Stenothoe irinae, Marin & Sinelnikov, 2018

Marin, Ivan & Sinelnikov, Sergey, 2018, Two new species of amphipod genus Stenothoe Dana, 1852 (Stenothoidae) associated with fouling assemblages from Nhatrang Bay, Vietnam, Zootaxa 4410 (1), pp. 57-76 : 67-73

Stenothoe irinae sp. nov.

( Figs. 1 a , 7–9 )

Material examined. Holotype (ZIN 89319) male; paratype (ZIN 89320) female, paratype (ZIN 89321) male, paratype (ZIN 9385, 89386, 89387) dissected—Pacific Ocean, South China Sea, Vietnam, Nhatrang Bay, Tre Island, Dam Bay, 12°11'42.6"N 109°17'39.8"E, underwater surface of floating fish farm construction, 1 meter depth, SCUBA, coll. Marin I., Sinelnikov S., March 2014; 20 adult ovigerous females, 12 males, 15 juveniles (LEMMI)—Tre Island, Dam Bay, 12°11'42.6"N 109°17'39.8"E, underwater surface of floating fish farm construction, special washout from hydroids Macrorhynchia sp. and Gymnangium sp., 1 meter depth, SCUBA, coll. Marin I., Sinelnikov S., March 2014.

Description. Integument smooth, without robust setae and ridges ( Fig. 1 d ); latero-cephalic lobe not extending to apex of rostrum. Epimeral plates I–III extending backward, disto-ventral angle of epimeral plate III blunt.

Antenna. Antenna I ( Fig. 7 a, b ) with smooth articles; peduncular article I cylindrical, slightly tapering distally, unarmed, about twice longer than wide, as long as half of the length of head; article II cylindrical, smooth, without robust setae, more slender and slightly shorter than article I; article III short, smooth, cylindrical, slightly tapering distally, about half of the length of article II; flagellum consists of approximately 20 small smooth articles, accessory flagellum absent. Antenna II ( Fig. 7 c, d ) with smooth articles, shorter than antenna I; peduncular article I cylindrical, smooth, with small robust setae, about as long as wide, approximately 2.5 times shorter than article II and 3 times shorter than article III; article II elongated, cylindrical, about 3 times as long as wide, slightly shorter or equal to article II, thinner than article I, smooth, without robust setae; article III cylindrical, smooth, about 5 times longer than wide, thinner than article II; flagellum consists of 12–13 small smooth articles.

Mouthparts. Mandible ( Fig. 7 e ) robust and strong, well developed, without palp, incisor and lacinia mobilis minutely multi-dentate, lacinia mobilis broad; molar process reduced, but well marked, stiff-like. Maxilla I ( Fig. 7 f ) normal; inner lobe armed with 5 large stiff-like robust setae; outer lobe two-segmented, article I smooth, about half the length of article II, wider than article II; article II tapering distally, approximately 2.5 times as long as wide, with inner edge armed with a row of 7 robust setae. Maxilla II ( Fig. 7 g ) simple, with reduced plates, coalescent basally; inner margin armed with 2 simple robust setae; apical margin rounded and armed with 5 setae. Maxilliped ( Fig. 7 h ) with reduced outer plate, with small inner plate; article of endopod cylindrical, without projections; palp 4-articulate, penultimate article shorter than ultimate, with thick seta medio-distally ( Fig. 7 i ).

Gnathopods. Gnathopod I ( Fig. 8 a–c ) subchelate, significantly smaller than gnathopod II, especially in males. Gnathopod I in males ( Fig.8 b ) large, with well-marked chela; coxal plate I reduced, almost concealed by coxal plate II; basis about 5.5 times longer than wide, smooth; ischium about as long as wide, with 2 robust setae along ventral margin; merus short, rectangular, about 1.5 times longer as wide, about 1.5 times longer than ischium, bluntly produced disto-ventrally, armed with 3 strong robust setae along ventral margin, disto-ventral angle armed with 5 long simple setae and many short robust setae; carpus about 1.5 times as long as wide, widening distally, with disto-ventral angle is armed with simple robust setae, posterior lobe not developed; propodus about 1.5 times longer than wide, about 1.5 times longer than carpus, armed with several tufts of simple setae along the dorsal margin, palmar edge ( Fig. 8 c ) oblique, straight edge, armed with small teeth and robust setae, 4 large stout robust setae at the base of the segment; dactylus about 7–8 times longer than wide, slightly curved along the length of the distal part, with serrate cutting margin, sharp distally. Gnathopod I in females and males generally similar in size and structure, with more slender segments. Gnathopod II subchelate, significantly differs in males ( Fig. 8 e, f ) and females ( Fig. 8 d ). Gnathopod II in males ( Fig. 8 e, f ) with coxa continuously rounded both anteriorly and ventrally, lower edge with 3 simple robust setae; basal segment about 3.5 times as long as maximal width, widening distally, with several simple setae along ventral margin; ischium about as long as wide, with 2 robust setae ventrally, smooth; merus about as long as wide, with outer margin covered with numerous simple setae; carpus small, triangular, about as long as wide, without specific features; propodus slightly longer than basis, with almost parallel margins to longitudinal axis, smooth, about 3.5 times longer than wide, with dorsal margin convex, ventral margin straight, covered with numerous small simple setae, palmar corner ( Fig. 8 f ) with acute triangular tooth and triangular blunt projection armed with several simple setae; dactylus long, slender, about 9 times longer than maximal width, curved falcately, equal in the length to palm, smooth, with cutting margin smooth, not serrated, covered with a row of small simple setae. Gnathopod II in females ( Fig. 8 d ) generally similar to gnathopod I; coxal plates continuously rounded both anteriorly and ventrally, lower edge with 7–8 simple robust setae; basal segment about 3.5 times longer than maximal width, widening distally, smooth; ischium about as long as wide; merus short, as long as wide, about 1.5 times longer than ischium, bluntly produced disto-ventrally, armed with 2 short strong robust setae at disto-ventral angle and 2 short robust setae along ventral margin; carpus short, widening distally, with disto-ventral angle armed with simple robust setae, posterior lobe not developed; propodus about twice as long as wide, shorter than basis, dorsally unarmed or armed with 1–2 small robust setae, palmar edge oblique, sinuous, distal part armed with wide triangular teeth, with large stout robust setae at the base of the segment; dactylus about 8–9 times longer than wide, curved, with smooth cutting margin, sharpening distally.

Peraeopods. Peraeopods III–V with similar shape general for all representatives of the genus. Peraeopod III equal in length to the peraeopods IV–VI, slightly longer than peraeopod VII; coxal plates elongated in shape; basis about 4–5 times longer than wide, armed with simple setae along the distal upper margin; ischium about as long as wide, the lower margin with a simple seta; merus about twice as long as wide, about two times as long as ischium, with pronounced disto-dorsal angle; carpus about twice as long as wide, approximately the same length of merus, with several short robust setae on the upper side and 1 or 2 short robust setae along the ventral margin; propodus elongated, 3.5–4 times longer than wide, with several short robust setae along ventral margin, and several small setae along dorsal margin; dactylus simple, curved, 4–5 times longer than wide, about 1/3 of the length of propodus. Peraeopod IV with extension of coxal plate ( Fig. 9 c ), typical for the genus; coxal plates smooth and unarmed. Peraeopods VI and VII almost similar; basis with enlarged outer plate, about as long as wide, inner side of basal segment with 4 setae; ischium about as long as wide, without robust setae and setae; merus 1.5 times as long as wide, 1.5 times longer than ischium, with slightly pronounced disto-dorsal angle; carpus about 1.5–2 times as long as wide, equal to merus, with 2 short robust setae along ventral margin, and 1 robust seta disto-ventrally, upper side unarmed; propodus elongated, 4.5 times longer than wide, with 5 short robust setae along ventral margin, and a pair of long robust setae at disto-ventral angle; dactylus simple, curved, 3 times longer than wide, about 3 times shorter than the propodus.

Uropods. Uropod I ( Fig. 9 d ) biramous, peduncle about 1.5 times longer than rami, armed with 3 dorsal and 3 ventral robust setae; rami about 5 times as long as wide, the outer ramus thinner than inner, with 1 dorsal robust seta, inner ramus with 2 dorsal robust setae. Uropod II ( Fig. 9 e ) biramous, peduncle about 1.5 times longer than rami, rami equal in length; outer ramus about 4 times as long as wide, with 1 robust seta; inner ramus about 4 times as long as wide, with 1 robust seta. Uropod III ( Fig. 9 f ) uniramous, ramus 2-articulate, length ratio of peduncle and articles is 1.8:1:1, peduncle with 2 dorsal robust setae; article I of ramus with a pair of robust setae distally, article II with a series of dorsal bumps.

Telson ( Fig. 9 g ) entire, gently rounded, tapering distally, armed with 3 pairs of relatively stout short sublateral robust setae, without additional pair of setae.

Measurements. The maximum tbl. of observed female is 5.0 mm; tbl. of male—7.0 mm.

GenBank (NCBI) accession numbers. MH128325 .

Remarks. Stenothoe irinae sp. nov. clearly belongs to the Stenothoe “ gallensis ” species complex (after Krapp- Schickel 2006, 2015) by the distally sharpening distal segment of outer lobe of maxilla I, the specific shape of gnathopod II in males (disto-ventral angle armed with a triangular blunt projection) and the presence of an upwardly curved and distal article of the ramus of uropod III, characteristically sculptured with dorsal bumps in male. This species group presently includes more than 15 species distributed both in the Atlantic and Pacific basins and was even reported by Winfield et al. (2006) from soft bottoms of the Sigsbee abyssal plain, Gulf of Mexico, at 3635 m depth (Krapp-Schickel 2015). At the same time, Stenothoe irinae sp. nov. is geographically well separated from other closely related species, being the first species of Stenothoe “ gallensis ” species complex described from South-Eastern Asia, the second species of the genus known from Vietnam (see above) and only the third species from the species complex known in the Pacific Ocean. The two other Pacific species are Stenothoe dentirama Hirayama & Takeuchi, 1993 from Fukushima, Japan ( Hirayama & Takeuchi 1993) and S. crenulata Chevreux, 1908 described from Île Mangareva, Polynesia (the type locality) and Sanur-Bali, Indonesia (Krapp-Schickel 2015). The reports of the “ gallensis ” species group from the Indian Ocean are Stenothoe himyara Krapp-Schickel , TABLE]. Morphological features separating the species within Stenothoe " gallensis¨ species complex.

2015 from Port Sudan, Red Sea, Stenothoe irakiensis Salman, 1985 (considered as a junior synonym of S. gallensis ) from the Arabian Gulf, Stenothoe andamanensis Krapp-Schickel, 2015 from Andaman Islands and Stenothoe gallensis Walker, 1904 from Sri Lanka and some other localities (see Krapp-Schickel 2015: Fig. 1 ).

The new species can be clearly separated from the sibling Indo-West Pacific species by the fused articles of maxilla II, the shape of disto-ventral palmar margin armed with two well marked blunt teeth and long dactylus (reaching carpus) of gnathopod II in males and females, the presence of three pairs of dorsal submarginal robust setae on the telson (as well as the absence of dorsal setae) and the armature of the distal article of the rami of uropod III in males (see Table 1).

From the possibly closest species, Stenothoe gallensis , the species clearly differs by minute but important morphological features such as the completely fused articles of maxilla II ( Fig. 7 g ) (versus a well separated 2- segmented maxilla II) and the proximal position of dorsal robust setae on the telson ( Fig. 9 g ) (versus a more central position in S. gallensis ) and the absence of a pair of dorsal setae on the telson ( Fig. 9 g ) (versus their presence in S. gallensis ). From Stenothoe dentirama , the new species differs by the slender antennas I and II ( Fig. 7 a–d ), a clearly different form of gnathopod II in males ( Fig. 8 e, f ) (vs. similar in males and females in S. dentirama ), a different shape of maxilla II ( Fig. 7 g ), the proximal position of dorsal robust setae on the telson ( Fig. 9 g ) (vs. the central position in S. dentirama ) and the absence of a pair of dorsal setae on telson ( Fig. 9 g ) (vs. present in S. dentirama ).

From Stenothoe crenulata , the new species differs by a more slender and developed incisor process of mandible, a completely fused maxilla II ( Fig. 7 g ) (vs. separated in S. crenulata ), a more slender gnathopod II in males possessing a moderately convex dorsal margin and significantly slender dactylus ( Fig. 8e, f ) (vs. gnathopod II in males of S. crenulata from Pacific with a swollen dorsal margin as well as deep and a stouter dactylus (Krapp- Schickel 2015: Fig.4 )).

All other Indo-West Pacific species (e.g. S. himyara and S. andamanensis ) as well as other members of the “ gallensis ” species group can be clearly separated from Stenothoe irinae sp. nov. by a different shape of the propodus of gnathopod II in males, the armature of telson and the rami of uropod III in males (see Table 1).

Living coloration. The body and appendages generally transparently whitish; sternites and pleurae with small black dots, flagella and antennas I–II and eyes reddish ( Fig. 1 d ).

Etymology. The species is named after Dr. Irina A. Zhurbina , the Head of Department of Youth Programs at Russian Foundation for Basic Research (RFBR). This department of RFBR supported many interesting scientific projects and by naming this species we would like to thank the Foundation.

Ecology. Similar to previous species (see above, in Ecology of Stenothoe nhatrangensis sp. nov.), we suppose that Stenothoe irinae sp. nov. is associated with hydroids as we found the species in large number only in washouts from separate colonies of hydroids Macrorhynchia sp. and Gymnangium sp. Specimens of Stenothoe irinae sp. nov. have never been found together with specimens of Stenothoe nhatrangensis sp. nov. in washouts from neither fouling communities nor separate hydroid colonies.

Distribution. The species is presently known from the type locality—Vietnam, Nhatrang Bay, 12°11'42.6"N 109°17'39.8"E. Probably, the species is widely distributed along the coast of Vietnam in fouling communities forming on marine artificial constructions.

Figures

FIGURE 1. Alive coloration of female (a) and male (b, c) of Stenothoe nhatrangensis sp. nov., (LEMMI) and male of Stenothoe irinae sp. nov. (d).

FIGURE 7. Stenothoe irinae sp. nov., male (LEMMI) (a, b, e–h), female (LEMMI) (c–d): a, b—antenna I; c, d—antenna II; e—mandible; f—maxilla I; g—maxilla II; h—maxilliped; i—same, distal segments.

FIGURE 8. Stenothoe irinae sp. nov., female (ZIN 89385) (a, d), male (ZIN 89386) (b, c, e, f): a—gnathopod I; b—same, distal segments; c—same; distal part of propodal segment and dactylus; d, e—gnathopod II; f –same; distal part of propodal segment and dactylus.

FIGURE 9. Stenothoe irinae sp. nov., male (ZIN 89387): a—peraeopod III; b—propodus and dactylus of peraeopod III; ccoxal part of peraeopod IV; d—uropod I; e—uropod II; f—uropod III; g—telson.

FIGURE4. Stenothoe nhatrangensis sp.nov., male (ZIN 89382):a, b—peraeopod III; c, d—peraeopod IV; e,f—peraeopod V.