Sessilogoga perfuga Kolbasov & Savchenko 2024, sp. nov.

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