Macrostomum bicaudatum Wang, Sun & Zhang

Sun, Ting, Zhang, Lv, Wang, An-Tai & Zhang, Yu, 2015, Three new species of freshwater Macrostomum (Platyhelminthes, Macrostomida) from southern China, Zootaxa 4012 (1), pp. 120-134 : 128-131

Macrostomum bicaudatum Wang, Sun & Zhang , n. sp.

( Figs.10–11 )

Material examined. Holotype (PLA-Ma00024): permanent slides of specimen stained by the H.E. method. Paratype (PLA-Ma00025~00027): permanent slides of specimen stained by the H.E. method. All specimens are deposited in IZCAS. A dozen of M. bicaudatum n. sp. were collected from dead leaves at the bottom of an artificial lake of Shenzhen University campus (22°31' 44"N, 113°55' 52"E) ( Fig.1 ) in December 2006, at the water depth of about 80 cm. Lotuses, as well as other aquatic plants, fishes, snails, shrimp and zooplankton were originally transferred from a wetland of Foshan City, Guangdong Province, China when this artificial lake was constructed. This species was also found by the authors on July 20, 2014 at the root of a water hyacinth in a tributary of Dongjiang River, Huizhou, Guangdong Province (23°8' 59.74"N, 114°22' 26.27"E, altitude 14 m, water temperature 24°C, pH 7.65).

Etymology. The specific epithet refers to the shape of the tail end of this new species, which is of a fishtail shape.

Description. The body is flat. The length and width of its body is 1000.8 ± 210.8 µm (n = 4) and 259 ± 59.7 Μm (n = 4), respectively. Its head is bluntly round and the tail has 2 lappets arranged in a fishtail shape ( Figs. 10 A and 11A). It has dense and short rod-like adhesive glands (ad) ( Figs. 10 C and 11A).

M. bicaudatum n. sp. is hermaphroditic. The false vesicula seminalis (fvs), vesicula seminalis (vs) and vesicula granulorum (vg) are arranged in a lateral triangle shape ( Figs. 10 A & B, 11A & B). The penis stylet (ps) bends from the right side to the left side planimetrically, and then bends to the male gonopore (mg) in an auricle-shape ( Figs. 10 B, 11B–D). The curved-line (marked as ‘a’) and straight-line (marked as ‘b’) distances between the base and distal end of penis stylet (ps) are 160 ± 3.05 Μm (n = 3) and 70 ± 6.01 Μm (n = 3) ( Fig. 4 D). The diameters of basal and distal parts of penis stylet (ps) are 11 ± 0.35 Μm (n = 3) and 4 ± 0.6 Μm (n = 3), respectively. The upper margin of the distal part is longer than the lower margin, while the horseshoe-shape lower margin is thickened ( Figs. 10 B, 11B–D). The features of the female reproductive system are identical to those of other species within the genus of Macrostomum .

Remarks. Macrostomum bicaudatum n. sp. is most similar to M. phocorum Marcus, 1954 in external morphology and M. semicirculatum Ax, 2008 in penis stylet morphology. However, M. phocorum is much bigger, with a length of 1,500 Μm and width of 300 Μm. It has a rounded head, and the tail is bifurcated with affiliated adhesive glands (ad) at the tip. Its penis stylet (ps) is matroos pipe-shaped. In contrast, the tail of M. bicaudatum n. sp. is bifurcated and its penis stylet is in auricle-shaped. Therefore, although similar in external features, the two species differ in body size as well as morphology of the penis stylet.

In M. semicirculatum Ax, 2008 , the penis stylet (ps) is bent in a semicircular-shape. The diameters of basal and middle parts of penis stylet (ps) is 20 Μm and 3~4 Μm, respectively. The base of penis stylet (ps) is funnel-shaped. The tail of M. semicirculatum is arc-shaped. In contrast, the penis stylet (ps) of M. bicaudatum n. sp. is auricleshaped. As a whole, it is much slender than the penis stylet of M. semicirculatum , and is slightly swollen at anterior 1/10 position of the base region. The tail has 2 significant fishtail-shape lappets. Therefore, it differs remarkably from M. semicirculatum morphologically.

Discussion. The taxonomic study of Chinese turbellarians started relatively late. At the end of the 20th century, only 21 species of freshwater turbellarians were recorded ( Zhao et al. 2011). Since the 21st century, 30 more species of turbellarians have been described, including 18 species of Rhabdocoela (Wang 2004; Wang & Li 2005; Wang & Wu 2005a, 2005b; Wang & Deng 2006; Wang & Wu 2008; Zhang et al. 2010; Wang & Sun 2011; Lai et al. 2014; Lu et al. 2013; Xia et al. 2014; Zhang et al. 2014), 3 species of Tricladida ( Yu et al. 2013; Chen et al. 2015a; Chen et al. 2015b), 2 species of Prolecithophora ( Gao et al. 2011; Ma et al. 2014), 1 species of Lecithoepitheliata ( Peng et al. 2007), 1 species of Acoela ( Sun & Wang 2014), and 6 species of Macrostomum (Wang et al. 2004, Wang & Luo 2004; Wang 2005; Zhao et al. 2011).

Macrostomum inhabits a variety of habitats including clean to eutrophic, and freshwater and marine waters. In this contribution, all specimens of the three new species were collected in pollution-free freshwater environments. Among the Macrostomum that have been described in China, M. intermedium ( Tu, 1934) has been reported from a pond of Tsinghua University, Beijing only; M. tuba (Wang et al., 2004) is widely distributed in Anhui, Guangdong and Fujian provinces; M. xiamensis (Wang & Luo 2004) is distributed in Xiamen; M. sinensis , M. acus and M. obtusa (Wang 2005) are distributed in Guangdong; M. saifunicum ( Zhao et al. 2011) is widely distributed in Beijing, Anhui, Hunan, Jiangxi and Guangdong. The above 7 species of Macrostomum were all collected from pollution-free habitats with highly diverse aquatic invertebrates. In terms of biogeographic distribution, M. intermedium is distributed in the Palaearctic region; and M. saifunicum and M. tuba are distributed worldwide. The other 4 Macrostomum species and the three new species in this study are all distributed in the freshwater areas of the Oriental region. Due to rapid population expansion and economic development, the local distribution of these Macrostomum species is restricted to protected water zone and the brooks in remote areas, which suggests that they are susceptible to the widespread pollution of freshwater environment. The three new species reported in this study are all originated from southern China, a subtropical area with highly diverse plants and animals. Therefore, further investigation on the diversity of turbellarians is warranted in the future study.

Figures

FIGURE 1. The locations of three new species of Macrostomum in China

FIGURE 4. Macrostomum heyuanensis Wang & Sun, n. sp., ventral view: (A) body in whole; (B) profile of the tail; (C) copulatory apparatus; (D) penis stylet; (E) sperm.

FIGURE 10. Macrostomum bicaudatum Wang, Sun & Zhang, n. sp., dorsal view: (A) whole body photographed in life; (B) penis stylet; (C) permanent slide stained by the H. E. method.

FIGURE 11. Macrostomum bicaudatum Wang, Sun & Zhang, n. sp., ventral view: (A) whole body; (B) copulatory apparatus; (C) stylet; (D) distal end of stylet.