postaxial side overlapping postaxial parts of next 3 segments when antennule is folded (Fig. 9A). Third segment trapezoidal, narrowing somewhat distally, without setae. Cuticle on outer side of segments 1–3 bearing dense ctenate scales (Fig. 9A, B). Fourth segment trapezoidal, narrowing towards preaxial margin, with 2 stout conspicuous smooth ‘fork setae’ on preaxial margin far beyond reach of sixth segment’s movable claw, and massive proximal seta ~3 times larger than distal seta (Figs 2D, E, 9D). Fifth segment 1.5–2.0 longer than wide, bluntly tapering due to oblique but distally slightly convex preaxial margin, bearing 4 short, simple setae (Figs 2D, E, 9A, C). Sixth segment slightly shorter than fifth and armed with sensory and grasping structures (Figs 2C–E, 9C–F). Proximal sensory process short, located on postaxial margin at basal third of segment (Figs 2D, E, 9C, F), bearing 3 unequal terminal setae and 1 longer, swollen and blunt subterminal aesthetasc. Short, curved claw arising from distal end of segment (Figs 2D, E, 9C, E); claw’s concave margin smooth (Fig. 9E). Three small setae at base of claw: two lateral and one on anterior margin. Retracted claw sheathed by grooved claw guard (Figs 2D, E, 9C, E), latter about 90 μm long, with wide flange on outer side, thin, membranous apical hood, and 3 small terminal setae (Fig. 9E).

Oral cone conspicuous, about 390–500 μm long (Figs 2B, 3A, 8B, 10A), formed by cone-shaped labrum surrounding piercing mouth parts (Figs 3A, 10A). External cuticle of labrum with ctenate scales in proximal part (Fig. 10A, B); posterior margins free, unfused (Fig. 3A). Unpaired medial languette arising posterior to mouth, with sharp tip and sharp subterminal spine on anterior margin (Fig. 3B). Mandibles formed as lanceolate stylets (Fig. 3C, D); cutting edge of each bearing approximately 60–62 simple or complex and multifid teeth; neck of mandible bearing small, sharp denticles; distal part with row of 26–28 recurved teeth on posterior margin. Maxillules consisting of wide basal half and narrow distal half (Figs 3E, 10A, C); cutting edge bearing row of rod- or needle-shaped denticles flanked on anterior (outer) side by row of small, simple hair-like filaments, denticles decreasing in length towards distal end and absent at tip. Maxillae (Figs 3F, 10D, E) thin, fused at base; tips rather simple, not harpoon-shaped, without posterior hook-shaped prong.

All thoracopods natatory, biramous, and of similar length except for shorter sixth pair(Fig. 4). Seminal receptacles found in lateral proximal parts of coxae of thoracopods 2–5 (Fig. 4B–E), consisting of 4–5 long ampulliform sacs per leg, external opening(s) not observed. First thoracopod (Fig. 4A) with elongate protopod composed of coxa and basis, and 2-segmented rami with exopod much longer and thicker than endopod. Thoracopods 2–5 all with 3-segmented endopods and 2-segmented exopods (Fig. 4B–E). Protopod of thoracopod 6 (Fig. 4F) narrow, and both rami 2-segmented. Setation of thoracopods of female holotype and male paratype summarized in Table 1. Coxa and basis of thoracopod 1 without setae; proximal segment of exopod without long setae, second segment with 20 long, plumose setae; basal segment of endopod bearing 2 long, plumose setae distally, distal segment with 3 terminal plumose setae (Fig. 4A). Coxae and bases of thoracopods 2–4 (Fig. 4B–D) all densely fringed with fine, hair-like structures on both inner and outer margins and bearing 1 large outer distal seta in position ‘1’; thoracopods 5, 6 without setae in this position (Fig. 4E, F), inner distal coxal seta absent in position ‘9’ in all thoracopods. Bases of thoracopods 2, 4 with row of 5 plumose setae along distal part of inner margin (position ‘8’), 3 such setae in thoracopod 3 and 1 seta in thoracopod 5. Numbers of setae along outer margin of proximal exopodal segment (position ‘3’) of thoracopod 2 (Fig. 4B) significantly greater than in thoracopods 3 and 4 (Fig. 4C, D). Distal segments of both rami more or less similarly armed with long, plumose setae in all 4 limbs, these setae mostly about 1/4–1/3 as long as limb. In thoracopod 6 (Fig. 4F), coxa and basis unarmed, and both rami with long, plumose apical setae (2, 3) on distal segment and 1 subdistal seta on basal segment of endopod.

Male (Figs 1C, 5, 6, 7, 11, 12). Most male structures differing little if at all from those in females; only those showing sexual dimorphism described in detail here.

Carapace bivalved, laterally flattened ellipsoid, in mature males 1.17–1.58 mm long, 0.62–1.15 mm high, posteriodorsal region not produced (Figs 1C, 5A, 11A). Dorsal margin almost straight or slightly convex; anterior, ventral, and posterior margins rounded. Exterior surface of carapace valves smooth, without setae and big pores (Fig. 11A). Gut diverticulum of simplified W-shape, without lateral branches. Lobed, W-shaped, white testis with thick and short often bifurcated secondary branches arising from both anterior and posterior primary branches (Figs 1C, 5A). Frontal filament complex (not observed in females) with one ramus (anterior?) with 4–5 short aesthetascs and bulbous basal appendix (Fig. 11B). Cuticular armament of mantle similar to that in female (Fig. 11C–E).

Body of male resembling that of female (Figs 5B, 11A). Cephalon with antennules and oral cone. Trunk consisting of 6 limb-bearing thoracic somites; first abdominal somite bearing conspicuous bifurcated penis much larger than that of female (Figs 5E, 11A, G), 3 limbless abdominal somites, and telson bearing telsonic spines and furcal rami of roughly same shape and proportions as in female. Structure and form of thoracomeres similar to that in female, and lateral epaulets again absent on sixth thoracomere. Cuticular armament of ctenate scales on telson similar to that in female (Fig. 11F). Telsonic spines without denticles (Figs 6F, 11F); furcal rami 6.5 times longer than basal height, setation same as in female, small denticles (7, absent in female) along ventral margin in proximal half (Figs 6F, 11F).

Antennules of male (Figs 5C, D, 12) as in female, except slightly longer aesthetasc of proximal sensory process in sixth segment and less massive proximal seta of ‘fork setae’ in fourth segment. Oral cone and mouth parts also similar to those of female (Fig. 6A–E). Thoracopodal setation of examined male (Table 1, Fig. 7) differing from that of holotype female (Fig. 4) mostly in presence of setae (3) on outer distal margin of basis of thoracopod 2, setal number of basal segment of exopod of thoracopods 3 and 4 (9 instead 4 in female and 2 instead 6 respectively), presence of seta on outer margin of basal segment of exopod and absence of seta on inner margin of basis of thoracopod 5.

Tergite of penis-bearing seventh trunk somite without pleural extensions (Figs 5B, 11A, G). Penis originating from ventral side of somite, altogether about 700–1100 μm long, more than 4–4.5 times as long as its segment, and consisting of 3 parts: basal shaft, transitional region, and rami (Figs 5B, E, 11A, G). Basal shaft cylindrical, 250–340 μm long, transitional region slightly swollen, 200–300 μm long, with unpaired short, conical process extending from its upper (anterior) side. Two vermiform rami originating posteriolaterally and narrowing toward their tips, with rare short setae on basal and middle parts, sparse long setae on distal part, and long setae at tip (Figs 5E, 11G).

Host-parasite relationship and distribution

Similar to the recently described Sessilogoga captiva (Kolbasov et al., 2020), specimens of Sessilogoga perfuga sp. nov. were observed crawling within the host’s gastrovascular system as endoparasites. They were also able to leave the host via mouth opening of polyps and freely swim around. This is the first species of an ascothoracidan found associated with an antipatharian from Malaysia. Previously three ascothoracidan species were described from antipatharians in West Pacific. These are Sessilogoga elongata from Guam and Sessilogoga captiva and Synagoga arabesque Kolbasov, Petrunina, Ho & Chan, 2019 from Taiwan (Grygier 1990, Kolbasov et al. 2019, 2020). Additionally, the fourth species, Synagoga millipallus Grygier & Ohtsuka, 1995, was described from plankton off Okinawa, South Japan (Grygier & Ohtsuka 1995).

Remarks

The new species, Sessilogoga perfuga sp. nov., is the third known species of this genus. The description of S. elongata was based mostly on two males (Grygier 1990), so only limited comparison of females is possible for this species. Sessilogoga perfuga sp. nov. can be easily distinguished from S. captiva in having longer furcal rami, which are ~ 6 times longer than high in females and 6.5 in males compared to those of S. captiva where length/height ratio is about 5.0 in females and 4.0 in males. The ‘fork’ setae on the fourth antennular segment are more massive in the new species in comparison to that in S. captiva . The fifth antennular segment in S. captiva has 3–4 setae along the preaxial margin, while the new species has 4–5 setae there. The maxillules are bifid in S. captiva, but not in the new species. In the thoracopods, the female of Sessilogoga perfuga sp. nov. differs from that of S. captiva in having 4–5 instead of 4–6 seminal receptacles, 1 but not 2 outer distal setae on the coxa (position 1) in thoracopods 2, 3 and 4, and none versus 1 seta at that position in thoracopod 5; more than twice as less setae at position 3 on thoracopods 2, 3 and the absence of inner distal seta on the coxa (position 9) in all thoracopods, which is present on thoracopod 2 in S. captiva . The male differs from that of S. captiva in having 1 instead of none outer distal setae on the coxa (position 1) in thoracopods 2, 4, and 3 outer distal setae on the basis (position 2) versus 1 at that position in thoracopod 2.

The morphology of new species, Sessilogoga perfuga sp. nov., is very similar to that of S. elongata . They can be distinguished by thoracopods and antennules setation, structure of telsonic spines and length/height proportion of furcal rami. The second antennular segment in S. elongata has a tuft of thin proximal setae on postaxial margin absent in the new species. In the thoracopods, the male of Sessilogoga perfuga sp. nov. differs from that of S. elongata in having 1 seta instead of 2 setae on the first endopodal segment (position 6–7) and lacking an inner seta on the basis (position 8) of thoracopod 1; in having 3 outer setae instead 0 on the basis (position 2), about three times as less outer setae on the first exopodal segment (position 2, 12 instead 29–30) in thoracopod 2; and 4 instead of 8 distal setae on the second exopodal segment (position 4) of thoracopod 3. The telsonic spines are denticulate in S. elongata, but smooth in both sexes of Sessilogoga perfuga sp. nov. Finally, S. elongata has longer furcal rami, which are 7.5–9.0 times longer than their basal height; those of Sessilogoga perfuga sp. nov. are shorter, with length/height equal to about 6.5 in males and 6.0 in females.