Melita megacheles Giles, 1885 by monotypy.

Eyes small. Antenna 1, accessory flagellum pluriarticulate. Mandibular palp article 3 rectolinear or clavate. Coxae small, relatively short, coxa 1 dilated, produced forward. Gnathopod 1 (male) subchelate. Gnathopod 2 subchelate and greatly larger than gnathopod 1. Pereopods 6–7, dactylus elongate, falcate. Uropods 1 – 2 biramous; rami slighty subequal; peduncle with ventrodistal process. Uropod 3 uniramous. Telson entire.

Cheiriphotis contains 16 species: Cheiriphotis australiae Stebbing, 1910; Cheiriphotis delloyei Pirlot, 1934; Cheiriphotis durbanensis K.H. Bamard, 1916; Cheiriphotis erythraeus Ruffo, 1969; Cheiriphotis geniculata K.H. Barnard, 1916; Cheiriphotis madagascarensis Ledoyer, 1979; Cheiriphotis mediterranea Myers, 1983; Cheiriphotis megacheles (Giles, 1885); Cheiriphotis minima Ledoyer, 1982; Cheiriphotis neotropicalis Valerio-Berardo, 2007; Cheiriphotis pediformis Myers, 1995; Cheiriphotis quadrichelatus Ortiz & Lalana, 1997; Cheiriphotis rotui Myers, 1989; Cheiriphotis walkeri Stebbing, 1918; Cheiriphotis williamsoni Salman & Jabbar, 1990;

Holotype. ♂, THAILAND, Lower Gulf of Thailand, Talet Bay (09˚18'39.5"N, 99˚46'46.4"E), seagrass bed (associated with Thalassia hemprichii), 24 September 2008, Puttapreecha, R., PSUZC-CR-0264.

Allotype. ♀, collected with holotype, PSUZC-CR-0265 (adult female, 4.16 mm)

Other material. Same data as for holotype, UKMMZ-1446 (5♂; 15♀); PSUZC-CR-0266 (5♂; 20♀)

Male (holotype). Total body length 3.5 mm (from tip of rostrum to apex of telson). Body rather slender and subcylindrical. Head subequal in length to first 2 pereonites; rostrum not developed; inferior antennal sinus short and concave, about 0.3 times of head length; eye distinct. Antenna 1 slightly longer than antenna 2, ratio of peduncular article 1–3 as 5:9:8; article 1 slender, with 2 postero-marginal setae; flagellum with 10 articles, 0.7 times as long as peduncle; accessory flagellum with 4 articles, last article scale-like. Antenna 2 peduncle slender; article 1–4 in ratio of 2:5:4:2; inner margin of article 4 and 5 with long postero-marginal setae; article 5 shorter than 4; flagellum short with long setae, subeaqual in length to peduncular article 5, composed of 7 articles, last article scale-like.

Upper lip or labrum round and broad, with small depression in the middle and pubescent on each lobe. Lower lip inner lobe small and pubescent, mandibular process well developed; outer plate with a group of finger-like setae on the inner face of the outer lobe, covered with thin hair-like setae. Mandible, both incisors with 5 teeth; lacinia mobilis armed with 4 teeth on the left side and 5 teeth on the right side; molar process columnar, ridged distally; palp 3-articulate with ratios of 1:3:3, article 1 with 2 marginal setae, article 2–3 with apical and marginal setae. Maxilla 1, inner plate small with 2 apical setae, outer plate with 8 apical and marginal serrate robust setae; palp extending beyond outer plate, biarticulate with 6 apical serrate robust setae. Maxilla 2, inner plate with 19 slender marginal setae; outer plate larger than inner plate with 20 slender setae. Maxilliped, inner plate broad and short, reaching half of outer plate, apically provided with 4 conate setae and fine setae; outer plate broad, almost reaching palp article 2 with 7 conate setae; palp 4-articulate with ratio of 3:5:2:1.

Pereon. Gnathopod 1 subchelate, smaller than gnathopod 2; coxal plate subtriangular, produced anteriorly with long fine setae on anteroventral corner; length ratio of articles from basis to dactylus about 14:5:6:13:10:9; basis slender, broader distally, posterior margin bearing long setae; ischium short, subrectangular with apical setae; merus subtriangular with posteromarginal setae, longer than ischium; carpus longer than propodus with plumose setae on posterior margin; propodus shorter than dactylus, palm oblique with a robust seta at the proximal half, surface of palm toothed; dactylus slightly longer than palm, falcate, inner margin with a robust seta, . Gnathopod 2 subchelate; coxal plate short and wide, subrectangular, length ratio of articles from basis to dactylus about 9:5:8:8:19:19; basis robust, nearly as long as wide, broader distally, anterior margin straight, both sides naked; ischium subrectangular; merus longer than ischium; carpus distal and anterior margin fused with propodus; propodus enlarged, as long as wide, anterior margin with a row of plumose setae, posterior margin with short setae; palm transverse, with 4 blunt teeth and one acute palmar corner; dactylus slightly longer than palmar margin, inner margin smooth.

Pereopod 3 slender and elongate; coxal plate small and suboval, with 3 plumose setae on anterior side; length ratio of articles from basis to dactylus 10:3:6:2:6:4; basis slender, distally extended; ischium short, subrectangular; merus longer than carpus, slightly produced anterodistally; carpus subrectangular, medially broad, posterior margin setose; propodus subrectangular; basis – propodus bearing plumose setae on both sides; dactylus falcate, long and thin, shorter than propodus. Pereopod 4 rather similar to pereopod 3, coxal plate suboval with plumose setae on ventral side; length ratio of articles from basis to dactylus about 10:2:5:3:6:4; basis slender; ischium short, subrectangular; merus longer than carpus, slightly produced anterodistally; carpus subquadrate, shorter than propodus; basis to propodus with plumose setae on both margins; propodus long and narrow; dactylus long and thin, shorter than propodus. Pereopod 5 shorter than pereopod 6 and 7; coxa bilobed; length ratio of articles from basis to dactylus about 14:2:3:3:6:3; basis subrectangular with plumose setae on both margins; ischium shortest with posteromarginal plumose setae; merus subequal to carpus, with posteromarginal plumose setae and 1 anterodistal seta; carpus with posteromarginal plumose setae; propodus with 4 robust setae along posterior margin; dactylus short, strongly curved. Pereopod 6 elongate, 1.5 times as long as pereopod 5; coxa posteriorly produced with rounded lobe; length ratio of articles from basis to dactylus about 5:2:3:3:5:2; basis oval with plumose setae on both margins; ischium short with plumose setae on anteroventral corner; merus oblong, with plumose setae on both margins; carpus shorter than propodus, bearing long setae; propodus slender with marginal robust setae and setose posterodistally; dactylus falcate. Pereopod 7 elongate, 1.6 times as long as pereopod 5; coxa short and wide, subtriangular, anteriorly produced; length ratio of articles from basis to dactylus about 13:5:7:7:11:6; basis posteriorly produced, bearing plumose setae on both margins; ischium short and subquadrate with plumose setae on anterodistal corner; merus elongate with plumose setae on both sides; carpus subequal to merus, both margins with sparse setae; propodus slender, longer than merus, distally extended; bearing setae on both margins and one robust seta on anterodistal corner; dactylus falcate, with one thin seta at 2/3 from proximal end.

Pleon. Pleopods 1–2 well developed; peduncles subcylindrical, longer than broad and fringed with several plumose setae and a pair of retinaculae on the inner margin; inner ramus subequal to peduncle with 9–10 articles, outer ramus shorter than inner ramus, both rami with facial setae.

Pleopod 3 similar to pleopod 1 and 2 except the tip of outer ramus modified; bearing long setae with sparse setule and having three additional modified setae with three forked tips respectively, outer ramus longer than inner ramus.

Uropod 1 longest, extending beyond uropods 3; peduncle longer than both rami, beset with robust setae, peduncular apex bearing 3 posteroventral robust setae; outer and inner margins of both rami lined with a row of robust setae, distal margin rounded and bearing several robust setae. Uropod 2 not reaching uropod 3, peduncle shorter than rami, both outer and inner margins with a row of robust setae; outer ramus slightly longer than inner one, both rami lined with a row of robust setae and distal margin bearing several short and long robust setae. Uropod 3 uniramous, peduncle extended with robust seta on apex, subequal to ramus; apically 3 robust setae and 2 setae. Telson subtrapezoidal, distally excavated with long simple setae near both distal corners.

Female. (allotype). Total body length 4.2 mm (from tip of rostrum to apex of telson). – (sexually dimorphic characters).

Antenna 1 flagellum with 12 articles.

Pereon. Gnathopod 2 subchelate, smaller than that of male, basis to propodus setose; basis more slender, about 2.3 times as long as broad; carpus subtriangular, as long as broad; propodus suboval, longer than carpus, palm oblique and defined by a large bifid robust seta, palmar margin convex, distal end covered with sparse setae; dactylus curved with 5 inner marginal short setae. Coxa 4 and 5 longer than those of male.

Pleopod 3 without modified tip of outer ramus.

Figure 1.

Map of sampling area

Figure 2A.

Photography of Cheiriphotis trifurcata sp. n. i holotype, male, (PSUZC-CR-0264), 3.47 mm. ii allotype, female, (PSUZC-CR-0265), 4.16 mm. Talet Bay, Lower Gulf of Thailand.

Figure 2B.

Cheiriphotis trifurcata sp. n., holotype, male, (PSUZC-CR-0264), 3.47 mm. Talet Bay, Lower Gulf of Thailand. All scales represent 0.2 mm.

Figure 2C.

Cheiriphotis trifurcata sp. n., holotype, male, (PSUZC-CR-0264), 3.47 mm. Talet Bay, Lower Gulf of Thailand. All scales represent 0.2 mm.

Figure 2D.

Cheiriphotis trifurcata sp. n., holotype, male, (PSUZC-CR-0264), 3.47 mm. Talet Bay, Lower Gulf of Thailand. All scales represent 0.1 mm.

Figure 2E.

Cheiriphotis trifurcata sp. n., holotype, male, (PSUZC-CR-0264), 3.47 mm. Talet Bay, Lower Gulf of Thailand. Scales for U1– U3 and T represent 0.1 mm; PL1 – PL3 represent 0.2 mm.

Figure 3A.

Cheiriphotis trifurcata sp. n., allotype, female, (PSUZC-CR-0265), 4.16 mm. Talet Bay, Lower Gulf of Thailand. All scales represent 0.2 mm.

Figure 3B.

Cheiriphotis trifurcata sp. n., allotype, female, (PSUZC-CR-0265), 4.16 mm. Talet Bay, Lower Gulf of Thailand. All scales represent 0.1 mm.

Figure 3C.

Cheiriphotis trifurcata sp. n., allotype, female, (PSUZC-CR-0265), 4.16 mm. Talet Bay, Lower Gulf of Thailand. All scales represent 0.1 mm.

Figure 3D.

Cheiriphotis trifurcata sp. n., allotype, female, (PSUZC-CR-0265), 4.16 mm. Talet Bay, Lower Gulf of Thailand. Scale for U1-U3 and T represents 0.1 mm; remaining represents 0.2 mm.

The specific name “trifurcata” is from latin ‘tri = three’ and ‘furcated = forked’, referring to the distinct three forked tips of the modified setae on the outer ramus in male pleopod 3.

Even Cheiriphotis trifurcata shows a distinct character, with the presence of the three additional modified setae in male pleopod 3 and each seta equipped with three forked tips, but this character might be overlooked in other species. Besides, the general characters in the present species are closely related to Cheiriphotis williamsoni, Cheiriphotis neotropicalis, Cheiriphotis mediterranea and Cheiriphotis walkeri especially in the; 1) fused carpus-propodus of male gnathopod 2; 2) propodus with transverse palm and; 3) uropod 3 uniramus. Further examination on the present species also indicated that Cheiriphotis trifurcata can be distinguished from Cheiriphotis williamsoni by the male gnathopod 1 which has the carpus longer than the propodus and the palm of male gnathopod 2 which bears 4 blunt teeth and 4 blunt teeth and one acute palmar corner. The present species also differs from Cheiriphotis neotropicalis in the carpus of male gnathopod 1 which is longer than the propodus and the propodus of male gnathopod 2 as long as broad in contrast to Cheiriphotis neotropicalis where the carpus of gnathopod 1 is subequal to propodus and propodus of gnathopod 2 is broader than long.

Cheiriphotis trifurcata shares a character of epimeron 2 with plumose setae on the ventral margin of epimera 2 with four known congeners, Cheiriphotis erythraeus, Cheiriphotis mediterranea, Cheiriphotis williamsoni and Cheiriphotis neotropicalis. The former can be distinguished from Cheiriphotis erythraeus by the carpus of the male gnathopod 1 which is partly fused with the propodus, the transverse palm which has 4 blunt teeth and the uniramus uropod while in the latter the carpus of the male gnathopod 1 is not fused with the propodus, the palm is medially V-shaped excavated with two teeth on both sides and the uropod is biramus. Cheiriphotis trifurcata is easily separated from Cheiriphotis mediterranea by the distally expanded peduncle of uropod 3 (vs. peduncle of uropod 3 not expanded distally).

To date, only one species of Cheiriphotis (i.e. Cheiriphotis megacheles)has been reported from the Andaman Sea and the South China Sea (Imbach 1967 and Rabindranath 1971). The absence of robust setae along the palm of gnathopod 1, the unfused carpus-propodus of the male gnathopod 2, a rounded epimeron 2, and the biramus uropod 3 in Cheiriphotis megacheles readily differentiates that species from the present one.