On a new species of Macrobrachium Spence Bate (Decapoda: Palaemonidae) from Ayeyarwady River, Myanmnar

Macrobrachium myanmarum sp. nov. was found from near Min Kun (local name Min Gon) at Mandalay, Ayeyarwady (Irrawaddy) River (22.043N & 96.043E), Myanmar. It is a small-sized prawn showing close relation with Arachnochium kulsiense (Jayachandran et al. 2007) and A. mirabile (Kemp 1917). This species can at once be identified from both the species by the characters: medium-sized and highly-elevated and arched rostrum, extending as far as distal segment of antennular peduncle or behind, upper margin with 11–15 teeth of which three (rarely 4) teeth post-orbital; ventral margin without teeth (rarely one); second chelate legs unequal, right leg larger (sometimes left); major leg in which carpus with proximal part narrow and distal end broadened, subequal to merus, palm and fingers; propodus slightly more than the combined length of merus and carpus; fingers very slender, almost equal to palm, fixed finger a bit shorter than movable finger, cutting edges with 2–6 weak denticles at proximal cutting edges, distal denticle situated at about 1/3rd distance from base; ischium, merus, carpus, propodus, palm and dactylus in the ratio 9.62 : 19.25 : 24.06: 47.06 : 23.53 : 23.53, respectively (related to the total length of pereopod); minor leg with minute tubercles in larger specimens; ischium, merus, carpus, propodus, palm and fingers in the ratio 13.1 : 23.68 : 22.37 : 40.8 : 15.8 : 25.0, respectively (related to total length of pereopod); palm slightly swollen and shorter than fingers, fingers slender and curved with a wide gap when closed. Maximum size recorded for the species is 56mm male and 44mm female.


Diagnosis
Macrobrachium having the medium-sized, highlyelevated and arched rostrum, extending as far as distal segment of antennular peduncle or behind, upper margin Image 1. Collection station -Min Kun from Ayeyarwady (Irrawaddy) River, Myanmar J TT with 11-15 of which three (rarely 4) teeth post-orbital; ventral margin generally without teeth (rarely with one); second chelate legs unequal, right leg larger (sometimes left leg); major leg in which carpus with proximal part  narrow and distal end broadened, subequal to merus, palm and fingers; propodus slightly more than the combined length of merus and carpus; fingers very slender, almost equal to palm, fixed finger a bit shorter than movable finger, cutting edges with 2-6 weak denticles at proximal cutting edges, distal denticles at about 1/3 rd distance from base; ischium, merus, carpus, propodus, palm and dactylus in the ratio: 9.62: 19.25: 24.06: 47.06: 23.53: 23.53 respectively (related to total length of pereopod); minor leg with tubercles in large specimens; ischium, merus, carpus, propodus, palm and fingers in the ratio 13.1: 23.68: 22.37: 40.8: 15.8: 25.0, respectively (related to total length of pereopod); palm swollen and slightly shorter than fingers

Description
Rostrum medium-sized, extending as far as distal segment of antennular peduncle or behind, highly elevated and arched and tip directed forwards (in younger specimens rostrum less elevated), upper margin with 11-15 teeth of which three teeth (rarely 4) behind the orbit, proximal most and second teeth more widely separated than the remaining series teeth of uniform distance between them, proximal six teeth anteriorly directed and remaining teeth directed upwards, thick and long bunches of setae present in between teeth; ventral margin curved upwards, one minute tooth in holotype at the level of 10 th dorsal tooth (generally absent); setae longer and closely set in both upper and lower margins (Figs. 1 A,B Image 2,4) Carapace generally smooth but with small tubercles on antero-lateral side, about 43.0 per cent of total length, orbit sunken, antennal spine sharp, not placed at edge; hepatic spine sharp; a distinct groove present below hepatic spine, pterygostomian region not sharp (Figs. 1A,B Image 2,4) Eyes developed. Telson slender, distal end sharply pointed and extends as far as or beyond the level of the outer spine of uropodal exopod; upper margin with two pairs of feeble spines, proximal pair situated at about 53 per cent and not in a line and distal pair closer to proximal pair (76 per cent), distal end with two pairs of spines, outer pair smaller and inner pair slender, longer and sharp, eight long plumose setae present in between the inner pair of spines (Fig. 3B).
Antennular peduncle three segmented, extends as far as 1/3 rd of merus of major second cheliped and beyond merus of minor second legs, middle segment shortest, antero-lateral spine of basal segment reaches beyond middle but not to tip of 2 nd segment of antennular peduncle (Fig. 1 D); disto-lateral spine of antennal scale J TT sharply pointed, subdistal in position and extends just in front of distal end of merus of 2 nd cheliped (Fig. 1E).
First chelate legs slender, extends up to tip of antennal scale; ischium slightly broader; merus slender; carpus 1.60 times longer than propodus; palm cylindrical, 1.3 times longer than fingers; fingers slender, equal sized with tufts of setae on outer margin (Fig. 2B).
Second chelate legs with tubercles in bigger specimens, unequal, right leg larger (sometimes left); major leg 1.7 times the size of total length and 2.4 times the size of minor leg; ischium flat; merus cylindrical; carpus with proximal part narrow and distal end broadened, subequal to merus, palm and fingers;

J TT
propodus with maximum width at distal palm and slightly more than the combined length of merus and carpus; fingers very slender almost equal to palm, fixed finger a bit shorter than movable finger, movable finger curved, cutting edges with 2-6 weak denticles at proximal cutting edges, distal denticle at about 1/3 rd distance from base; ischium, merus, carpus, propodus, palm and dactylus in the ratio 9.62: 19.25: 24.06: 47.06: 23.53: 23.53, respectively (related to total length of pereopod) (Fig. 2D, Image 3). Minor leg with tubercles in larger specimens; ischium, merus, carpus, propodus, palm and fingers in the ratio 13.1: 23.68: 22.37: 40.8: 15.8: 25.0, respectively (related to total length of pereopod); palm swollen and slightly shorter than fingers; fingers slender with a wide gap when closed (Fig. 2C, Image 3).
Fifth pleura of the abdomen unilobed at posterolateral region as in the genus. Pleopods comparatively shorter than in other species of the genus; second pleopod with basis and rami almost same sized; appendix masculina long, folded structure with very stiff setae along its border and extends up to 2/3 distance of endopod. Uropodal exopod bears an accessory spine almost the length of major one and free borders with long setae (Fig. 3A).
Variation in growth: Young specimens: rostrum not much elevated as in adult, second chelipeds slender, without tubercles below the size of total length up to 48mm (Fig. 1 C). Male (big sized 56.0mm sized -recently collected): second pereopods unequal, major leg 2.4 times longer than minor leg; ischium, merus, carpus, palm and dactylus in the ratio 9.62: 19.25: 24.07: 23.52: 23.52, respectively; palm broader, fingers subequal and very slender, movable finger longer and slightly curved and with six small denticles of which distal one roughly 1/3 rd distance from the base; minor leg with ischium, merus, carpus, palm and fingers in the ratio 14.10: 23.08 ; 21.79: 15.38: 25.64, respectively; palm swollen; fingers slender and curved with a wide gap when closed; palm and fingers possess stiff long setae (Image 3). Variations in the growth is shown in image 4. Table 1 provides detailed morphometric measurements of the specimens.
Colouration: Body generally bluish coloured with red

Remarks
The new species is closely related to Arachnochium kulsiense (Jayachandran, Lal Mohan & Raji, 2007) and A. mirabile (Kemp, 1917). M. myanmarum sp. nov. is characterized by the presence of a medium sized highly elevated rostrum which extends as far as the distal segment of antennular peduncle or behind. The dorsal margin is curved with 11-15 of which 3 (rarely 4) postorbital in position. Branchiostegal groove extending slightly behind hepatic spine. The second pereopods are unequal. The major legs 1.7 times longer than the total length and 2.4 times the total length of minor leg. The fingers of major leg are slender and almost equal to palm and bear two to six minute denticles at proximal cutting edges of which the distal denticles situated at about 1/3 rd distance from base whereas in the minor leg the palm is swollen and subequal to fingers and carpus. In large males the fixed finger is a bit smaller than movable finger and movable finger curved. It possesses large number of eggs. In A. kulsiense rostrum is long which extends as far as the tip of antennal scale and the upper margin highly elevated with 9-12 teeth of which two or three are post-orbital in position. The second cheliped in which palm is shorter than fingers and carpus. It possesses a few large eggs (up to 20). A. mirabile is characterized by a highly elevated short rostrum with a formula of 13-16 of which 4-6 teeth post-orbital in position and 1-2 ventral teeth. Dorsal teeth not uniformly spaced. Branchiostegal suture not extending behind hepatic spines. Second pereopods subequal in length and similar in form with palm subcylindrical, fingers slender and much longer than palm and without denticles on the cutting edges, chela 1 ¾ times as long as carpus, palm swollen, smooth and less than ¾ as J TT long as carpus. The present new species can at once be identified on the basis of its highly elevated curved rostrum with specific rostral formula and also nature, proportion of segments, ratio between carpus, palm and fingers of major leg and denticles on cutting edges. Fifth pleura of abdomen is unilobed as in the genus. A comparison of characters of related species is given in Table 2. Morphological variations during growth are shown in image 4.
The very slender fingers of the major second chelate leg of the present species shows some resemblance with that of M. lar (Fabricius, 1798). The rostral formula and general shape of the rostrum of the two species differ considerably. In M. lar chela is 3.5 times as long as carpus and palm of uniform thickness and longer than to twice as long as carpus. Carpus is shorter than merus. In the present new species chela is about 2.0 times as long as carpus and palm shorter than carpus and distal region with maximum thickness. Carpus longer than merus (Chace & Bruce 1993).
Wowor & Ng (2010) have created a new genus, namely, Arachnochium, to accommodate M. mirabile and M. kulsiense. This new genus is characterized by the presence of elongated fourth and fifth pereiopods, large blunt tip triangular median process on T4, without transverse plate in T5, without wide median process in T8, bilobed nature of postero-lateral region of fifth abdominal pleura and longer inner spine on exopod of uropod. The present new species does not possess any of the above characters and hence it is appropriate to retain it in the genus Macrobrachium Bate, 1868 Khin et al. (2018) have reported M. kulsiense Jayachandran, Lal Mohan & Raji (= A. kulsiense) from Myanmar and is a misidentification. www.threatenedtaxa.org The Journal of Threatened Taxa (JoTT) is dedicated to building evidence for conservation globally by publishing peer-reviewed articles online every month at a reasonably rapid rate at www.threatenedtaxa.org. All articles published in JoTT are registered under Creative Commons Attribution 4.0 International License unless otherwise mentioned. JoTT allows allows unrestricted use, reproduction, and distribution of articles in any medium by providing adequate credit to the author(s) and the source of publication.