Diversity and distribution of the cladocerans (Crustacea, Branchiopoda) in Thailand

Abstract An updated checklist of the cladoceran fauna from inland aquatic habitats in Thailand (a high-diversity hotspot in Southeast Asia), based on published cladoceran records found in literature is presented. The checklist updates nomenclature and species distributions, especially habitat preferences. A total of 138 valid recorded species is relatively high. However, the estimators indicate that more species are expected to be found with more research. The north-eastern and southern regions of Thailand are well-studied regions of high species richness with 100 and 96 cladoceran species, respectively, whereas the northern and eastern regions have large research gaps that should be studied further. Moreover, each habitat type seems to have a unique cladoceran community as the similarity values amongst them are mostly low (Sorensen similarity index < 0.50). Therefore, it is suggested that habitats with unique characteristics, such as peat swamps, stream and cave pools, are worthy of further exploration. If the current records of cladoceran diversity in Thailand confirms a high diversity of this animal in the tropical region, then the geographical distribution of each species can be properly explained.


Introduction
Thailand is a biodiversity hotspot in Southeast Asia. Few species of freshwater zooplankton have been reported previously, but high diversity is currently shown in various groups, including copepods, rotifers and cladocerans. Research in Thailand on cladocerans began with Boonsom in 1984 and researchers have started to pay more attention to this group of zooplankton since 1997, with more research published. A total of 34 research papers and five research reports have been published, with the majority of studies covering taxonomy and diversity (Boonsom 1984, Pholpunthin 1997, Sanoamuang 1998, Korovchinsky 2000, Pipatcharoenchai 2001, Saeng-aroon 2001, Sa-ardrit 2002, Kotov and Sanoamuang 2004, Kotov et al. 2005a, Kotov et al. 2005b, Maiphae 2005, Maiphae et al. 2005, Sa-ardrit and Beamish 2005, Sanoamuang and Faitakum 2005, Korovchinsky and Sanoamuang 2008a, Maiphae et al. 2008, Chittapun et al. 2009, Maiphae and Janpriang 2009, Maiphae et al. 2010, Choedchim and Maiphae 2012, Meksuwan et al. 2012, Korovchinsky and Sanomuang 2013, Van Damme and Maiphae 2013, Tiang-nga et al. 2016, Sinev et al. 2017, Alonso et al. 2019, Jantawong and Maiphae 2020, Tiang-nga et al. 2020, Plangklang and Athibai 2021, Tiang-nga et al. 2021, Sinev et al. 2023) and many fewer covering ecology and aquaculture (Kotov et al. 2013a, Choedchim et al. 2017, Manklinniam et al. 2018. After almost four decades of intensive study on the diversity of cladoceran in bodies of water in Thailand, 138 species have been identified. However, the taxonomic status of some recorded species has changed given that the taxonomical ranking of these species has changed greatly due to enhanced understanding of their evolution, along with the application of more tools (Van Damme et al. 2005, Van Damme et al. 2010, Van Damme et al. 2011). In addition, it seems that many more species have been discovered in recent years despite its being a relatively well-studied region. This is probably due to the high diversity of microhabitats, with some yet to be surveyed. Therefore, we revise and update the checklist of the cladocerans in Thailand in this paper in light of recent insights into their taxonomy and nomenclature, including analysis of the species diversity and the ecological and geographical distribution of this group. This research contributes to existing knowledge on this important component of freshwater biota in Thailand and offers suggestions for how this knowledge gap could be filled in the future.

Materials and method
In the present study, a checklist of cladoceran species in Thailand was compiled from the existing 39 research papers and research reports, as mentioned above. The updated names of each species were presented and used for all analyses and the species names used in previous publications were provided. Data on biogeographical distribution are mostly drawn from literature, as shown in Table 1. Occurrences are identified in eight large biogeographical regions (Palearctic, Afrotropical, Oriental, Nearctic, Neotropical, Australian, Pacific and Antarctic), as described in Segers (2007). Table 1.
List of cladoceran species, their habitat occurrence and distribution in Thailand. (Abbreviation codes: c = canal, d = dam, e = estuary, f = floodplain, ff = fish field, l = lake, m = marsh, mi = mine, ml = man-made lake, p = pond, po = pool, ps = peat swamp, r = river, rc = roadside canal, re = reservoir, rf = rice field, sf = saline rice field, st = stream, sw = swamp, tp = temporary pond, w = wastewater treatment pond, wf = waterfall, N = north, NE = northeast, W = west, E = east, C = central, S = south, Aus = Australian, Afr = Afrotropical, Nea = Nearctic, Neo = Neotropical, Ori = Oriental, Pal = Palearctic; Reference codes: 1 = Boonsom 1984, 2 = Pholpunthin 1997, 3 = Sanoamuang 1998, 4 = Saeng-aroon 2001, 5 = Pipatcharoenchai 2001, 6 = Sa-ardrit 2002, 7 = Kotov and Sanoamuang 2004, 8 = Sa-ardrit and Beamish 2005, 9 = Kotov et al. 2005a, 10 = Kotov et al. 2005b, 11 = Maiphae 2005, 12 = Maiphae et al. 2005, 13 = Sanoamuang and Faitacum 2005, 14 = Sinev et al 2007, 15 = Sinev and Sanoamuang 2007, 16 = Maiphae et al. 2008, 17 = Korovchinshky and Sanoamuang 2008a, 18 = Chittapun et al. 2009, 19 = Maiphae and Janpriang 2009, 20 = Maiphae et. al. 2010, 21 = Meksuwan et al. 2012, 22 = Choedchim and Maiphae 2012, 23 = Sinev and Kotov 2012, 24 = Kotov et al. 2013a, 25 = Van Damme and Maiphae, 2013, 26 = Van Damme et al. 2013, 27 = Sinev and Sanoamuang 2013, 28 = Korovchinsky and Sanoamuang 2013, 29 = Tiang-nga et al. 2016, 30 = Choedchim et al. 2017, 31 = Sinev et al. 2017, 32 = Manklinniam et al. 2018, 33 = Alonso et al. 2019, 34 = Jantawong and Maiphae 2020, 35 = Korovchinsky 2000, 36 = Tiang-nga et al. 2020, 37 = Tiang-nga et al. 2021, 38 = Plangklang and Athibai 2021, 39 = Sinev et al. 2023 angustatus (Sinev 2009 rostrata (Sanoamuang 1998 For the data analysis-to answer the following research questions: (1) whether some geographical regions in Thailand were more diverse in cladoceran species than others, regardless of the differences in sampling efforts and (2) whether some habitat types were richer than other types-we divided all the records in Thailand into six geographical regions comprising the north (N), northeast (NE), west (W), east (E), central (C) and south (S) (Fig. 1), based on natural drainage, including landforms and drainage. Northern Thailand is a mountainous area where high mountains are incised by steep river valleys and upland areas that border the central plain. Like the north, the geography of the western region is characterised by high mountains and steep river valleys. The northeast region is a large plateau basin that is extremely flat in some parts with a few low, rugged and rocky hills. Unlike the other areas, the northeast has a long dry season. The central region is a large lowland basin formed by the accumulation of sediment, sand, rocks and mud. The geography of the eastern region is characterised by short mountain ranges alternating with small basins of short rivers that drain into the Gulf of Thailand. Southern Thailand, part of a narrow peninsula, is distinctive in terms of climate, terrain and resources. We also categorised all habitats into 22 types comprising canal, dam, estuary, floodplain, fish field, lake, marsh, mine, man-made lake, pond, pool, peat swamp, river, roadside canal, reservoir, rice field, saline rice field, stream, swamp, temporary pond, wastewater treatment pond and waterfall. The definitions for each habitat type are noted in Suppl. material 1. The species richness estimators, including jackknife1, jackknife2 and bootstrap, were analysed by a species accumulation curve using the EstimateS programme. The jackknife estimator is suitable and tends to reduce the bias in small data samples. In addition, bootstrap is a simple method used to derive estimates of standard errors and confidence intervals for complex estimators of the distribution. Therefore, both estimators were analysed to confirm that the trends in the evaluation results were consistent. In addition, Sorensen's Similarity Index, which is a statistic used to gauge the similarity of two samples, was used to explore the similarities in species composition amongst regions and habitat types. The index was calculated with Microsoft Excel 2016.
Only five species (3.62%)-Diaphanosoma excisum, D. sarsi, D. volzi, Latonopsis australis and Moina siamensis-were found in all regions and many species were restricted to only one region. One species, Bosmina fatalis, has been found only in the W region, while three species (2.17%) have been found only in the N region (Ilyocryptus cf. bhardwaji, I. raridentatus and I. thailandensis) and nine species ( (Fig. 2).
Sorensen's Similarity Index indicated that the E and N regions showed the highest similarity in terms of cladoceran species composition (0.75), followed by the S and the NE (0.72) and the W and the NE (0.64), whereas the least similarity was found between the S and the N, which were of equal value, along with the S and the E (0.11) (Suppl. material 2).
The highest species richness was found in swamps and lakes (77 species each), followed by ponds (60 species), peat swamps (55) and rivers (54 species each), whereas estuaries showed the lowest species richness (one species) (Fig. 4). Sorensen's Similarity Index showed that the similarity of cladoceran species composition was less than 0.50 between most habitat types, whereas only 45 pairs from 231 pairs of different habitats showed a similarity of more than 0.50. Pools and dams had the highest similarity of cladoceran species composition (0.81), followed by mines and dams (0.75), swamps and peat swamps (0.72) and pools and mines (0.69), whereas no similarities (0) were found in 28 pairs of different habitat types (Suppl. material 3).
According to the general species accumulation curve, the sampling effort (in this case, the number of research papers) is considered insufficient given that the observed values of S (138) align with those calculated in the bootstrap estimator (152.94) and the asymptote estimates of the jackknife 1 (170.31) and jackknife 2 (183.20) variation indicators (Fig. 3).
Twenty-four species were found in various habitat types (> 10 habitats); Ephemeroporus barroisi, Dunhevedia crassa and Ilyocryptus spinifer occurred in most habitat types (16). In contrast, 28 species were found in only one habitat type: eight species were found only in The Venn diagram shows the number of cladocerans restricted to each region and shared between regions. Number in bracket represents number of total species in that region.

Species richness
Since being poorly known in Thailand 30 years ago, the number of identified and studied cladoceran species has continued to increase. More intensive diversity studies in various types of microhabitats, including the taxonomic revision of some species, have led to more species being recorded. A total of 38 new records have been identified during the past 15 Cladocerans species richness in each habitat type. Abbreviation codes; see Table 1.
years compared to the records of Maiphae et al. (2008). Of these, 15 species are described from Thailand and eight species are endemic to Thailand. In addition, 16 synonymies were detected in previous records (Table 1). The total number of species identified in Thailand is relatively high and accounts for approximately 45% (about 298 species) of all records in Southeast Asia , Tiang-nga et al. 2020. In addition, the species richness of cladocerans in Thailand is relatively high compared with records from other countries in Southeast Asia, as shown by the following statistics: Malaysia has about 104 species (Korovchinsky 2013, Sinev andYusoff 2015); Indonesia has about 118 species ; the Philippines has about 55 species , Pascual et al. 2014,Lopez et al. 2017; Laos has about 80 species (Kotov et al. 2013b, Siboualipha et al. 2020; Cambodia has about 60 species (Tanaka and Ohtaka 2009); Vietnam has about 130 species (Sinev and Korovchinsky 2013, Sinev 2014, Sinev and Irina 2021. These differences are not only because more sampling sites were explored in Thailand, but also because the studied sites included a high diversity of habitat types (22 types). However, the estimator indices indicated that the present number of records is an underestimate and that more species could be discovered in Thailand with more research, particularly in less studied regions (i.e. the N, W and E). Currently, high species richness is found in the W, despite relatively few sites being sampled in comparison to the size of the area. The W region comprises mountain ranges and plains and is similar to the N region. Therefore, it would be interesting to explore more sites and microhabitats, especially peat swamps, streams and cave pools, as the discovery of more species is expected.
The N region of Thailand is relatively large. However, few studies have been conducted despite all cladoceran microhabitats being represented. Researchers have focused on the Ilyocryptidae (Kotov and Sanoamuang 2004), Sididae (Korovchinsky and Sanomuang 2013) and Moinidae (Alonso et al. 2019) families following their interest in taxa reported from the N. Likewise, only the Sididae and Moinidae families (Alonso et al. 2019) have been researched in the E region. This is one reason why the cladoceran compositions in these two regions have a high level of similarity. Likewise, the NE and S regions also show numerous similarities in cladoceran composition due to a similar research focus. The E region is the smallest in Thailand, but it is the most interesting to investigate due to its diverse geography (river basins and coastal areas with a mountain range in the middle). However, the N and E regions have a large research gap that could be targeted by further studies researching their species diversity. The distribution pattern for the species and range boundaries of each species could then be tentatively outlined and more extensive zoogeography could be analysed.

Geographical distribution
Present records show that the proportion of commonly distributed species is less than that of restricted species. Only Diaphanosoma excisum, D. sarsi, D. volzi, Latonopsis australis and Moina siamensis were found in all regions. Of the other species, Bosminopsis deitersi and Ephemeroporus barroisi are also common, as they are distributed in every region, except the N, which might be because studies are lacking in that region, as mentioned previously. Korovchinsky (1992) made it clear that Sididae, especially the genus Diaphanosoma, contribute substantially to all continents. In addition, amongst Moinidae, Moina is much more common in the limnetic zone of tropical lakes. These small and transparent species are relatively immune to fish predation (Dumont 1994), which could explain their wide distribution, especially in oriental and circumtropical regions that have a high abundance of planktivorous fish. Bosminopsis deitersi is a species known for its multicontinental range and broad ecological requirements (Garibian et al. 2021

Microhabitat distribution
Lakes and swamps are heterogeneous environments that harbour the highest cladoceran diversity and include high-richness habitats. A total of 77 species are found in these habitats, accounting for about 56% of the known cladoceran species in Thailand. Two biologically rich lakes in Thailand, Kud-Thing Lake and Thale-Noi Lake, are Ramsar sites where fauna thrive. Apart from cladocerans, other groups of zooplankton, fish, birds and aquatic plants have high diversity in these lakes. Kud-Thing Lake is a large natural lake connected to the Mekong River and Thale-Noi Lake is connected to Songkhla Lake ( Ramsar Sites Information Service 2023). These geographical characteristics provide complex lake structures that enable organisms to live in several microhabitats and ecological niches.
It was also found that similar habitat structures led to similar cladoceran compositions. Pools, dams and mines are permanent man-made habitats that show a high similarity of cladoceran compositions. Swamps and peat swamps, which are natural habitats mostly covered with aquatic plants, also showed high similarity in cladoceran composition. Some types of habitats, such as estuarine waters, have unique structures, leading to low similarity with other habitats. The species found in these unique habitats, such as Salinalona sarasinorum, warrant further study, particularly in other research fields, such as ecophysiology. Some cladoceran habitats have scarcely been studied, including peat swamps, streams and cave pools. Thailand has several small and large cave systems in each region. Copepods are a good example of organisms that are well studied in cave pools and high numbers of copepods are seen in this harsh habitat (Watiroyram 2021, Sanoamuang and Watiroyram 2021). It is expected that some yet-to-be-discovered cladoceran species may be present.
Although the taxonomy and distribution of most cladoceran species are now clearly understood, further studies should be carried out to reach a plateau. To determine the actual species richness of the country and gain a greater understanding of the ecological and biogeographical distribution of cladocerans, increased sampling efforts should be directed at less-studied habitats, such as peat swamps, streams, cave pools and groundwater. In addition, the habitats on islands in the Thai-Malaysia Peninsula would also be interesting to explore and are anticipated to contribute greatly to a better understanding of the biogeographical distribution of this animal in Southeast Asia. Moreover, it would be interesting to further integrate both morphological and genetic diversity given that cryptic species are assumed to be widely distributed in nature and amongst biogeographical regions (Pfenninger and Schwenk 2007) and that their discovery and description are pivotal to the correct assessment of actual biodiversity patterns. Since we now know that the cladoceran community in Thailand could somehow be representative of tropical countries, it would be interesting to use the cladoceran species as a model to study functional traits and as bioindicators to measure the health of aquatic environments. This would meet the purpose of this updated checklist, which aims to contribute to more aspects of cladoceran research in tropical regions.