Cheiriphotis trifurcata, new species (Crustacea, Amphipoda, Corophiidae, Protomedeiinae) from the Seagrass Bed of the Lower Gulf of Thailand

Abstract A new species of corophiid Amphipoda, Cheiriphotis trifurcata, collected from the seagrass bed of the Lower Gulf of Thailand, is described. Cheiriphotis trifurcata is characterized by its trifurcated tip of the modified setae on the outer ramus in male pleopod 3. In this paper, the new species is fully described and compared with related species and a complete key of the 16 valid species in the genus Cheiriphotis is given.


Introduction
Species of the genus Cheiriphotis Walker, 1904, are predominant and widespread in both marine circumtropical and warm-temperate waters of the world. Of the 15 valid species that have been described so far, only C. megacheles (Giles, 1885) was reported in the Andaman Sea while the Gulf of Thailand has no reports of amphipods in this genus (e.g. Angsupanich and Kuwabara 1995;Angsupanich et al. 2005;Ariyama et al. 2010;Bussarawich 1985;Chilton 1925;and Ruensirikul et al. 2007).
Cheiriphotis megacheles was first described from the Bay of Bengal in 1885 by Giles, and later reported from Sri Lanka (Ceylon) by Walker in 1904. However, the specimens from the two localities are clearly distinctive in male gnathopod 1 and uropod 3 which Walker (1904) concluded as an ontogeny. Salman and Jabbar (1990) redescribed C. megacheles based on material collected from the north-west of the Arabian Gulf and found that the variation between Giles's and Walkers' specimens is not an ontogeny, instead they are two distinct species. In the present study we provide a detailed description and illustration of both male and female species of Cheiriphotis trifurcata new species collected from the seagrass bed area. This description represents the first record of the genus Cheiriphotis in the Gulf of Thailand. A key for the genus Cheiriphotis is also presented.

Material and methods
Amphipods were collected using a 20×20 cm 2 Ekman's grab in a seagrass bed of Talet bay ( Figure 1). The sites were visited at low tide and amphipods were collected from the subtidal zone. Seagrass and sediment were sieved with a 0.5 mm sieve. Amphipod specimens were sorted out and fixed in formalin for 1 week and then stored in 70% alcohol. In the laboratory, the animals were examined using a compound microscope and later selected for dissection. The appendages were examined and figures were produced using an Olympus CH30 light microscope with a camera lucida. The following abbreviations are used: A, antenna; G, gnathopod; HD, head; LL, lower lip; MD, mandible; MX, maxilla; MP, maxilliped; P, pereopod; Pl, pleopod; T, telson; U, uropod; UR, urosome; UL, upper lip; r, right; l, left; ♂, male; ♀, female. The type material of the new species is deposited at Prince of Songkla University Zoological Collection (PSUZC) and the Universiti Kebangsaan Malaysia Muzium Zoologi (UKMMZ), Malaysia. duced 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.
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.
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. Etymology. 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.
Remarks. 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 C. williamsoni, C. neotropicalis, C. mediterranea and C. 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 C. trifurcata can be distinguished from C. 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 C. 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 C. 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, C. erythraeus, C. mediterranea, C williamsoni and C. neotropicalis. The former can be distinguished from C. 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 C. 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. C. megacheles) has been reported from the Andaman Sea and the South China Sea (Imbach 1967 andRabindranath 1971). The absence of robust setae along the palm of gnathopod 1, the unfused carpuspropodus of the male gnathopod 2, a rounded epimeron 2, and the biramus uropod 3 in C. megacheles readily differentiates that species from the present one.