Macrobrachium assamense (Tiwari, 1955)

Cai, Y., Naiyanetr, P. & Ng, P. K. L., 2004, The freshwater prawns of the genus Macrobrachium Bate, 1868, of Thailand (Crustacea: Decapoda: Palaemonidae), Journal of Natural History 38 (5), pp. 581-649 : 618-621

Macrobrachium assamense (Tiwari, 1955)

(figures 13, 14A–F)

Palaemon assamensis assamensis Tiwari, 1955a: 190 [type locality: Someswari river , near Siju, Garo Hill, Assam, India]; 1955b: 234; 1968: 297.

Palaemon assamensis peninsularis ; Tiwari, 1955a: 190 [type locality: Nerbudda river at Khetgaon, Mandla District, Madhya Pradesh, India]; 1955b: 234; 1968: 298 .

Macrobrachium assamense assamense ; Cai and Ng, in press.

Material examined

West Thailand. One W, cl 17 mm (CU 1997.87), Tak, coll. P. Naiyanetr, 17 May 1985 ; 1 W, 18.5 mm ( ZRC), Tak, coll. P. Naiyanetr, 17 May 1985 ; 1 W, cl 18.7 mm, 1 X, cl 15 mm (CU 1997.167), Suam Pheng, Ratchaburi, no date.

Description

Rostrum (figure 13A) straight, high, reaching to distal end of scaphocerite, tip slightly upturned; rostral formula: 2+5–7/3–4. Teeth placed more widely posteriorly than anteriorly. Antennal spine below inferior orbital angle; hepatic spine smaller than antennal spine, lying behind and slightly below antennal spine. Antennular peduncle about 0.6 times as long as carapace. Carapace smooth.

Fourth thoracic sternite with small median process (figure 14C). Telson (figure 14E) with two pairs of dorsal spines, ending in a small acute median point, dorsolateral spines small, smaller than dorsal spines, intermediate spines well developed, with about six pairs of long plumose setae. First to third abdominal sternites (figure 14D) with transverse ridge and a median tooth, that of second abdominal sternite most prominent, that of first abdominal sternite less prominent, that of third one smaller than previous two. Preanal region with small ridge, smooth.

Eyes (figures 13A, 14A) well developed, cornea longer, broader than stalk. Basal segment of antennular peduncle broad. Stylocerite distinctly pointed, reaching middle of basal segment. Antenna (figure 14F) with stout basicerite, with strong distoventral tooth, carpocerite reaching to about 0.4 times of scaphocerite length. Scaphocerite slender, about 3.2 times as long as wide, with straight outer margin.

Epistome as in figure 14B, bilobed by deep depression. Mouthparts typical of genus.

First pereiopod (figure 13B) reaching with entire chela beyond distal end of scaphocerite, propodus 1.8 times as long as chela; chela with fingers as long as palm. Second pereiopods of male (figure 13C, D) unequal in length, similar in form. Major second pereiopod distinctively shorter than body length, reaching with length of carpus beyond distal end of scaphocerite, carpus 3.2 times as long as broad, slightly shorter than merus, distinctively shorter than palm; fingers short, as long as palm, with distinctive longitudinal grooves, densely clothed by velvety setae, not gaping even when closed, cutting edge with two pairs of small teeth at proximal one-third. Third pereiopod (figure 13E, F) reaching end of scaphocerite, propodus 7.6 times as long as broad, 2.3 times as long as dactylus; dactylus 3.5 times as long as wide, terminating in small claw.

Uropodal diaeresis with a spine (figure 14G) about half length of outer angle.

Remarks

Tiwari (1955a, 1955b, 1968) recognized two subspecies, mamely M. a. assamense , from Assam and Myanmar, and M. a. peninsularis, from the Indian Peninsula. According to Tiwari (1955a, 1955b, 1968), the nominal subspecies differs from the peninsular subspecies by the longer rostrum (0.6–0.8 times as long as carapace versus 0.5–0.6 times) and the elongated carpus (3.5–4.0 times as long as broad versus 3.0–4.5 times). Two lots of specimens from western Thailand agree well with the diagnosis of M. assamense by Tiwari (1955a, 1955b, 1968). We are unable to assign them to either subspecies. The rostrum, about 0.7 times as long as carapace, fits with the nominal subspecies, but the carpus, about 3.2 times as long as broad, fits with the peninsular subspecies. There is a possibility that the Thai specimens may represent a third subspecies. Since the material we have on hand is not substantial enough to be confident and we have not seen the types of M. assamense , we prefer to merely refer the Thai specimens to M. assamense sensu lato for the time being. With regards to the second pereiopod, which has the distinctive longitudinal grooves densely clothed by velvety setae, M. assamense was assigned to the M. hendersoni species group (Tiwari, 1955a, 1955b, 1968). However, because of the higher and longer rostrum, together with the less prominent teeth along the cutting edge of the fingers, it can be easily separated from other members in this group.