Pleusymtes japonica (Gurjanova, 1938)

Ivan, Marin, Sergey, Sinelnikov & Agniya, Sokolova, 2013, Ecological remarks and re-description of the hermit crab-associated pleustid amphipod Pleusymtes japonica (Gurjanova, 1938) (Crustacea: Amphipoda: Pleustidae: Pleusymtinae) from the Russian coasts of the Sea of Japan, Zootaxa 3640 (4), pp. 581-588 : 582-583

Pleusymtes japonica (Gurjanova, 1938)

( Figs. 1–4 )

Sympleustes japonicus Gurjanova, 1938: 320 , fig. 34 [type locality: Petrova Is., Sea of Japan].

Material examined.— 3 large ovigerous females and 10 juveniles, the Sea of Japan, Russia, the Peter the Great Bay, Furugelma Island, Western Bay, depth 3–6 meters, on the shells occupied by hermit crabs Pagurus ochotensis and overgrown by hydroids Hydractinia sp., SCUBA, coll. I. Marin, 15 June 2012.

Brief description. —Female. Medium-sized amphipods with smooth body, max. bl. 6-7 mm. Rostrum peaklike, reaching basal peduncular article of antenna I. Eyes large, well marked. Antenna I ( Fig. 2 a) with basal peduncular article oblong, smooth, about half of the head length, about twice longer that the second and third peduncular articles, with short naked flagellum about 3 times longer than peduncle. Antenna II ( Fig. 2 b) with basal peduncular article oblong, covered with small setae distally, about twice longer than the second and equal to the third peduncular articles, with short naked flagellum about twice longer than the peduncle; accessory flagellum absent. Labrum rounded. Mandible ( Fig. 2 c) with strong and long palp, consisting of 3 articles, the distal article armed with strong sharp apical setae ( Fig. 2 c); with lacinia mobilis and incisor process well developed, serrate; incisor process robust, distally armed with short blunt teeth; lacinia mobilis consists of 8 – 10 fused blunt teeth of different sizes. Maxilla I ( Fig. 2 d) with inner plate small, with ventral margin armed with thin simple setae and with 6–9 long apical spines; outer plate longer than inner, about twice longer than wide, smooth, armed with 6–9 long sharp spines distally. Maxilla II normal, without special features, slender; outer and inner plates normal, each distally armed with simple setae. Maxilliped ( Fig. 2 e) normal; inner plates small, partly separated, rounded distally; outer plate reduced, with 4-articulate palp bearing long simple setae along inner lateral margins of segments, all articles are equal by length; propodal article armed with numerous short simple setae along inner margin, dactylus distally pointed, sharp, slightly curved, covered with tiny simple setae along lateral inner margin. Pereon smooth. Gnathopod I ( Fig. 3 a) subchelate, equal to gnathopods II ( Fig. 3 b); coxa short, subrectangular; basis elongated, about 3 times as long as wide; ischium subquadrate, slightly longer than wide, equal to merus, with small rounded distodorsal inner lobe; merus subquadrate with distoventral produced angular lobe bearing a row of long simple setae; carpus triangular in shape, slightly longer than wide, with ventral margin armed with produced lobe with a tuft of long simple setae; propodus (palm) robust, rectangular, about 1.5 times longer than wide, with several large robust setae (“locking teeth”) at the middle of the margin, central part of the margin armed with short thin setae, disto-ventral part of palmar margin convex; dactylus long, curved, about 5 times longer than wide, reaching the locking tooth on palmar margin of propodus, with smooth inner lateral margin. Gnathopod II ( Fig. 3 b) subchelate, generally similar and equal to gnathopod I; coxa short, subrectangular; basis elongated, about 3 times longer than wide; ischium subquadrate, about as long as wide, with small rounded distodorsal inner lobe; merus elongated, about twice longer than wide, with distoventral angle oblique, armed with a tuft of long simple setae; carpus short, triangular, with well marked distoventral rounded lobe, bearing numerous long simple setae; propodus (palm) rectangular, about 1.7 times longer than wide, without several series of locking teeth at the middle of the margin, central part of the margin armed with short thin setae, disto-ventral part of palmar margin convex; dactylus long, curved, about 5 times longer than wide, reaching the locking tooth on palmar margin of propodus, with smooth inner lateral margin. Pereiopod III normal, without specific morphological features; with smooth segments. Pereiopod IV ( Fig. 4 a) with large oval coxal plate having a district basal notch; basis segment elongated, about 3 times longer than wide; ischium subquadrate, about as long as wide; merus elongated, rectangular, about 3 times longer than wide, with 3 short short spines along lateral margins, distodorsal angle produced, triangular; carpus elongated, about twice longer than wide, equal to merus, with several short slender spines along ventral margin, distoventral angle armed with 2 short slender spines; propodus ( Fig. 4 b) rectangular, about 3.5 times as long as wide, with 3 single short spines along ventral margin and a pair of distoventral spines, distoventral angle not oblique; dactylus relatively robust, about 3 times longer than wide, curved, pointed distally, with smooth inner lateral margin bearing numerous small simple setae. Pereiopods V – VII similar to pereiopod VI. Urosome smooth; epimeral plate III with sharply produced distoventral angles, with several small simple setae along ventral margin. Pleopods biramous, with peduncles about half of the rami length; rami subequal, lined with long simple setae. Uropod I longer than uropod II, biramous; peduncle slightly longer to the length of rami. Uropod II slightly longer than uropod III, biramous, with peduncle longer than rami; rami subequal. Uropod III ( Fig. 3 c) biramous, distoventral angle of basal article armed with short spine; the outer rami is about twice longer then the inner rami. Telson ( Fig. 3 d) tongue-shaped, bluntly produced, about twice longer than wide, without submarginal spines or setae.

Males of the species are unknown.

Coloration. —Antenna I and II, somites and coxal plates I–IV, somites V, VIII–X are brown; bright yellow stripes are indicated articulations of body somites; other body parts and appendages are whitish translucent; eyes white.

Ecological remarks. —Amphipods were firstly seen underwater on the surface of gastropod shells occupied by hermit crabs Pagurus ochotensis Brandt, 1851 (Crustacea: Decapoda : Paguridae ) and thoroughly overgrown by hydroids of the genus Hydractinia van Beneden, 1841 (Cnidaria: Hydrozoa: Hydractiniidae ), possibly Hydractinia sp. “ echinata (Fleming, 1828)”. Amphipods were usually observed sitting close to the shell aperture; some specimens were sitting close with the head turned to the direction of the feeding appendages of hermit crabs; no amphipods were found inside observed shells. Amphipods stayed clinging to the surface of shells even shells were removed from water. Photographs from Rudnaya Bay ( Fig. 1a ) showed that the species is also associated with Pagurus middendorfii Brandt, 1851 and possibly with other large species of hermit crabs in the area (for example, Pagurus brachiomastus (Thallwitz, 1892)) .

Distribution. —The Sea of Japan: Rudnaya Bay (present paper), Petrova Island (Gurjanova, 1938), Furugelma Island (present paper); at the range of depths from 3 (present paper) to 62 meters (Gurjanova, 1938).

Taxonomical remarks.—Three species currently referring to the genus Pleusymtes (Barnard, 1969) are known from the Peter the Great Bay of the Sea of Japan: Pleusymtes suberitobia (Gurjanova, 1938) (Gurjanova, 1938: 318) , Pleusymtes uncigera (Gurjanova, 1938) (Gurjanova, 1938: 320) and Pleusymtes japonica (Gurjanova, 1938) (Gurjanova, 1938: 320) .

Gurjanova (1951) suggested the key to separate these species:

1. Distoventral margin of coxal plates I and II sharply produced, hook-like........... Pleusymtes uncigera (Gurjanova, 1938) – Distoventral margin of coxal plates I and II not produced, rectangular............................................ 2 2. Telson and ventral margin of epimeral plate III with strong setae. Telson with 2 pairs of setae..........................

................................................................... Pleusymtes suberitobia (Gurjanova, 1938) 3. Telson and ventral margin of epimeral plate III unarmed........................ Pleusymtes japonica (Gurjanova, 1938)

All species were collected simpatrically, on the same depths (Gurjanova, 1938), except Pleusymtes suberitobia (Gurjanova, 1938) referred as a distinctive participant of the symbiosis Suberites domunculus + Pagurus ” (Gurjanova, 1938: 320). At the same time within the group of genera variation of such morphological features as sharply produced distoventral margin of coxal plates I and II, the presence or absence of setae on telson or ventral margin of epimeral plate III can be possibly explained by the intraspecies variations. Re-examination of the type material will confirm the validity or synonymy of these three species but the deposition of the type material of these species remains unknown.

Figures

FIGURE 2. Pleusymtes japonica (Gurjanova, 1938), female: a — antennula, b — antenna, c — mandible, d — maxilla, e — maxilliped.

FIGURE 3. Pleusymtes japonica (Gurjanova, 1938), female: a — gnathopod I, b — gnathopod II, c — uropods III, d — telson.

FIGURE 4. Pleusymtes japonica (Gurjanova, 1938), female: a — pereiopod IV, b — distal segments of pereiopod IV.

FIGURE 1. a — Pleusymtes japonica (Gurjanova, 1938) on hermit crab Pagurus middendorfii Brandt, 1851 in Rudnaya Bay, Sea of Japan (the specimen was not collected; photo A. Shpatak); b, d, e, f — Pleusymtes japonica (Gurjanova, 1938) on the shell occupied by hermit crab Pagurus ochotensis Brandt, 1851 at Furugelma Island, Sea of Japan (the shell is overgrown by hydroids of Hydractinia sp.); c — a pair of Pleusymtes japonica (Gurjanova, 1938) in the shell occupied by Pagurus ochotensis Brandt, 1851; g — fixed specimen of Pleusymtes japonica (Gurjanova, 1938); h — alive specimen of Pleusymtes japonica (Gurjanova, 1938). Arrows indicates specimens of amphipods. Scale— 1 mm.