J OURNAL OF C RUSTACEAN B IOLOGY, 32(5), 762-768, 2012
SPECIES COMPOSITION, REPRODUCTION, AND BODY SIZE OF MUD CRABS,
SCYLLA SPP., CAUGHT IN URADO BAY, JAPAN
Cynthia Yuri Ogawa 1 , Katsuyuki Hamasaki 1,∗ , Shigeki Dan 2 ,
Yasuhiro Obata 2 , and Shuichi Kitada 1
1 Department
of Marine Biosciences, Tokyo University of Marine Science and Technology,
Konan, Minato, Tokyo 108-8477, Japan
2 Tamano Laboratory, National Research Institute of Fisheries and Environment of Inland Sea, Fisheries Research Agency,
Tamano, Okayama 706-0002, Japan
ABSTRACT
Species composition, reproduction and body size of mud crabs, genus Scylla de Haan, 1833, were investigated using gill nets from October
2008 to October 2009 in Urado Bay, Japan. Three mud crab species were identified in the area, with S. paramamosain Estampador, 1949
being the dominant species (74% of the catch), followed by S. serrata (Forskål, 1775) (23%) and S. olivacea (Herbst, 1796) (3%). We
found temporal changes in species composition with abundances of S. serrata increasing towards the summer season. Ovigerous females
of S. paramamosain and S. olivacea occur between January and October, peaking in the warm (May-July) and rainy (June-July) seasons,
but we found no berried females of S. serrata. The female-biased sex ratios of S. paramamosain and S. serrata, and the larger body size of
females during the period from autumn (November) to early spring (April), suggest that large gravid females may have migrated offshore
before the spawning season. The mean body size of females and males of S. paramamosain and S. serrata tended to increase and the
mating activity was high between May and October, showing the moulting and growth season.
K EY W ORDS: mating, maturity, mud crabs, offshore migration, sex ratio
DOI: 10.1163/193724012X649787
I NTRODUCTION
Mud crabs, genus Scylla de Haan, 1833, are a group of four
species that inhabit littoral zones of shallow estuaries and
coastal waters throughout the Indo-Pacific region (Keenan
et al., 1998; Keenan, 1999). They are an important component of artisanal fisheries in many countries over their
area of occurrence. There is a great demand for consumption and the price of mud crabs in local and export markets is increasing. Therefore, mud crab aquaculture and fishery have gained worldwide importance; however, the mud
crab aquaculture industry has developed with a dependence
on wild-caught juveniles (Keenan and Blackshaw, 1999; Le
Vay, 2001; Fielder and Allan, 2004), because reliable larviculture technologies have not yet been developed (Fielder
and Allan, 2004; Le Vay et al., 2008). Thus, in many regions,
fishing pressure that targets all size classes from juvenile
to adults has increased (Le Vay et al., 2001; Walton et al.,
2006), resulting in decreased sizes and abundances (Kosuge,
2001; Lebata et al., 2007). Fishing regulations that prohibit
the capture of immature crabs are indispensable for sustainable utilisation of mud crab resources (Overton and Macintosh, 2002; Walton et al., 2006; Hamasaki et al., 2011a).
Although mud crabs are typically associated with mangroves in subtropical and tropical areas (Keenan et al., 1998;
Le Vay, 2001; Walton et al., 2006; Lebata et al., 2007), their
distribution extends to temperate zones in Japanese waters
∗ Corresponding
where no mangroves grow (Ito, 2000; Imai et al., 2004;
Obata et al., 2006). Japanese temperate waters are the northern limit of the distribution of mud crab species. They support commercially important fisheries on a local scale in
small bays and inlets, such as Hamana Bay in Shizuoka
Prefecture and Urado Bay in Kochi Prefecture (Fig. 1)
(Hamasaki and Kitada, 2008; Hamasaki et al., 2011b). However, little is known about the biology of each mud crab
species in Japanese temperate waters. The aim of this study
was to examine the species diversity and composition, reproduction and the size of mud crab species in the commercial
catch in Urado Bay, which is a shallow semi-enclosed bay
of ∼7 km2 with some rivers, located in the southern central
area of Shikoku Island, Japan (Fig. 1).
M ATERIALS AND M ETHODS
Crab Collection and Measurement
In Urado Bay, the mud crab fishery use gill nets with a
stretched mesh size of ∼15 cm. We employed two fishers
to record the following data on a daily basis from October
2008 to October 2009: the species, sex and carapace width
(CW) including the anterolateral spines to the nearest 1 mm
of each specimen of mud crab captured, as well as the
occurrence of ovigerous females and mating pairs. Mud crab
males can mate soft-shelled females just after moulting.
In the present study, no distinction between pre- and post-
author; e-mail: [email protected]
© The Crustacean Society, 2012. Published by Brill NV, Leiden
DOI:10.1163/193724012X649787
OGAWA ET AL.: COMPOSITION, REPRODUCTION, AND BODY SIZE OF SCYLLA SPP.
Fig. 1.
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Map showing Urado Bay, Kochi Prefecture, Shikoku Island, Japan. The location of Hamana Bay is indicated.
copulatory embrace was made, and all males and females
caught in an embrace were treated as a mating pair. Species
identification was performed according to Keenan et al.
(1998) and Keenan (1999).
Previous observations suggested that Scylla paramamosain Estampador, 1949 is the dominant species in Urado Bay
(Obata et al., 2006). Therefore, to assess the gonadal development of this species, monthly sampling was performed
from October 2008 to September 2009 and a total of 128
females and 128 males were obtained from the fishers. All
females had no egg mass on their pleon. Each crab was
humanely sacrificed by being dipped in ice-cold seawater,
and the CW was measured using a vernier calliper to the
nearest 0.1 mm. To identify the maturity stage of crabs, the
widest portion of the fifth pleomere (AW) of females, and
the highest point of the crushing chela (CH) excluding the
dorsal spines of males, were measured. After body measurements, crabs were dissected to remove the gonads which
were weighed to the nearest 0.1 g.
Data Analysis
From the fishery data collected, the following data were
summarised on a monthly basis: species composition, percentage of the number of each crab species to the total number of crabs; size (CW) frequency distribution and mean values of CW for each sex; sex ratio, ratio of the number of
males to the total number of crabs; ovigerous rate, percentage of ovigerous females to the total number of mature females >105 mm CW (smallest ovigerous female); mating
rate, percentage of the number of mating pairs to the total
number of females. The gonado-somatic index (GSI) was
calculated as GSI = (GW)/(CW3 ) × 106 , where GW is go-
nad weight and CW is carapace width. The AW and CH were
plotted against CW to establish the increase in their values
that deternined maturity (Walton et al., 2006).
The difference in the mean body sizes between species
was assessed with a pairwise t-test without assuming equal
variances based on Welch’s method. The monthly mean
values of CW and GSI were compared with one-way
analysis of variance (ANOVA) without assuming equal
variances based on Welch’s method. A binomial test was
used to test the null hypothesis for sex ratio in each month
(H0 ; sex ratio = 0.5). The relationship between the body
size of male and female crabs in mating pairs was evaluated
by a Pearson’s correlation coefficient. All statistical analyses
were performed using the R language (R Development Core
Team, 2011), and the level of significance was assessed at
α = 0.05.
R ESULTS
Three species of mud crab, S. paramamosain Estampador,
1949, S. serrata (Forskål, 1775) and S. olivacea (Herbst,
1796), were identified in the fishing activity in Urado
Bay. The total number of crabs caught by the two fishers
was 2200. Scylla paramamosain was the dominant species,
accounting for 74% of the total catch, followed by S.
serrata (23%) and S. olivacea (3%). Abundances of S.
paramamosain fluctuated seasonally between 48% in July
(summer) and 100% in February (winter), i.e., the other two
species, especially S. serrata, increased towards the summer
season (Fig. 2).
The CW of most of the S. paramamosain, S. serrata,
and S. olivacea was between 90 and 160 mm, 100 and
200 mm, and 70 and 140 mm, respectively (Fig. 3), showing
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Fig. 2. Species composition as a percentage (%) of each mud crab species
towards the total number of crabs per month. Secondary y-axis corresponds
to the total number of crabs.
that S. serrata was the largest and S. olivacea the smallest
(pairwise t-test; P < 0.0001). There were no evident modal
progressions in the monthly size-frequency distributions for
S. paramamosain and S. serrata (see on-line supplementary
material; Fig. S1). However, significant seasonal fluctuation
was detected in the mean CW values of the females and
males (ANOVA; female S. paramamosain, F12,192.3 =
14.79, P < 0.0001; male S. paramamosain, F12,87.4 = 6.84,
P < 0.0001; female S. serrata, F11,60.4 = 4.00, P < 0.001;
male S. serrata, F10,42.4 = 2.37, P = 0.025) (Fig. 4). The
mean CW value of females of S. paramamosain was large at
129-138 mm between November and April; then it decreased
to 116 mm in May and increased again to 126 mm in
October. The mean CW value of males of S. paramamosain
fluctuated around 120-125 mm until June; then it increased
to around 130-135 mm in August. A similar trend of the
Fig. 3. Carapace width (CW) distribution frequency of females (a) and
males (b) of Scylla spp.
Fig. 4. Monthly mean values of carapace width (CW) of males and
females. A, Scylla paramamosain; B, S. serrata. Error bar represent the
standard error of the mean.
mean CW change was observed for females and males of S.
serrata. Monthly changes in the sex ratio were also observed
for S. paramamosain and S. serrata (Fig. 5). Overall trends
showed that the proportion of males largely decreased from
October to January/March, and then it linearly increased
towards May. Analyses of seasonal changes in the CW and
sex ratio were not performed for S. olivacea due to the low
catch of this species.
A total of 32 ovigerous females of S. paramamosain and
15 of S. olivacea were captured. No berried S. serrata were
Fig. 5. Sex ratio, as the ratio of the number of males to the total number
of crabs per month for Scylla paramamosain and S. serrata. Marks in the
graph (∗ for S. paramamosain and + for S. serrata) show a month when the
null hypothesis (H0 ; the sex ratio = 0.5) was rejected by the binomial test.
OGAWA ET AL.: COMPOSITION, REPRODUCTION, AND BODY SIZE OF SCYLLA SPP.
Fig. 6. Occurrence of ovigerous females of Scylla paramamosain and
S. olivacea represented as a percentage (%) of ovigerous versus the total
number of mature females (> smallest ovigerous female) per month.
recorded. Ovigerous females of S. paramamosain occurred
from January to October, peaking from May to July (Fig. 6).
Ovigerous females of S. olivacea were found from March
to September. The smallest ovigerous female captured for
both species was 105.0 mm CW and the largest S. olivacea
was 128.0 mm CW while the largest S. paramamosain was
145.0 mm CW.
Mating pairs were observed for all three species: 116, 18,
and 2 pairs for S. paramamosain, S. serrata, and S. olivacea,
respectively. Moreover, an interspecific copulatory pair was
found between a female S. serrata (120 mm CW) and a
male S. paramamosain (145 mm CW). Although the mating
rate was higher in S. paramamosain than S. serrata, similar
seasonal changes were observed in the occurrence of mating
pairs for both species; almost all mating pairs were recorded
from May to October (Fig. 7). Mating pairs of S. olivacea
were found in May and October. The relationship between
the body size of males and females caught in a couple
indicated for all three species that mating tended to occur
in a male with a smaller female and the female body size
tended to increase with increasing male body size (Pearson’s
correlation coefficient; S. paramamosain, r = 0.322, P <
0.001; S. serrata, r = 0.586, P = 0.011) (Fig. 8).
Based on the pleon shape of 128 females of S. paramamosain, 16 crabs with narrow pleonal flaps were classified as immature (83.5-103.4 mm CW) and 112 crabs with
Fig. 7. Occurrence of mating pairs of Scylla paramamosain and S. serrata
as a percentage (%) of the number of mating pairs to the total number of
females per month.
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Fig. 8. Relationship between the carapace width (CW) of males and
females for Scylla paramamosain, S. serrata and S. olivacea caught in a
mating pair. The line represents the equal sizes between mating pairs.
broad U-shaped pleonal flaps as mature (106.0-154.0 mm
CW), while the relationship between CW and CH values was
not able to identify morphologically mature males (Fig. 9).
The GSI values of mature females ranged between 0.30
and 39.40 and significantly fluctuated with month (ANOVA;
F11,32.8 = 18.03, P < 0.0001) (Fig. 10). The GSI of females
began to increase from October and remained high from December to April. Females with low GSI values occurred from
April (mainly May) to September; however, higher GSI values were recorded from July to September (mainly in July).
The GSI values of males, whose maturity stage could not be
determined, ranged between 0.32 and 4.66, and they did not
Fig. 9. Relationships between carapace width (CW) and the fifth pleomere
width (AW) of females (a) and chela height (CH) of males (b) of Scylla
paramamosain.
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JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 32, NO. 5, 2012
Fig. 10. Gonado-somatic index (GSI) of females (a) and males (b) of
Scylla paramamosain. The line represents the change in mean value of each
month.
largely fluctuate with month compared with the GSI values
of females (ANOVA, F11,23.6 = 2.63, P = 0.024) (Fig. 10).
D ISCUSSION
In Urado Bay, S. paramamosain was dominant, followed by
S. serrata and S. olivacea, as previously reported in this bay
(Obata et al., 2006) and Hamana Bay, Japan (Ito, 2000).
Thus, the predominance of S. paramamosain over the other
two mud crab species is common at their northern limit
of distribution. Although similar dynamics were observed
in the mean CW value, sex ratio and mating rate between
S. paramamosain and S. serrata, the species composition
changed seasonally in Urado Bay. Increased abundance of
S. serrata towards the summer season may be related to
temperature adaptation of this species because the estimated
lower critical temperatures for egg development of S. serrata
are known to be higher than those of S. paramamosain
(Hamasaki, 2002, 2003).
Body sizes of the three mud crab species caught in Urado
Bay generally agreed with those recorded from subtropical
and tropical areas (Overton and Macintosh, 2002; Hamasaki
et al., 2011a; Ogawa et al., 2011): S. serrata is the largest
followed by S. paramamosain and S. olivacea. Increased
mean CW values and higher mating activity from May to
October suggest that this period is the moulting and growth
season for S. paramamosain and S. serrata.
Ovigerous females of S. paramamosain and S. olivacea
were caught in Urado Bay, peaking in the warm (May-July)
and rainy (June-July) seasons (Takeda, 1957). The estuarine
conditions in the spawning season of temperate mud crab
populations coincided with those of tropical and subtropical populations summarized by Le Vay (2001). The highly
variable GSI values in/after July may indicate gonadal development of recently moulted mature females and/or rematuration of spawned females of S. paramamosain. Similar
temporal changes in GSI values and occurrence of ovigerous
females were known for the portunid crab Portunus trituberculatus (Miers, 1876) in Japanese temperate waters (Imai et
al., 1998; Hamasaki et al., 2004).
It has been reported for mud crab species that few
ovigerous females are found in estuarine habitat, and that
females migrate offshore to spawn (Hyland et al., 1984;
Hill, 1994; Le Vay, 2001; Koolkalya et al., 2006; Hamasaki
et al., 2011a). In Urado Bay, however, berried females
were observed for S. paramamosain and S. olivacea, but
not for S. serrata. Even though female S. paramamosain
can spawn in the bay, the female-biased sex ratios and
the larger mean CW values of females during the period
from autumn (November) to early spring (April), suggest
increased offshore migration by large gravid females that
had developed ovaries before the spawning season. In S.
serrata, the absence of berried females and population
dynamics similar to S. paramamosain strongly suggest
offshore migration from the bay by females. For S. olivacea,
the lack of data on the sex ratio and mating rate because of
the small number caught did not allow us to infer an offshore
migration by females, although the higher ovigerous rate
(100%) of this species may indicate that most females laid
eggs in the bay.
Hill (1994) suggested two possible reasons for an offshore
spawning migration by Scylla females: intolerance of larvae to estuarine conditions, especially low salinity, and improved dispersal of larvae. Larval culture experiments have
demonstrated that the appropriate salinity for rearing larvae
was >25h for S. paramamosain (Ogawa et al., unpublished
data) and S. olivacea (Baylon, 2011). We did not observe
the egg developmental stage of berried females; therefore,
it is not known whether the larvae hatched inside the bay.
If hatching occurs in the bay, it can be expected from the
egg incubation period (Hamasaki, 2002; Ates et al., 2011)
and water temperatures (Takeda, 1957), that the main hatching season would occur after June. The depth in Urado
Bay is commonly 10 m and the seawater layer with >25h
was found under ∼2 m depth (Yamamoto and Imai, 1956;
Kimura et al., 1997). Therefore, in Urado Bay, estuarine
conditions with moderate low salinity may allow females
of S. paramamosain and S. olivacea to spawn and possibly hatch their eggs inside the bay. According to Keenan et
al. (1998), S. serrata is the most widespread and dominant
species in oceanic conditions with full salinity (>34h). Dan
and Hamasaki (2011) suggested that the appropriate salinity for rearing S. serrata larvae was 30-35h. Larval salinity
adaptation may stimulate S. serrata females to spawn outside the bay.
Morphological maturity of females in S. paramamosain
samples occurred from 106.0 mm CW. It was similar to the
OGAWA ET AL.: COMPOSITION, REPRODUCTION, AND BODY SIZE OF SCYLLA SPP.
smallest size of ovigerous females (105.0 mm CW) of this
species. For males of S. paramamosain, maturity size could
not be determined based on chela morphology because of
lack of smaller specimens. However, body sizes of mating
pairs may indicate the size at functional maturity of male
crabs (S. paramamosain 95 mm CW; S. serrata 140 mm
CW). The sizes of immature and mature individuals generally overlapped in mud crab species (Overton and Macintosh, 2002; Walton et al., 2006; Hamasaki et al., 2011a;
Ogawa et al., 2011); e.g., morphologically pubertal moulting occurred between 108.8 and 114.6 mm CW and 98.4 and
111.0 mm CW for females and males of S. paramamosain,
respectively, in Don Sak, Bandon Bay, Thailand (Hamasaki
et al., 2011a). The size at sexual maturity is important as
the basis for setting a minimum landing size as a fishing
regulation and the variation in size at sexual maturity has
been shown to occur not only between species, but also between populations of the same mud crab species (Walton et
al., 2006; Hamasaki et al., 2011a). Therefore, further studies
are needed to determine the size at sexual maturity for mud
crab species at the northern limit of their distribution.
In the present study, a copulatory pair between a female
S. serrata and a male S. paramamosain was recorded.
This suggests that natural hybrids may occur between
different mud crab species. Indeed, Imai and Takeda (2005)
demonstrated by genetic and histological analyses using
a specimen collected in Wakayama Prefecture, Japan, the
existence of a natural hybrid male mud crab having a sterile
testis that appears to have been produced by cross-breeding
between a female S. olivacea and a male S. serrata. Thus,
a natural hybrid mud crab has been found; however, its
occurrence may be less common because of the very low
frequency of interspecific copulatory pairs (0.7% in all
mating pairs).
The results of this study give evidence that there is a
synchronism between reproductive events. The mating season is evidenced by the presence of mating pairs, followed
by the increase in the GSI values resulting in the spawning season and the occurrence of ovigerous females. Furthermore, reproductive activities influenced the body size
of the catch in the bay. However, actual evidence of migration and the hatching places of females are lacking. Therefore, to better understand the complete reproductive cycle of
mud crab species at the northern limit of their distribution,
further studies such as mark-recapture experiments and offshore sampling of crabs are needed.
ACKNOWLEDGEMENTS
We gratefully acknowledge the help of H. Nagano for collecting fishery data
and crab samples during the surveys. We would like to thank the editors and
anonymous reviewers for their valuable comments and suggestions.
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R ECEIVED: 3 February 2012.
ACCEPTED: 9 May 2012.
S UPPLEMENTARY M ATERIAL
OGAWA ET AL.: COMPOSITION, REPRODUCTION, AND BODY SIZE OF SCYLLA SPP.
S1
Fig. S1-1. Size-frequency distributions of Scylla paramamosain males and females in monthly catches by gill nets from October 2008 to October 2009 in
Urado Bay, Kochi Prefecture, Japan. Abbreviations: nF, number of females; nM, number of males.
S2
JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 32, NO. 5, 2012
Fig. S1-2. Size-frequency distributions of Scylla serrata males and females in monthly catches by gill nets from October 2008 to October 2009 in Urado
Bay, Kochi Prefecture, Japan. Abbreviations: nF, number of females; nM, number of males.