Macrobrachium amplimanus Cai & Dai, 1999

Hanamura, Yukio, Imai, Hideyuki, Lasasimma, Oulaytham, Souliyamath, Pany & Ito, Sayaka, 2011, Freshwater prawns of the genus Macrobrachium Bate, 1868 (Crustacea, Decapoda, Palaemonidae) from Laos, Zootaxa 3025, pp. 1-37 : 3-6

Macrobrachium amplimanus Cai & Dai, 1999

( Fig. 2 )

Macrobrachium amplimanus Cai & Dai, 1999: 231 , figs. 14–16; Cai et al. 2004: 637. Macrobrachium dienbienphuense: Li et al. 2007: 84 (part).

Material examined. Luang Prabang Province: Ou River, Ngoi District, 3 males (CL 13.4–17.8 mm), Feb 2001, coll. O. Lasasimma (LARReC 0004); Ou River, Ngoi District, 1 ovig. female (CL 10.6 mm), Feb 2001, coll. O. Lasasimma (ex LARReC 0006); Ou River, Ngoi District, 1 male (CL 14.3 mm), Feb 2001, coll. O. Lasasimma (ex LARReC 0007); Xuang River, Na Pho Village, St. 2, 1 male (CL 12.8 mm), 2 females (CL 9.4 mm, 10.3 mm), 28 Jan 2008, hand net, coll. S. Ito et al.; Xuang River, Na Pho Village, St. 3, 1 male (CL 17.7 mm), 29 Mar 2008, hand net, coll. S. Ito et al.; Xuang River, Na Pho Village, St. 2, 2 males (CL 13.5 mm, 15.0 mm), 1 ovig. female (CL 9.8 mm), 27 Apr 2008, hand net, coll. S. Ito et al.; Xuang River, Na Pho Village, St. 1, 2 males (CL 11.5 mm, 13.5 mm), 3 ovig. females (CL 10.0– 12.5 mm), 24 May 2008, hand net, coll. S. Ito et al.; Xuang River, Na Pho Village, St. 1, 1 male (CL 17.0 mm), 24 May 2008, hand net, coll. S. Ito et al. Champasak Province: Khone Island, 2 males (CL 9.5 mm, 11.3 mm), 2 females (CL 11.3 mm, 14.0 mm), 2 ovig. females (CL 9.5 mm, 10.1 mm), 11 July 2009, set net, coll. S. Ito & Y. Niimura.

Diagnosis. Rostrum ( Fig. 2 a) reaching mid-length to anterior end of antennular peduncle or occasionally extending slightly beyond antennular peduncle, 0.38–0.52 times as long as carapace, dorsal margin weakly convex, directed slightly ventrad, with dorsally 10–13 teeth including 3–6 placed posterior to orbital margin, ventral margin with 1–3 teeth; antennal lobe moderately developed, antennal spine marginal, apex reaching to or extending slightly beyond antennal lobe; hepatic spine slightly smaller than antennal spine, placed distinctly below level of antennal spine.

Abdomen with sixth somite 1.30–1.50 times as long as fifth, pre-anal carina well marked and highly elevated; pleuron of fourth somite rounded posteroventrally, that of fifth somite somewhat angulated. Telson 1.4–1.7 times as long as sixth abdominal somite, posterior end sub-triangular with 2 pairs of ordinary sub-terminal spines and 2 pairs of dorsolateral spines, anterior pair of which spines are placed just posterior to mid-length.

Epistome ( Fig. 2 b) with anteromedian projection (tri-lobed), but median projection occasionally hardly noticeable. Antennal scale 0.5–0.6 times as long as carapace, 2.6–2.9 times as long as wide. Third maxilliped barely reaching anterior end of antennal scale, distal segment slightly shorter than penultimate one.

First pereopod extending beyond antennal scale by half to full length of carpus and onwards, fingers about 0.7 times as long as palm. Second pereopods distinctly unequal in length, covered with long velvety setae on palm, fingers, and, occasionally, on anterior part of carpus: major leg ( Fig. 2 a, c, d) as long as or slightly longer than total body length (from the apex of the rostrum to the posterior end of the telson), extending beyond antennal scale by carpus and onwards; merus as long as palm, 2.3–3.0 times as long as high, with minute spinules particularly on lateral and anterior surfaces; carpus cup-shaped, about half length of merus and again as long as or slightly shorter than anterior high, with minute spinules particularly on lateral and dorsal surfaces, marked transverse groove present along anterodorsal margin; palm 0.7–1.0 times as long as fingers, 1.6–1.9 times as wide; fingers with narrow gape when closed, with 13–17 teeth on cutting edges; minor leg extending beyond antennal scale by carpus and onwards, palm as long as or slightly shorter than merus, 0.6–0.9 times as long as fingers, 1.6–2.2 (normally <2.0) times as long as wide. Third pereopod ( Fig. 2 e) with propodus 2.4–3.0 times as long as dactylus, with 5 or 6 sparsely situated spines on posterior margin.

Appendix masculina ( Fig. 2 f) about twice length of appendix interna, with numerous stiff setae on anterolateral margin as well as mesial surface. Uropodal exopod sub-equal in length to endopod, movable spine on diaeresis ( Fig. 2 g) shorter than lateral projection.

Egg. Non-eyed eggs measuring 0.9–1.25 x 1.3–1.5 mm.

Remarks. Li et al. (2007) suggested that Macrobrachium amplimanus , as well as M. eriocheirum Dai, 1984 and M. pilosum Cai & Dai, 1999 , should be junior synonyms of the morphologically variable M. dienbienphuense Dang & Nguyen, 1972 . In fully grown males examined in this study, the palm of the major second pereopod for specimens referred to M. amplimanus is noticeably greater than that attributed to M. dienbienphuense and M. eriocheirum (cf. Cai & Dai 1999), and the structure of the carpus of the second pereopod also differed ( Table 2 ). To confirm this, the 16S rRNA mitochondrial gene was sequenced for typical specimens. The results of analyses support the recognition of three genetically different groups ( Fig. 5 ). Hence, M. amplimanus , M. dienbienphuense , and M. eriocheirum are regarded as good species (see also “Remarks” under M. dienbienphuense ).

Macrobrachium amplimanus and M. eriocheirum occur in abundance in the Xuang River, Luang Prabang Province, northern Laos, with M. amplimanus normally found in faster-running streams than M. eriocheirum , suggesting that the two species use different microhabitats within the same river (Ito pers. obser.).

Cai & Dai (1999) and Cai et al. (2004) noted that M. amplimanus has a comparatively short rostrum, falling slightly short of the mid-length of the third segment of the antennular peduncle. The rostrum of the Laotian specimens, however, showed some minor variations in length as it fully reaches the mid-length to slightly overreach the anterior end of the antennular peduncle. Moreover, our specimens consistently had a pair of distoventral spines at least on the propodus of the third pereopod.

In specimens collected from the Champasak Province, southern Laos, the median lobe on the epistome was poorly developed (somewhat similar to bi-lobed form) and, as a consequence, superficially resembled M. hirsutimanus ( Tiwari, 1952) sensu Cai et al., 2004 . However, these authors described M. amplimanus as having a proportionately longer propodus of the third pereopod (2.7–3.1 times the length of dactylus vs. 2.0– 2.5 in M. hirsutimanus ), a poorly developed median tooth on the third abdominal somite (vs. well developed), a sharp preanal carina (vs. rounded ridge), and comparatively small-sized eggs (0.9–1.0 x 1.25–1.4 mm vs. 1.0–1.25 x 1.4–1.7 mm). As such, the present materials should also be referred to M. amplimanus .

TABLE 1. Specimens of the Macrobrachium dienbienphuense species groups used for DNA analyses. * Macrobrachium rosenbergii and M. lanchesteri were chosen as out-groups.

Examined so far for Laotian specimens, 44% of specimens (N=46) were right-handed (right second pereopod larger than left one) (Hanamura et al. unpubl. data).

Distribution. Yunnann Province, south-western China, Thailand, and Laos (Cai & Dai 1999; Cai et al. 2004; present study).

Figures

FIGURE 2. Macrobrachium amplimanus, male (CL 17.0 mm) (a, b, g), male (CL 14.8 mm) (c), and male (CL 17.8 mm) (d – f) collected from Xuang River, Luang Prabang Province, Laos: a, entire body; b, epistome; c, right second major pereopod, mesial view; d, chela of right second major pereopod, denuded, mesial view; e, propodus and dactylus of right third pereopod, lateral view; f, appendix masculina, mesial view; g, uropodal diaeresis, lateral view.

FIGURE 5. Genetic relationship of three species of the “ Macrobrachium dienbienphuense ” group inferred from the mitochondrial gene analysis (16 S rRNA).