Rotomelita longipropoda, Wongkamhaeng, Koraon, Coleman, Charles Oliver & Pholpunthin, Pornsilp, 2013

Wongkamhaeng, Koraon, Coleman, Charles Oliver & Pholpunthin, Pornsilp, 2013, Maeropsis paphavasitae and Rotomelita longipropoda, two new species (Crustacea, Amphipoda) from Lower Gulf of Thailand, ZooKeys 307, pp. 15-33 : 19-23

Rotomelita longipropoda sp. n.

Type material.

Holotype. ♂ (1.65 mm), THAILAND, Lower Gulf of Thailand, Phangan Island (9°41'48"N, 100°0'2"E), seagrass bed (associated with Thalassia hemprichii and Halophila ovalis ), 1 July 2009, Bantiwiwatkul, N., PSUZC-CR-0195.

Allotype. ♀, collected with holotype, PSUZC-CR-0196 (gravid female, 1.66 mm) Other material. Same data as for holotype, PSUZC-CR-0197 (5♂; 5♀)

Description.

Based on male holotype. Total body length 1.65 mm (from tip of rostrum to apex of telson). Body, rather slender and subcylindrical. Head, slightly shorter than first 2 pereonites; rostrum not developed; inferior antennal sinus shorter than eyes, concave, about 0.2 times head length; eye distinct. Antenna 1, longer than antenna 2, ratios of peduncular articles 1-3 2:2:1; article 1 slender; flagellum with 16 articles plus 1 rudimentary article, 2 times as long as peduncle; accessory flagellum with 3 articles, last article scale-like. Antenna 2, peduncle slender; gland cone fleshy, short, not reaching to end of peduncular article 3; articles 2-5 ratios 1:1:3:3; inner margin of article 4 and 5 with sparse setae; article 5 slightly shorter than 4; flagellum short with sparse setae, longer than peduncular article 5, composed of 7 articles, last article scale-like.

Upper lip (labrum), round and broad. Lower lip, inner lobe small, outer lobe pubescent, mandibular process well developed. Mandible, left incisor with 6 teeth and right incisor with 5 teeth; lacinia mobilis armed with 4 teeth on both sides; molar triturative; palp 3-articulate with ratios 1:2:3, article 3 with 3 apical setae. Maxilla 1, inner plate triangular, small, with 3 apical robust setae, outer plate with 8 apical serrate robust setae; palp 2-articulate, article 1 shorter than 2, article 2 apical margin with 4 robust setae. Maxilla 2, inner plate with 5 slender apical setae; outer plate subequal to inner plate, with 8 slender setae. Maxilliped, inner plate narrow, short, reaching half of outer plate, apically provided with 6 plumose setae and 3 stout robust setae; outer plate broad, subrectangular, almost reaching palp article 3, with 5 apical and marginal setae and 2 apical robust setae; palp 4-articulate, with ratios 4.5:4:4:3, article 4 unguiform.

Pereon. Gnathopod 1, subchelate, smaller than gnathopod 2; coxal plate subrectangular; length ratios of articles from basis to dactylus about 10:4:4:7:6:4; basis slender, posterior margin bearing long setae; ischium short, subrectangular with short posteromarginal setae; merus subrectangular with long and short posteromarginal setae; carpus trapezoidal with long setae on posterior margin; propodus subtriangular, shorter than carpus, palm transverse, produced, with sparse setae, posterodistal corner minutely serrate, slightly bending inwards; dactylus shorter than palm, falcate. Gnathopod 2, subchelate; coxal plate short and wide, subrectangular, length ratios of articles from basis to dactylus about 14:5:5:6:16:12; basis slender, 3 times as long as wide, anterior margin straight; ischium subrectangular, subequal to merus; carpus triangular; propodus enlarged, oval, 2 times as long as wide, palm strongly oblique with sparse setae; dactylus falcate, slightly shorter than palmar margin, inner margin smooth, the inner surface not excavate.

Pereopod 3, slender and elongate; coxal plate small and subtrapezoidal, with 5 fine setae on anterioroventral margin; length ratios of articles from basis to dactylus 10:2:6:5:6:2; basis slender; ischium short, subrectangular; merus slightly longer than carpus, produced anteriorly; carpus slender with apical setae; propodus subrectangular; basis to propodus bearing sparse setae on both sides; dactylus falcate, shorter than propodus. Pereopod 4, similar to pereopod 3, coxal plate subrectangular with fine setae on anteroventral margin; length ratios of articles from basis to dactylus about 12:2:6:5:6:2; basis slender; ischium short, subrectangular; merus produced anterodistally; carpus slender, subequal to propodus; basis to propodus with sparse setae on both margins; propodus long and narrow; dactylus falcate and short. Pereopod 5, shortest; coxa bilobed; length ratios of articles from basis to dactylus about 8:2:5:4:6:3; basis subrectangular with short fine setae on posterior margin; ischium shortest; merus with a posterodistal robust seta and 1 anterodistal seta; carpus with distal robust setae on both sides; propodus with 4 distal long fine setae; dactylus short and curved. Pereopod 6 elongate, 1.6 times as long as pereopod 5; coxa posteriorly produced with rounded lobe; length ratios of articles from basis to dactylus about 12:3:8:7:11:4; basisposterior margin straight with minute castellations, with fine setae on both margins; ischium short with fine setae on anteroventral corner; merus oblong, with robust setae on posterior margins and posterodistal corner; carpus subrectangular with 3 anterodistal robust setae; propodus oblong, slender with marginal robust setae, setose posterodistally; dactylus falcate. Pereopod 7, subequal to pereopod 6; coxa short and wide, semicircular; length ratios of articles from basis to dactylus about 12:3:8:8:14:5; basis posteriodistally produced, bearing fine setae on posterior margin; ischium short and subquadrate; merus elongate with robust setae on both sides; carpus subequal to merus, bearing 3 anterodistal robust setae; propodus oblong, longer than merus, bearing robust setae on both margins and anterodistal corner; dactylus falcate.

Pleon. Epimera 1-3 each with small posterodistal tooth. Uropod 1, peduncle with 2 distal robust setae; rami slightly shorter than peduncle, armed with 3 apical robust setae. Uropod 2, peduncle subequal to rami; rami subequal, outer ramus with a marginal robust seta and apex armed with several long and short robust setae. Uropod 3, biramous; inner ramus minute, pointed apically, bearing 1 apicomedial robust seta; outer ramus biarticulate, with marginal robust setae, distal article short. Telson, broader than long, cleft, lobes very broad and apically truncate, each lobe with 2 apical setae.

Female. (allotype) (sexually dimorphic characters). Total body length 1.66 mm (from tip of rostrum to apex of telson).

Pereon.Gnathopod 1, subchelate, smaller than gnathopod 2; coxal plate subrectangular; length ratios from basis to dactylus about 10:4:4:6:5:4; propodus subtriangular, longer than dactylus, palm transverse with short marginal setae; dactylus falcate, tapering distally. Gnathopod 2, subchelate; length ratios from basis to dactylus about 10:5:5:6:8:5; basis slender, 2.5 times as long as wide, anterior margin weakly produced with sparse setae; ischium subrectangular; carpus triangular; propodus suboval, 1.6 times as long as wide, palm oblique, defined by 2 robust setae; dactylus falcate, slightly shorter than palmar margin, inner margin smooth.

Etymology.

The specific name “longipropoda” is from latin 'longi = long’ and 'propoda = propodus’, referring to the relatively long propodus of male gnathopod 2 compared to congeners.

Remarks.

Rotomelita longipropoda new species shares some characteristics with Rotomelita lokoa Barnard, 1977 and Rotomelita ana Barnard, 1977 from Hawaii by having stalked coxal gills; weakly sexually dimorphic gnathopod 1 and 2; coxa 1 not expanded distally; coxa 4 proximally excavated, smooth pleon segments 1-3 and urosomite 1, and a deeply cleft telson, with truncate lobes.

However, the new species can be easily distinguished from its congeners by having eyes (vs. lacking eyes in Rotomelita lokoa and Rotomelita ana ); antenna 1 peduncle without robust setae (vs. with 2 robust setae on ventroproximal margin in Rotomelita lokoa and Rotomelita ana ); male gnathopod 2 with relatively long propodus (2.7 times as long as carpusvs, 1.6 times in the two other species) and the uropod 3 rami are shorter (2.2 times as long as peduncle) compared to the other two species (3.3 times as long as peduncle).

Rotomelita longipropoda is also similar to other Rotomelita subgroup i.e. Nainaloa Karaman & Barnard, 1979 and Tegano Barnard & Karaman, 1982. Rotomelita longipropoda can be distinguished from Nainaloa as follows: Rotomelita longipropoda gnathopod 1 palm has inner surface not excavate (inner face of propodus excavate in Nainaloa ); pediculate gills (figure 8) (simple gill in Nainaloa ); article 2 of pereopods 5-7 not lobed (lobed in Nainaloa ) and truncate lateral cephalic lobes (prominent in Nainaloa ). Besides, Rotomelita longipropoda is also allied to member of genus Tegano occurring in Indo-Pacific. They differ as follows: Rotomelita longipropoda has mandibular palp article 3 longer than article 2, bearing 3 apical setae (article 3 reduced with 1 apical seta in Tegano ) and the telson short, cleft, lobes very broad and apically truncate (tapering and apically point in Tegano ).

In terms of ecology Rotomelita , Nainalao and Tegano are recorded from fresh to brackish water ( Barnard 1977; Bousfield 1971 and Lowry and Springthorpe 2009) while Rotomelita longipropoda was collected from seagrass beds in a salinity range of 28-32 ppt. However, there is a small creek located 1 km northern of seagrass patch. The amphipods also can be considered as a brackish species. It is the first record of this genus from West Pacific.