Macrobrachium spelaeus, Cai & Vidthayanon, 2016

Cai, Yixiong & Vidthayanon, Chavalit, 2016, Macrobrachium spelaeus, a new species of stygobitic freshwater prawn from Thailand (Decapoda: Palaemonidae), Raffles Bulletin of Zoology 64, pp. 117-122 : 117-121

Macrobrachium spelaeus , new species

( Figs. 1–3 )

Material examined. Holotype: male, cl 13.7 mm, ( NIFI SH 00176 ), Tham Phra Wangdaeng , Baan Chompu southwest of Thung Salaeng Luang National Park, Pitsanulok Province, lower north Thailand, 100 m. from the cave entrance: 16.8379°N 100.877°E, 28 August 2002, coll. C. Vidthayanon. Paratypes: 1 male, cl 11.7mm ( OUMNH. ZC.2016-01-022) , 1 female, cl 11.9mm ( ZRC 2016.0049 ), same locality as holotype .

Description. Rostrum ( Fig. 1A, B ) straight, reaching distal end of scaphocerite, rostral formula: 4-5+6-7/2-3. Teeth more widely spaced on anterior than postorbital region. Antennular peduncle about 0.4 times as long as carapace. Carapace smooth, inferior orbital margin moderately produced. Antennal spine sharp ( Fig. 1D ), situated slightly lower than inferior orbital angle, reaching slightly beyond carapace margin. Hepatic spine smaller, situated behind and substantially below antennal spine. Branchiostegal suture running from base of hepatic spine to carapace margin.

Fourth thoracic sternite ( Fig. 1F ) without median process. Eighth thoracic sternum ( Fig.1G ) with narrowly separated anterolateral lobes, without median process. Abdomen smooth ( Fig. 3 ), glabrous, first to third pleurites broadly rounded, fourth and fifth pleurites feebly produced posteriorly, fourth pleurite sub-triangular, fifth pleurite sub-rectangular, sixth abdominal somite 1.5 times as long as fifth, with posteroventral angle feebly produced, subacute. Telson ( Fig. 1J ) moderate, stout, 3.0 times as long as wide, 1.6 times as long as sixth abdominal segment, with 2 pairs of dorsal spines, ending in a median projection, lateral spines of distal margin slightly larger than dorsal spines, intermediate spines well developed, with long plumose setae. First 3 abdominal sternites ( Fig. 1H ) with transverse ridge and a median tooth each, with first one very prominent, much more developed than the other two. Second one triangular in form. Third one much smaller, pointed. Inter-uropodal sclerite armed with well-developed preanal carina ( Fig. 1I ).Ocular stalk and cone reduced, 0.12 times of carapace length, facets present, pigment visible, anterior end only reaching middle of basal segment of antennular peduncle. Stylocerite pointed, reaching 0.8 times of second segment. Antenna with stout basicerite and prominent distoventral tooth. Carpocerite reaching to about 0.35 times of scaphocerite length. Scaphocerite ( Fig. 1C ) slender, 0.6 times as long as carapace, about 3.0 times as long as wide, with straight outer margin.

Epistome ( Fig. 1E ) bilobed by a shallow triangular depression. Mouth parts typical of the genus. Third maxilliped with robust endopod, ultimate segment reaching to anterior end of antennal peduncle; ultimate segment slightly shorter than penultimate segment; exopod short, with numerous plumose setae distally.

First pereiopods ( Fig. 2A ) very slender, reaching beyond scaphocerite by entire chela and 0.2 carpus length, equal in length, similar in form, carpus 1.8 times as long as chela, chela with fingers slightly shorter than palm. Second pereiopods ( Fig. 2B, C ) not sexually dimorphic; both cylindrical, similar in form, unequal in length. Both covered with tufts of velvety setae on fingers and distal half of palms. Major one with smooth or with indistinct spinules, subequal to body length, reaching beyond distal end of scaphocerite by both entire carpus and chela; merus as long as palm, both segments distinctly longer than palm but shorter than fingers; capus 0.65 times as long as palm; palm slightly inflated, fingers with no gap when closed, with a row of about 15 small teeth on each side of cutting edge. Third pereiopods ( Fig. 1D, E ) slender, reaching beyond scaphocerite by entire dactylus, propodus 10 times as long as broad, 3.4 times as long as dactylus; dactylus 4.5 times as long as wide, terminating in an unguis. Fourth pereiopods slender, slightly longer than third pereiopods, similar in form. Fifth pereiopods most slender, longest, reaching beyond scaphocerite by entire dactylus. Endopod of male first pleopod about 0.45 times as long as exopod, weakly broadened distally, slightly curved mesially. Appendix masculina of male second pleopod longer than appendix interna, with spiniform setae on dorsal surface. Appendix interna of male second pleopod slender, reaching to 0.6 length of appendix masculina.

Uropodal diaeresis ( Fig. 1K ) with a movable spine, distinctly shorter than outer angle.

Colouration (live). Yellowish pale cephalothorax with pale yellow and brown visceral mass, rostrum translucent, eyes black, abdomen opaque with brownish hue dorsally. Hairs on chelae pale brown, antennas and appendages opaque.

Habitat. Macrobrachium spelaeus is known only from shallow (0.3–1 m depth) subterranean streams in the type locality, over 100 m from the cave entrance, at Tham Phra Wang Daeng of the Tham Phra Karst in Klong Chompu area of Thung Salaeng Luang National Park.

Etymology. The new species is named spelaeus (L., cavedwelling), after its stygobitic habit.

Remarks. Mcrobrachium spelaeus , new species, represents the first stygobitic Macrobrachium species found in Thailand. With the existence of the tufts of velvety setae on the dactylus of second pereiopods, short carpus in second pereiopods, relatively short rostrum which does not exceed the end of the scaphocerite, Macrobrachium spelaeus , new species, should be referred to the Macrobrachium pilimanus species group (cf. Cai et al., 2004). To date, there are 15 species recognised in the Macrobrachium pilimanus group, namely M. pilimanus ( De Man, 1879) , M. leptodactylus ( De Man, 1892) , M. hirsutimanus ( Tiwari, 1952) , M. dienbienphuense Dang & Nguyen, 1972 , M. poeti Holthuis, 1984 , M. eriocheirum Dai, 1984 , M. ahkowi Chong & Khoo, 1987 [= M. johnsoni Chong & Khoo, 1987 ], M. gua Chong, 1989 , M. forcipatum Ng, 1995 , M. platycheles Ou & Yeo, 1995 , M. pilosum Cai & Dai, 1999 , M. amplimanus Cai & Dai, 1999 , M. sirindhorn Naiyanetr, 2001 , M. kelianense Wowor & Short, 2007 , and M. empulipke Wowor, 2010 . With its reduced eyes, M. spelaeus could be easily separated by all the other members of the M. pilimanus group except M. poeti . Macrobrachium poeti was described from several caves in Gunung Sewu, Java, Indonesia. Macrobrachium spelaeus could be distinguished from M. poeti (cf. Holthuis, 1984) by having more teeth on the lower margin of the rostrum (2 or 3 vs. 1 in M. poeti ); the shorter merus of the second pereiopods (as long as palm vs. distinctly longer than palm in M. poeti ); the larger number of teeth on the cutting edges of the fingers (15 vs. several in M. poeti ), the slender propodus in third pereiopods (3.4 times as long as dactylus vs.3 times in M. poeti ). Compared to epigean species of the group, M. spelaeus morphologically resembles M. dienbienphuense Dang & Nguyen, 1972 , a species originally described from northern Vietnam, but also reported from China, Laos, and Thailand ( Cai et al., 2004; Hanamura et al., 2011), especially when taking into account of the form of second pereiopods. Besides the eyes, M. spelaeus could be separated from M. dienbienphuense (cf. Dang & Nguyen, 1972; Cai et al. 2004, Hanamura et al. 2011) by the form of rostrum (straight vs. convex); shorter major second pereiopods in male specimens (as long as body length vs. distinctly longer in M. dienbienphuense ) less setae/pubescence in second pereiopods (see fig. 3 vs. fig. 3 in Dang & Nguyen 1972); fewer number of teeth on the cutting edges of the fingers (15 vs. 18–32 in M. dienbienphuense ).

Comparative specimens examined. Macrobrachium dienbienphuense : 10 males, cl 9.0–11.0 mm, 5 ovigerous females, cl 6.7–8.3 mm, eggs 1.3 × 1.0 mm ( ZRC 2000.2693 ), Central Thailand, Lop Buri Province, Chai Badan , coll. Y. Cai, 20 June 1998 ; 9 males, cl 10.2–11.7 mm, North-east Thailand, Phibun, Mangsahan, Ubon Rachathani Mae Nam Moon, Mangsahan afternoon market, coll. Y. Cai et al., 16 June 1998 .

Figures

Fig. 1. Macrobrachium spelaeus, new species. A, B, cephalothorax; C, scaphocerite; D, antennal spine; E, epistome; F, fourth thoracic sternite; G, eighth thoracic sternum; H, 1st–3rd abdominal sterna with transverse ridge and median tooth; I, preanal carina; J, telson; K, uropodal diaeresis. Scale bars = 4 mm [A–C]; 1.5 mm [D]; 0.8 mm [E, H, I]; 1.2 mm [G, J]; 1.0 mm [F]; 0.5 mm [K] (holotype: male, cl 13.7 mm, NIFI SH 00176).

Fig. 2. Macrobrachium spelaeus, new species. A, first pereiopods; B, C, second pereiopods; D, E, third pereiopods, Scale bars = 2 mm [A, C]; 4 mm [B]; 1.5 mm [D]; 0.5 mm [E] (holotype: male, cl 13.7 mm, NIFI SH 00176).

Fig. 3. Colour photograph of Macrobrachium spelaeus, new species (paratype, male, cl 11.7mm, OUMNH.ZC.2016-01-022)