Macrobrachium yui Holthuis, 1950

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 : 29-32

Macrobrachium yui Holthuis, 1950

( Figs. 16 , 17 )

Palaemon brevicarpus var. heterochirus Yu, 1936: 305 , figs. 1, 2.

Macrobrachium yui Holthuis, 1950: 211 : Liu et al. 1990: 123, fig. 20; Naiyanetr 1998: 34 (list): Cai & Dai 1999: 237: Li & Liang 2002: 708, 715; Cai et al. 2004: 611, fig. 10; Li et al. 2007: 142, fig. 57.

Macrobrachium yeti Dang, 1975: 67 , fig. 1; 1980: 386, fig. 221.

Material examined. Houaphan Province: Et River, Et District, 12 males (CL 10.3–13.5 mm), 13 females (CL 14.0–17.0 mm), 27 Jan 2009, hand net, coll. S. Ito et al. Sayabouri Province: Tributary of Heung River, 6 males (CL 16.2–18.9 mm), 10 females (CL 18.2–26.5 mm), 25 June, 2009, hand net, coll. S. Ito et al. Luang Namtha Province: Mi River, Viengphoukha District, 1 male (16.0 mm), definite data unknown, coll. Somphuthone Phimmachak. Luang Prabang Province: Ou River, Ngoi District, 12 males (CL 24.0– 35.5 mm), 6 females (CL 22.2– 29.0 mm), 12 Aug 2000, coll. Somboon, Kaenchanh & Tinthip (LARReC C 0001); Ou River, Ngoi District, 2 males (CL 18.0 mm, 20.0 mm), 19 females (CL 15.5–20.5 mm), Sept 2000, coll. O. Lasasimma (LARReC C 0002); Ou River, Ngoi District, 1 male (CL 17.0 mm), 3 females (CL 15.5-20.0 mm), Feb 2001, coll. O. Lasasimma (LARReC C0003); Ou River, Ngoi District, 7 females (CL 12.5-19.0 mm), Feb 2001, coll. O. Lasasimma (LARReC C 0005); Ou River, Ngoi District, 2 males (CL 16.5 mm, 19.0 mm), 4 females (CL 16.0–19.0 mm), Feb 2001, coll. O. Lasasimma (LARRec C 0008); Xuang River, Na Pho Village, 4 males (CL 25.8–38.4 mm), Feb 2001, coll. O. Lasasimma (LARReC 0010); Xuang River, Na Pho Village, 10 males (CL 26.8–35.5 mm), 13 females (CL 19.0–30.0 mm), 28 July 2007, trap, coll. S. Ito et al.; mouth of rock cave in Xuang River, Na Pho Village, more than 50 juvs. (CL 1.6–1.8 mm), 6 Nov 2007, trap, coll. S. Ito et al.: mouth of Kok-dua, Ou River, Ngoi District, 4 males (CL 26.5–33.5 mm), 6 females (CL 23.8–26.5 mm), 22 July 2008, coll. S. Ito et al.; Houy-gun, Ou River, Ngoi District, 10 males (CL 24.0–30.0 mm), 23 July 2008, coll. S. Ito et al.; Ken-gun Cave, Khan River, Xieng Ngeun District, 7 males (CL 25.0– 30.5 mm), 4 females (CL 19.0–23.0 mm), 25 July 2008, hand net, coll. S. Ito et al.

Diagnosis. Rostrum ( Fig. 16 ) barely reaching mid-length of third peduncular segment of antennule, dorsal margin weakly rounded and armed dorsally with 8–13 teeth including 3–5, commonly 4, placed posterior to orbital margin, ventrally with 2–5, commonly 2 or 3, teeth; antennal spine sub-marginal, apex extending as far as or slightly beyond antennal lobe; hepatic spine as large as antennal spine, situated posteriorly, somewhat below level of antennal spine.

Antennal scale 0.45–0.5 times as long as carapace, slightly less than 2.5 times as long as wide.

Sixth abdominal somite slightly longer than fifth; fourth pleuron rounded posteroventrally and fifth one forming sub-acute angle; pre-anal carina sharp, sub-triangular in shape. Telson slightly less than 2 times as long as sixth abdominal somite, posterior margin sub-triangular, with small projection, with 2 pairs of ordinary sub-terminal spines of unequal length, dorsal median margin of telson weakly grooved, with 2 pairs of dorsolateral spines, anterior pair situated at slightly posterior to mid-length.

First pereopod extending beyond antennal scale by distal two-thirds of carpus and onwards, fingers sub-equal or slightly shorter than palm. Second pereopods in males ( Figs. 16 , 17 a) sub-equal or slightly unequal in length; major leg (mostly in left side) extending beyond antennal scale by carpus and onwards; carpus shorter than merus; fingers slightly shorter than palm, with some 20 small teeth on cutting edges; minor leg extending beyond antennal scale by distal one-third of carpus and onwards. Second pereopods in females sub-equal in length, extending beyond antennal scale by distal one-third of carpus and onwards. Third pereopod ( Fig. 17 b) with propodus 3.9–4.1 times as long as dactylus, latter somewhat robust basally.

Appendix masculina ( Fig. 17 c) about twice length of appendix interna, with long stiff setae on anterior margin and also on mesial surface. Uropodal exopod slightly longer than endopod, movable spine present in juveniles ( Fig. 17 g), reducing its size with growth, then completely absent in adult specimens ( Fig. 17 d).

Remarks. Cai et al. (2004) mentioned that, in the early life stages, this species has a movable spine on the diaeresis of the uropodal exopod, although adult specimens lack this character. Their finding is confirmed in our study, as early juveniles in fact have a distinctly longer spine than the lateral projection ( Fig. 17 g), which becomes short with growth and is completely lost in specimens larger than 15 mm in carapace length ( Fig. 16 d). The appendix masculina starts to develop at about a size of 4–5 mm in carapace length.

Unlike other species in the present study, breeding females were not encountered in the open riverine water despite extensive surveys. Due to their relatively high commercial value, M. yui (“Khoung-Bo” means “cave prawn” in the local dialect) has been intensively exploited by local people in northern Laos. Local people catch larger adult specimens using a set net installed at steep streams leading to caves ( Fig. 18 ). Hence, mature individuals of this species were assumed to migrate up to a cave adjoining the main river to spawn. To support this, the junior authors (SI, OL, PS) conducted a diel sampling at a small stream flowing out from a possible spawning cave in the Xuang River, Luang Prabang Province. This survey yielded a large number of early stage juveniles at night ( Fig. 17 e–g). In addition, they successfully produced larvae from this prawn in the laboratory from adult females caught in the open waters. These findings indicate that mating of this species had occurred in the open riverine waters before they arrive to the spawning cave. Their unique ecology will be reported elsewhere.

Distribution. Macrobrachium yui has been recorded previously from northern Vietnam, Yunnan Province, south-western China, and northern Thailand (see Cai et al. 2004). The present study confirmed its presence in several localities of northern Laos.

Figures

FIGURE 16. Macrobrachium yui, male (CL 31.5 mm) collected from Xuang River, Luang Prabang Province, Laos: entire body, lateral. Scale bar = 10 mm.

FIGURE 17. Macrobrachium yui, male (CL 31.3 mm) (a), male (CL 29.0 mm) (b), male (CL 30.0 mm) (c, d), and early juvenile (CL 1.9 mm) (e – g) collected from Xuang River, Luang Prabang Province, Laos: a, basal part of fingers of right second pereopod, mesial view; b, propodus and dactylus of left third pereopod, lateral view; c, appendix masculina, mesial view; d, right uropodal diaeresis, lateral view; e, juvenile anterior part of carapace and cephalic appendages, lateral view; f, juvenile distal end of telson, dorsal view; g, juvenile left uropodal diaeresis, lateral view.

FIGURE 18. Prawn fishery ground for Macrobrachium yui in the Xuang River, Luang Prabang Province, Laos: many set nets made from bamboo are being installed at a steep stream leading to a possible spawning cave.