A New Species of Land-locked Freshwater Shrimp Genus Caridina (Decapoda: Atyidae) from Middle Mekong Basin, Thailand

Abstract

A newly discovered freshwater shrimp in the genus Caridina H. Milne-Edwards, 1837 from Thailand is described here as C. panhai sp. nov. The distinctiveness of the new species was supported by both morphological investigation and molecular analysis based on a concatenated dataset of two mitochondrial genes (16S rRNA and COI). Diagnostic characters of the new species include the strongly convex rostrum, very slender carpus of second pereopod (8.11-10.04 times as long as wide), and the reduced epipod on second and third pereopods. Intriguingly, females of C. panhai sp. nov. possess a small number of very large eggs (22-36 eggs per individual), which is characteristic of landlocked species. The new species is the first landlocked Caridina species discovered in Thailand, and its range is restricted to the Middle Mekong Basin in the northeast part of the country. By including historical records, there are 15 species of Caridina found in Thailand.

Full text

Tropical Natural History, Supplement 7 (2023): 229–241
Date of publication: 6 May 2023
©2023 by Chulalongkorn University
A New Species of Land-locked Freshwater Shrimp Genus Caridina
(Decapoda: Atyidae) from Middle Mekong Basin, Thailand
KONGKIT MACHAROENBOON1, CHIRASAK SUTCHARIT2,
WARUT SIRIWUT1 AND EKGACHAI JERATTHITIKUL1*
1Animal Systematics and Molecular Ecology Laboratory, Department of Biology, Faculty of Science, Mahidol University,
Bangkok 10400, THAILAND
2Animal Systematics Research Unit, Department of Biology, Faculty of Science, Chulalongkorn University, Bangkok 10330, THAILAND
*Corresponding author. Ekgachai Jeratthitikul (ekgachai.jer@mahidol.ac.th)
Received: 18 January 2023; Accepted: 3 April 2023
http://zoobank.org/urn:lsid:zoobank.org:pub:9F378666-8565-4A15-ACD6-5DF9C44F5B33
ABSTRACT.– A newly discovered freshwater shrimp in the genus Caridina H. Milne-Edwards, 1837 from Thailand is described here as
C. panhai sp. nov. The distinctiveness of the new species was supported by both morphological investigation and molecular analysis based on
a concatenated dataset of two mitochondrial genes (16S rRNA and COI). Diagnostic characters of the new species include the strongly
convex rostrum, very slender carpus of second pereopod (8.11–10.04 times as long as wide), and the reduced epipod on second and third
pereopods. Intriguingly, females of C. panhai sp. nov. possess a small number of very large eggs (22–36 eggs per individual), which is
characteristic of land-locked species. The new species is the first land-locked Caridina species discovered in Thailand, and its range is
restricted to the Middle Mekong Basin in the northeast part of the country. By including historical records, there are 15 species of Caridina
found in Thailand.
KEYWORDS: crustacean, freshwater fauna, molecular phylogeny, taxonomy, Southeast Asia
INTRODUCTION
The freshwater shrimp genus Caridina H. Milne
Edwards, 1837, which comprises over 300 valid
species worldwide, is one of the most diverse groups in
the family Atyidae De Haan, 1849 (De Grave et al.,
2015; de Mazancourt et al., 2021; GBIF, 2023). In
general, Caridina can be found in a variety of fresh-
water habitats such as headwater streams, lakes, ponds,
and lowland rivers, as well as in brackish water and
brine of coastal areas (Page et al., 2007; Cai et al.,
2007; De Grave et al., 2008; Bernardes et al., 2017; de
Mazancourt et al., 2021). In terms of ecology, they can
be categorized into two groups based on their life
history, namely amphidromous species and land-locked
species (Lai and Shy, 2009; Han et al., 2011; Yatsuya
et al., 2013). An amphidromous species spawns nume-
rous small-sized eggs, and their larvae require saline
water to develop and migrate to coastal areas after
hatching. Such behavior gives them relatively high
dispersal ability and allows them to widely expand
their range. In contrast, a land-locked species produces
a small number of large-sized eggs and lives in
freshwater for its entire life, without migrating to the
sea. Their larvae require less time to develop to the
juvenile stage. In addition, they tend to colonize the
same habitats as the parents or places nearby (Hama-
saki et al., 2021). These factors result in relatively low
dispersal ability for land-locked species, which are
typically endemic to a particular area (Page et al.,
2008; Bauer 2013; Yatsuya et al., 2013; de Mazancourt
et al., 2021).
The systematics of Caridina in mainland Southeast
Asia has received attention from several authors in
recent decades (i.e., Ng and Choy, 1990; Johnson,
1961a, b; Yeo et al., 1999; Cai and Ng, 1999, 2000; Li
and Liang, 2002; Cai et al., 2007; Do et al., 2020,
2021). However, Thailand still lacks a robust
taxonomic revision of this genus, as well as some basic
knowledge in other aspects including their biology,
geographic distribution, and evolution. Based on
literature records, 14 Caridina species have been
reported from Thailand, namely C. typus H. Milne
Edwards, 1837; C. laevis Heller, 1862; C. gracilirostris
De Man, 1892; C. sumatrensis De Man, 1892; C.
weberi De Man, 1892; C. gracillima Lanchester, 1901;
C. brachydactyla De Man, 1908; C. propinqua De
Man, 1908; C. macrophora Kemp, 1918; C. penin-
sularis Kemp, 1918; C. tonkinensis Bouvier, 1919; C.
lanceifrons Yu, 1936; C. temasek Choy and Ng, 1991;
and C. johnsoni Cai et al., 2007. The majority of these
were described from the southern and central regions
of Thailand (Lanchester, 1901; Kemp, 1918; Kamita,
1966; Tiwari and Pillai, 1971; Junk, 1977; Cai et al.,
2007; Cai and Ng, 2007; Naiyanetr, 2007; Do et al.,
2021), while other parts of the country have mostly
been neglected from checklists and surveys.
Recent field surveys in the Middle Mekong Basin of
northeastern Thailand have yielded an unknown
Caridina species that possesses a unique strongly
convex rostrum, very slender second pereopod chela

TROPICAL NATURAL HISTORY, SUPPLEMENT 7 (2023)
230
TABLE 1. List of samples used in phylogenetic analyses, with specimen vouchers and GenBank accession numbers. n/a = data not
available. References: 1 = Page et al. 2007, 2 = Shih and Cai, 2007, 3 = Bernardes et al., 2017, 4 = de Mazancourt et al. 2017, 5 = de
Mazancourt et al., 2018, 6 = de Mazancourt et al., 2019, 7 = de Mazancourt et al., 2020, 8 = Do et al., 2020, 9 = Xu et al., 2020, 10 =
Klotz et al., 2021.
Taxon
Locality
Voucher ID
16S
COI
Reference
Caridina panhai sp. nov.
Akat Amnuai, Sakon
Nakhon, Thailand
MUMNH-CAR507-F1
(paratype)
OQ092406
OQ107450
This study
Caridina panhai sp. nov.
Akat Amnuai, Sakon
Nakhon, Thailand
MUMNH-CAR507-F2
(paratype)
OQ092407
OQ107451
This study
Caridina panhai sp. nov.
Akat Amnuai, Sakon
Nakhon, Thailand
MUMNH-CAR507-M1
(paratype)
OQ092405
OQ107452
This study
Caridina panhai sp. nov.
Bueng Khong Long Lake,
Bueng Kan, Thailand
MUMNH-CAR115-F1
OQ092408
n/a
This study
Caridina acutirostris
Sulawesi, Indonesia
ZMB_30212–1
MT769137
MT769192
10
Caridina acutirostris
Sulawesi, Indonesia
ZMB_30212–2
MT769138
MT769193
10
Caridina africana
South Africa
CA1628
MK189899
MK190058
6
Caridina appendiculata
Australia
CA1670
MH497534
MK190059
6
Caridina appendiculata
Australia
CA1708
MK189903
MK190063
6
Caridina brachydactyla
Indonesia
CA1131
MH497502
MK190011
6
Caridina brevicarpalis
Matepono River,
Guadalcanal, Solomon
Island
GUC881
DQ478485
n/a
1
Caridina brevicarpalis
Solomon Islands
CA1521
MK189887
MK190049
6
Caridina buehleri
Solomon Islands
CA1519
KY350244
MK190047
6
Caridina caerulea
Sulawesi, Indonesia
ZMB_28062–1
MT769139
MT769194
10
Caridina elongapoda
Malaysia
CA1899
MK189924
MK190085
6
Caridina ensifera
Sulawesi, Indonesia
ZMB_28055–1
MT769141
MT769196
10
Caridina gracilipes
Borneo
CA1673
MH497535
n/a
5
Caridina gracilipes
Australia
CA1694
MH497540
n/a
5
Caridina gracilirostris
Kolombangara Island
CA1497
MT303886
n/a
7
Caridina gracilirostris
Australia
CA1678
MT303887
n/a
7
Caridina gueryi
Sulawesi, Indonesia
CA1161
KY350241
n/a
4
Caridina jeani
Vanuatu
CA1370
MK189867
MK190028
6
Caridina lanceifrons
Dongfang, Hainan, China
65
MT446450
MN701605
9
Caridina lanceifrons
Dongfang, Hainan, China
66
MT446451
MN701606
9
Caridina leucosticta
Japan
CA1913
MH497557
n/a
5
Caridina lilianae
Sulawesi, Indonesia
ZMB_29807–1
MT769154
MT769208
10
Caridina longicarpus
New Caledonia
CA1557
MK189892
MK190051
6
Caridina longidigita
Sulawesi, Indonesia
ZMB_28061–1
MT769159
MT769213
10
Caridina macrodentata
Vanuatu
CA1374
MK189869
MK190030
6
Caridina marlenae
Sulawesi, Indonesia
ZMB_29519–10
MT769165
MT769219
10
Caridina neglecta
Vella Lavella, Solomon
Island
CA1703
MT303889
n/a
7
Caridina neglecta
Kolombangara Island
CA1938
MT303890
n/a
7
Caridina opaensis
Sulawesi, Indonesia
ZMB_29008–1
MT769179
MT769233
10
Caridina poso
Sulawesi, Indonesia
ZMB_29621–1
MT769181
MT769236
10
Caridina propinqua
Rathgama Lake, Southern
Province, Sri Lanka
2309SL
AY708117
n/a
1
Caridina propinqua
Giang Thanh, Kien Giang,
Vietnam
ZMB_30260
MT526817
MT526836
8
Caridina sarasinorum
Sulawesi, Indonesia
ZMB_28056–1
MT769184
MT769239
10
Caridina schenkeli
Sulawesi, Indonesia
ZMB_29159
AM747699
AM747774
10
Caridina typus
Sarawak, Malaysia
CT_Sar02
KY069345
KY069464
3
Caridina typus
Seychelles
MEFGL_CT_Sey01
KY069374
KY069493
3
Caridina villadolidi
Mindoro, Philippines
ZMBunid1227
KY436222
n/a
3
Caridina weberi
Kolombangara Island
CA1516
MT303920
n/a
7
Caridina weberi
Papua New Guinea
CA2244
MT303921
n/a
7
Caridina zhujiangensis
Dong’ao Island, Zhuhai,
China
isolate_22
MT446448
MN701603
9
Neocaridina ketagalan
Taipei, Taiwan
NCd4
AB300167
AB300180
2
Neocaridina palmata
Ha Quang, Cao Bang,
Vietnam
ZMB 30256
MT526825
MT526843
8
Neocaridina saccam
Taipei, Taiwan
NCd9
AB300169
AB300181
2
Neocaridina denticulata
Kinmen, Taiwan
NCd1
AB300173
AB300187
2
Neocaridina spinosa
Tongan, Fujian, China
NCo1
AB300174
AB300188
2

MACHAROENBOON ET AL.—A NEW CARIDINA FROM MIDDLE MEKONG BASIN
231
and carpus, and very large-sized eggs. An integrative
approach involving detailed morphological exami-
nation and molecular analyses confirms the taxonomic
validity of this taxon. In this study, we thus propose it
as an additional new species, and provide a description
herein. This is the first Caridina species discovered in
Thailand since Kemp (1918) originally described C.
macrophora and C. peninsularis from southern
Thailand.
MATERIALS AND METHODS
Specimen sampling
Freshwater shrimps were collected by triangular dip
net from aquatic habitats (i.e., streams, rivers, and
ponds) from the Middle Mekong Basin in northeastern
Thailand during field surveys between 2018 and 2020
The live habitus specimens were photographed to
document their body coloration. The methods used for
euthanasia followed Cooper (2011) under the protocol
approved by Mahidol University-Institute Animal Care
and Use Committee (approval number MU-IACUC
2018/004). Specimens were preserved in 75% and 95%
(v/v) ethanol for long-term storage and molecular
work, respectively. Voucher specimens were deposited
into the Mahidol University Museum of Natural
History (MUMNH), Department of Biology, Faculty of
Science, Mahidol University, Thailand. Specimens
were observed under ZEISS Stemi 305 stereo micro-
scope and Olympus CH30 microscope. Specimens
were photographed by digital eyepiece (Dino-Lite
AM423X). Morphological characters were measured
via DinoCapture software v.2.0 and reported in milli-
meters. All measurements were taken from ovigerous
females except first and second pleopods, which were
observed using male shrimps.
DNA extraction and PCR method
For molecular work, genomic DNA was extracted
from the abdominal muscle using NucleoSpin tissue kit
(MACHEREY-NAGEL, Germany). Two mitochon-
drial genes were used as genetic markers, including a
fragment of approximately 520 bp of the large ribo-
somal subunit rRNA gene (16s rRNA) and a fragment
of 820 bp of the protein-coding Cytochrome c oxidase
subunit I (COI). Two sets of primers, 16Samod (5’–
AAA AAC TAT TTG TCC GTC TTC AT–3’) and
16Sbmod (5’–GGT CTG AAC TCA AAT CAT GTA
AA–3’) for 16s rRNA gene, and COI–F–Car (5’–GCT
GCT AAT TTT ATA TCT ACA G–3’) and COI–R–
Car (5’–TGT GTA GGC ATC TGG GTA ATC–3’) for
COI gene, were used in PCR (von Rintelen et al.,
2007). DNA amplification was performed using T100
thermal cycle (Bio-Rad Laboratories, USA), with the
following conditions: 5 min at 94 °C; 30 cycles of
denaturation for 60 s at 94 °C, annealing for 45 s at 46–
50 °C, and elongation for 90 s at 72 °C; pre-denatura-
tion for 3 min at 94 °C; and post-elongation for 4 min
at 72 °C. The final total PCR volume was 30 μl, con-
sisting of 15 μl of EmeraldAmp PCR Master Mix (TA-
KARA BIO INC.), 1.5 μl of both forward and reverse
primers, 9 μl of distilled water, and 3 μl of the template
DNA. The newly obtained nucleotide sequences in this
study were deposited in the GenBank database under
accession numbers OQ092405–OQ092408 for 16s
rRNA, and OQ107450–OQ107452 for COI.
Phylogenetic analyses
The data matrix used in phylogenetic analyses
included sequences from four individuals of the
unknown species and other Caridina species retrieved
from the GenBank database consisting of 44 sequences
from 29 species in order to confirm the taxonomic
validity of the unknown species. Five species of atyid
freshwater shrimp genus Neocaridina Kubo, 1938 were
utilized as the outgroup. Details of specimens used in
molecular analyses are shown in Table 1. Sequences
were aligned automatically in MAFFT v.7.49 (Katoh
and Standley, 2013) by using L-INS-i algorithm for
both 16S rRNA and COI genes.
Phylogenetic trees were estimated using the
maximum likelihood (ML) and Bayesian inference (BI)
methods through the online CIPRES Science Gateway
server (Miller et al., 2010). The best-fit models of
nucleotide substitution for each gene were determined
by ModelFinder (Kalyaanamoorthy et al., 2017) as
implemented in IQ-Tree 2.1.2 (Nguyen et al., 2015) for
ML analysis and jModeltest v.2.1.10 (Darriba et al.,
2012) based on Bayesian Information Criterion for BI
analysis. The selected best-fit models are presented in
Table 2. The ML trees were executed in IQ-Tree 2.1.2
(Nguyen et al., 2015), with 1,000 bootstrap replica-
tions; the BI analyses were conducted in MrBayes
v3.2.6 (Ronquist et al., 2012), using 10,000,000 gene-
rations of Metropolis-coupled Markov chain Monte
Carlo (MCMC), with tree sampled every 2,000 gene-
rations, and the first 25% of obtained trees were
discarded as burn-in. Tracer v.1.6 was used to check
the stationary conditions of all parameters (Rambaut et
al., 2018).
TABLE 2. Best-fit models of nucleotide substitution for each
gene partition used in phylogenetic analyses.
Molecular
marker
Codon
position
Substitution model
IQ–TREE
MrBayes
16S
-
TVM+G4
HKY + I + G
COI
1
TIM3e+FQ+I+G4
SYM + G
COI
2
HKY+I
HKY + G
COI
3
TIM2+F+G4
GTR + G

TROPICAL NATURAL HISTORY, SUPPLEMENT 7 (2023)
232
RESULTS
SYSTEMATICS
Family Atyidae De Haan, 1849
Genus Caridina H. Milne Edwards, 1837
Caridina panhai sp. nov.
http://zoobank.org/urn:lsid:zoobank.org:act:9F378666-8565-4A15-
ACD6-5DF9C44F5B33
(Figs 1–5)
Type materials.– THAILAND • Holotype: ovigerous
♀, carapace length 3.70 mm, MUMNH-CAR507-H1;
Nam Chan Reservoir, Akat Amnuai District, Sakon
Nakhon Province; collected on 18 Jan 2020; 17.5983
°N, 104.0126°E. • Paratypes: 9 ovigerous ♀, MUMNH
-
CAR507-F1–F9; 6 ♂, MUMNH-CAR507-M1–M6;
same collection data as for holotype.
Other materials examined.– THAILAND • 5♀, 3♂,
MUMNH-CAR115; Bueng Khong Long Lake, Bueng
Khong Long District, Bueng Kan Province; collected
on 9 Apr 2018; 17.9603°N, 104.0376°E. • 3♀, 3♂,
MUMNH-CAR153; small canal near road no. 2094,
Seka District, Bueng Kan Province; collected on 6 Jan
2018; 17.83552°N, 103.9400°E. • 3♀, 2♂, MUMNH-
CAR173; Nong Loeng Reservoir, Phon Charoen Dis-
trict, Bueng Kan Province; collected on 6 Jan 2018;
18.0341°N, 103.6301°E. • 3♀, 1♂, MUMNH-CAR
369; collected on 05 Jan 2018; small canal near road
no. 5027, Phon Phisai District, Nong Khai Province;
17.9764°N, 103.0845°E.
Etymology.– The specific name of the new species,
C. panhai sp. nov. is named in honor of Prof. Dr.
Somsak Panha (Chulalongkorn University, Thailand), a
famous Thai taxonomist, for his dedication and sub-
stantial contributions to the systematics of invertebrates
in Thailand and Southeast Asia.
Description
Cephalothorax and cephalic appendage (n = 7).
Carapace length 2.54–3.51 mm (median 3.09), width
1.75–3.30 mm (median 2.36). Rostrum (Fig. 1A, C)
slender, strongly convex between base to near proximal
2/3, then curved upward to the tip, reaching near or
slightly beyond the end of the third segment of
antennular peduncle, 0.74–1.00 (median 0.84) times as
long as carapace, rostral formula (2–3) + 9–15 / 1–4.
Antennal spine placed below inferior orbital angle.
Pterygostomian margin subrectangular. Eye well
developed, anterior end reaching to 0.42–0.70 (median
0.55) of first segment of antennular peduncle.
Antennular peduncle (Fig. 1A, C) 0.72–0.84 (median
0.79) times as long as carapace, first segment 1.19–
1.68 (median 1.38) times as long as second segment,
second segment 1.47–1.83 (median 1.69) times as long
as third segment. Tooth on distolateral margin of first
segment of antennular peduncle prominent and acute.
Stylocerite reaching to 0.76–0.86 (median 0.80) of first
segment of antennular peduncle. Scaphocerite 3.09–
3.53 (median 3.19) times as long as wide, distal margin
with short plumose setae.
Branchial formula. Podobranch on second
maxilliped reduced to a thin plate. Third maxilliped
possesses one small and one large arthrobranch.
Pleurobranchs present on all pereopods.
Mouthparts (n = 3). Mandible without palp (Fig.
2E), incisor process with 5–6 irregular teeth, two rows
of setae placed along inner margin. Molar process
truncated. Lower lacinia of maxillula sub-rectangular
(Fig. 2F), with many rows of plumose setae. Upper
lacinia elongate, inner margin straight, with numerous
conical spinules and elongate distal spinules. Palp
elongate, with few distal spinules. Scaphognathite of
maxilla wide anteriorly (Fig. 2C), tapering posteriorly,
ending with a tuft of setae, short marginal setae placed
along proximal triangular process. Anterior margin
with long plumose setae. Palp slender. Upper and
middle endites subdivided with many rows of marginal
setae. Lower endite partly fused with middle endite,
with relatively longer marginal setae. Palp of first
maxilliped rounded (Fig. 2D), ending with finger-like
projection and with few plumose setae. Caridean lobe
broad, with marginal plumose setae. Exopod well-
developed with distally marginal plumose setae.
Ultimate and penultimate segments of endopod indis-
tinctly divided, with marginal setae. Ultimate and
penultimate segments of endopod of second maxilliped
incompletely divided (Fig. 2B). Inner margin of
ultimate and penultimate segments with simple and
pappose setae. Exopod long and slender, with a tuft of
long setae at tip. Third maxilliped with epipod (Fig.
2A). Ultimate segment of endopod with a row of strong
spinules at proximal 2/3 of posterior margin, ending
with one large claw, 7–10 spiniform setae on distal 1/3
of posterior margin, 0.80–0.86 (median 0.83, n=7)
times as long as penultimate segment. Exopod long and
slender, with a tuft of long setae at tip.
Pereopods (n = 7). Epipod present on first two
pereopods but reduced in second pereopod. Epipod on
third pereopod absent or sometimes reduced. Epipod
on last two pereopods always absent. Chelae of first
and second pereopods well-developed (Fig. 1E, F).
First pereopod slender (Fig. 1E); chela 3.20–3.69
(median 3.39) times as long as wide, 0.98–1.19
(median 1.07) times as long as carpus; tips of fingers

MACHAROENBOON ET AL.—A NEW CARIDINA FROM MIDDLE MEKONG BASIN
233
FIGURE 1. Caridina panhai sp. nov. A. ovigerous female. B. top view of carapace. C. cephalothorax and cephalic appendages of
female and D. male specimens. E. first pereopod. F. second pereopod. G. dactylus of third pereopod. H. third pereopod. I. dactylus of
fifth pereopod. J. fifth pereopod. Drawings were taken from paratype MUMNH-CAR507-F9 (A, B, E–J); paratype MUMNH-CAR507-
F8 (C), and paratype MUMNH-CAR507-M6 (D). Scale bar: A–D, H, J = 1 mm; E, F = 0.5 mm; G, I = 0.1 mm.

TROPICAL NATURAL HISTORY, SUPPLEMENT 7 (2023)
234
FIGURE 2. Caridina panhai sp. nov. A. third maxilliped. B. second maxilliped. C. maxilla. D. first maxilliped. E. mandible. F.
maxillula. G. preanal carina. H. uropodal diaeresis. I. telson. J. distal end of telson. K. first pleopod. L. second pleopod. Drawings were
taken from paratype MUMNH-CAR507-F9 (A–J); paratype MUMNH-CAR507-M6 (K, L). Scale bar: I, J = 0.5 mm; A–H, K, L = 0.25
mm.

MACHAROENBOON ET AL.—A NEW CARIDINA FROM MIDDLE MEKONG BASIN
235
rounded, with tuft of setae near tip; dactylus 1.19–1.55
(median 1.31) times as long as palm; carpus excavated
distally, 3.47–4.24 (median 3.72) times as long as
wide, 1.24–1.42 (median 1.34) times as long as merus;
merus 3.10–4.05 (median 3.45) times as long as wide,
1.06–1.33 (median 1.21) times as long as ischium.
Second pereopod extremely slender (Fig. 1F); chela
very long, 6.03–7.05 (median 6.37) times as long as
wide, 0.69–0.77 (median 0.75) times as long as carpus,
tips of fingers round, with tuft of setae near tip;
dactylus 0.90–1.40 (median 1.08) times as long as
palm; carpus very slender, 8.11–10.04 (median 8.82)
times as long as wide, 1.62–1.89 (median 1.76) times
as long as merus; merus 4.52–6.33 (median 5.68) times
as long as wide, 0.93–1.26 (median 1.06) times as long
as ischium.
Third pereopod not sexually dimorphic (Fig. 1H);
dactylus with 8–10 spinules on flexor margin (Fig.
1G), 4.50–6.04 (median 5.28) times as long as wide
(including terminal claw), terminating with one large
claw; propodus with numerous spiniform setae on
lateral and posterior margin, 10.03–15.82 (median
13.04) times as long as wide, 2.66–3.34 (median 3.09)
times as long as dactylus; carpus with 3–4 spiniform
setae on posterior margin of outer surface, the distal
seta largest, the other setae minute, 5.72–7.01 (median
6.31) times as long as wide, 0.52–0.60 (median 0.57)
times as long as propodus; merus with three large
spiniform setae on posterior margin of outer surface,
8.08–10.21 (median 9.31) times as long as wide, 1.76–
2.19 (median 1.91) times as long as carpus; ischium
with one spiniform seta.
Fifth pereopod slender (Fig. 1J); dactylus with 57–
68 spinules on flexor margin (Fig. 1I), 5.25–7.21
(median 5.83) times as long as wide (including
terminal claw), terminating with one large claw;
propodus with numerous spiniform setae on posterior
margin, 11.81–14.37 (median 13.41) times as long as
wide, 2.29–2.76 (median 2.55) times as long as
dactylus; carpus with 3–4 (mode 3) spiniform setae on
posterior margin of outer surface, the distal seta largest,
the other setae minute, 4.48–5.61 (median 4.97) times
as long as wide, 0.42–0.51 (median 0.48) times as long
as propodus; merus with two large spiniform setae on
posterior margin of outer surface, 6.75–8.70 (median
7.60) times as long as wide, 1.49–1.92 (median 1.66)
times as long as carpus; ischium without spiniform
seta.
Pleopods (n = 6). Endopod of male first pleopod
subtriangular (Fig. 2K), wider proximally, 2.06–2.50
(median 2.21) times as long as width, 0.28–0.30
(median 0.29) times exopod length, without appendix
interna. Appendix masculina of second male pleopod
rod-shaped (Fig. 2L), with numerous setae, 0.66–0.81
(median 0.70) times as long as endopod (including
distal setae). Appendix interna very slender, reaching
0.59–0.69 (median 0.64) times appendix musculina
length.
Abdomen (n = 7). Sixth abdominal somite 0.63–
0.79 (median 0.75) times carapace length, 1.84–2.15
(median 2.02) times as long as fifth somite, 0.94–1.27
(median 1.03) times as long as telson (Fig. 1A). Telson
2.58–3.03 (median 2.74) times as long as wide, with 3–
4 pairs of dorsal spiniform setae and one pair of
dorsolateral spiniform setae (Fig. 2I). Distal margin of
telson subtriangular, without posteromedian projection,
frequently with one median moveable plumose seta
and 4–5 pairs of moveable plumose setae, subequal in
length (Fig. 2J). Preanal carina stout, with a few setae,
without a spine (Fig. 2G). Uropodal diaeresis with 7–
12 short moveable spiniform setae (Fig. 2H).
Eggs. Ovigerous females with few eggs (22–36
eggs, n = 5). Size of eye-developed eggs 0.94–1.06 ×
0.60–0.67 mm (n = 20).
Coloration.– In living specimens (Fig. 3), body brown
to thistle green, furnished with numerous scattered
black spots. Cephalic region and first to second dorsal
somites darker than other parts. Each somite with a
dorsal dark stripe. Eggs greenish brown.
Distribution and habitat.– Based on the materials in
this study, the distribution of C. panhai sp. nov. is
limited to the Songkhram River and tributaries that
drain into the Middle Mekong River, in northeast
Thailand (Fig. 4). This species can be found in lentic
freshwater habitats, such as marsh, pond, lake, and
paddy field. They were found living on aquatic
vegetation in shallow areas, where the bottom substrate
was mud and clay.
Phylogenetic results.– The molecular analyses
indicated C. panhai sp. nov. as distant phylogenetically
from other members of Caridina, with high support
(100 ML bootstrap value and 1.00 BI Bayesian
posterior probability; Fig. 5), and nested it as a sister
clade to C. propinqua s.s. Page et al. (2007).
Remarks.– The new species shares the characteristics
of a short-rostrum and slender second pereopod carpus
with C. excavatoides Johnson, 1961; and C. temasek.
However, it can be distinguished from these three
species by having a longer rostrum that reaches beyond
the end of antennular peduncle (vs rostrum that reaches
between the base and the end of the third segment of
antennular peduncle), much more slender second
pereopod chela (6.03–7.50 times as long as wide vs
3.0–5.3 times as long as wide), and larger eggs (0.94–

TROPICAL NATURAL HISTORY, SUPPLEMENT 7 (2023)
236
FIGURE 3. Living female of Caridina panhai sp. nov. Photos was taken from A. MUMNH-CAR507_P1 and B. MUMNH-
CAR507_P2.

MACHAROENBOON ET AL.—A NEW CARIDINA FROM MIDDLE MEKONG BASIN
237
1.06 × 0.60–0.67 mm vs 0.40–0.85 × 0.25–0.54 mm).
In addition, the first pleopod of the new species also
lacks appendix interna (vs present in C. temasek), and
the third pereopod is not sexually dimorphic (vs
sexually dimorphic in C. thambipilaii) (Johnson,
1961a; Choy and Ng, 1991; Cai et al., 2007).
Caridina panhai sp. nov. closely resembles C.
laevis, but can be distinguished by having a longer
rostrum that reaches slightly beyond the end of the
third segment of the antennular peduncle (vs reaching
between the middle of second segment to the middle of
third segment of antennular peduncle); fewer dorsal
teeth (9–15 vs 15–22), fewer postorbital teeth (2–3 vs
4–5); more slender first pereopod carpus (3.47–4.24
times as long as wide vs 2.6–2.7 times), and much
more slender second pereopod carpus (8.11–10.04
times as long as wide vs 6.5–7.4 times) (Heller, 1862;
De Man, 1892; Pillai 1964).
Caridina panhai sp. nov. is also similar to C.
tonkinensis Bouvier, 1919. However, the new species
differs from C. tonkinensis by the absence of a spine on
preanal carina (vs presence), more slender P2 carpus
(8.11–10.04 times as long as wide vs 4.4 times as long
as wide), fewer spiniform setae on fifth pereopod
dactylus (57–68 vs approximately 80), the feature of
plumose setae at distal end of telson (subequal in
length vs two median pairs shorter than the others), and
larger eggs (0.94–1.06 × 0.60–0.67 mm vs 0.68–0.75 ×
0.47–0.48 mm) (Bouvier, 1919; Cai et al., 2007).
The new species is very similar to C. excavata
Kemp, 1913 from India by sharing a very slender
second pereopod chela, the shape of rostrum, and
large-sized eggs. However, the new species differs
from C. excavata by having more convex rostrum;
fewer number of ventral teeth (1–4 vs 2–8); more
slender first pereopod carpus (3.47–4.24 times as long
as wide vs 3 times), much longer second pereopod
carpus (8.11–10.04 times as long as wide vs 5.5 times),
greater number of spiniform setae on fifth pereopod
dactylus (57–68 vs 40–50), and greater number of
dorsal spiniform setae on telson (3–4 pairs vs 2 pairs)
(Kemp, 1913). In addition, the distribution range of the
new species is limited to the Middle Mekong Basin in
northeast Thailand, whereas C. excavata is dispersed
along the Brahmaputra River in Assam State, India
(Kemp, 1913). Both species carry large-sized eggs, and
are characterized as land-locked species (Yatsuya et al.
2013; de Mazancourt et al., 2021). Thus, they are each
distributed in their own limited area, since land-locked
species spend their whole life in freshwater without
migration, and tend to colonize in or near parental
habitats (Bauer 2013; Hamasaki et al., 2021). Unfor-
tunately, the molecular data of C. excavata was not
available in the current study. However, considering
the significant differences in morphology and their
geographic distributions, there is no doubt that they are
distinct species.
FIGURE 4. Map of Thailand showing the distribution of Caridina panhai sp. nov. (Black star for the type locality and black circles for
other materials). Maps were generated using QGIS v3.24.3 by compiling, river and lake topology from the HydroSHEDS database
(https://www.hydro sheds.org), and map raster data from the NASA EARTHDATA (https://www.earthdata.nasa.gov/).

TROPICAL NATURAL HISTORY, SUPPLEMENT 7 (2023)
238
DISCUSSION
Caridina panhai sp. nov. would be included in the
C. nilotica group based on morphology alone, because
they possess several similar characters, such as a
rostrum that extends beyond the end of third antennular
peduncle, a dorsal hump on the third abdominal somite,
and few moveable setae on the uropodal diaeresis
(usually less than 15) (Richard and Clark, 2005, 2014;
de Mazancourt et al., 2018, 2020; Cai, 2020). How-
ever, the molecular analyses indicated that this new
species split apart from the C. nilotica group (Fig. 5).
FIGURE 5. Maximum Likelihood tree of Caridina and outgroups based on 16S rRNA and COI genes. Numbers at each node show ML
bootstrap value followed by Bayesian posterior probability.

MACHAROENBOON ET AL.—A NEW CARIDINA FROM MIDDLE MEKONG BASIN
239
In fact, it differs from members of the C. nilotica group
in having reduced epipods on the second and third
pereopods (vs usually present on first four pereopods)
(Cai et al., 2007; de Mazancourt et al., 2018, 2020).
According to the current phylogenetic tree,
C. panhai sp. nov. is nested with C. propinqua sensu
Page et al. (2007), but with a great distance between
the two, and their morphology is very different (De
Man, 1908a; Cai et al., 2007). This could be a
consequence of taxon sampling bias. The overall
sampling of the genus Caridina is far from complete,
especially for the mainland Southeast Asia taxa.
Further work requires more comprehensive sampling,
including genetic data from species such as C. laevis
and C. excavata, in order to accurately examine
phylogenetic relationships with this new species.
This species is the first endemic land-locked Cari-
dina species to be discovered in northeast Thailand.
Thus, there are now 15 species of Caridina known to
exist in Thailand. At present, several species of
Caridina such as C. apodosis Cai and Ng, 1999 and C.
striata von Rintelen and Cai, 2009 are severely
threatened due to habitat destruction and overhar-
vesting for aquarium trades (De Grave et al., 2015; de
Mazancourt et al., 2021; IUCN, 2022). Extensive
information regarding systematics, biology, and
geographical range of each species will be essential for
conservation management in the future. Moreover,
Thailand is a part of several significant freshwater
ecoregions, which are the Lower and Middle Salween
Basins, Middle Mekong Basin, Mae Khlong Basin,
Chao Phraya Basin, and Malay Peninsula Basin (Abell
et al., 2008). Future field surveys and phylogenetic
studies in all of these regions will allow us to better
understand the evolution and biogeography of Caridina
in mainland Southeast Asia.
ACKNOWLEDGMENTS
We would like to express our sincere gratitude to R.
Chanabun, R. Srisonchai, S. Klomthong, and A.
Chaowvieng for facilitating the field survey, and D.J.
Anderson for grammar checking and comments. Our
grateful thanks also extend to reviewers for their useful
comments and suggestions. This study was supported
by the Center of Excellence on Biodiversity (BDC-
PG4-163009).
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