DSpace at VNU: Food sources of macro-invertebrates in an important mangrove ecosystem of Vietnam determined by dual stable isotope signatures

Journal of Sea Research 72 (2012) 14–21

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Journal of Sea Research
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Food sources of macro-invertebrates in an important mangrove ecosystem of Vietnam
determined by dual stable isotope signatures
Nguyen Tai Tue a, b,⁎, Hideki Hamaoka b, Atsushi Sogabe c, Tran Dang Quy d, Mai Trong Nhuan d, Koji Omori b
a

Graduate School of Science and Engineering, Ehime University, 2‐5 Bunkyo-cho, Matsuyama, Japan
Center for Marine Environmental Studies, Ehime University, 2‐5 Bunkyo-cho, Matsuyama, Japan
Graduate School of Biosphere Science, Hiroshima University, 1-4-4 Kagamiyama Higashi-Hiroshima 739‐8528, Japan
d
Faculty of Geology, Hanoi University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam
b
c

a r t i c l e

i n f o

Article history:
Received 12 February 2012
Received in revised form 24 April 2012
Accepted 9 May 2012
Available online 16 May 2012
Keywords:
Stable Isotopes

Invertebrate
Food Sources
Mangrove Ecosystem
Vietnam

a b s t r a c t
Dual stable isotope signatures (δ13C and δ15N) were applied to determine the contribution of mangrove materials and other organic carbon sources to the invertebrate community in an ecologically important mangrove ecosystem of Vietnam. We have analyzed 181 specimens of 30 invertebrate species and found δ13C
and δ15N ranging from − 14.5 to − 26.8‰ and from 1.3 to 12.1‰, respectively. From taxa measured for stable
isotopes, polychaete, gastropods, bivalves, and grapsid crabs living in mangrove forest showed relative low
δ13C values, while fiddler crabs inhabiting in the land–water ecotone showed the highest δ13C values. The
δ13C showed that just a few mangrove inhabitants directly relied on the mangrove materials. The wide ranges
of δ13C and δ15N signatures indicated that the invertebrates utilized heterogeneous diets, comprising benthic
microalgae, marine phytoplankton, particulate organic matter, sediment organic matter, mangrove detritus,
and meiofauna and rotten animal tissues as the supplemental nutrient food sources. Moreover, the significant
correlation between δ13C values and body sizes of invertebrates showed that snails Littoraria melanostoma
and Terebralia sulcata, bivalve Glauconome virens, and portunid crab Scylla serrata exhibited ontogenetic shifts
in diets. The present study showed that adjacent habitats such as tidal flat and mangrove creeks seem to contribute an important microalgal food resource for invertebrates and highlighted the need for conservations of
mangrove forests and the adjacent habitats.
© 2012 Elsevier B.V. All rights reserved.

1. Introduction
Mangrove forests have often been regarded as one of the most
productive ecosystems in the (sub)tropical coastal waters. They are
characterized by the high biodiversity of invertebrates and fish
(Nagelkerken et al., 2008; Sasekumar et al., 1992). Several hypotheses
have been proposed for the explanation of the high biodiversity of
the invertebrates and fish in the mangrove ecosystem, including
(1) mangroves characterize by the structural complexity of pneumatophores and/or prop roots which provide shelter from predators for
invertebrates and fish (Kon et al., 2009), and (2) mangroves produce
large amounts of organic matter that form the basis of the estuarine

food webs (Hogarth, 2007; Nagelkerken et al., 2008; Odum and
Heald, 1972). Therein, the later hypothesis has long been debated between non-stable isotopic (i.e., stomach content analysis, fecal analysis, and direct observation (Michener and Lajtha, 2007)) and isotopic

⁎ Corresponding author at: 790‐8577 Center for Marine Environmental Studies,
Ehime University, 2‐5 Bunkyo-cho, Matsuyama, Japan. Tel.: + 81 89 927 9643, + 81
902 894 1610 (Cell); fax: + 81 89 927 9643.
E-mail address: (N.T. Tue).
1385-1101/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.seares.2012.05.006

studies. Based on stomach content analysis several studies have demonstrated that the mangrove detritus contributes a significant amount
of organic carbon fueling detrital-based food webs (Nanjo et al., 2008;
Nordhaus et al., 2011; Odum and Heald, 1972). Nevertheless, the isotopic studies have failed to confirm the ingestion of mangrove materials in
the estuarine food webs (France, 1998; Rodelli et al., 1984), apparently
because the mangrove detritus is too refractory and the simple ingestion of mangrove detritus does not indicate any direct assimilation of
that material (Fry and Ewel, 2003).
Invertebrates that play important roles in the mangrove structure
processes (Cannicci et al., 2008), organic carbon dynamics (Robertson
and Daniel, 1989), biogeochemical processes (Kristensen, 2000), are
preyed upon foraging fishes during high tides (Kruitwagen et al., 2010),
and serve as important links between mangrove detritus and estuarine
secondary production (Lee, 2008). The gastropods and brachyuran
crabs are the most dominant invertebrate groups in the mangrove ecosystem. They play an important role in leaf litter turnover, for example,
sesarmid crabs can consume approximately 70% of the total annual litter
fall from the forest floor (Robertson and Daniel, 1989), significantly
retaining mangrove organic production and reducing direct export.
Moreover, recent isotopic studies pointed out that several invertebrate
species rely on the mangrove materials and such use of mangrove carbon

N.T. Tue et al. / Journal of Sea Research 72 (2012) 14–21

depends on the morphological characteristics of mangrove forests
(Bouillon et al., 2004) and/or microhabitats (Kon et al., 2007).
Stable isotopes have been used to track organic matter flows and
to measure the food web structures, and ontogenetic shifts in diets
(Michener and Lajtha, 2007). Stable isotopes of carbon (δ 13C) and
nitrogen (δ15N) have frequently applied for tracing the flow of organic
matters in lake (Post, 2002), estuarine (Peterson et al., 1985), and marine (Michener and Lajtha, 2007) food webs. Therein, δ13C can be
used to evaluate the ultimate sources of carbon for an organism when
the isotopic signatures of the sources are different (Post, 2002). The
δ13C is enriched approximately 0.5–1‰ in the animal relative to its
diet. The basic assumption for estimation of trophic position is based
on a conservative enrichment of 3–4‰ δ15N in a consumer relative to
its diet (Michener and Lajtha, 2007). Stable isotope signatures (δ13C
and δ15N) are therefore powerful methods for determining the ingested
food sources and the food assimilation over long period of a consumer.
Stable isotope measurements have been applied to trace mangrove
detritus in the estuarine environment (Dittmar et al., 2001), to estimate
the assimilation of mangrove detritus by a specific species such as snail
(Penha-Lopes et al., 2009; Slim et al., 1997) and fiddler crabs (France,
1998). Yet, stable isotope analysis has been rarely applied to identify
food sources of a large range of invertebrate taxa in the mangrove ecosystem (i.e., Bouillon et al., 2002a, 2004; Kon et al., 2007; Kruitwagen
et al., 2010), particularly in developing countries such as Vietnam.
In the present study, we have analyzed dual stable isotope signatures (δ 13C and δ 15N) of 181 specimens of 30 invertebrate species
for figuring out the question what degree of mangrove materials is assimilated by invertebrates in an ecologically important mangrove
ecosystem of Vietnam. The objectives of the present study are (1) to
determine the utilization of food sources by invertebrates and (2) to
examine the question of whether these invertebrates exhibit the ontogenetic shifts in diets.
2. Materials and methods

2.1. Study area
The present study was conducted in an estuarine mangrove ecosystem of Xuan Thuy National Park (XTP) in northern Vietnam (Fig. 1). The
detailed description for the XTP has been shown elsewhere (Tue et al.,
2012, 2012). Briefly, XTP covers a total wetland area of 12,000 ha, of
which about 3000 ha are covered by mangrove forests. The most abundant mangrove species are Sonneratia caseolaris, Kandelia obovata,

15

Aegiceras corniculatum, and Avicennia marina. The mangrove ecosystem
provides a broad array of ecosystem services such as maintaining high
biodiversity, storm protection, erosion mitigation, local climate regulation, fishery production, and the subsistence livelihood. The economic
values of the mangrove ecosystem have been estimated from
31,565,720 to 34,620,100 VND/year (price in 2002, currency exchange
rate, US$1= VND15,300) (Nhuan et al., 2003). Therein, invertebrate
production contributes an important economic value and livelihood
for local communities, estimating VND30,000 per local person/day by
selling crabs, shrimps, and oysters (Nhuan et al., 2009). In the year
1989, XTP is declared as the first wetland of International importance
of Southeast Asia (). In the year 2004, through
UNESCO XTP is designated to be the most important sub-zone of
the Red River Delta Biosphere Reserve ().
The characteristics of seasons, tides, salinity, and other environmental conditions are well described elsewhere (Tue et al., 2012). Briefly,
the XTP has a distinct monsoon climate with a rainy season from June
to October, and a dry season from November to May. The tides are characterized with tidal amplitudes ranging from 1.5 to 1.8 m, and the maximum and minimum tidal levels are 3.6 m and 0.5 m, respectively.
The salinity ranges from 12.5 to 25.6‰ for the rainy season and from
21.1 to 26.5‰ for the dry season.
2.2. Field sampling
Field work was conducted from 28 January to 10 February 2008.
Invertebrates were collected at two sites in the XTP (Fig. 1a). The
sampling transect is started at the tidal flat and creek bank toward

the dense mangroves at sites 1 (Fig. 1b) and 2 (Fig. 1c), respectively.
The benthic invertebrate specimens were manually collected by hand
during low tides. Total invertebrate specimens were 181, belonging to
30 species (Table 2). The species of polychaete, bivalves (Geloina coaxans
and Laternula truncata), gastropods, and grapsid crabs were collected
within mangrove forests. The crabs of Leucosiidae, Ocypodidae families,
and portunid crab (Scylla serrata) were collected from fringe mangrove
forests, tidal flat, and creek banks. Prawns and portunid crab (Charybdis
helleri) were collected from Tra Creek by the gill nets during spring and
ebb tides. The number of invertebrate samples is shown in Table 2. These
invertebrates were selected in the present study because they are
predominant and high economic values in the XTP (Cuong and Khoa,
2004).
In general, the benthic microalgae (BMA) production is low in the
mangrove forests due to the high tannin concentration and low light

Fig. 1. Sampling sites within the mangrove ecosystem of Xuan Thuy National Park, Vietnam (a), the sampling transect at site 1 (b) and at site 2 (c). In each sampling transect the
habitats and invertebrate samples are shown (see the sampling methods for details). Refer to Table 2 for acronyms of macro-invertebrate samples.


16

N.T. Tue et al. / Journal of Sea Research 72 (2012) 14–21

penetration (Alongi, 1994). However, the BMA production has been
considered as an important food source for invertebrates in the mangrove ecosystem (Bouillon et al., 2002a; France, 1998). In present
study, the BMA samples were collected at five sites in the tidal flat
and creek bank to examine the contribution of this organic matter
source to the invertebrates. The BMA samples were extracted from
conspicuous layers on surface sediments according to the procedure

described by Bouillon et al. (2002a).
The invertebrate and BMA samples were packed in labeled polyethylene bags and immediately stored in iceboxes, transported to
the laboratory for further processing and were frozen at −20 °C
until analysis.
2.3. Sample preparation and analysis
In the laboratory, the polychaete species were kept alive for 24 h
in filtered seawater to allow them to excrete their gut contents. The
invertebrate specimens were washed by distilled water and wiped
with paper. And then, the carapace width of crabs and prawns and
shell size of mollusks were measured. The invertebrates were dissected and only their muscle tissues were taken for stable isotope analysis. The muscle tissues were dried at 60 °C for 24 h, and then ground
to a fine powder by an agate mortar and pestle.
Post et al. (2007) showed that δ 13C can be altered by changes in
lipid contents of invertebrates. In the present study, the lipid content
is extracted from the invertebrate tissues prior to stable isotope analysis. The pulverized muscle tissue was then placed in an Eppendorf
tube and immersed in a 2:1 chloroform:methanol solution for 24 h
to extract lipids. After lipid extraction, the muscle tissue was dried
in an electric oven at 60 °C for 24 h. BMA was treated with 1 N HCl
for δ 13C analysis according to the procedure for sediments described
by Tue et al. (2012), and sub-sample of BMA for δ 15N analysis was
not treated with acid.
The muscle tissues and BMA samples were packed in tin capsules
and analyzed for stable isotope signatures (δ 13C and δ 15N) by using a
stable isotope mass spectrometer (ANCA-GSL; Sercon Inc., UK). Stable
isotopic compositions are expressed by δ values, which are measured
as a ratio of the heavy to light isotopes in a sample relative to a standard
À
À
by an equation: δX ‰Þ ¼ Rsample =Rstandard −1Þ Â 1;000, where X is the
isotope value in permil (‰) and R is the ratio of the heavy to the light
isotope of the sample (Rsample) to a standard (Rstandard). For stable

carbon and nitrogen isotope ratios, R is 13C/12C and 15N/14N, respectively. The standards were Pee Dee Belemnite (PDB) limestone carbonate
for δ13C and atmospheric nitrogen for δ15N. During analysis processes,
L-histidine was used as certified reference material. Analytical errors
were 0.1‰ for δ 13C and 0.2‰ for δ 15N, respectively.
Linear regression analysis was used to examine the size-specific
shifts in δ 13C and δ 15N within each invertebrate species. The significance level of linear regression was 0.05 (p b 0.05) for each statistical
procedure.
3. Results and discussion
3.1. Background data for potential organic carbon food sources of
invertebrates
The characteristics of major primary producers and other potential
organic carbon food sources of invertebrates in the XTP have been
reported elsewhere (Tue et al., 2012, 2012). The stable isotope
compositions of the potential food sources are sumarized in Table 1.
The δ13C of the potential food sources ranged from −29.9± 0.5 to
−20.2± 0.6‰. The δ 13C values were increased as follows: mangrove
leaves, mangrove sediments, tidal flats, creek bank and bottom sediments, particulate organic matter (POM), marine phytoplankton, and
BMA. The δ15N values of the potential food sources ranged from 0.7 ±
0.6 to 3.9 ± 0.9‰. The δ 15N values showed an increasing trend from

Table 1
Stable carbon and nitrogen ratios of the potential organic carbon food sources in the
mangrove ecosystem of Xuan Thuy National Park, Vietnam.
Organic matter sources

ACR

δ13C

δ15N

n

References

0.9
0.6
0.2
0.3
0.9
0.6
0.4

9
9
8
3
24
5
12

Tue et al.
(2012)

3.9
3.3

0.9
1.1

5
5

3.4

1.2

13

Mean

SD

Mean

SD

− 27.2
− 27.3
− 29.9
− 21.2
− 23.9
− 20.2
− 25.9

0.9
0.6
0.5
0.5
0.8

0.6
1.4

1.6
0.7
2.4
3.6
2.5
2.3
4.3

Cbs
Bcs

− 24.1
− 24.0

1.2
0.9

Tfs

− 24.2

1.0

Mangrove leaves
Kandelia obovata
Aegiceras corniculatum
Sonneratia caseolaris

Marine phytoplankton
Creek POM
Benthic microalgae
Mangrove sediments

Mang
Kao
Aec
Soc
Phyto
POM
BMA
Msed

Adjacent habitat
sediments
Creek bank sediments
Bottom creek
sediments
Tidal flat sediments

Ased

This study
Tue et al.
(2012)

Tue et al.
(2012)

ACR: acronym; n: number of samples; SD: 1 standard deviation.

mangrove leaves, though to BMA, POM, phytoplankton, and to creek
bank and mangrove sediments. The stable isotope compositions of
BMA from the present study were consistent with previous reports in
mangrove ecosystems from Coringa Sanctuary, India (Bouillon et al.,
2002a, 2004) and Sikao Creek, Thailand (Kon et al., 2007).
3.2. Food sources of invertebrates in the mangrove ecosystem
3.2.1. Stable isotope compositions of invertebrates
From the taxa measured for stable isotopes, the δ 13C values of invertebrates ranged from −14.5 to −26.8‰ (Table 2, Figs. 2 and 3).
The lowest δ 13C values were expressed in two polychaete species,
followed by bivalves L. truncata and G. coaxans, grapsid crabs Episesarma
versicolor and Metopograpsus messor, snails Cassidula aurisfelis, Cerithidea
ornata, and Littoraria melanostoma, and grapsid crab Sesarma dehaani.
Highest δ 13C values were expressed in ocypodid crabs Uca acuta,
Uca borealis, Uca flammula, grapsid crab Metaplax elegans, and followed
by ocypodid crab Uca urvillei, slug Onchidiidae spp., crab of family
Leucosiidae, ocypodid crab Uca arcuata, the portunid crab C. helleri, the
grapsid crab Metaplax longipes, prawns, and the portunid crab S. serrata
(Table 2, Fig. 2).
The δ15N values ranged from 1.3 to 12.1‰, the grapsid crab M.
elegans expressed the lowest average δ15N value, followed by the snails
L. melanostoma, C. ornata, C. aurisfelis, the grapsid crabs M. messor and S.
dehaani, slug Onchidiidae spp., and bivalves L. truncata and G. coaxans.
Highest δ15N values were expressed in prawns of Palaemonidae and
Penaeidae families, and polychaete (Diopatra neapolitana), followed
by the portunid crabs C. helleri and S. serrata, polychaete (Nephthys
polybranchia), fiddler crabs, and tidal flat bivalves Glauconome virens
and Ensis magnus (Fig. 2, Table 2).
3.2.2. Food sources of invertebrates in mangrove ecosystem

3.2.2.1. Polychaete. The polychaete N. polybranchia had the δ 13C value
in the range of mangrove leaves (Fig. 2). The δ 13C value of polychaete
D. neapolitana was similar to that of mangrove sediments or more
enriched 1–2‰ than that of mangrove leaves. These results indicated
that two polychaete species may obtain the carbon food sources from
mangrove materials such as decomposed leaves on the forest floor and/
or scavenge mangrove detritus in the mangrove sediments. δ13C signatures of polychaete confirmed that the feeding guild of D. neapolitana is
herbivore and/or scavenger (Fauchald and Jumars, 1979), and suggested
the feeding guilds of N. polybranchia may be a motile subsurface depositfeeder. More depleted in 13C of polychaete (i.e., N. polybranchia) compare


N.T. Tue et al. / Journal of Sea Research 72 (2012) 14–21
Table 2
Stable isotope signatures (δ13C and δ15N) of invertebrates in Xuan Thuy National Park,
Vietnam.
Species

Polychaete
Diopatra neapolitana
Nephthys polybranchia
Gastropods
Onchidiidae sp.
Cassidula aurisfelis
Dostia violecea
Littoraria melanostoma
Terebralia sulcata
Nassarius olivaceus
Cerithidea ornata
Bivalves
Ensis magnus

Glauconome virens
Laternula truncata
Geloina coaxans
Crabs
Grapsidae
Metopograpsus messor
Sesarma dehaani
Episesarma versicolor
Metaplax elegans
Metaplax longipes
Leucosiidae
Ebalia malefactrix
Ocypodidae
Uca urvillei
Uca flammula
Uca borealis
Uca acuta
Uca arcuata
Portunidae
Scylla serrata
Charybdis helleri
Prawns
Palaemonidae
Macrobrachium rosenbergii
Penaeidae
Metapenaeus joyneri
Penaeus merguensis
Penaeus monodon

ACR

L (mm)

δ15N

δ13C

Mean SD

Mean SD

Mean

n
SD

0.8 − 25.5 0.1
− 26.8

Dn
Np

52.7
42.0

2.1 11.3
1.0
8.7

On

Ca
Dv
Lm
Ts
No
Co

40.3
25.2
16.6
20.6
26.1
16.0
23.0

2.0
2.2
1.9
3.6
4.9
1.0
1.4

6.2
5.6
7.3
4.1
9.3
8.3
6.8

Em
Gv
Lt
Gc

52.1
30.3
49.7
28.3

6.5
4.2
1.2
1.9

7.5
9.2
5.7
7.1

0.8
0.5
0.8
0.3

Mm
Sd
Ev
Me

Ml

11.8
17.7
18.0
7.9
16.0

0.7
1.2
0.5
3.2

5.0
7.8
8.7
3.3
9.9

1.0 − 22.7
0.9 − 22.6
− 23.8
0.7 − 16.7
0.3 − 18.5

Ebm 18.2

0.7

9.3

0.6 − 17.6 1.1

6

Uu
Uf
Ub
Ua
Uar

13.7
25
25.5
21.5
25.7

2.1

8.0
9.0
8.9
9.0
8.8

0.6 − 17.1 0.5
− 16.3
− 15.7
− 14.5
0.5 − 19.0 0.9

3
1
2
2
7

Ss
Ch

49.5
51.8

18.0 10.0
14.4 10.6

Mr

74.9

9.3 10.9

Mj
81.6
Pm 62.6
Pmo 57.1

6.5 11.0
14.1 11.1
4.3 10.4

0.5
0.5
2.7

3
2

− 17.7
− 22.8
− 19.2
− 22.1
− 19.6
− 20.6
1.3 − 22.4

2
1.1
3
1.5 11
0.6 10
2.5 18
2
0.7
5

− 21.8
− 21.3
− 24.6
− 24.3

0.2
6
0.5 11
0.7
3
0.3
4

0.7
3.5
2.8
0.9

0.9
0.8

6
3
1
1.3
6
1.1 11

0.5 − 19.4 1.8 17
0.6 − 17.6 1.0
6

0.3 − 19.5 1.2

8

0.3 − 19.8 0.3
6
0.6 − 19.0 1.2 11
0.3 − 17.9 0.6
5

ACR: acronym; n is number of samples; mean, and mean and SD values are given
where n = 2, and n ≥ 3, respectively; L: body length, for crab is carapace length, and
other species is total body length.

17

to that of other benthic fauna was also reported in Bouillon et al.
(2002b), apparently a preferential food source of polychaete living in
dense mangrove forests was the mangrove detritus. The polychaete
δ15N values were high (Table 2, Fig. 2), indicating that they occupied
higher trophic position in the benthic food web. The trophic position
suggested these polychaete species were also nourished on the enriched
15
N food sources (i.e., animal tissues). The Nephtyids can feed on small
invertebrates, including mollusks, crustaceans, and other polychaete.
The Onuphids may nourish by the rotten animal tissues as the supplemental nutrient food sources (Fauchald and Jumars, 1979). Nevertheless,
this explanation needs to be confirmed by an experimental determination of the variation of isotopic compositions of polychaete.
3.2.2.2. Gastropods. Two individuals of slug Onchidium spp. had an average δ13C value (−17.7‰) close to that of the BMA, and δ 15N values
were higher from 3 to 4‰ than that of BMA (Fig. 2). The isotopic compositions obviously indicated that Onchidium spp. grazed on the conspicuous layer of BMA on surface sediments.
The mangrove snails showed a wide range of δ 13C, from −23.7 to
−16.4‰ (Fig. 3a). The wide range of δ 13C signatures indicated the
heterogeneous diets of snails (Penha-Lopes et al., 2009; Rodelli et al.,

1984) that can be explained by several factors, such as (1) feeding guilds
(Guest et al., 2004), (2) ontogeny (Fratini et al., 2004; Slim et al., 1997),
and (3) microhabitats (Fratini et al., 2004; Penha-Lopes et al., 2009).
As shown in Fig. 3a, the δ13C values of the snails were within the
ranges of the BMA, marine phytoplankton, POM, and sediment organic
matter of adjacent habitats or slightly higher than the mangrove sediments. The δ13C values from the present study indicated the preferential
BMA, marine phytoplankton, and POM over the mangrove leaves of the
mangrove snails. These findings are consistent with the results of diets
of Terebralia sulcata and L. melanostoma from mangrove ecosystems of
Okinawa, Japan (Meziane and Tsuchiya, 2000) and Coringa Wildlife
Sanctuary, India (Bouillon et al., 2002a), respectively. The food sources
of snail T. sulcata from the present study were contradictory with the
stomach content analysis that reported for mangrove snails of genus
Terebralia such as T. palustris (Fratini et al., 2004; Penha-Lopes et al.,
2009; Slim et al., 1997). Slim et al. (1997) showed that the stomach
of adult and juvenile snail T. palustris contained up to 62.5% and 19% of
mangrove materials, respectively. The contradictory results between
the two methods can be explained by the snails that ingested the mangrove litters but not always assimilated (Fry and Ewel, 2003; Lee et
al., 2001). Hence, the mangrove materials probably did not form an
important contribution to energy intake by these mangrove snails.

Fig. 2. Dual isotope plot of the δ15N and δ13C (mean ± SD, ‰) values of invertebrates from the mangrove ecosystem of Xuan Thuy National Park, Vietnam. The point denotes the
mean value and error bar denotes 1 SD. Boxes indicate the range of stable isotope compositions of the potential organic carbon food sources of invertebrates. Refer to Tables 1
and 2 for acronyms of the potential organic carbon food sources and invertebrates, respectively.


18

N.T. Tue et al. / Journal of Sea Research 72 (2012) 14–21

Fig. 3. Frequency distribution of δ13C values of invertebrate groups from the mangrove ecosystem of Xuan Thuy National Park, Vietnam. The gray bar indicates the ranges of the
potential organic carbon food sources as referred in Table 1.

Subsequently, fecal materials of snails may contain large undigested
mangrove materials (Lee et al., 2001). In the ecological aspect, the
fecal materials can be quickly increased in bacteria biomass and nutrient
contents that can be easily assimilated by other deposit-feeders. As a
result, snails play as an important linkage between mangrove primary
producer and other invertebrates (Lee, 2008).
Mangrove snails change their microhabitats with the life stages
(Fratini et al., 2004; Penha-Lopes et al., 2009). Juveniles prefer the adjacent habitats such as creek bank and tidal flat and fringe mangrove
forest. They will migrate into the landward zone after reaching to
adult stages (Fratini et al., 2004; Penha-Lopes et al., 2009). In addition,
many deposit feeders shift their diets during the development to meet
their nutrient demands (Hentschel, 1998). The ontogenetic shift in
diets was reported for the snail of genus Terebralia (i.e., T. palustris)
from mangrove forests of Gazi Bay, Kenya (Slim et al., 1997), Inhaca
Island, Mozambique (Penha-Lopes et al., 2009) and Malindi, Kenya
(Fratini et al., 2004). These authors reported that the juvenile snails
are detritivorous and the adults are mainly leaf-litter consumers. Our results showed that δ13C was significantly negatively correlated with the
shell size of L. melanostoma (regression line: δ13C = −0.1 × shell_size
− 20.1; R²= 0.39, p b 0.05, Fig. 4a) and of T. sulcata (regression line:
δ13C = −0.44 × shell_size− 8.1; R² = 0.73, p b 0.05, Fig. 4b). These results demonstrated that the ontogenetic shift in diets may also occur
with these snail species. The small snails may graze on enriched in 13C
food sources such as the BMA, marine phytoplankton, and POM.
They will shift to depleted 13C food sources such as sediment organic
matter or mangrove materials when they reach to adult stages. For
the L. melanostoma species, they are generalist grazers, grazing on surfaces of the substrata non-selectively (Lee et al., 2001). In the present
study, the snails L. melanostoma were found abundant on mangrove
roots and stems where they ingested the plant tissues and filamentous

algae on the plant surface. Unfortunately, we could not measure the stable isotope compositions of the filamentous algae on mangrove roots
and stems. However, the stable isotopic values of the filamentous algae
from Bouillon et al. (2002a) suggested that the snail L. melanostoma
fed on mixed diet that is probably composed of filamentous algae and
mangrove materials, with the latter sources increasing as with the size
of L. melanostoma. The trophic dimorphism between juveniles and adults
has been explained by a difference in radula morphology of gastropods
(Slim et al., 1997).
3.2.2.3. Bivalves. Four filter feeding bivalve species can be separated by
the δ13C signatures (Table 2, Figs. 2 and 3b). The δ13C values of two bivalves G. coaxans and L. truncate living in the dense mangrove forests

were in the range of POM and mangrove sediments (Table 2; Fig. 2).
The mangrove detritus proportion in POM was >50% during the flood
tide in the small creeks of this mangrove ecosystem (Tue et al., 2012).
As a result, the mangrove detritus may contribute a significant proportion to diets of these mangrove bivalves. This finding is consistent with
the report on the diet of bivalve G. coaxans in the Okinawa mangrove
ecosystem (Bachok et al., 2003).

Fig. 4. Correlation between shell size and δ13C (pb 0.05) of gastropod and bivalve species
L. melanostoma (a), T. sulcata (b), and G. virens (c).


N.T. Tue et al. / Journal of Sea Research 72 (2012) 14–21

The δ 13C signatures of two bivalves G. virens and E. magnus living
in the tidal flat were in the ranges of marine phytoplankton and
BMA, and slightly enriched than that of POM. Thus, they probably
had major diets from the POM, and the microalgae, consisting of marine phytoplankton (Rodelli et al., 1984) and BMA resuspension from
surface sediments by hydrodynamics (Cognie et al., 2001; Kang et al.,
1999). Interestingly, the measurements of δ 13C values and shell sizes

were significantly negatively correlated for individuals of G. virens
species (regression line: δ 13C = − 0.1 × shell_size − 18.6; R² = 0.57,
p b 0.05, Fig. 4c), implying the ontogenetic shift in diets of the G. virens
from the microalgal diets to POM containing high proportion of mangrove detritus. The difference in food resource utilization of juvenile
and adult G. virens may relate to the mechanisms of selective feeding,
strength of inhalant flows, siphon sizes, and lipid contents of filterfeeding bivalves (Kang et al., 1999).
3.2.2.4. Crabs. The stable isotope compositions of these brachyurans
varied with the habitats, feeding behavior, and taxonomy (Table 2;
Fig. 2). In general, the crabs fed on variety of food resources, consisting
of BMA, marine phytoplankton, POM, and sediment organic matters
from adjacent habitats and mangrove forests (Table 1; Figs. 2 and 3c).
For grapsid crabs, the average δ13C value of M. messor was −22.7 ±
0.9‰ (n= 6), indicated that the grapsid crab M. messor did not directly
rely on the mangrove leaves. The Metopograpsus spp. has been reported
to be an omnivorous that nourished animals, plant materials, and inorganic sediments (Poon et al., 2010). However, our results may not support the omnivorous feeding guild of the M. messor in the XTP due to
the low δ15N values in the muscle tissues (Table 2). In addition, δ13C
was slightly higher than sediment organic matters of mangrove and
adjacent habitat sediments, and POM (Fig. 2). The stable isotope compositions thus suggested that the grapsid crab M. messor fed on a bulk
mixture of sediment organic matters with high proportion of mangrove
detritus.
The δ 13C of sesarmid crab S. dehaani was slightly enriched relative
to that of the mangrove sediment organic matters (Table 2, Fig. 2),
and the δ 15N was 3.5‰ higher relative to these substrates. The isotopic compositions demonstrated that the sesarmid crab S. dehaani fed
on the organic matters in the mangrove sediments, which composed
of the marine phytoplankton and mangrove detritus, with the later
sources being predominant (Tue et al., 2012).
Only one specimen of sesarmid E. versicolor was collected in the
present study, thus it is not obvious to indicate the food sources of
this species. However, the stable isotope compositions from the present study are similar to those reported for the E. versicolor from Indian
mangrove forest (Bouillon et al., 2002a) and Indonesian mangrove forest (Nordhaus et al., 2011). These patterns suggested that the sesarmid

E. versicolor may have alike diets in different mangrove forests. The diets
of E. versicolor included sediment organic matter, other invertebrates,
carrion, and mangrove litter thereof (Nordhaus et al., 2011).
The δ 13C values of two Metaplax species and the pebble crab Ebalia
malefactrix were higher than those of other sesarmid and grapsid
crabs (Table 2, Fig. 2). The high δ 13C values indicated that mangrove
organic carbon was not significantly contributed to diets of these crabs.
The δ13C signatures of two Metaplax species showed that they nourished
the BMA, e.g., benthic diatoms and cyanobacteria (Bouillon et al., 2002a).
Moreover, the species M. longipes had significantly higher δ15N than the
species M. elegans (pb 0.05). The δ15N values demonstrated that the
species M. longipes fed on a higher trophic position than the species
M. elegans. The stable isotope compositions showed that the species
M. longipes ingested the BMA, animal carrions, and juveniles of gastropods and bivalves.
For fiddler crabs (Uca spp.), δ13C values varied from −20.6 to
−14.3‰, with the most enriched in 13C for U. acuta species (δ 13C values
of two individuals were −14.3‰ and −14.6‰). However, δ 15N of the
Uca spp. species had a small range, varying from 7.3 to 9.5‰. The
small range of δ15N indicated that these fiddler crabs fed on the same

19

trophic level. The fiddler crabs are conspicuous residents of the land–
water ecotone in the mangrove ecosystem (France, 1998). They are deposit feeders, ingesting organic matter from the exposed mud at low
tide (Hogarth, 2007). Therefore, the δ 13C values suggested that they
nourished nutrients from surface sediments with a major food source
from the BMA (Fig. 2). Two species U. arcuata and U. urvillei collected
from the tidal flat at site 1 (Fig. 1b) were enriched in 13C compared to
the BMA, referring to the preferential diets from the BMA. The species
U. borealis was collected from tidal flat and two species U. flammula

and U. acuta were collected from the creek bank (Fig. 1c), had δ13C
values higher from 2.6 to 4.6‰ than that of the BMA, suggesting that
they may not rely only on the BMA food source. Moreover, the δ15N
values of these fiddler crabs were 5.0 to 7.2‰ greater than that of the
BMA. The 15N enrichment has probably ruled out the substantial dietary
contribution from the BMA, given the expected fractionation 3–4% enrichment in δ 15N per trophic level (Post, 2002). Obviously, these fiddler
crabs nourished other food sources with/without BMA such as bacteria
(France, 1998), and ciliate protozoa and nematodes (Hogarth, 2007). In
agreement with our study, the fiddler crabs in the mangrove ecosystems of Malaysia (Rodelli et al., 1984), Puerto Rico (France, 1998), and
Coringa Wildlife Sanctuary, India (Bouillon et al., 2002a) were highly
selective for the BMA and other food sources (i.e., bacteria, ciliate protozoa, and nematodes) than the mangrove detritus.
The stable isotope compositions of two portunid crabs showed
that they are among the top predators in the mangrove ecosystem
(Fig. 2). In Thailand mangrove forests, the portunid crabs feed on
the slow moving benthic animals, including bivalves, snails, other
crabs, and polychaete (Thimdee et al., 2004). Our results suggested
that the portunid crabs probably relied on the various types of invertebrates (i.e., M. longipes, T. sulcata, Nassarius olivaceus, Dostia violecea,
and G. virens). Moreover, our findings showed that δ13C exhibited a significant negative correlation with carapace width >30 mm of portunid
crab S. serrata (regression line: δ13C = −0.1 × carapace_width − 14.4;
R² = 0.6, p b 0.05, Fig. 5). This pattern suggested that the larger individuals S. serrata extensively fed on other invertebrates in the lower trophic
levels (i.e., C. ornata, S. dehaani, and M. messor) and microphytobenthos
for enough nutrient demand.
3.2.2.5. Prawns. Stable isotope composition for three penaeid shrimps
and Macrobrachium rosenbergii species were tightly clustered (Fig. 2),
which indicated of an assessment to the similar food resources. As
shown in Fig. 3d, the major food sources of these penaeid shrimps
were BMA, marine phytoplankton, POM, and sediment organic matters.
In addition, the δ15N signatures of the penaeid shrimps were highest
that are indicative of the top predators among the invertebrates in
the mangrove ecosystem of XTP. The high δ15N signatures were indicative of a feeding on other small preys such as juveniles of crabs, gastropods, and bivalves. In agreement with our study, Chong and Sasekumar

(1981) showed that penaeid shrimps are opportunistic omnivorous and
are known to feed on a variety of food — depending on the locality and
availability of food items.

Fig. 5. Correlation between carapace width and δ13C (pb 0.05) of portunid crab S. serrata.
The square symbols are the values of individuals with carapace width b 30 mm, and these
individuals were not used to calculate the regression between carapace width and δ13C.


20

N.T. Tue et al. / Journal of Sea Research 72 (2012) 14–21

4. Conclusions
The dual stable isotope signatures (δ 13C and δ 15N) were applied to
identify the utilization of food sources by invertebrates in an ecologically important mangrove ecosystem of Vietnam. The results showed
that the invertebrates had heterogeneous diets and just a few mangrove
inhabitants such as polychaete, gastropods, bivalves, and grapsid crabs
directly relied on the mangrove detritus. In addition, fiddler crabs living
in the land-water ecotone were highly selective for the BMA and other
food sources (i.e., bacteria, ciliate protozoa, and nematodes) than the
mangrove detritus. The present study supports other isotopic studies
that invertebrates assimilate a greater production of high nutrient food
sources such as BMA, marine phytoplankton, meiofauna, and animal
carrions in the mangrove ecosystem. The ontogenetic shifts in diets
were identified for several invertebrate species, consisting of snails
L. melanostoma and T. sulcata, bivalve G. virens, and portunid crab
S. serrata. On the basis of the δ13C and δ15N signatures, feeding guilds
of invertebrates could be divided into low trophic position, consisting
of snails, bivalves, grapsid crabs (exclude: M. longipes) and high trophic

position (polychaete, grapsid crab M. longipes, portunid crabs, and
prawns). This study revealed that the snails may play as a linkage between mangroves and other invertebrates in the high trophic positions,
and the estuarine food webs (Proffitt and Devlin, 2005). In addition, the
land–water ecotone such as tidal flats and creek banks seem to contribute an important microalgal food resource for invertebrates (e.g., fiddler
crabs and prawns). These results highlight the need for conservations of
mangrove forests and other habitats such as tidal flat and mangrove
creek systems.

Acknowledgments
The authors are grateful to the staff of Hanoi University of Science,
Vietnam for supporting this research. We express our sincere thanks
to anonymous reviewers for their critical reviews and comments
which significantly improved this manuscript. This work was
supported by the “Global COE Program” from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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