Small dams fragment assemblages of diadromous and freshwater decapods in Hong Kong lowland streams

Abstract

Dam construction has fragmented and substantially altered streams globally, obstructing migrations between coastal and freshwater habitats by diadromous animals. We undertook a territory-wide survey of decapod species (shrimps and crabs) in Hong Kong, southern China, examining spatial and seasonal variability in assemblage composition, and the impacts of barriers, across 24 lowland streams. Thirteen diadromous and 10 primary (i.e., non-migratory) freshwater decapod species were recorded, considerably more than reported elsewhere on the Chinese mainland. While some decapods are proficient climbers, six diadromous species were confined to unobstructed streams. Dams (0.3–8.7 m high) reduced total richness, but had stronger effects on diadromous species (mean richness fell from 4.9 to 3.2 species). Mean species richness of both total and diadromous decapods were lowest in streams with dams > 2 m tall (reductions of 6.4 to 3.8 and 4.9 to 2.2, respectively). Decapod assemblage structure was significantly different above and below dams, reflecting the restriction of primary freshwater species to reaches above dams, and diadromous species to reaches downstream of dams. Our findings underscore the need for improved knowledge of the diversity of diadromous animals in China and tropical East Asia, as well as better understanding of mitigation measures to improve dam passage by these animals.

Full text

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Hydrobiologia (2025) 852:2871–2886
https://doi.org/10.1007/s10750-025-05801-9
PRIMARY RESEARCH PAPER
Small dams fragment assemblages ofdiadromous
andfreshwater decapods inHong Kong lowland streams
JefferyC.F.Chan · JiaHuanLiew ·
DavidDudgeon
Received: 8 November 2024 / Revised: 10 January 2025 / Accepted: 13 January 2025 / Published online: 10 February 2025
© The Author(s) 2025
Abstract Dam construction has fragmented and
substantially altered streams globally, obstructing
migrations between coastal and freshwater habitats
by diadromous animals. We undertook a territory-
wide survey of decapod species (shrimps and crabs)
in Hong Kong, southern China, examining spatial
and seasonal variability in assemblage composi-
tion, and the impacts of barriers, across 24 lowland
streams. Thirteen diadromous and 10 primary (i.e.,
non-migratory) freshwater decapod species were
recorded, considerably more than reported elsewhere
on the Chinese mainland. While some decapods are
proficient climbers, six diadromous species were
confined to unobstructed streams. Dams (0.3–8.7 m
high) reduced total richness, but had stronger effects
on diadromous species (mean richness fell from 4.9
to 3.2 species). Mean species richness of both total
and diadromous decapods were lowest in streams
with dams > 2m tall (reductions of 6.4 to 3.8 and 4.9
to 2.2, respectively). Decapod assemblage structure
was significantly different above and below dams,
reflecting the restriction of primary freshwater spe-
cies to reaches above dams, and diadromous species
to reaches downstream of dams. Our findings under-
score the need for improved knowledge of the diver-
sity of diadromous animals in China and tropical East
Asia, as well as better understanding of mitigation
measures to improve dam passage by these animals.
Keywords Tropical· Diadromy· Southern China·
Shrimps· Crabs· Barriers
Introduction
Most decapod crustaceans are confined to marine
environments, but a significant (~ 21%) proportion
occurs in fresh water (De Grave etal., 2023; DecaNet,
2025). Over 3600 species are confined to fresh water
throughout their lives, mainly among the Astacidea,
Brachyura, and Caridea (De Grave et al., 2023;
DecaNet, 2025). Some of them have suppressed the
planktonic larval stage and instead produce relatively
few, large eggs that undergo direct development,
Handling editor: Koen Martens
Supplementary Information The online version
contains supplementary material available at https:// doi.
org/ 10. 1007/ s10750- 025- 05801-9.
J.C.F.Chan· D.Dudgeon(*)
School ofBiological Sciences, The University ofHong
Kong, HongKongSAR, China
e-mail: ddudgeon@hku.hk
J. C. F. Chan
e-mail: jefferychan@connect.hku.hk
J.C.F.Chan· J.H.Liew
Science Unit, Lingnan University, HongKongSAR, China
e-mail: JiaHuan.Liew@utas.edu.au
J.H.Liew
School ofNatural Sciences, University ofTasmania,
Hobart, Australia
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then hatching as a miniature adult (e.g., Mashiko,
1983; Ng, 1988). Among other decapods found in
fresh water, some species (e.g., varunid crabs) have
a diadromous life cycle involving migrations between
marine and freshwater environments and broods of
many small eggs (Kobayashi & Matsuura, 1995).
Certain shrimp genera (Macrobrachium and Carid-
ina, for example) include representatives of both life
cycle types (e.g., Hayashi & Hamano, 1984; Jalihal
etal., 1993). All ~ 220 species of diadromous caridean
shrimps (Atyidae, Palaemonidae, and Xiphocaridae)
are amphidromous, with planktonic larvae drifting
downstream to the sea before migrating to upstream
as juveniles (de Mazancourt & Ravaux, 2024). In
contrast, some varunid crabs are catadromous, with
adults migrating from fresh water to the sea to breed;
subsequently, juveniles ascend streams where they
grow and mature (Cumberlidge & Ng, 2009).
Diadromous decapods are widespread geographi-
cally, with the greatest species richness in the tropics
and subtropics (Cumberlidge & Ng, 2009; de Mazan-
court & Ravaux, 2024). They serve as both prey and
macro-consumers in food webs (March etal., 2002),
affecting litter decomposition (Crowl et al., 2001)
and sediment dynamics (Pringle etal., 1993). Addi-
tionally, commercial and artisanal fisheries are based
on diadromous decapod species, such as the Chinese
mitten crab Eriocheir sinensis and river shrimp Mac-
robrachium vollenhovenii (Cheng etal., 2008; Ofori-
Darkwah etal., 2020), while others, most notably M.
rosenbergii, are used in aquaculture (New & Valenti,
2008).
While freshwater decapod species are exposed
to an array of multiple anthropogenic threats (e.g.,
Dudgeon, 2019), their diadromous counterparts are,
arguably, more at risk due to the combination of
pressures they encounter during migrations between
the marine and freshwater realms (Jarvis & Closs,
2019). For instance, habitat degradation and over-
fishing have led to collapses in annual yields of the
varunid Eriocheir sinensis in the Yangtze River estu-
ary (Xue et al., 2016). Man-made barriers, particu-
larly dams, might also block essential migrations
of diadromous animals and fragment populations
(Dudgeon, 2020; He etal., 2024). The consequences
for ecosystem services can be significant: the Diama
Dam in Senegal obstructed migrations by Macrobra-
chium vollenhovenii, a predator of snails that host
schistosome parasites, resulting in schistosomiasis
outbreaks in upstream areas (Sokolow etal., 2014),
with 400 million people at risk in other dammed west
African drainages (Sokolow etal., 2017). Although
some decapod species are capable climbers (see
Hongjamrassilp etal., 2021), the richness and abun-
dance of diadromous migrants is generally reduced
upstream of dams (Miya & Hamano, 1988; Benstead
etal., 1999; Frotté etal., 2020), suggesting impaired
upstream movement, whereas reservoirs and entrain-
ment into pumping infrastructure (e.g., turbines) are
obstacles for larvae traveling downstream (Benstead
etal., 1999; Jarvis & Closs, 2019).
Hong Kong Special Administrative Region (hence-
forth, HKSAR) is located on the tropical fringes of
Southern China, where primary freshwater decapods
such as Macrobrachium and Caridina shrimps play
a significant role in the breakdown of allochthonous
leaf litter and can contribute substantially to sec-
ondary production in streams (Mantel & Dudgeon,
2004a, 2004b; Yam & Dudgeon, 2006). However,
comparatively little is known about the diversity or
importance of diadromous decapod species in the ter-
ritory. The species richness of diadromous fishes in
HKSAR is high, but varies greatly among different
regions of the territory and between the wet and dry
seasons, with significant reduced richness in streams
fragmented by low-head dams (Chan et al., 2024).
Diadromous decapod species may exhibit similar
patterns, but it is not known if their climbing ability
might offset the barrier effects of dams. The present
study was a territory-wide survey of decapods in
coastal lowland steams of HKSAR intended to docu-
ment spatial and seasonal variability in assemblage
structure. Particular emphasis was placed on compar-
ing the effects of dams on diadromous decapod spe-
cies relative to the more sedentary primary freshwater
crabs and shrimps.
Materials andmethods
All surveys and statistical analyses follow protocols
established for sampling diadromous fishes by Chan
et al. (2024), where details of the survey sites and
environmental conditions are given (see also Sup-
plementary file 1). In brief, surveys were conducted
across 24 coastal streams in HKSAR (1114 km2;
22°17′N, 114°10′E; Fig.1; TableS1): four on Hong
Kong Island (HKI), 10 on Lantau Island (LTI), and
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10 in the mainland New Territories (NT). Streams
sampled and their codes (H1-N24; the first letter indi-
cates region; H = HKI; L = LTI; N = NT) correspond
to those in Chan etal. (2024). All streams were sur-
veyed once during the wet season (April–September
2022) with a subset of seven streams surveyed again
during the dry season (November 2022–March 2023)
to detect seasonal differences in diadromous assem-
blages and account for the monsoonal climate of
HKSAR (Dudgeon & Corlett, 2011). Three of those
seven streams (see Table S1) were also surveyed
repeatedly on three occasions during both the dry
and wet seasons to examine the extent of intra-sea-
sonal assemblage variation. These three streams were
selected to maximize spatial coverage of HKSAR,
with priority given to sites with the greatest species
richness according to wet season surveys.
Eight of the surveyed streams were free-flowing,
13 were fragmented by low-head dams (hereafter,
simply dams), two were fragmented by waterfalls
(11.5 and 12.3m high), and one had two dams and
a 10.2m tall waterfall (TableS1). The incidence of
barriers varied among regions, with HKI the least
obstructed (two dams in one stream), followed by LTI
(10 dams in six streams) and NT (nine dams and three
waterfalls in eight streams).
Survey methods
The main survey was conducted during the wet sea-
son, which has been reported to be the peak period
of abundance for diadromous decapods in East Asia,
including Caridina spp. and Macrobrachium nippon-
ense in Japan (Yatsuya etal., 2012, 2013) and Eri-
ocheir japonica in Taiwan (Chen etal., 2014). During
Fig. 1 Map of study area, the Hong Kong Special Adminis-
trative Region of China (HKSAR), showing the three regions
surveyed. Exact locations of sampling sites are not displayed as
some diadromous decapods are poached locally. Please contact
the corresponding author if you require further information
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each visit to the 24 streams, 400m of stream course
was surveyed, immediately upstream of the high tide
level at the stream mouth. The 400 m length was
based on trial surveys of diadromous decapod spe-
cies undertaken in 2021 and its suitability was later
confirmed by examination of species accumulation
curves (see below). The sampling reach was divided
into four continuous 100 m transects, which were
each sampled for 30 min. Adjustments were made
when barriers were present within a transect. For
example, if a barrier was encountered 60 m along
a 100 m transect, the sections above and below the
barrier were sampled for 18min (60m) and 12min
(40 m), respectively. On each visit, the survey
was conducted during the day (between 14:00 and
17:30h) and repeated that night (between 18:30 and
22:00h) to account for diurnal and nocturnal species.
Decapod species were captured using hand nets
(30 × 30 cm, 2-mm mesh size). Two surveyors
worked along either side of the stream reach in each
100 m transect before sampling in midstream. Nets
were swept over the stream bed, with particular care
taken to collect animals lurking within vegetation
(including emergent and submerged macrophytes),
trailing roots, and leaf litter. A cast net (1.8m radius;
mesh size 6mm) was deployed three times in pools
deeper than 1.5 m to supplement collections made
with hand nets. In sufficiently deep streams and deep
pools, single-pass snorkel surveys were conducted a
day after netting to determine if any additional spe-
cies had eluded capture. The snorkel surveys followed
the procedure used by Tsang & Dudgeon (2021) for
stream fishes in HKSAR. Decapod species were col-
lected in accordance with the Hong Kong Agricul-
ture, Fisheries, and Conservation Department permit
((168) AF GR CON 11/17 pt.6). All captured deca-
pods were identified to species level (following Wong
et al., 2021; Chow et al., 2022). When necessary,
photographs were taken, or specimens collected, for
detailed examination in the laboratory. Life cycle cat-
egorization of each species followed de Mazancourt
& Ravaux (2024) for shrimps and primary literature
sources (e.g., Wong etal., 2021) for crabs.
Species accumulation curves
Species accumulation curves were drawn up for each
stream to examine whether the sampling effort ade-
quately reflects the whole assemblage of diadromous
species present. The presence of diadromous decapod
species was pooled for each 100m transect, and sam-
ple-based species accumulation curves with the Mao
Tau estimator (see Colwell etal., 2012) were used to
assess the adequacy of sampling effort in each stream.
Analyses were carried out in the R environment (ver-
sion 1.4.1106; R Core Team, 2021) using the func-
tion ‘specaccum’ from the vegan package (Oksanen,
2010).
Data normalization and rarefaction
To account for the imbalance in sampling effort (four
streams surveyed on HKI but 10 each on LTI and
NT), species richness data from LTI and NTI were
normalized to match HKI. This was done by first
pooling decapod species presence data in each region.
Then, rarefaction was conducted in R using the func-
tion ‘rarefy’ from the vegan package (Oksanen,
2010), thereby providing the expected species rich-
ness of a specified region based on Hurlbert (1971).
Rarefaction was performed for all decapod species
(irrespective of life cycle, hereafter referred to as total
decapods) and diadromous decapod species (hereafter
referred to as diadromous decapods) only.
Influence of regions on assemblage composition
All analyses utilized species presence/absence data
and the Sørensen index to calculate the ratio of unique
and shared species among samples (Bakker, 2024).
Assemblages of total decapods and diadromous deca-
pods were analyzed separately. Non-metric multidi-
mensional scaling (NMDS) was first used to compare
species composition among regions with differences
tested by one-way analysis of similarities (ANO-
SIM; Bakker, 2024). If a significant difference was
detected, the similarity percentages breakdown (SIM-
PER) procedure (Bakker, 2024) was used to identify
species making major contributions to regional dif-
ferences. All multivariate analyses were performed in
the R environment, with the vegan package (Oksanen,
2010) for NMDS, ANOSIM, and SIMPER, and the
‘factomineR’ package (Lê etal., 2008) for PCA.
Influence of barriers on assemblage composition
Assemblages of total decapod and diadromous deca-
pods were analyzed separately. NMDS and ANOSIM
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were used to compare and test for significant differ-
ences in the composition of decapod species in the
first (0–100m) and last transect (300–400m) of unob-
structed (seven) and dam-obstructed (13; Fig.2a–b)
streams. If a significant difference was detected, SIM-
PER was used to identify species making major con-
tributions to differences between transects. The three
streams with waterfalls (Fig.2c) were excluded from
these analyses, but descriptions of their decapod spe-
cies assemblages are given below.
Results
Evaluating survey coverage
Species accumulation curves based on hand-net sam-
ples had reached an asymptote by the third 100 m
transect (i.e., 300m upstream; FiguresS1a–c) in all
24 study streams, with one exception: a single mitten
crab Eriocheir japonica was found in the final tran-
sect of Stream L6, but was not present in the three
downstream transects (FigureS1b). Cast-netting con-
ducted in deep pools did not yield any species that
were not recorded during hand-net surveys, and no
additional species were recorded during single-pass
snorkel surveys. Overall, these findings confirm that
hand-net samples from four 100-m transects in each
stream yielded representative collections of decapod
assemblages.
Assemblage composition
Twenty-three species of decapods were recorded
during the wet season surveys (Table 1); 13 were
diadromous (10 amphidromous, 3 catadromous; Fig-
ureS2). The five families collected were dominated
by shrimps: Atyidae (eight species), Palaemonidae
(seven), Sesarmidae (three), Varunidae (three), and
Potamidae (two). The highest richness of total deca-
pods was recorded in H2 (10 species; Table2), H4
and L9 (nine species each), and N21 and N24 (eight
species each). Two unobstructed streams (H2 and
H4) had the highest richness of diadromous decapods
(seven species; Table2), followed by L9 and N17 (six
species), and H3, L7, N21, and N24 (five species).
No decapods were found in H1 and so the site was
excluded from subsequent analyses. In the 23 streams
where decapods were present, diadromous species
comprised over half (mean = 67%) of the decapods
encountered. However, only Macrobrachium for-
mosense was present in L11 and L13. This shrimp
was the most widespread decapod (occurring in 96%
or streams sampled), followed by Caridina elongap-
oda (75%) and Eriocheir japonica (58%; Table 2).
The other 11 diadromous decapods occurred in fewer
Fig. 2 Example of barriers
from Hong Kong streams: a
dam < 2m tall; b dam > 2m
tall; and c waterfall > 10m
tall. Photographs by J.C.F.
Chan
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than half of the streams, and six were confined to a
single stream: M. lar (H2); Varuna litterata (H4),
M. lantau (L9), M. nipponense (L10), C. gracilipes
(L14), and C. sp. (N17).
Seasonal dynamics
All decapods recorded were present during the
wet season, and dry season surveys of a subset of
seven streams did not yield any additional spe-
cies (Table S2). There was little seasonal variation
in assemblage composition and, of the eight deca-
pod species collected from streams sampled in both
seasons, only one (M. lar) was not found in the dry
season. Nor was there any evidence of intra-seasonal
variation during wet season surveys of two (L13 and
N20) out of three streams sampled repeatedly, but H2
differed in that M. lar was not found on one out of
three occasions, although the presence of the other
six decapod species in this stream was unchanged.
Regional distribution
Total decapod richness was highest on LTI (16 spe-
cies), followed by NT (13) and HKI (12) (Table2).
After data normalization to account for survey effort,
HKI had the highest richness, followed by LTI (11
species) and NT (10 species). Nine decapod species
were present in all regions, while euryhaline Chiro-
mantes haematocheir was present in both HKI and
LTI, with the remaining 13 species not shared among
regions. More decapod species were restricted to LTI
(six species) than NT (four) and HKI (three). Rich-
ness of diadromous decapods was highest on LTI (10
species), followed by HKI (nine), and NT (eight).
After data normalization to account for survey effort,
Table 1 Freshwater decapod species recorded during 2022 wet season surveys of 24 coastal streams in HKSAR
Names in bold are diadromous species. Region abbreviations: HKI Hong Kong Island, LTI Lantau Island, NT New Territories. Life
cycles according to Wong etal. (2021) and de Mazancourt & Ravaux (2024)
Family Species Life cycle Frequency of occur-
rence % (Number of
streams)
Region
ATYIDAE Caridina cantonensis (Yü, 1938) Primary 92 (22) HKI, LTI, NT
C. elongapoda (Liang & Yan, 1977) Amphidromous 75 (18) HKI, LTI, NT
C. gracilipes (De Man, 1892) Amphidromous 4 (1) LTI
C. lanceifrons (Yu, 1936) Primary 13 (3) NT
C. serrata (Stimpson, 1860) Primary 29 (7) HKI, LTI, NT
C. trifasciata (Yam & Cai, 2003) Primary 4 (1) NT
C. typus (H. Milne Edwards, 1837) Amphidromous 21 (5) HKI, LTI, NT
C. sp. Amphidromous 4 (1) NT
PALAEMONIDAE Macrobrachium equidens (Dana, 1852) Amphidromous 13 (3) HKI, LTI, NT
M. formosense (Spence Bate, 1868) Amphidromous 96 (23) HKI, LTI, NT
M. laevis (Zheng, Chen & Guo, 2019) Primary 4 (1) LTI
M. lantau (Chow, Chan & Tsang, 2022) Amphidromous 4 (1) LTI
M. lar (Fabricius, 1798) Amphidromous 4 (1) HKI
M. meridionale (Liang & Yan, 1983) Amphidromous 29 (7) HKI, LTI, NT
M. nipponense (De Haan, 1849) Amphidromous 4 (1) LTI
POTAMIDAE Sinolapotamon anacoluthon (Kemp, 1918) Primary 4 (1) LTI
Nanhaipotamon hongkongense (Shen, 1940) Primary 4 (1) HKI
SESARMIDAE Chiromantes haematocheir (De Haan, 1833) Euryhaline 13 (3) LTI, NT
Orisarma patshuni (Soh, 1978) Euryhaline 4 (1) NT
Hemigrapsus takanoi (Asakura & Watanabe, 2005) Euryhaline 4 (1) LTI
VARUNIDAE Eriocheir japonica (De Haan, 1835) Catadromous 58 (14) HKI, LTI, NT
Varuna litterata (Fabricius, 1798) Catadromous 4 (1) HKI
V. yui (Hwang & Takeda, 1986) Catadromous 46 (11) HKI, LTI, NT
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HKI had the highest richness (nine species), followed
by LTI (eight) and NT (seven). Seven diadromous
decapods were present in all three regions, while
the other six species were not shared among regions
(Table2). More diadromous decapods were restricted
to LTI (three species) than HKI (two) or NT (one).
The NMDS biplot of total decapods did not reveal
any regional differences, with all three ellipses over-
lapping (Fig.3a: ANOSIM R2 = 0.02, p = 0.40). The
composition of assemblages in H3 and L7 were the
same, as were those in L6, N15, and N20. The NMDS
biplot of diadromous decapods likewise did not differ
among regions (Fig.3b: ANOSIM R2 = 0.04, p = 0.3)
and assemblages in three pairs of streams (H3 and
L7; L11 and L13; L12 and N16) were the same, as
was the case for another group of six streams (L6, L8,
N15, N18, N20, and N22).
Barrier effects on diadromous species
Among non-diadromous decapods, only Nanhaipota-
mon hongkongense was confined to a free-flowing
stream, and the presence of other nine species were
relatively unaffected by barriers (Table3). In com-
parison, nearly half (six species) of diadromous deca-
pods were confined to unobstructed streams and M.
lantau was recorded below dams (Table 3). Of the
other six diadromous species, five were able to trav-
erse waterfalls and dams, but V. yui only traversed
dams. The mean proportion of diadromous decapods
was the highest in unobstructed streams (74% of the
total assemblage) and was slightly lower in streams
with waterfalls (63%) or with dams (62%).
Total decapod mean richness declined with bar-
rier presence: unobstructed streams (6.4 species,
Table 2 Decapod
assemblages in coastal
streams in HKSAR
surveyed during the 2022
wet season; proportions
in bold represent streams
where at least half of the
species recorded were
diadromous
Stream code Diadromous decapod
richness
Total decapod rich-
ness
Proportion diadro-
mous decapods (%)
Hong Kong Island
H1 0 0 0
H2 7 9 78
H3 5 6 83
H4 7 10 70
Lantau Island
L5 2 3 67
L6 3 4 75
L7 5 6 83
L8 4 6 67
L9 6 9 67
L10 3 4 75
L11 1 3 33
L12 2 3 67
L13 1 4 25
L14 4 6 67
New Territories
N15 3 4 75
N16 2 4 50
N17 6 7 86
N18 3 5 60
N19 3 4 75
N20 3 4 75
N21 5 8 63
N22 4 5 80
N23 3 6 50
N24 5 8 63
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Fig. 3 NMDS ordination plots with ellipses enclosing assem-
blages of decapod species in streams within each region of
HKSAR for a total decapods (stress value = 0.14) and b dia-
dromous decapods (stress value = 0.14). Abbreviations:
HKI = Hong Kong Island; LTI = Lantau Island; NT = New Ter-
ritories; HKI & LTI = Hong Kong Island and Lantau Island
overlapping assemblage; LTI & NT = Lantau Island and New
Territories overlapping assemblages
Table 3 Presence of diadromous decapods in relation to natural and man-made barriers along 24 study streams (13 of them unob-
structed) in HKSAR
*Species present below waterfalls but absent above
#Present above both dams and a waterfall within a single stream
Free-flowing streams only Below dams Above & below dams Above & below
dams & waterfalls
Non-diadromous
Nanhaipotamon hongkongense Caridina trifasciata Macrobrachium laevis C. cantonensis
Chiromantes haematocheir Sinolapotamon anacoluthon C. lanceifrons
Hemigrapsus takanoi C. serrata
Orisarma patshuni
Diadromous
C. gracilipes M. lantau V. yui* C. elongapoda
C. sp. C. typus
M. equidens M. meridionale
M. lar M. formosense#
M. nipponense Eriocheir japonica
Varuna litterata
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range 4–10); streams with dams (5 species, range
3–9); streams with waterfalls (6 species, range 4–8);
and stream with a waterfall and dams (6 species).
The number of dams in a stream did not have consist-
ent impacts on mean richness: one dam (4.8 species,
range 3–8); two dams (5.8 species, range 3–9); and
three dams (1 species, 1 stream). However, the height
of the first barrier had more discernible impacts:
streams with dams < 2 m (5.8 species, range 3–9);
streams with dam > 2m (3.8 species, range 3–4); and
streams with waterfalls > 10 m (6.0 species, range
4–8).
Mean richness of diadromous decapods was more
than 30% higher in unobstructed streams (4.9 species,
range 3–7) than those with dams (3.2 species, range
1–6) and were lowest in the streams with dams and
a waterfall (3 species), although the effect of water-
falls only (4.0, range 3–5) were less marked. As with
total decapods, an increase in the number of dams
was not invariably associated with a greater reduction
in mean species richness, and streams with a single
dam had a lower mean richness (2.5 species, range
1–4) than streams with two (4.0 species, range 1–5)
or three dams (3.0 species). The height of the most
downstream barrier affected richness of diadromous
decapods: streams with dams < 2 m in height had
slightly lower mean richness (3.9 species, range 1–6)
than unobstructed streams (4.9 species), but rich-
ness fell to only 2.2 (range 1–3) species when dams
were > 2 m in height. For comparison, streams with
taller waterfalls (> 10 m in height) had comparable
mean richness (3.7 species, range 3–5) to streams
with low (< 2m) dams.
Up- vs downstream differences in streams with and
without dams
There were no significant differences in total decapod
assemblages between the first and last transects in
unobstructed streams (ANOSIM, R2 = 0.07, p = 0.23,
Fig.4a), but the assemblages in the first and last tran-
sects of dam-obstructed streams were well separated
(ANOSIM, R2 = 0.38, p = 0.02). In general, down-
stream assemblages were more varied than upstream
Fig. 4 NMDS ordination plots of first and last transects of a
total decapod and b diadromous decapod species assemblages
in 20 streams of HKSAR. Stress value = 0.10. Overlapping
points with both colors indicate first and last transects with
identical assemblages
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sections, with downstream assemblages having eight
(of 13 streams) identical assemblages. SIMPER
revealed that 100% of pairwise differences among
the first and last transects of dam-obstructed streams
were driven by 10 species (Table 4) with two dia-
dromous decapods (C. elongapoda and E. japonica)
and one primary freshwater shrimp (C. cantonensis)
ranked as top contributors. Caridina elongapoda and
E. japonica were recorded more frequently in down-
stream transects, whereas C. cantonensis more often
occurred upstream.
The NMDS biplot revealed that diadromous deca-
pod assemblages differed between the first and last
transects of dam-obstructed streams (ANOSIM,
R2 = 0.37, p = 0.05; Fig. 4b) and were more species-
rich downstream, but this was not the case in unob-
structed streams (ANOSIM, R2 = 0.06, p = 0.24).
However, there was considerable overlap in species
assemblages within and between dam-obstructed
streams (Fig. 3b). More than half of the dam-
obstructed streams had identical upstream assem-
blages, with the similarity driven by M. formosense,
which was found in all streams and was frequently the
only diadromous species present (as represented by
the major point of convergence in Fig.3b). Two other
diadromous decapods, C. elongapoda and Eriocheir
japonica, also traversed dams and were present in the
most upstream transect in some streams. SIMPER
revealed that 100% of pairwise differences among
first and last transects of dam-obstructed streams were
driven by seven species (Table4), with C. elongapoda
and E. japonica contributing to the majority (62%) of
variation. Moreover, streams with fish passes in dams
(L5 and N15) had mixed results and only allowed the
passage of strong climbers (M. formosense, M. merid-
ionale, C. elongapoda, E. japonica).
Discussion
Decapod species richness and composition
A total of 30 species are now known from streams
in HKSAR, including seven species that were not
encountered during the present surveys but have been
reported in the literature. Because our surveys were
confined to lowland streams (all transects surveyed
were < 72 m ASL), we did not record six primary
Table 4 Results of
SIMPER analysis
comparing diadromous and
total decapod assemblages
between first and last
transects of 13 dam-
obstructed streams in
HKSAR during the 2022
wet season
Species Average
contribution (%)
Frequency of occurrence (%) Cumulative
contribution (%)
First transect Last transect
Total assemblage
C. elongapoda 11.9 83.3 16.7 23.4
C. cantonensis 10.3 33.3 83.3 43.5
E. japonica 7.8 50.0 16.7 58.8
V. yui 6.3 50.0 0.0 71.3
C. serrata 5.6 0.0 33.3 82.3
C. typus 2.0 16.7 0.0 86.2
Hemigrapsus takanoi 1.8 16.7 0.0 89.6
Chiromantes haematocheir 1.8 16.7 0.0 93.1
Ma. equidens 1.8 16.7 0.0 96.5
M. lantau 1.8 16.7 0.0 100.0
Diadromous assemblage
Caridina elongapoda 16.2 83.3 16.7 37.8
Eriocheir japonica 10.2 50.0 16.7 61.5
Varuna yui 8.5 50.0 0.0 81.4
Caridina typus 2.7 16.7 0.0 87.6
Macrobrachium equidens 2.7 16.7 0.0 93.8
M. lantau 2.7 16.7 0.0 100.0
M. formosense 0.0 100.0 100.0 100.0
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freshwater decapod species (hereafter referred to as
freshwater decapods) known from higher elevations
or inland wetlands: Caridina logemanni, C. ngan-
keeae, Neocaridina palmata, Macrobrachium venus-
tum (Chow etal., 2018, 2022, 2024), Nanhaipotamon
aculatum (Stanton & Leven, 2016), and Somma-
niathelphusa zanklon (Ng & Dudgeon, 1992). Fur-
thermore, the catadromous crab, Eriocheir hepeuen-
sis was not found in the present study as it is largely
confined to marine habitats (Wong et al., 2021).
Nearly half of the freshwater decapods in HKSAR
(1114 km2) are diadromous, and the tally of 14 spe-
cies recorded is substantially higher than neighboring
Shenzhen (six species in 2050 km2; Huang & Mao,
2021), Macau (one species in 118 km2; Zhou et al.,
2021), or Hainan Island (eight species in 35,400 km2;
Guo & He, 2008; Cai, 2014; Hao etal., 2021). These
findings suggest that HKSAR is a potential hotspot
for diadromous decapods, although we acknowledge
this could be an artefact of the limited sampling of
these animals in coastal China. Species richness
was comparable to Puerto Rico (17 species in 9104
km2; Perez-Reyes et al., 2013), but lower than Tai-
wan (approximately 39 species in 36,197 km2; Shy &
Yu, 1999; Chou etal., 2020) or the oceanic Solomon
Islands (24 species of atyid shrimps in 28,446 km2; de
Mazancourt etal., 2020). Importantly, species accu-
mulation curves from the present study reached an
asymptote in all streams surveyed, indicating that the
samples of diadromous decapods were representative.
The high richness of diadromous decapods in
HKSAR was unexpected, as lowland watercourses
have been greatly degraded by dams, channelization,
historic pollution, and invasion by non-native spe-
cies (Dudgeon, 1996; Tsang & Dudgeon, 2021; Chan
etal., 2023). This has led to population declines of
some freshwater animals; for instance, the endemic
Caridina apodosis has not been reported since
its description by Cai & Ng (1999). However, the
streams sampled in the present study appeared unpol-
luted and none were channelized, although dams were
present along 13 out of 24 streams. Despite this, six
species of diadromous decapods (nearly half the total)
were encountered in a single stream only. These ‘rare’
species included C. gracilipes, an atyid recorded in
HKSAR for the first time, as well as a new species of
palaemonid, M. lantau (Chow etal., 2022).
The substantial proportion of ‘rare’ species observed
may be because HKSAR is positioned at the northern
edge of the Tropic of Cancer where tropical and tem-
perate species mix (Dudgeon & Corlett, 2011). The
same mixing phenomenon is also likely to explain the
high richness of diadromous fishes locally (Chan etal.,
2024). Five diadromous decapods appeared to be at
their distributional limits (GBIF.org, 2023). Three of
them (C. elongapoda, C. propinqua, and M. equidens)
were tropical species at the northern limits of their
range, while two others (M. formosense and E. japon-
ica) were temperate species at their southern range lim-
its. Of the marginal species, only C. propinqua had a
restricted distribution in HKSAR, and the remaining
species were widespread locally. In comparison, diadro-
mous fishes at their range limits tended to be rare, with
10 of 14 species recorded at single localities, indicating
that they could be susceptible to local extinction. This
is supported by the fact that four diadromous fishes for-
merly present in HKSAR (Plecoglossus altivelis, Rhy-
acichthys aspro, Sicyopterus longifilis, Sicyopus zoster-
ophorus) were not encountered during comprehensive
surveys in 2022 and 2023 (Chan etal., 2024). Unfor-
tunately, comparable historic records for decapods are
lacking, so we are unable to determine whether there
have been changes in their richness or distribution in
streams in HKSAR.
Temporal and spatial variations
There were no seasonal variations in total decapod
assemblages, in part because primary freshwater
decapods in HKSAR are present year-round (e.g.,
Mantel & Dudgeon, 2004a; Yam & Dudgeon, 2006).
However, there was likewise no seasonal variations in
the occurrence of diadromous species notwithstand-
ing territory’s monsoonal climate (HKO, 2023) and
seasonal alteration of sea currents (Morton & Wu,
1975). The only exception was M. lar, which was
not recorded during the dry season despite being
encountered in two out of three wet season surveys.
This shrimp is locally rare, with no more than a single
individual recorded in each survey, so their absence
in dry season surveys may have been a chance event.
The absence of decapod species seasonality in
HKSAR contrasts markedly with diadromous fishes
that were almost twice as rich during the wet season
(27 vs 14 species; Chan etal., 2024). As both taxa are
predominantly amphidromous and live in streams as
adults (71 and 77% of diadromous fishes and deca-
pods, respectively), this inconsistency in seasonal
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trends cannot be attributed to life-cycle differences
and, instead, may reflect a higher tolerance of deca-
pods for reduced flows during the dry season. For
example, an ability to store water in brachial cham-
bers (Skudlarek etal., 2011) allows decapods to crawl
overland (Hongjamrassilp etal., 2021; Herborg etal.,
2003) making them less susceptible than fishes to
droughts or intermittent surface flows.
There were no significant differences in total deca-
pod assemblages in different regions of HKSAR,
which were all dominated by three diadromous spe-
cies (M. formosense, Eriocheir japonica, Varuna yui)
and a primary freshwater shrimp (C. cantonensis).
This spatial homogeneity is in agreement with evi-
dence of panmixia in local populations of C. elon-
gapoda and C. typus (Ma etal., 2021), and the broad
regional geographic distributions and limited genetic
structuring of other diadromous decapods, such as
M. lar (Castelin et al., 2013) and E. japonica (Xu
etal., 2009), found in HKSAR. In contrast, however,
there were marked differences in the diadromous fish
assemblages on Hong Kong Island, Lantau Island,
and the New Territories (Chan etal., 2024). We can
offer no obvious explanation for the disparity between
their distribution patterns of diadromous fishes and
their decapod counterparts.
Effects of barriers on species richness and
assemblages
Overall, mean richness of total decapods in streams
declined in the presence of dams—an effect driven
by nearly half of diadromous species that were con-
fined to unobstructed streams (see Table3). When
categorized by height, dams exceeding 2 m in
height had fewer species of decapods than streams
with dams < 2m tall (3.8 vs 5.8 species) or streams
that had waterfalls (6.0 species), even though some
were over 10m tall. The presence of dams reduced
mean richness of diadromous decapods (from 4.9 to
3.2 species), which is consistent with findings from
Japan (Suzuki etal., 1993) and Puerto Rico (Hol-
mquist etal., 1998). Likewise, dams over 2m tall
resulted in lower mean richness (2.2 species) than
small dams (3.9 species) or waterfalls (3.7 species).
Macrobrachium formosense, E. japonica, and, occa-
sionally, C. elongapoda and M. meridionale were
the only diadromous decapods found upstream of
dams > 2m tall. However, C. typus and V. yui were
capable of traversing small dams and waterfalls,
although the remaining five species were restricted
to free-flowing streams, indicating likely interspe-
cific differences in climbing ability.
Both total decapod assemblages, and those of
diadromous species, differed significantly above
and below dams. Upstream assemblages of total
decapods were more complex (Fig. 4) due to the
persistence of primary freshwater species (e.g.,
C. cantonensis, C. serrata, and C. lanceifrons)
that were sometimes absent from transects below
dams. However, assemblages of diadromous deca-
pods upstream of dams tended to be depauperate
and similar to each other, often including M. for-
mosense only, while those below dams were more
speciose; the same trend has been recorded in Japan
(Satake & Ueno, 2013). Species recorded above
dams, such as E. japonica and M. formosense, are
known to be proficient climbers (Kobayashi, 2003;
Watanabe & Kano, 2009). Diadromous decapods
in HKSAR ascended further upstream than diadro-
mous fishes in Hong Kong streams, with M. for-
mosense present in the most upstream transect of
all 13 dammed streams, compared to only one fish,
Eleotris oxycephala, found in the last transect of a
single dammed stream. Although 10 species (36%)
of diadromous fishes were able to ascend at least
one dam, most were unable to ascend other dams
further upstream.
However, our focus on presences/absence data
may have underestimated the effects of dams,
since population sizes can be reduced upstream
of dams even though some individuals are able to
access these sites (Richardson etal., 2004; Olivier
et al., 2013). While the ecological consequences
of changes in decapod assemblages resulting from
dams in Hong Kong streams has yet to be evaluated,
diadromous decapods can affect stream food webs
(Yam & Dudgeon, 2006; de Mazancourt & Ravaux,
2024), litter decomposition (Crowl etal., 2001; Yam
& Dudgeon, 2005; Bobeldyk & Ramírez, 2007),
and sedimentation (Pringle etal., 1993). Important
indirect consequences of dams that blocked move-
ment by Macrobrachium spp. in West African rivers
included reduced predation pressure on snails that
were intermediate hosts for schistosomiasis, thereby
contributing to increased human parasite burdens
upstream (Sokolow etal., 2014).
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Conclusion
Despite its small area, HKSAR is a surprisingly
rich assemblage of 30 freshwater decapods, exceed-
ing neighboring regions and comparable to some
of those that are substantially larger. Moreover,
the tally of 14 diadromous decapods represents the
highest regional richness in China, indicating that
HKSAR may be a hotspot for diadromous biodi-
versity in the region, given the additional presence
of 32 diadromous fishes (Chan etal., 2024). China
has experienced widespread coastal degradation
(Tian etal., 2016) and, in the absence of informa-
tion about the conservation status of diadromous
animals in the nation, preservation of the assem-
blages in HKSAR would be prudent. More gener-
ally, diadromous decapods are understudied glob-
ally and regionally, with much baseline ecological
information yet to be collected. Long-term popula-
tion assessments of these animals, including factors
affecting recruitment, will be needed to inform their
conservation and management.
Dams in HKSAR obstruct both diadromous and
primary freshwater decapods and significantly reduce
species richness upstream. Fish passes have been
presumed to facilitate passage of decapods, but have
mixed responses in maintaining connectivity (Fièvet,
2000; Bauer, 2011). Indeed, although it is often
assumed that these animals can ascend dams or other
barriers (Bauer, 2011; Hongjamrassilp et al., 2021),
our findings shows that not all decapod species are
capable climbers. In cases where dam removal is not
possible, rigorous evaluation of fish passes before
and after installation will be needed in order to assess
dam porosity for all candidate species.
Acknowledgements We thank Alex Lau, Billy Lam, Yip
Hon Tim, Chiu Yui Hong, Samuel C. L. Ho, and Sandy S. M.
Yau for help in the field; Bi Wei Low and Juan Diego Gaitán-
Espitia for providing valuable advice on analyses; and Uva Y.
Y. Fung, Samuel C.L. Ho, Valentin de Mazancourt, and Koen
Martens for helpful comments that improved the manuscript.
Funding This study was partially supported by the Lantau
Conservation Fund (Grant Number: LCF-RE-2021-02).
Data availability Species localities are hidden for conserva-
tion purposes. For access to data please contact jeffe rychan@
conne ct. hku. hk.
Declarations
Conflict of interest Jeffery C. F. Chan developed the research
concept, study design, data collection, data analysis, and drafted
the manuscript. Jia Huan Liew and David Dudgeon developed
the research concept and study design, edited the manuscript,
and supervised the study. The authors declare that there are no
competing financial or non-financial interests.
Open Access This article is licensed under a Creative Com-
mons Attribution 4.0 International License, which permits
use, sharing, adaptation, distribution and reproduction in any
medium or format, as long as you give appropriate credit to the
original author(s) and the source, provide a link to the Crea-
tive Commons licence, and indicate if changes were made. The
images or other third party material in this article are included
in the article’s Creative Commons licence, unless indicated
otherwise in a credit line to the material. If material is not
included in the article’s Creative Commons licence and your
intended use is not permitted by statutory regulation or exceeds
the permitted use, you will need to obtain permission directly
from the copyright holder. To view a copy of this licence, visit
http://creativecommons.org/licenses/by/4.0/.
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