Taxonomy notes and new occurrence data of four species of atyid shrimp (Crustacea: Decapoda: Atyidae) in Vietnam, all described from China

Abstract Background Freshwater shrimp of the family Atyidae De Haan, 1849 have been studied in Vietnam for more than a century. A total of 24 species of atyid shrimps from the genera Caridina H. Milne Edwards, 1837, Neocaridina Kubo, 1938, Atyopsis Chace, 1983 have been recorded from Vietnam. With 22 species, the majority are from the genus Caridina. In 2013, Karge and Klotz mentioned the occurrence of four yet undescribed species belonging to Paracaridina Liang, Guo & Tang, 1999 in Vietnam without taxonomic details. In general, studies of freshwater atyids in Vietnam are limited and most Vietnamese taxa await a taxonomic revision. The available data do not fully reflect their estimated species diversity in the country and distribution data are deficient. Here, we focus on four species of atyid shrimps from two genera, viz. Caridinacantonensis Yu, 1938, C.lanceifrons Yu, 1936, C.serrata Stimpson, 1860 and Neocaridinapalmata (Shen, 1948), all described from China and have been reported to occur in Vietnam. The previous reports on the occurrence of these species in Vietnam are largely unreliable due to taxonomic confusion. To contribute to the knowledge of these taxa, we provide the first verified distribution records of the four species in the country with some taxonomic remarks. New information This study shows the first taxonomically-verified distribution data of four atyid shrimp species originally described from China, but also reported from Vietnam, albeit under various species names and, in some cases, erroneously. These data allow the first meaningful discussion of the distribution in light of the reproductive strategy of these four species and, in conjunction with the taxonomic remarks, will contribute to the knowledge of these taxa. As a result of this research and data from previous studies, we now consider all four species as widespread and non-endemic, but land-locked (with a complete freshwater life cycle). In Vietnam, all four species are confined to the northern half of the country. Beyond Vietnam, we provide the first records for Caridinalanceifrons from southern Thailand, which suggests a major sampling gap in Indochina.


Introduction
Freshwater shrimps of the family Atyidae (De Haan 1849) have a global distribution in the tropics and subtropics. Atyids are old (von Rintelen et al. 2012) and diverse with ca. 440 species (De Grave et al. 2015). The highest diversity of atyids is found in Southeast Asia (here including southern China) with more than 210 species in 13 genera (De Grave et al. 2008). Atyid shrimps are, alongside palaemonids, the only diverse group of shrimps in freshwater (De Grave et al. 2015) and play an important ecological role as primary consumers and aid in detritus decomposition (Dudgeon and Wu 1999, Crowl et al. 2001, Williams 2002, Yam and Dudgeon 2006. Most recent taxonomic research on atyids -traditional and integrative -has focused on Southeast Asia, but the atyid fauna of Indochina including Vietnam has been largely neglected. Virtually all of the few studies in this region have been conducted in Vietnam and all are purely taxonomic. At present, 24 described atyid species from four genera, including Caridina Milne Edwards, 1837 (Milne Edwards 1837), Paracaridina Liang, Guo & Tang, 1999(Liang et al. 1999), Neocaridina Kubo, 1938(Kubo 1938 and Atyopsis Chace, 1983(Chace 1983, have been recorded from Vietnam (Bouvier 1904, Bouvier 1905, Bouvier 1919, Bouvier 1925, Dang 1967, Dang 1975, Dang et al. 1980, Cai 1996, Nguyen 1999, Li and Liang 2002, Nguyen 2003, Dang and Do 2007a, Dang and Do 2007b, Do and Dang 2010, Dang and Ho 2012, Karge and Klotz 2013, Do et al. 2020, Do et al. 2021. The majority (22 species) come from the genus Caridina (Karge and Klotz 2013, Do et al. 2020, Do et al. 2021). The number of species is likely going to increase when more studies are done. However, the taxonomic status of all species described by Vietnamese taxonomists is hard to verify, as all types have been lost due to lack of care (the species described by Dang (1975), Dang et al. (1980), Dang and Do (2007a), Do and Dang (2010)).
The taxonomic confusion in atyids in general and Vietnamese taxa, in particular, does not only make it difficult to obtain accurate estimates of species diversity, but also prevents the use of existing data for biogeographic or evolutionary studies, as species distribution cannot be reliably assessed.
Four species, Caridina cantonensis Yu, 1938, C. lanceifrons Yu, 1936, C. serrata Stimpson, 1860 and Neocaridina palmata (Shen, 1948), were originally described from China (Stimpson 1860, Shen 1948, Yu 1936, Yu 1938. Today, these are known to be widely distributed in China (Liang 2004). Vietnamese authors published taxonomic studies on the atyid shrimps of Vietnam including descriptions of new species, such as Caridina flavilineata Dang, 1975, C. vietriensis Dang & Do, 2007, C. pseudoflavilineata Do & Dang, 2010 and Caridina denticulata vietnamensis Dang, 1967. These authors also recorded Caridina cantonensis and C. serrata from Vietnam (Dang 1975, Dang et al. 1980, Dang and Do 2007a, Dang and Do 2007b, Dang and Ho 2012. However, Cai (1996) synonymised Caridina denticulata vietnamensis with Neocaridina palmata palmata (Shen, 1948), Li and Liang (2002) and Liang (2004) considered Caridina flavilineata Dang, 1975 as junior synonyms of C. lanceifrons Yu, 1936(Li and Liang 2002, Liang 2004). Dang and Ho (2012) resurrected C. flavilineata and N. vietnamensis as distinct species compared to the species described from China. Based on the description and the figures in Dang (1975), Dang et al. (1980), Dang and Do (2007a), Do and Dang (2010), we found that three species of the genus Caridina ( C. flavilineata, C. vietriensis and C. pseudoflavilineata) are similar to C. lanceifrons Yu, 1936, while Neocaridina vietnamensis is hardly distinguishable from N. palmata (Shen, 1948). In order to contribute to the taxonomic debate on the atyid shrimp fauna of Vietnam and China, only a proper revision could really solve it. A sampling campaign was conducted to cover as many locations in Vietnam as possible, especially in the northern part of the country. Our paper aims to address the relevant taxonomic issues and provide new occurrence data in Vietnam of four species of atyid shrimps originally described from China.

Materials and methods
We collected atyid specimens in different habitats including rivers, streams, lakes and caves from the mainland to islands throughout Vietnam between 2003 and 2020. The collected sites were partially chosen for their accessibility (Fig. 1, Suppl. material 1). The collecting methods included catching by hand-nets both during the daytime and at night. At each site, at least 10 specimens were collected. To document their live colouration, specimens were then photographed with a Nikon D5600 Digital Camera and afterwards preserved in 90% ethanol. During the field surveys, other activities included observing living animals in their natural habitats, recording coordinates and photographing habitats. Additionally, we interviewed local people on their personal knowledge of the shrimp fauna to gain further information on species composition and distribution in local streams and rivers. All collected samples were identified to species level, based on existing descriptions. The specimens were dissected and observed by using a Leica MZ 12 stereomicroscope. Voucher specimens are deposited in the Institute of Ecology and Biological Resources (IEBR), Vietnam Academy of Science and Technology (VAST), Ha Noi, Vietnam, the Museum für Naturkunde (MfN), Berlin, Germany (collection codens: ZMB), Ha Noi University of Science (HUS) and Oxford University Museum of Natural History (OUMNH). In addition to the data of the collected samples in Vietnam, we used records of the four species from China and other areas from the collection of Museum für Naturkunde (MfN), Berlin, Germany and the literature. For the latter, we only used records that could be georeferenced with reasonable accuracy and precision (< ca. 1 km radius across point locality) from authors who are experts on atyids. Based on these criteria, all records from Shen (1948), Zheng (1989), Cai and Yuan (1996), Cai and Ng (1999), Cai (2014) have been included. Liang (2004) contains a number of additional localities, but we have refrained from including them as they could not be georeferenced and the maps are not precise enough. The maps for Caridina cantonensis and Neocaridina palmata are thus not complete for China.
In the description, all terminology used follows Chace (1997). The abbreviation cl is used for carapace length, measured from the postorbital margin to the posterior median margin of the carapace in mm. The rostral formula, used in the present study, is defined as: number of dorsal teeth on the carapace posterior to the orbital margin + the number of dorsal teeth on the rostrum anterior to the orbital margin/number of ventral teeth of the rostrum. Cephalothorax and cephalic appendages. Carapace length 3.5-4.8 mm. Rostrum straight, slightly bent downwards, reaching to middle or the end of second segment of antennular peduncle, 0.38-0.46 (median 0.43) times as long as carapace, rostral formula: 4-5+9-12/3-5 (n = 10), teeth normal (Fig. 2a). Suborbital angle acute, completely fused with antennal spine; pterygostomian margin rounded (Fig. 2a). Eyes well developed with globular cornea, anterior end reaching to 0.7 times length of basal segment of antennular peduncle (Fig. 2a). Antennular peduncle 0.54-0.67 (median 0.60) times as long as carapace; basal segment 1.38-1.83 (median 1.57) times as long as second segment, second segment 1.4-2.0 (median 1.55) times as long as third segment (Fig. 2b). Stylocerite reaching almost to middle (0.4 times length) of second segment of antennular peduncle (Fig. 2b). Scaphocerite reaching beyond distal end of antennular peduncle, 3.0-4.0 (median 3.46) times as long as wide (Fig. 2a,c).
Pleopods. Endopod of male first pleopod extending to 0.6 times length of exopod, rectangular in shape, anterior part folded backwards, 2.0-2.73 (median 2.53) times as long as proximal width, inner margin concave, outer margin concave or slightly straight, rounded distally, long plumose setae on outer and distal margins, medium-length setae on inner margin; with appendix interna exceeding terminal margin of endopod by 0.3 of its length (Fig. 3e). Appendix masculina of male second pleopod slender, subcylindrical, reaching to proximal 0.5 times endopod length, 4.7 times as long as distal width, finger-shaped, slightly widened distally with some short spiniform setae on outer surface and some long spiniform setae on distal surface; appendix interna at middle of appendix masculina, narrow, small, extending about 0.5 times length of appendix masculina (Fig. 3f). Caridina cantonensis Yu, 1938;male, cl 4.1 mm (ZMB 30253 Colouration. Body yellowish, few irregular small black spots present on ventrolateral parts of carapace and abdominal somites (Fig. 4).

Distribution
Cu Lao Cham, in north-central Vietnam (Fig. 1a). Guangdong, Guangxi and Hong Kong, China (Liang 2004). Dang et al. (1980) recorded C. cantonensis from Chi Lang District, Lang Son Province, North Vietnam, near the border to China. We could neither find this species amongst our samples surrounding this area nor from many survey sites in the mainland of northern Vietnam. Our samples from Lang Son Province are not just a distinct species compared to C. cantonensis, especially in the anterior region of the endopod of male first pleopod (straight vs. folded backwards), but belong to an undescribed species. As this species has not been known from China until now, a description would be out of the focus of this paper. However, we discovered C. cantonensis at Cu Lao Cham Island off the central coast of Vietnam (Fig. 1a). The population in Vietnam showed no differences when compared to the original description (Yu 1938) and the later redescriptions in Cai and Ng (1999) and Liang (2004), except the lower number of spiniform setae on flexor margin of the dactylus of the fifth pereiopod (25-28 vs. 38-45, Fig. 3c, cf. Yu (1938): fig. 8h). In China, this species is known from Guangdong and Guangxi Provinces and from Hong Kong Island. It is only found in streams and, based on egg-size (Table 1), it can be considered as land-locked (Dudgeon 1987). In our study, this species was only found on one small island in central Vietnam, far from the Live colouration of Caridina cantonensis Yu, 1938, collected on Cu Lao Cham Island, Hoi An City, Quang Nam Province.
Pleopods. Endopod of male first pleopod extending to 0.4 times length of exopod, subtriangular in shape, 2.09-2.88 (median 2.32) times as long as proximal width, rounded distally, long plumose setae on outer and distal margins, medium-length setae on inner margin; with appendix interna exceeding terminal margin of endopod by 0.5 times its length (Fig. 6e). Appendix masculina of male second pleopod slender, subcylindrical, reaching to proximal 0.6 times endopod length, 6.67 times as long as distal width, finger-shaped, with some short spiniform setae on outer surface and some long spiniform setae on distal surface; appendix interna at the middle of appendix masculina, narrow, small, extending about 0.5 times length of appendix masculina (Fig.  6f).

Distribution
This species is widely distributed in northern Vietnam (no further south than Quang Nam Province in central Vietnam). In China, the species were found in Fujian, Guangdong, Guangxi, Hainan and Hunan (Liang 2004). Our study also provides the first record for Caridina lanceifrons from Thailand, where it has been found in the southern Provinces of Nakhon Si Thammarat and Satun (Fig. 1b). This apparently disjunct distribution seems indicative of a major sampling gap in Thailand, Laos and potentially Cambodia. However, the absence of this species from southern Vietnam is unlikely to be an artefact, as this part of the country has been surveyed extensively by the first author. This is the species with the widest distribution range when compared to other land-locked species in Vietnam (Fig. 1b).

Ecology
This species inhabits mountain streams and rivers. We could not collect any specimens of this species from estuarine environments. In addition, based on egg-size (Table 1), we consider it as a land-locked species.

Taxon discussion
The populations in Vietnam showed no significant difference when compared to the original description of Caridina lanceifrons Yu, 1936in Yu (1936 or the re-descriptions by Liang (2004) and Cai (2014).

Three species described from Vietnam have been considered synonyms of Caridina lanceifrons:
According to Li and Liang (2002), Caridina flavilineata Dang, 1975 is a junior synonym of Caridina lanceifrons Yu, 1936. Dang and and Dang and Ho (2012) a b Figure 7.
Live colouration of Caridina lanceifrons Yu, 1936; collected in Pu Mat National Park, Nghe An Province.
a: Body yellowish b: Female body dark green with dorsal yellow stripe.
commented that, although C. flavilineata is very close to C. lanceifrons, they can be distinguished by differences in some morphological characteristics, especially in body colour, egg size, appendix masculina, appendix interna and rostrum. By examination of many specimens of C. flavilineata collected throughout North Vietnam, including from the area of the type locality (ZMB 30390, ZMB 33800), we found that a black or dark green body colour, especially a yellow stripe, is only present in females and is not consistently found in every population; the egg size is similar to C. lanceifrons (0.9-0.95 × 0.5-0.6 compared to 0.84-0.9 × 0.56-0.62), the appendix masculina, the appendix interna and the rostrum did not show any significant differences. After checking many specimens from Vietnam, Liang (2004) also stated that there is no difference between these species. Caridina lanceifrons is very common in south China, especially in Hainan, Guangdong, Guangxi and (presumably) Fujian (Liang 2004). In the Longzhou River that flows through North Vietnam, this species is abundant. Therefore, it can move upstream and is widely distributed throughout North Vietnam. Based on specimens collected from many sites in Vietnam and the descriptions by Dang (1975) and Yu (1936), we agree with Li and Liang (2002) and Liang (2004) and Cai (2014) that C. flavilineata Dang, 1975 is a junior synonym of Caridina lanceifrons Yu, 1936. Dang andDo (2007a) described Caridina vietriensis from North Vietnam. According to these authors, this species differs from C. flavilineata in the rostrum, the endopod of the male's first pleopod and body colour. However, they did not point out these differences clearly. They also did not compare Caridina vietriensis with C. lanceifrons. Based on the descriptions given by Dang and Do (2007a) and examination of specimens (ZMB 33823) collected from Phu Tho Province, the area of the type locality and surrounding zones, we here consider C. vietriensis as a synonym of C. lanceifrons. Do and Dang (2010) also described Caridina pseudoflavilineata from the same locality as C. haivanensis Do & Dang, 2010 at Hai Van mountain pass. Unfortunately, we could not find this species again at the type locality during two of our surveys. These authors did not compare C. pseudoflavilineata with C. lanceifrons, but they compared it with C. flavilineata. They suggested that C. pseudoflavilineata can be distinguished from C. flavilineata by the shorter and sharper rostrum, the higher number of spiniform setae on the dactylus of the fifth pereiopod and uropodal diaeresis, the shape of endopod of male first pleopod and the larger eggs. However, based on the description and illustrations by these authors and the examination of specimens collected at some distance from the type localities, Thua Thien-Hue, Da Nang and Quang Nam, we here conclude that Caridina pseudoflavilineata Do & Dang, 2010 is identical to C. lanceifrons. The rostrum reaching to the end of the third segment of antennular peduncle with a sharpened tip, the subtriangular endopod of male first pleopod, the dactylus of the fifth pereiopod with 25-28 spiniform setae and uropodal diaeresis with 14-15 movable spiniform setae are the characteristics of C. lanceifrons (Do and Dang (2010): figs. 3-4, cf. Yu (1936): figs. 5-6, Liang (2004: fig. 128, Cai (2014) : Fig. 6). There is not any obvious character distinguishing these two species, except that the eggs of C. lanceifrons are slightly smaller than those of C. pseudoflavilineata (0.9-0.8 × 0.5-0.6 mm vs. 1.1 × 0.5 mm).

Pereiopods
Pleopods. Endopod of male first pleopod extending to 0.7 times length of exopod, subrectangular in shape, 2.45-2.75 (median 2.6) times as long as proximal width, inner margin concave, outer margin slightly convex, rounded distally, long plumose setae on outer and distal margins, medium-length setae on inner margin; with appendix interna exceeding terminal margin of endopod by 0.4 times its length (Fig. 9e). Appendix masculina of male second pleopod slender, finger-shaped, not widened distally, reaching to proximal 0.7 times endopod length, 3.8 times as long as distal width, some short spiniform setae on outer surface and some long spiniform setae on distal surface; appendix interna at the middle of appendix masculina, extending about 0.5 times length of appendix masculina (Fig. 9f).
Distribution: Caridina serrata Stimpson, 1860 was recorded from Hong Kong Island only by Cai and Ng (1999), but was later also reported from Lantau island, Hong Kong and (as C. nanaoensis) from Nanao Island, China (Klotz and von Rintelen 2014). Here, we present the first record of this species in Cu Lao Cham Island from central Vietnam (Fig. 1c).
Ecology: Although this species up to now was only recorded from streams on small islands (which are on the continental shelf, i.e. would have been connected to the mainland during glacial maxima; see, for example, Voris 2000), it can be considered a land-locked species, based on egg-size (Table 1). Dang et al. (1980) recorded two subspecies of Caridina serrata, namely C. serrata serrata and C. serrata cucphuongensis. In 1999, Cai and Ng revised the C. serrata species group and stated that Caridina serrata serrata Dang, 1980 does not belong to this species because of the length of the rostrum and it is also not a member of the C. serrata species group as the stylocerite is not over-reaching the basal segment of the antennular peduncle. Caridina serrata cucphuongensis was discussed as being different from C. serrata  and was elevated to full species rank ). Dang and Do (2007a) described C. pseudoserrata from North Vietnam and assigned the specimens identified as C. serrata serrata by Dang et al. (1980) to their new species.   Figure 10.

Distribution
In Vietnam, this species is distributed in the mountainous areas of the northeast and some parts of northwest Vietnam (Fig. 1d). In China, the species occurs in the following a b c d Figure 12.
a: Male first pleopod with appendix interna reduced as a small protrusion at base of inflated part b: Male first pleopod with appendix interna as short finger at base of inflated part c: Male second pleopod d: Appendix masculina and interna of male second pleopod.

Ecology
Living in mountain streams and based on egg-size, this species can be considered a land-locked species (Table 1). Dang (1967) described Caridina denticulata vietnamensis from northern Vietnam. In subsequent publications, he raised this subspecies to species level as C. vietnamensis (Dang 1975, Dang et al. 1980. Cai (1996) synonymised it with Neocaridina palmata palmata (Shen, 1948). Subsequently, Dang and co-authors moved it to the genus Neocaridina (N. vietnamensis) (Dang and Do 2007b, Dang and Ho 2012. Li and Liang (2002) followed Cai (1996) to consider C. denticulata vietnamensis as junior synonyms of Neocaridina palmata palmata (Shen, 1948). However,  and Dang and Ho (2012) stated that N. vietnamensis can be distinguished from N. palmata or a subspecies of this species by the endopod of the male first pleopod (round shape and stable vs. variable); appendix masculina of male second pleopod (perpendicular vs. rounded) and appendix interna (reaching nearly to the tip of appendix masculina vs. not reaching to the tip of appendix masculina) and larger egg size (1-1.25 × 0.65-0.85 mm) (Dang andHo 2012). These authors also mentioned the differences in the endopod of the first pleopod and the appendices masculina and interna. Nevertheless, Liang (2004) stated that the shape of the endopod of the first pleopod is very variable in N. palmata, even in the specimens Figure 13.

Taxon discussion
Live colouration of Neocaridina palmata palmata (Shen, 1948); collected from Phia Oac-Phia Den National Park, Cao Bang Province, details of the samples are given in the supplementary data.
collected from the same localities. According to this author, N. palmata can be distinguished from N. denticulata De Haan, 1849 by the third pereiopod. In both sexes, it differs in length and shape (vs. length and shape of third pereiopod being similar between male and female); the dactylus and distal portion of the propodus of the third pereiopod of male is inflated (vs. the dactylus and distal portion of the propodus of the third pereiopod of the male is not inflated); the propodus of the third pereiopod of the male is curved (vs. the propodus of the third pereiopod of the male is straight). He also noted that this species lives in streams, rivers, reservoirs, ditches and small ponds. It is distributed in the middle reaches of the Yangtze River and central China, south and southwest China and North Vietnam. Due to its wide distribution, humid climate and different living environments, the appendix masculina often shows morphological variation and subspecies differentiation. Our specimens collected from different sampling sites in northeast Vietnam showed that all of them belong to the subspecies N. palmata palmata.

Analysis
We clarified four species of atyid freshwater shrimps in Vietnam, three from the genus Caridina ( C. cantonensis, C. lanceifrons and C. serrata) and one from the genus Neocaridina (N. palmata palmata) that were originally described from China (Table 1). We considered Caridina flavilineata Dang, 1975, C. vietriensis Dang & Do, 2007and C. pseudoflavilineata Do & Dang, 2010 as synonyms of C. lanceifrons Yu, 1936 and Neocaridina vietnamensis Dang, 1975 as a synonym of N. palmata palmata (Shen, 1948). Egg-size and distribution data (Table 1) suggest abbreviated larval development and a complete life cycle in freshwater (Vogt 2013). In general, in Vietnam, all four species are confined to North Vietnam (Fig. 1). It is rather surprising that the only species amongst these extending further south (Caridina lanceifrons in southern Thailand) does not occur in southern Vietnam.