A NEW FRESHWATER PRAWN OF THE GENUS MACROBRACHIUM (DECAPODA, CARIDEA, PALAEMONIDAE) FROM THE HIGHLANDS OF SOUTH VIETNAM BY NGUYEN VAN XUÂN1) Faculty of Fisheries, University of Agriculture and Forestry, Thu Duc, Ho Chi Minh City, Vietnam ABSTRACT A new species of the genus Macrobrachium, M. thuylami, was discovered in the highlands of South Vietnam. A description, illustrations, and notes on habitat and economic importance of this species are provided. RÉSUMÉ Une nouvelle espèce du genre Macrobrachium, M. thuylami a été découverte dans une région montagneuse du Sud Vietnam. Une description et des illustrations, ainsi que des notes sur l’habitat et l’importance économique de cette espèce sont fournies. INTRODUCTION During a trip in Duc Lap, a district of Daklak Province about 300 km northeast of Ho Chi Minh City, we had an opportunity to buy some prawns of the genus Macrobrachium. These seem to belong to an as yet undescribed species, and this is described below. The new species, Macrobrachium thuylami, is characteristic in having the antepenultimate segment of the third maxilliped serrate at the distal half of the outer margin. Notes on habitat and economic importance are provided. The abbreviation tl. is used for total length, measured from the tip of the rostrum to the tip of the telson in the fully stretched specimen; cl. is used for carapace length excluding the rostrum. The material discussed here is deposited in the collection of the National Museum of Natural History (RMNH), Leiden, Netherlands. For identification of the prawn, papers of Holthuis (1950), Naiyanetr (1993), and Xuân (2003) were used. 1) e-mail: [email protected] © Koninklijke Brill NV, Leiden, 2006 Crustaceana 79 (1): 1-9 Also available online: www.brill.nl 2 NGUYEN VAN XUÂN DESCRIPTION Macrobrachium thuylami spec. nov. (figs. 1-3) Not: Macrobrachium thuylami Xuân, 2004: 19 [nomen nudum]. Material. — Bought on a local market in Duc Lap, a district in the highlands of South Vietnam, ◦ ◦ at 12 26 N 107 39 E (about 300 km northeast of Ho Chi Minh City), altitude about 690 m, 3 December 1999; 16 males (tl. 42.5-69.5 mm), 9 non-ovigerous females (tl. 39-50 mm), 1 ovigerous female (tl. 42 mm), 4 post-ovigerous females (tl. 43-51 mm). According to the local saleswoman, the specimens were taken from the Dak R’To river, a mountain river at about 690 m altitude, nearby. Type. — The holotype is a male with total length 53 mm, collected 3 December 1999. It is preserved in the National Museum of Natural History, Leiden, Netherlands (RMNH D 51179). Part of the paratypes (2 males, tl. 53 and 52 mm, 1 ovigerous female, tl. 51 mm) are in the same museum. The rest is in the collection of the author. Description. — The integument is firm. The rostrum is well developed and presents some variation: it is directed downwards, with the tip somewhat up- turned (fig. 1a) or nearly straight with the tip somewhat turned-up (fig. 1b), or nearly straight and directed slightly downwards; it fails slightly to reach the end of the scaphocerite or overreaches it slightly (in young males). The upper margin is somewhat sinuous or nearly straight and bears 8 to 9 teeth of which 2 (rarely 3) are situated on the carapace, the third tooth may be placed just over, before, or somewhat behind the orbital margin. The first tooth usually is slightly more distant from the second tooth than the second is from the third. In old males all dorsal teeth are immovable, some of the proximal (3 to 4) teeth are semi-movable in young specimens. The five or six proximal teeth are usally rather regularly spaced over the upper margin and the two or three distal teeth may be less widely spaced than the proximal ones. Sometimes the intervals between the three teeth in the middle portion of the upper margin are smaller than those between the distal and proximal teeth. In some other specimens the upper teeth are irregularly spaced from one another (i.e., the intervals between the distal teeth, the proximal teeth, and those in the middle are not equal). The lower margin bears a double row of setae; there are 2 or 3 teeth on the distal half of the lower margin. The carapace is smooth and covered with tiny, shallow pits in not fully grown males and in females. In old males the branchiostegal and lower regions of the carapace are covered with spinules. The carapace is 1.20 to 1.52 times as long as the rostrum. The antennal spine is larger than the hepatic spine and placed at a higher level; the branchiostegal suture does not extend posteriorly beyond the hepatic spine. In males, a triangular median spine is present on the first, second, and third abdominal sternites. The surface of the abdomen is smooth, with tiny pits, and is normal in shape; the fourth abdominal pleura end posteriorly in an almost right angle or are rounded;.