Streptocephalus diversity in Myanmar, with description of a new species (Branchiopoda, Anostraca)

Abstract The diversity of anostracans in Myanmar is poorly known. A series of biodiversity surveys had been conducted in Myanmar, and two species of Streptocephalus were collected in the central dry zone. Streptocephalus sirindhornae Sanoamuang et al., 2000 is reported in Myanmar for the first time, and Streptocephalus shinsawbuae sp. n. is described as new. Streptocephalus shinsawbuae sp. n. belongs to the S. dichotomus group and is similar to S. simplex Bond, 1934 and S. sahyadriensis Rogers & Padhye, 2014, but can be distinguished by the form of the male antennal posterior primary ramus and anterior primary ramus apex and egg ornamentation. Streptocephalus dichotomus has been reported from Myanmar in the past but was not found in this survey.


Introduction
The monogeneric Streptocephalidae Daday, 1910 is the largest anostracan family, composed of 65 species (Rogers 2013;Rogers and Padhye 2014), distributed across Africa, Eurasia, Australia, and North America (Daniels et al. 2004). Streptocephalus diversity in Asia has been examined in detail, with eleven species were reported from the Middle East to Taiwan Rogers andPadhye 2014, 2015;Shu et al. 2015). Six species are regarded as valid in India (Rogers and Padhye 2014;. Four species have been reported from Southeast Asia, although this region as a whole is poorly studied . Streptocephalus javanus Brehm, 1955 has been found and described from the island of Java (Vaas 1952;Brehm 1955), Streptocephalus sirindhornae and S. siamensis have been described from Thailand by Sanoamuang et al. (2000) and Sanoamuang and Saengphan (2006), respectively. Only Streptocephalus dichotomus has been reported from Myanmar previously (Belk and Brtek 1995;Sanoamuang et al. 2000).
The Southeast Asia Biodiversity Research Institute (Chinese Academy of Sciences) and the Forest Research Institute, Myanmar, conducted a series of biodiversity surveys in Myanmar from 2015 to 2017. Two Streptocephalus species were collected during these efforts: the first records of S. sirindhornae Sanoamuang et al., 2000 from Myanmar and a species new to science.

Materials and methods
Specimens were collected by a hand held dip net and preserved in 95% alcohol in the field. Specimens were examined under a stereo microscope (Zeiss Stemi 508) and a compound microscope (Olympus CX31) in the laboratory. All drawings were made using a camera lucida and images were taken by ToupCam microscope digital camera inside the compound microscope, the egg image ( Fig. 4D) was taken at different focal planes and combined automatically by Toupview to increase the depth of focus. The distribution map was produced by ArcGIS based on the GPS information which was collected in the field using a Garmin eTrex 309. Terminology follows Rogers et al. (2013), but to prevent confusion, parallel morphological terminology from Maeda-Martínez et al. (1995) and Sanoamuang et al. (2000) is marked in brackets. All specimens examined were deposited in the Kunming Natural History Museum of Zoology, Kunming Institute of Zoology, Chinese Academy of Sciences.  Allotype. KIZ-CR 2016002, female, same data as holotype. Paratypes. One male (SEABRI-CR 2016001) and one female (SEABRI-CR 2016002) deposited in Freshwater Biodiversity Laboratory, Southeast Asia Biodiversity Research Institute, Chinese Academy of Sciences, Myanmar, same data as holotype.

Order
Type locality. (Fig. 1A  Diagnosis. Streptocephalus shinsawbuae sp. n. is a member of the "S. dichotomus" species group, and can be distinguished from its congeners by the following characters: base of second antenna distal antennomere expanded, subquadrate, basal projection absent; antennal appendage with long peduncle, with one (rarely two) fleshy papilla(e), distal geniculations with 5-7 spines; anterior primary ramus with a digitiform basoposterior spine, ending distally as a triangular, lamellar projection, anterior ramus posterior branch with a subdistal and shallow notch; posterior ramus biramous, posteriolateral branch with two groups of crenulations, posterior primary ramus with two longitudinal rows of spines, distal tenth slightly curved anteriorly; egg with large, basically pentagonal polygons, separated by vertical ridges.
Description. Male. (Fig. 1C) Body length (from anterior margin of head to posterior margin of telson, not including cercopods) from 14.5 mm to 20.5 mm, average 17.7 mm.
Posterior ramus ("finger") biramous and longer than anterior ramus. Posterior margin in lateral view near rami confluence with a shallow emargination. Posterio- lateral branch ("lower finger" in Sanoamuang et al. 2000) arcuate, broadly curved to ~160°, with apex bent nearly 90° distally, nearly attaining primary ramus ("upper finger") apex. Posteriolateral branch anterior margin subcrenulate in basal third, crenulate proximally in distal third. Posterior primary ramus ("upper finger") straight, directed distally, subequal in length to peduncle, with distal tenth slightly curved anteriorly. Anterior surface with two longitudinal rows of spines (Fig. 4C). The lateral spine row bears small, wide based spines, from branch confluence to three fourths the length of the ramus. The medial row bears hyaline spines in a series of medial hyaline lamellae, larger than the lateral spines, with tumid bases and aciculate apices. Medial spines increase in size gradually along proximal three fourths, and become more slender and arcuate in distal fourth. Most lamellae developed and connected, with apical half rotating to the medial side of the branch. The medial spine row distal apex ends subdistally on branch.
Labrum large, triangular, middle compress, apex directed posteriorly. Mandible, first and second maxillae as typical for the genus.
Eleven pairs thoracopods, increasing in size from the first pair to the fifth pair, then decreasing posteriorly. The structures of praeepipodites and epipodites typical for genus. Fifth thoracopod endite I and II with closely set, long plumose marginal setae. Endite I submargin with three widely spaced spines, the distal two are single, the proximal one with a basal spinule. Endite II submargin with two closely spaced spines, distal spine short, proximal spine long. Endite III-V with 3, 2, 2 long plumose setae and 2, 2, 1 spine(s), respectively, with small setae in proximal half. Endopodite broad, margin distal half with sparse plumose setae, each with 1-6 basal spinulae. Exopodite linguiform, margins with closely set plumose setae, longest distally, most setae with basal spinule. Epipod oval, without setae and spines, prae-epipod broadly oval, margins with small hooks.
Genital segments smooth, with lateral linguiform outgrowths. Gonopod (Fig. 3C) cylindrical, with a basomedial spiniform outgrowth, bearing four denticles medially. Everted gonopod elongate, distal end expanded, extending to the distal margin of abdominal segment IV, with a lateral, longitudinal row of spines from base to apex.
Abdomen and cercopods as typical for the genus. Female. Body smaller than male, body length from 14.0 to 17.5 mm, average 15.4 mm (Fig. 1D).
First antennae 2.2 times length of eye plus peduncle and 1.6 times length of second antennae, apex blunt, with three subequal long setae. Second antennae (Fig. 3F) broad, oval, smooth, apex round, margins bearing short sparse setae. Thorax smooth. Thoracopods as in male.
Brood pouch (Fig. 3E) elongate, fusiform, extending to the middle or apex of abdominal segment V in most specimens, less frequently extending to segment IV or segment VI. Egg (Fig. 4D) subspherical, approx. 200 μm in diameter, with large, basically pentagonal polygons, separated by vertical ridges, polygons approx. 40 μm in diameter, and with broad floors.
Etymology. The specific epithet shinsawbuae refers to Queen Shin Sawbu (1453-1460) who facilitated more than 50 years of peace in Myanmar.
Ecology. During the sampling at the type location in June, 2017, the pond had a water temperature of 37.6 °C, a pH of 8.3, conductivity of 117μS/cm, and the dissolved oxygen was 5.9 mg/L. One species of clam shrimp, Cyzicus pilosus Rogers, Thaimuangphol, Saengphan, and Sanoamuang, 2013 was also collected.
Remarks. Streptocephalus shinsawbuae sp. n. is a member of the "S. dichotomus" species group, which includes S. dichotomus Baird, 1860, S. echinus Bond, 1934, S. longimanus Bond, 1934, S. sahyadriensis Rogers & Padhye, 2014, S. simplex Gurney, 1906, and S. sirindhornae Sanoamuang et al., 2000. This group is separated from all other Streptocephalus in that the posterior ramus (finger) is biramal. Of the six species in this group, S. shinsawbuae sp. n. is readily separated from other congers by the single papilla on the antennal appendage peduncle. Of approximately 120 male specimens of S. shinsawbuae sp. n., only one male from Magway (20°11'55.78"N, 95°22'0.62"E) had two papillae. This papilla in all other species of group is absent, or numbers three or more.
Streptocephalus shinsawbuae sp. n. is most similar to S. sahyadriensis. Both species have two longitudinal rows of spines on the antennal appendage posterior ramus (finger), and the anterior primary ramus (thumb) bears a small basoposterior spine. However, they can be separated by: (1) the shape of the posterior primary ramus (upper finger), which is straight in the proximal nine tenths, with the apex arcing anteriorly in S. shinsawbuae sp. n. vs. arcing distolaterally in the distal third 90° in S. sahyadriensis; (2) the posterior ramus posteriolateral branch (lower finger) has two groups of crenulations along the anterior margin in S. shinsawbuae sp. n. vs. only one group subdistally in S. sahyadriensis; (3) the anterior primary ramus apex shoulder is triangularly acute in S. shinsawbuae sp. n. vs. rounded in S. sahyadriensis; (4) the anterior primary ramus (thumb) basoposterior spine is digitiform in S. shinsawbuae sp. n. vs. triangular in S. sahyadriensis.
Streptocephalus shinsawbuae sp. n. is similar to S. simplex in having unbranched posterior primary ramus (upper finger), and acute anterior primary ramus (shoulder) apex, but they can be separated by: (1) the posterior ramus posteriolateral branch (lower finger) having two crenulated areas along anterior margin in S. shinsawbuae sp. n. vs. smooth in S. simplex; (2) the anterior primary ramus (shoulder) apex is triangular in S. shinsawbuae sp. n. vs. parallel sided in S. simplex; (3) anterior primary ramus (thumb) bearing a basal digitiform spine in S. shinsawbuae sp. n. vs. absent in S. simplex.
The eggs of S. shinsawbuae sp. n. have pentagonal polygons, which are very similar to those of both S. echinus and S. longimanus. From S. simplex it can be readily distinguished by the triangle polygons. In addition, the egg ridges are broad and deep in S. shinsawbuae sp. n. vs. narrow and shallow in S. sahyadriensis.

Streptocephalus (Streptocephalus) sirindhornae Sanoamuang, Murugan, Weekers, & Dumont, 2000
Remarks. Streptocephalus sirindhornae is the most widely distributed member of the genus in Southeast Asia, with previous records from: Thailand, Laos, Cambodia, and China (Sanoamuang et al. 2000;Rogers et al. 2013;Shu et al. 2015). Some characters vary in different populations (Shu et al. 2013), and the materials collected from Myanmar is more similar to the Thai populations, with a deep depression on the posterior ramus ventral margin and unequal apical subrami on the posterior ramus.

Discussion
Streptocephalus shinsawbuae sp. n. is the seventh species described from the S. dichotomus species group. The number of antennal peduncle papillae was used to separate some Indian species of Streptocephalus (Belk and Esparza 1995). Velu and Munuswamy (2005) suggested that this character is important in Streptocephalus taxonomy. All of our material bears a single papilla, except one specimen from Magway which has two, the larger papilla as described, with the smaller thin, short, and bare. This one aberrant specimen aside, we think that the single peduncle papilla character is stable, and this character readily allows it to be distinguished from all subgeneric species. This arrangement of papillae appears to fill a gap in the S. dichotomus group where the number of papillae is either none (S. echinus) or three or more. Rogers and Padhye (2014) provided the keys for species of Asian Streptocephalus species. Streptocephalus shinsawbuae sp. n. would key out to couplet 8, which terminates with S. simplex and S. longimanus. We propose the following amendment to that key at couplet 6 to accommodate the new species: 6(5) Antennal peduncle papillae three or more; antennal appendage posterior ramus ("finger") with lateral sickle shaped lateral subramus inerm on proximal half, crenulate on distal half .  Rogers et al. (2013) predicted that six species of Streptocephalus may occur in Southeast Asia. Our survey found two Streptocephalus species in Myanmar, bringing the total known number for this nation to three. The diversity of large branchiopods is poorly known in most of Southeast Asia, but it is probably fairly rich, especially since five species (Rogers et al. 2016a, b) have been reported or described since the last comprehensive review of the region .