Stenothoe nhatrangensis, 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 : 58-67

publication ID	https://doi.org/ 10.11646/zootaxa.4410.1.3
publication LSID	lsid:zoobank.org:pub:C3C45A46-3761-4074-BC03-6A3432FDF1F7
DOI	https://doi.org/10.5281/zenodo.5998078
persistent identifier	https://treatment.plazi.org/id/8BBCBB85-7BB2-47FB-B80C-EF7BA4414B4A
taxon LSID	lsid:zoobank.org:act:8BBCBB85-7BB2-47FB-B80C-EF7BA4414B4A
treatment provided by	Plazi
scientific name	Stenothoe nhatrangensis
status	sp. nov.

Treatment

Stenothoe nhatrangensis View in CoL sp. nov.

( Figs 1 b–d View FIGURE 1 , 2–6 View FIGURE 2 View FIGURE 3 View FIGURE4 View FIGURE 5 View FIGURE 6 )

Material examined. Holotype (ZIN 89316) male; paratype (ZIN 89317) female, paratype (ZIN 89318) male, paratype (ZIN 89382, 89383, 89384) dissected—Pacific Ocean, South China Sea, Vietnam, Nhatrang Bay, Tre Island, 12°11'24.1"N 109°18'46.1"E, underwater surface of floating fish farm construction, 1 meter depth, SCUBA, coll. Marin I., Sinelnikov S., March 2014; 15 adult ovigerous females, 10 males, 20 juveniles—same locality and date as holotype; 15 adult ovigerous females, 15 males—same locality and date, special washout from hydroids Macrorhynchia sp. and Gymnangium sp.

Description. Body dorsally smooth, without robust setae and ridges ( Fig. 1 a, b View FIGURE 1 ); latero-cephalic lobe not extending to apex of rostrum.

Antenna. Antenna I ( Fig. 2 a View FIGURE 2 ) with smooth articles; peduncular article 1 cylindrical, slightly tapering distally, about 1.5 times longer than wide, as long as half of the length of head, with subdistal robust seta; article II cylindrical, smooth, without robust setae, more slender and slightly shorter than article I; article III short, smooth, cylindrical, slightly tapering distally, about 3 times shorter than article II; flagellum consists of approximately 19 smooth articles, accessory flagellum absent. Antenna II ( Fig. 2 a View FIGURE 2 ) with smooth segments, slightly overreaching antenna I; peduncular article I cylindrical, smooth, without 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 4 times as long as wide, slightly shorter than article III, thinner than article I, smooth, without robust setae; article III cylindrical, smooth, about 6 times longer than wide, thinner than article II, flagellum consists of 8–9 smooth articles.

Mouthparts. Mandible ( Fig. 2 b View FIGURE 2 ) strong, well developed, without palp; incisor and lacinia mobilis minutely multi-dentate, lacinia mobilis broad ( Fig. 2 c View FIGURE 2 ); incisor process reduced, but well marked, stiff-like, distally sharpen. Maxilla I ( Fig. 2 e View FIGURE 2 ) normal; inner lobe armed with 5 large stiff-like robust setae ( Fig. 2 f View FIGURE 2 ); outer lobe two-segmented, article 1 smooth, about 1.5 times shorter and wider than article II; article II widening distally, approximately twice as long as maximal width, with inner edge armed with a row of 8 robust setae. Maxilla II ( Fig. 2 d View FIGURE 2 ) simple, with reduced plates, fused basally; inner margin armed with 5 simple robust setae; apical margin rounded and armed with 3 setae. Maxilliped ( Fig. 6 h View FIGURE 6 ) with reduced outer plate, with small inner plate; article of endopod cylindrical, without projections; palp 4-articulate: article III shorter than article IV, with thick hairy setae distally.

Gnathopods. Gnathopod I subchelate, significantly smaller than gnathopod II; gnathopod I in males ( Fig. 3 c View FIGURE 3 ) large, subchelate, with well-marked palm; coxal plate I reduced, almost concealed by coxal plate II; basis about 3.5 times longer than wide, 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 5 strong robust setae along ventral margin, distoventral angle armed with 4 long simple long 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 ( Fig. 3 d View FIGURE 3 ) about twice as long as wide, about 1.5 times longer than carpus, armed with several tufts of simple setae along the dorsal margin, palmar edge oblique, armed with small teeth and robust setae, 4 large stout robust setae at the base of propodus; 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. Gnathopod II subchelate, significantly differs in males ( Figs. 1 b, c View FIGURE 1 , 3 a View FIGURE 3 ) and females ( Fig. 6 e View FIGURE 6 ). Gnathopod II in males ( Fig. 3 a View FIGURE 3 ) with coxal plates continuously rounded both anteriorly and ventrally, lower edge with 3 simple robust setae; basis about 4 times as long as maximal width, widening distally; ischium about as long as wide, without robust setae, smooth; merus triangular about as long as wide, with outer margin covered with numerous simple setae; carpus small, about as long as wide, without specific features; propodus slightly longer than basis, most expanded on base almost parallel to longitudinal axis, smooth, about 3 times longer than wide, with ventral edge concave, covered with numerous small simple setae, disto-ventral angle ( Fig. 3 b View FIGURE 3 ) with large blunt triangular tooth and triangular blunt projection armed with several simple setae; dactylus long, slender, about 10 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. 6 e View FIGURE 6 ) generally similar to gnathopod I ( Fig. 6 c–f View FIGURE 6 ); 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 distoventrally, armed with 1 short strong robust seta at disto-ventral angle; carpus short, widening distally, with distoventral angle armed with simple robust setae, posterior lobe not developed; propodus ( Fig. 6 f View FIGURE 6 ) about twice as long as wide, equal to basis, dorsally unarmed, palmar edge oblique, sinuous, with large stout robust setae at the base of the obturator; dactylus about 9–10 times longer than wide, curved, with smooth cutting margin, sharpening distally.

Peraeopods. Peraeopod III ( Fig. 4 a, b View FIGURE4 ) equal in length to peraeopods IV–VI ( Fig. 4 c, e View FIGURE4 ), longer than the peraeopod VII ( Fig. 5 a, b View FIGURE 5 ),; coxal plate elongated in shape ( Fig. 4 a View FIGURE4 ); basis ( Fig. 4 b View FIGURE4 ) about 4 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 and about two times as long as ischium, with pronounced distodorsal angle; carpus about twice as long as wide, approximately the same length of merus, with 4 short robust setae on the upper side and 1 short seta along the ventral margin; propodus elongated, about 3.5 times longer than wide, with 7 short robust setae along ventral margin, and 2 small setae along dorsal margin; dactylus simple, curved, about 4 times longer than wide, about 2 times shorter than the propodus; peraeopods III–V ( Fig. 4 e, f View FIGURE4 ) with similar structure; peraeopod IV ( Fig. 4 c View FIGURE4 ) with extension of coxal plates, typical for the genus; coxal plate smooth, unarmed. Peraeopod VI ( Fig. 5 a View FIGURE 5 ) with expanded basis, about as long as wide, inner side of basal segment with 5 setae; ischium about as long as wide, without robust setae and setae; merus about twice as long as wide, about 3 times longer than ischium, with pronounced disto-dorsal angle; carpus about 1.5–2 times as long as wide, about half of the length of merus, with 4 pairs of short robust setae along ventral margin, upper side unarmed; propodus elongated, about 3.5 times longer than wide, with 6 short robust setae along ventral margin, and 2 small setae along dorsal margin; dactylus simple, curved, about 4 times longer than wide, about 2 times shorter than the propodus. Peraeopod VII ( Fig. 5 b View FIGURE 5 ) generally similar to peraeopod VI, basis expanded.

Uropods. Uropod I ( Fig. 5 c View FIGURE 5 ) bi- and equiramous, peduncle about twice as long as rami, armed with 8 dorsal robust setae along dorsal surface; rami about 7 times as long as wide, the outer ramus with 5 dorsal robust setae, inner ramus with 4 dorsal robust setae. Uropod II ( Fig. 5 d View FIGURE 5 ) biramous, peduncle about 1.5 times longer than rami, outer ramus slightly shorter than inner ramus; outer ramus about 4.5 times as long as wide, with 3 dorsal robust setae; inner ramus about 6 times as long as wide, with 3 robust setae. Uropod III ( Fig. 5 e View FIGURE 5 ) uniramous, ramus 2- articulate, length ratio of peduncle and articles is 2:1:1, peduncle with 5 dorsal robust setae; article I with a pair of robust setae distally, article II unarmed, smooth.

Telson ( Fig. 5 f, g View FIGURE 5 ) entire, oval, gently rounded, tapering distally, armed with 4 pairs of relatively stout short robust setae.

Measurements. The maximum tbl. of observed females is 5.5 mm; tbl. of males—7.0 mm.

GenBank (NCBI) accession numbers. MH128323 View Materials , MH128324 View Materials .

Remarks. Stenothoe nhatrangensis sp. nov. is most morphologically similar to Stenothoe valida Dana, 1852 (the type locality of which is Rio de Janeiro, Atlantic Ocean), and according to Krapp-Schickel (2015) it is possible to suggest the Stenothoe “ valida ” species complex which is characterized by 1) distally expanded and broad distal segment of outer lobe of maxilla I; 2) specific form of disto-ventral margin of propodus of gnathopod II armed with a triangular blunt projection in males; 3) widened and extremely lengthened merus reaching the end of the carpus of peraeopods VI, VII; 4) straight and smooth distal segment of ramus of uropod III both in males and females. Such morphological features such as the broad distal segment of the outer lobe of maxilla I and the form of the disto-ventral margin of the propodus of gnathopods II in males clearly separate this species-complex from the Stenothoe “ gallensis ” species complex (see below).

The present species can be separated from S. valida both geographically (Atlantic Ocean for S. valida vs. Pacific Ocean for Stenothoe nhatrangensis sp. nov.) and morphologically showing a reduced but well marked stifflike molar process of the mandible ( Fig. 2 b View FIGURE 2 ) (vs. completely reduced in S. valida ), fused segments of maxilla II ( Fig. 2 d View FIGURE 2 ) (vs. clearly separated in S. valida ), slender propodus of gnathopod II in males ( Figs. 1 c View FIGURE 1 , 3 a View FIGURE 3 ) with almost parallel margins, with dorsal margin convex and ventral margin concave, specific form of disto-ventral margin of propodus of gnathopod II in males ( Fig. 3 b View FIGURE 3 ) (vs. propodus of gnathopod II in males swollen, with convex dorsal and almost straight ventral margin in S. valida ). Such minute morphological features are usual within species complexes of the genus Stenothoe (see Krapp-Schickel 2015; present paper, below), while the geographical separation also confirmed the separation of the new species for the specimens from Nhatrang Bay, Vietnam.

Living coloration. The body and appendages generally transparent whitish; sternites and pleurae with small black dots in males ( Fig. 1 b, c View FIGURE 1 ), with broader black horizontal bands on sternites and longitudinal bands on pleura in females ( Fig. 1 a View FIGURE 1 ); eyes, flagella and antennas I–II reddish.

Etymology. The species is named after the locality (Nhatrang Bay, Vietnam), where it was firstly collected.

Ecology. The specimens of Stenothoe nhatrangensis sp. nov. were collected in large mixed male and female groups of mature and juvenile specimens from fouling assemblages of local fishing farm floating platforms as well as numerous specimens also found in washouts from hydroids taken from floating platforms. Similar to some other representatives of the family Stenothoidae (Pirlot 1932, Krapp-Schickel 1993, Vader & Krapp-Schickel 1996, Krapp-Schickel & Vader 2015, Tandberg & Vader 2015) we suppose an obligate character of association of Stenothoe nhatrangensis sp. nov. with hydroids. Most specimens of the species were found in washouts from separate colonies of hydroids Macrorhynchia sp., Gymnangium sp. (Cnidaria, Hydrozoa, Aglaopheniidae ) or Sertularia sp. (Cnidaria, Hydrozoa, Setullaridae) abundant in fouling communities in the Bay. Such communities associated with artificial structures in Nhatrang Bay usually consist of a dense community of specific hydroids (representatives of the genera Macrorhynchia , Gymnangium , Aglaophenia (Aglaopheniidae) , Sertularia and others), and sponges and ascidians inhabited by smaller marine invertebrates, such as amphipods. The number of species reported from fouling is large and is widely distributed among the existing groups of organisms. Two other most abundant and numerous amphipod species, Caprella spp. (Crustacea, Amphipoda , Caprellidae ) and Podocerus sp. (Crustacea, Amphipoda , Podoceridae ), were also collected in large numbers from the washout of fouling communities as well as separate hydroids. Other large crustaceans known from these communities in Nhatrang Bay are pontoniine shrimps of the genus Rapipontonia Marin, 2007 (Crustacea, Decapoda , Palaemonidae , Pontoniinae ) (Marin et al. 2005, Marin 2007, 2009).

Distribution. The species is presently known only from the type locality—Nhatrang Bay, 12°11'24.1"N 109°18'46.1"E, Vietnam, South China Sea. Probably, the species is widely distributed along the coast of Vietnam in fouling communities forming on marine artificial constructions.