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Relationships between life history traits and sexual dimorphisms in two varunid crabs, Hemigrapsus takanoi Asakura & Watanabe, 2005 and H. Sinensis Rathbun, 1931 (Brachyura: Varunidae)

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Differences in life histories and mating systems can influence the strength of sexual selection, leading to different degrees of sexual dimorphism among allied species. To test this premise, we compared the life history traits (including operational sex ratios) between two brachyuran crabs (Hemigrapsus takanoiAsakura & Watanabe, 2005 and H. sinensisRathbun, 1931) that have different degrees of sexual dimorphism. In H. sinensis, both sexes have setal patches on their chelae, but patches are found only on the males of H. takanoi. Our field surveys showed that sexual dimorphism about body size (carapace width) differed between the two species, males being larger than females in H. takanoi, whereas females were slightly larger than males in H. sinensis. Ovigerous females of H. takanoi occurred from May to October, with two peaks in the abundance trend; ovigerous females of H. sinensis were found from March to June, with one peak of abundance when almost all females were ovigerous. The population density of H. takanoi in the field far exceeded that of H. sinensis. Although the operational sex ratio was highly biased toward males in both species due to the rarity of receptive females, the bias was more pronounced in H. takanoi than in H. sinensis, partly due to the longer receptive duration of H. sinensis females than that of H. takanoi females. The larger male biased operational sex ratio and higher population densities of H. takanoi (compared to H. sinensis) may explain interspecific differences in sexual dimorphism of morphological features, including the occurrence of the setal patch.
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Journal of Crustacean
Journal of
Crustacean Biology
Journal of Crustacean Biology, 37(1), 21–28, 2017. doi:10.1093/jcbiol/ruw011
Relationships between life history traits and
sexual dimorphisms in two varunid crabs,
Hemigrapsus takanoi Asakura & Watanabe, 2005
and H. sinensis Rathbun, 1931 (Brachyura: Varunidae)
AyaMiyajima1,2 and KeijiWada2,3
1Suma Aqualife Park, Kobe, 654-0049, Japan;
2Department of Biological Sciences, Nara Women’s University, Nara, 630–8506, Japan; and
3IDEA Consultants, Inc. Nanko-kita 1-24-22, Suminoe-ku, Osaka, 559–8519, Japan
Correspondence: A.Miyajima; email: b9k5jc@gmail.com
(Received 7 June 2016; accepted 19 November 2016)
ABSTRACT
Dierences in life histories and mating systems can influence the strength of sexual selection,
leading to dierent degrees of sexual dimorphism among allied species. To test this premise, we
compared the life history traits (including operational sex ratios) between two brachyuran crabs
(Hemigrapsus takanoi Asakura & Watanabe, 2005 and H. sinensis Rathbun, 1931) that have dier-
ent degrees of sexual dimorphism. In H. sinensis, both sexes have setal patches on their chelae,
but patches are found only on the males of H. takanoi. Our field surveys showed that sexual
dimorphism about body size (carapace width) diered between the two species, males being
larger than females in H. takanoi, whereas females were slightly larger than males in H. sinensis.
Ovigerous females of H. takanoi occurred from May to October, with two peaks in the abun-
dance trend; ovigerous females of H. sinensis were found from March to June, with one peak
of abundance when almost all females were ovigerous. The population density of H. takanoi in
the field far exceeded that of H. sinensis. Although the operational sex ratio was highly biased
toward males in both species due to the rarity of receptive females, the bias was more pro-
nounced in H. takanoi than in H. sinensis, partly due to the longer receptive duration of H.sinensis
females than that of H.takanoi females. The larger male biased operational sex ratio and higher
population densities of H. takanoi (compared to H. sinensis) may explain interspecific dierences
in sexual dimorphism of morphological features, including the occurrence of the setal patch.
Key Words: breeding cycle, interspecies comparison, operational sex ratio, sexual dimorphism
INTRODUCTION
Animal species vary in the degree of morphometric dieren-
tiation between males and females. This variation among taxa
is likely a result of dierences in the strength of sexual selec-
tion. Operational sex ratios (OSRs), the ratio of sexually recep-
tive males to sexually receptive females, and population densities
have often been proposed as factors determining the strength of
sexual selection (Emlen & Oring, 1977; Kvarnemo & Ahnesjö,
1996; Kokko & Rankin, 2006). The OSR is an important deter-
minant of the direction and intensity of competition for mates
(Emlen & Oring, 1977; Kvarnemo & Ahnesjö, 1996). If the OSR
deviates from unity, sexual selection is predicted to be stronger for
the more abundant sex (Kvarnemo & Ahnesjö, 1996). The biased
OSR may thus predict which sex will compete more intensely for
access to mates (Kvarnemo & Ahnesjö, 1996). If the male OSR
bias is moderate, male–male competition for mates may be less
intense; alternatively, female–female competition may be more
intense (Kvarnemo & Ahnesjö, 1996). Population densities deter-
mine inter and intrasexual encounter rates, thereby influencing
the pattern of mating dynamics (Krupa & Sih, 1993). Under the
high population density, male–male competition is considered to
be more intense because of frequent competitor encounter rates
(Bertin & Cézilly, 2005). Baeza & Asorey (2012) compared sexual
dimorphism in two porcelain crabs, Petrolisthes spinifrons (H. Milne
Edwards, 1837) and P. mitra (Dana, 1852). Petrolisthes mitra occurs
in dense aggregations within its sea urchin symbiotic partner. This
A. MIYAJIMA AND K. WADA
22
species exhibits strong sexual dimorphism in body and cheliped
traits, and males in dense aggregations compete strongly for mates.
Petrolisthes spinifrons is anemone-dwelling, but the crabs occur sin-
gly in their rare hosts, and both sexes develop chelipeds, thereby
reducing sexual dimorphism. These habitat conditions can make
interspecific dierence in local population densities, which lead to
interspecific dierence in the intensity of male–male competition.
Crustaceans OSRs are generally biased toward males since the
potential reproductive rate of males is likely higher than that of
females (Duy & Thiel, 2007). Quantifications of OSRs in natural
populations are nevertheless few (Orensanz et al., 1995; Moreau
& Rigaud, 2000; Correa & Thiel, 2003a; Brockerho & McLay,
2005a), likely because of the practical diculties encountered
when measuring relevant parameters in the field. OSRs may be
overestimated when few or no receptive females can be found dur-
ing searches in the reproductive season. For example, Brockerho
& McLay (2005a) found few receptive females of Hemigrapsus
sexdentatus (H. Milne Edwards, 1837) in natural population, but
more receptive females were found in field cage. Brockerho
& McLay (2005a) considered that they could not find receptive
females in natural population because females become receptive
for a very short time. Furthermore, most studies have focused on
single species that are usually strongly sexually dimorphic (Correa
& Thiel, 2003a; Brockerho & McLay, 2005a). There has been no
study that compares the OSRs between sexually dimorphic spe-
cies and non-dimorphic species.
Life history traits have been compared among species of brach-
yuran crabs (Knudsen, 1964; Orensanz et al., 1995; Wada et al.,
1996). These traits have been found to dier among even closely
related taxa. Reproductive parameter (e.g., breeding season, the
proportion of ovigerous females, and the number of broods) dif-
ferences are especially prominent. The reproductive parameters
can mediate the strength of sexual selection, leading to the sex-
ual dimorphism (Emlen & Oring, 1977). For example, Orensanz
et al. (1995) suggested that the timing of reproductive events,
i.e., multiple brooding vs. single brooding, aects the degree of
sexual dimorphism in adult claw size between the allied species,
Metacarcinus gracilis (Dana, 1852); intermediate level of dimor-
phism) and M.magister (Dana, 1852); low level of dimorphism).
To investigate relationship between sexual dimorphism and
life history traits, we focused on two varunids, Hemigrapsus taka-
noi and H. sinensis, which have dierence in the degree of sex-
ual dimorphism. Only males of H. takanoi bear hairy structures
composed of fine setae (the “setal patches”) in the gapes of their
chelae (Asakura & Watanabe, 2005). The species is commonly
found under intertidal boulders in the inner bays and estuaries of
East Asia. In contrast, both males and females of H. sinensis have
these setal patches. This species occurs in oyster beds and under
boulders in the intertidal/subtidal zones of East Asia (Sakai,
1976). Miyajima et al. (2012) also showed that the sexual dier-
ence in the relative increment of chela size is more pronounced
in H. takanoi than in H. sinensis. These dierences in the degree
of sexual dimorphism in the chelae were correlated with sexual
dierences in their use during social behaviors. In the males of H.
takanoi and both sexes of H.sinensis that have setal patches on the
chelae, individuals used their chelae more frequently than their
walking legs to grasp one another during intra-sexual interac-
tions. Female H. takanoi, which lack the setal patch, nevertheless
used their walking legs more frequently for touching and pushing
one another (Miyajima et al., 2012). As suggested by Orensanz
et al. (1995), the life history traits of these varunid crabs, espe-
cially their reproductive characteristics, may be related to dier-
ences in the degree of sexual dimorphism in social behavior and
morphology.
We investigated the life history traits (including reproductive
characteristics) of H. takanoi and H. sinensis, which dier in the
degree of sexual dimorphism in morphology and social behavior.
We also measured OSRs in the field and in the laboratory. We
expected a more prominent OSR bias toward males in H. takanoi,
the dimorphic species, than in H. sinensis. We used these data to
test the hypothesis that sex ratios and population densities influ-
ence interspecific dierences in the degree of sexual dimorphism.
MATERIALS AND METHODS
Monthly sampling
We quantitatively sampled H. takanoi and H. sinensis each month
from April 2014 to March 2015 in intertidal areas of two adja-
cent estuaries of the Nakano River (34°53’N, 136°35’E) and the
Tanaka River (34°43’N, 136°30’E) in Mie Prefecture, Japan.
Sampling was performed when the moon was full at low spring
tide in each month. Hemigrapsus takanoi specimens were collected
from 2–4 quadrats (50cm × 50cm or 30cm × 30 cm) deployed
on substrates comprising a mix of mud, oyster shells, and cobbles.
All oyster shells and cobbles inside the quadrats were turned over
to reveal the crabs. Because we rarely found H. sinensis on sub-
strates, specimens of H. sinensis were collected from oyster beds
attaching to the concrete retaining walls of the estuaries, adjacent
to the substrates. The substrates where we collected H. takanoi
was located just above water level during spring low tide, whereas
the retaining wall where H. sinensis was collected was just above
water level during neap low tide. We recorded the area of oyster
beds that were scraped clear to collect the crabs. Oyster scrap-
ing continued until we had collected about 50 individuals of H.
sinensis specimens on each sampling occasion. All crabs were sexed
and measured to the nearest 0.05 mm with hand calipers. We
recorded the presence of molted crabs and ovigerous females, and
classified crabs without gonopods or gonopores as juveniles. All
female gonopore opercula were checked under a portable binocu-
lar microscope to determine the proportion of receptive females
in the field population. Females were considered to be receptive
when the opercula were mobile and could be readily pushed
inward in a trapdoor manner (Brockerho & McLay, 2005a, b).
We calculated the following parameters for both species in each
month: density (number of individuals that were of mature size
per unit area), adult sex ratio (ASR, the ratio of males to females
that were of mature size), and the OSR (see details of calculation
below).
Laboratory rearing
We estimated the OSR by raising both species in the laboratory
during their breeding seasons. Aselection of crabs collected in the
field were taken to the laboratory and reared in a tank (48.0cm
× 48.0cm × 50.0cm high) (ARD 9.5-101A, AQUA Co., Tokyo,
Japan) with simulated tides. The animals were held in artificial sea-
water (Premium Salt, Gex Co., Osaka, Japan) diluted to salinities
of 16–23 (temperature: 21–25.5°C in the April-October period in
2014 and 8–13°C in the February-March period in 2015)under
natural daylight conditions. Hemigrapsus takanoi was reared in the
laboratory in April-May 2014; H. sinensis was held in the tanks in
the periods of April-May, 2014 and February-March, 2015. Crabs
were kept individually in plastic cups (6cm diameter × 13.5cm
high) to preclude interactions between individuals. Hemigrapsus
takanoi was fed TetraMin (Tetra Japan Co., Ltd., Tokyo, Japan);
H. sinensis was fed crushed bait for shrimps and crabs (Kyorin Co.,
Ltd., Hyogo, Japan). Food was supplied at intervals of three days.
We checked the operculum mobility of the females daily under a
binocular microscope to confirm their receptivity. Crabs were kept
captive for about one month and replaced with new individuals
collected during the monthly collections. The numbers of recep-
tive females on each day and the duration of receptivity were
recorded. The duration of receptivity of each female was desig-
nated as the period from the first day when we found the female
having mobile opercula to the last day when the opercula kept
LIFE HISTORIES AND SEXUAL DIMORPHISMS IN TWO VARUNIDCRABS
23
mobile; when the opercula was found recalcified, the female was
regarded as non-receptive. We checked the lunar cycle to know if
the female receptivity linked with tide.
Calculations ofOSR
OSRs were calculated by two procedures. In the first, we used
data collected in the monthly field sampling to calculate the tem-
poral OSR for each month through the breeding season (hereafter,
tOSR). The tOSR was calculated using the following expression:
tOSR M RF=+/( )M
where M is the number of mature-size, inter-molt males, and RF
is the number of receptive females in the field. In the second pro-
cedure, we used data collected in the laboratory to calculate the
estimated OSR (hereafter, eOSR) for each month through the
breeding season using the following expression:
eOSR M MeRF= +/( )
where eRF is the estimated number of receptive females in the
tanks calculated from the following expression:
eRF RF RD F F
labo fieldlabo
= ×[( )/ ]( /)Σ
where RFlabo is the number of receptive females on each day in the
rearing tank, RD is the number of days when at least one female
became receptive in rearing tank, Ffield is the number of females
collected in field, and Flabo is the number of reared females in the
laboratory.
Mature-sizecrabs
Females exceeding the minimum carapace width of ovigerous
individuals collected in the field were regarded as mature. We used
previously available data and preliminary observations of mat-
ing behavior in males to determine the minimum carapace width
of male sexual maturity for each species: 5.1 mm for H. takanoi
(N=186), 4.1mm for H. sinensis (N=81).
Data analysis
We used t-tests to identify significant gender dierences in carapace
width in each of the species. We also used t-tests to identify signifi-
cant dierences in the duration of female receptivity between spe-
cies. Mann-Whitney U-tests were used to compare OSRs between
H. takanoi and H. sinensis. For our investigation of ASRs in each
species in each month, we used a binomial test to compare the
numbers of males and females against an expected ratio of 1.0. We
compared densities between species and sexes using paired t-tests.
RESULTS
Monthly changes in size frequency distributions
The size frequency distributions of the two species are shown
in Figs. 1 and 2. The carapace widths of H. takanoi males
(9.29 ± 0.11 mm [mean ± SE, through the text], N = 1,415)
exceeded those of females (8.45 ± 0.08 mm, N = 1,451)
(t = 6.30, P < 0.001). The carapace widths of H. sinensis
females (7.17 ± 0.11 mm, N = 459) exceeded those of males
(6.72 ± 0.10 mm, N = 356) (t = –3.03, P = 0.002). Paired
t-tests detected significant dierences in carapace width in each
month between the sexes of H. takanoi (t = 4.08, P = 0.002),
(mean carapace width in males: 9.35 ± 0.37 mm, N = 12; in
females: 8.48 ± 0.22 mm, N = 12). In H. sinensis, carapace
widths in each month were not significantly dierent between
males (7.04 ±0.48mm, N=12) and females (7.31±0.61mm,
N=12) (paired t-test: t =–1.65, P = 0.06). The maximum cara-
pace widths of male and female specimens of H. takanoi were
24.25mm and 18.35mm, respectively. The respective maximum
Figure 1. Size frequency distribution of Hemigrapsus takanoi individuals
in the period of April 2014 to March 2015. Solid bars indicate ovigerous
females; the oblique line refers to juveniles of unknown sex.
A. MIYAJIMA AND K. WADA
24
widths in H. sinensis were 13.50 mm and 13.70 mm. The maxi-
mum male carapace width in H. takanoi in each month was sig-
nificantly larger (21.65± 0.52mm, N= 12) than that of females
(16.39±0.35mm, N = 12) (paired t-test, t = 9.75, P <0.0001).
The maximum male carapace width in H.sinensis in each month
was significantly smaller (10.00 ± 0.47 mm, N = 12) than that
of females (10.94 ± 0.51 mm, N = 12) (paired t-test, t =–2.60,
P=0.02). The minimum carapace widths of ovigerous females in
H. takanoi, and H. sinensis were 5.20mm and 5.15mm, respectively.
Breedingcycle
Ovigerous H. takanoi and H. sinensis females were collected from
May to October (Fig.1) and from March to June (Fig.2), respec-
tively. The monthly abundance trend for ovigerous females of H.
takanoi was bimodal: more than half of the females became oviger-
ous in June and in the period from September to October (Fig.3).
The abundance trend in ovigerous H. sinensis crabs was unimodal,
with one peak in the April-May period, when almost all females
became ovigerous (Fig.3).
ASR
The ASR of H. takanoi deviated from 1.0 in May (biased toward
males), September (biased toward females), and October (biased
toward females) of 2014, and in December (biased toward females)
and February (biased toward females) of 2015, but not in other months
(Table1). The ASR of H. sinensis was biased toward females from May
to July 2014, biased toward males from August to September 2014,
but not biased toward either sex in other months (Table1).
OSR
Because receptive females were extremely rare in the field, tOSR
was highly biased toward males in both species. The monthly
tOSR for H. takanoi during the breeding season (May to October
2014) was in the range 0.97–1.00 (mean = 0.99). In H. sinensis,
the monthly tOSR (April to May 2014, and March, 2015)was in
the range of 0.92–1.00 (mean = 0.97). A Mann-Whitney U-test
detected no significant dierence in tOSR between H. takanoi and
H. sinensis (U=8.00, P >0.05).
Our rearing procedure in the laboratory produced more recep-
tive females than the field collections. Receptive females tended to
occur near the times of spring and neap tides (Fig.4). The eOSR
of the two species in each month (Table2) deviated toward males
significantly more frequently in H. takanoi (0.95±0.01) than in H.
sinensis (0.86±0.04) (Mann-Whitney U-test, U=1.00, P <0.05).
Females of H. sinensis tended to remain receptive for signifi-
cantly longer periods (2.76±2.00days) than those of H. takanoi
(1.25±0.71days) (t-test: t=–4.87, P<0.001).
Temporal density changes
The mean densities (individuals m–2) in each month fluctuated
between 194.7 and 378.7 (280.5±18.4) in H. takanoi and between
15.3 and 105.1 (59.1±13.3) in H. sinensis. The densities of mature-
size H. takanoi increased in summer and decreased in winter (Fig.5).
In H. sinensis, the densities of mature-size individuals were higher
in the fall to winter period than in the spring to summer period.
Figure 3. Monthly changes in the proportional abundances of ovigerous
females of Hemigrapsus takanoi (solid line) and H. sinensis (dashed line).
Figure 2. Size frequency distribution of Hemigrapsus sinensis individuals in
the period April 2014 to March 2015. Solid bars indicate ovigerous females;
the oblique line refers to juveniles of unknown sex.
LIFE HISTORIES AND SEXUAL DIMORPHISMS IN TWO VARUNIDCRABS
25
The monthly density of H. takanoi was significantly higher than
that of H. sinensis (paired t-test; t=10.18, N=12, P<0.0001). The
densities in each month were not significantly dierent between
the sexes of H. takanoi (Fig.6, Table3). The densities of H. sinen-
sis females were nevertheless higher than those of males in May,
June, and July (Fig.6, Table3); in September, the density of males
exceeded that of females (Fig.6, Table3).
DISCUSSION
Our monthly observations detected gender dierences in setal
patch appearance and body size (males were larger) in H. takanoi,
whereas in H.sinensis females were slightly larger than males and
setal patch appearance was similar between sexes. Sexual dimor-
phism was therefore more prominent in H. takanoi.
Reproductive traits also diered between the two species.
Ovigerous females of H. takanoi were found from spring to early fall,
with two peaks in abundance. Females of H. sinensis became oviger-
ous in the late winter to spring period with a single high peak of
abundance. Thus, H. takanoi had a long breeding season with a mod-
erate level of synchrony in female receptivity, whereas H. sinensis had
a shorter breeding season but a higher level of synchrony in female
receptivity. The breeding seasons and seasonal frequencies of ovi-
gerous females in the varunid species H. sanguineus (De Haan, 1835),
H. penicillatus (De Haan, 1835), Gaetice depressus (De Haan, 1833), and
Cyclograpsus intermedius (Ortmann, 1894) resemble those of H. takanoi
(Pillay & Ono, 1978; Fukui, 1988). In contrast, Acmaeopleura parvula
(Stimpson, 1858), which also belongs to Varunidae, has fall to spring
breeding with two seasonal peaks (Fukui, 1988). Similar interspe-
cific dierences in breeding periods were reported by Wada et al.
(1996), who compared breeding seasons between Ilyoplax pingi (Shen,
Table1. Adult sex ratios (/+) in Hemigrapsus takanoi and H. sinensis. The binomial tests were used to identify significant ratio departures from 1.0.
*P < 0.05.
Year Month H.takanoi H.sinensis
Sex ratio Binomial test (P) Sex ratio Binomial test (P)
2014 April 0.48 (101/209) 0.34 0.41 (24/58) 0.12
May 0.67 (98/146) < 0.0001* 0.35 (18/51) 0.02*
June 0.53 (123/230) 0.87 0.28 (14/50) 0.001*
July 0.52 (106/205) 0.71 0.25 (8/32) 0.004*
August 0.52 (92/177) 0.73 0.75 (15/20) 0.02*
September 0.43 (96/225) 0.003* 0.84 (16/19) 0.002*
October 0.43 (104/244) 0.01* 0.57 (43/75) 0.12
November 0.50 (141/284) 0.48 0.51 (44/86) 0.46
December 0.37 (94/255) < 0.0001* 0.55 (34/62) 0.26
2015 January 0.50 (75/150) 0.53 0.41 (25/61) 0.10
February 0.43 (69/161) 0.04* 0.47 (48/103) 0.28
March 0.48 (74/154) 0.71 0.55 (46/83) 0.19
Figure 4. Proportions of receptive females of Hemigrapsus takanoi and H. sinensis in a laboratory tank.
A. MIYAJIMA AND K. WADA
26
1932) and I. dentimerosa (Shen, 1932), and by Knudsen (1964), who
compared Hemigrapsus nudus (Dana, 1851) and H. oregonensis (Dana,
1851). Ilyoplax dentimerosa and H. nudus had an early breeding period
extending from winter to early spring, when the abundances of ovi-
gerous females peaked. Ilyoplax pingi and H.oregonensis had breeding
periods similar to that of H. takanoi (Knudsen, 1964; Wada et al.,
1996). Wada et al. (1996) considered the unusual breeding period
of I. dentimerosa to be an adaptation to its upper intertidal zone
habitat, where conditions are frequently hot and dry. We found
that the major habitat of H. sinensis at our study site was within the
water-channel retaining walls of the estuaries at higher tidal eleva-
tions than the muddy bottom habitat of H.takanoi. Knudsen (1964)
suggested that the reproductive activities of H. nudus and H. oregon-
ensis were triggered by short day length and increasing water tem-
peratures, respectively. The breeding period of H. takanoi at our site
appeared to track a trend of increasing water temperatures, as in
most intertidal crabs. Hemigrapsus sinensis, however, began its period
of reproductive activity as day lengths fell, which was also the case
in H. nudus (Knudsen, 1964). There was nevertheless a time lag
between the period of shortest day length (December in Japan) and
the start of breeding in H. sinensis (receptive females first appeared
in February). Hence, the increase in day length after mid-winter
may have triggered reproductive activity.
Figure 5. Monthly change in the densities (± SE) of mature-sized indi-
viduals of Hemigrapsus takanoi (closed circles) and H. sinensis (open circles).
Figure 6. Monthly changes in the densities (± SE) of male (solid line) and
female (dashed line) crabs. Upper panel, Hemigrapsus takanoi; lower panel,
H. sinensis.
Table2. Numbers of receptive females observed in a laboratory tank and the estimated operational sex ratios (eOSR) of Hemigrapsus takanoi and H. sinensis.
The proportions of receptive females in each month are also given. RFlabo, number of receptive females on each day in the rearing tank; eRF, estimated
number of receptive females in the tank.
Year Month Species Rearing period Receptive females Total females Proportion of receptive females ΣRFlabo eRF eOSR
2014 April H. takanoi 26 1 18 0.06 2 6 0.94
May 26 7 24 0.29 9 3 0.97
June 27 9 32 0.28 13 4.35 0.97
July 27 5 33 0.15 8 6 0.94
August 26 9 30 0.3 9 3.19 0.97
September 26 23 31 0.74 28 8.32 0.92
October 26 2 34 0.06 6 4.12 0.96
November – –
December – –
2015 January – –
February – –
March – –
2014 April H. sinensis 26 15 25 0.6 16 3.11 0.89
May 26 4 22 0.18 5 1.5 0.92
June – –
July – –
August – –
September – –
October – –
November – –
December – –
2015 January – –
February 25 5 28 0.18 13 5.11 0.9
March 7 23 29 0.79 88 16.04 0.74
LIFE HISTORIES AND SEXUAL DIMORPHISMS IN TWO VARUNIDCRABS
27
Monthly changes in the ASR followed the temporal trends in
densities of males and females in both of the species we studied.
The ASR did not change in H. takanoi during the breeding season,
except during the months of May and September. In contrast, the
number of H. sinensis males fell to 50% of the female frequency
by the end of the breeding season (Table1). Males of this species
may have died earlier than the females or moved to other habitats
after reproducing. Gender dierences in mortality are known to
produce a skewed sex ratio (Andersson, 1994).
The OSR was highly biased toward males in both of the species
we studied. In our laboratory experiment, OSR values were sig-
nificantly more male-biased in H. takanoi than those of H. sinensis.
The synchrony of female receptivity was therefore greater in H.
sinensis than in H. takanoi, in agreement with our observations of
natural breeding cycles. In H. takanoi, the eOSR bias toward males
was smallest in September due to the high frequency of receptive
females (Table 2), the female-biased ASR and the female-biased
densities in that month (that densities were not significantly dif-
ferent between sexes). The high eOSR in H. takanoi during May
was related to the low frequency of receptive females (Table 2)
and to male-biases in ASR and density (dierences between sexes
were not significant) (Fig. 6, Tables 1, 3). We found no relation-
ship between eOSR, ASR, or density in H. sinensis. In March,
when the bias toward males in the eOSR was lowest, the ASR was
not biased toward either sex, but male and female densities were
similar (Fig.6, Tables 2, 3). In May, ASR and density were female-
biased, but the eOSR was highly biased toward males (Fig. 6,
Tables 2, 3). Factors aecting the OSR bias in H. takanoi thus
appeared to be the ASR, and male and female densities; however,
in H. sinensis, the major factor influencing the OSR bias appeared
to be the proportion of receptive females. We should introduce a
note of caution in these considerations of strong male-bias in the
OSR of the two species. We assumed that all mature-size males
with hard-shells were able to mate. In some species of crusta-
ceans, males deplete their sperm reserves during successive mat-
ings (Kendall & Wolcott, 1999; Correa & Thiel, 2003b; Sato &
Goshima, 2006). Males of the two species we studied were able to
copulate more than twice in succession with dierent females (A.
Miyajima, unpublished data), but the ejaculate sizes for each cop-
ulation remain unknown. It is therefore likely that not all males
included in our calculations of OSR were ready tomate.
When receptive females are rare, males compete for them, and
only the strongest males are able to successfully mate; however, as
females become more abundant over time, competition between
males decreases (Shuster & Wade, 2003). Furthermore, density
aects inter and intrasexual encounter rates (Krupa & Sih, 1993).
As population density increases, male–male competition should
become more intense due to the increasing encounter rates of
competitors (Berlin & Cezilly, 2005). Under such circumstances,
large males have advantages in the male–male competition for
access to females (Andersson, 1994). Our results fit with these
hypotheses; the male-biased OSR and population density are
more pronounced in H. takanoi, which has sexual size dimorphism
(males are larger than females) than in H. sinensis. Not only body
size but also chela size in crustaceans aects the outcome of male–
male competition (Lee & Seed, 1992; Lee, 1995; Sneddon et al.
1997; Mariappan et al., 2000). Sexual dimorphism in H. takanoi
was evident in carapace widths, chela sizes, and the appearances
of setal patches on the chelae. Males most often use chelae during
intra-sexual fighting, but females use walking legs more frequently
(Miyajima et al., 2012). These suggest that males of H. takanoi
fights aggressively for access to limited receptive females. In con-
trast, the high frequency of receptive H. sinensis females during the
breeding season and low population density may reduce fighting
among males. This combination of traits may have promoted the
evolution of sexual monomorphism in this species. The chelae of
H. sinensis are larger in males than in females (as in H. takanoi), but
chela size per unit body size of female crabs is higher in H. sinensis
than in H.takanoi (A. Miyajima, unpublished data). Since both the
males and females of H. sinensis frequently fight with their chelae
(Miyajima etal., 2012), not only males but also females of this spe-
cies may also fight for access to superiormales.
We found that females of H.sinensis had longer receptive period
than those of H.takanoi. Females of Hemigrapsus sexdentatus isolated
from males stay receptive for longer periods than females in prox-
imity to males (Brockerho & McLay, 2005a). Neohelice granulata
(Dana, 1851) females can adjust the duration of their receptivity
according to the fullness of seminal receptacle and habitat char-
acteristics (Sal Moyano et al., 2012). The population density of
H. sinensis was lower than that of H. takanoi through the repro-
ductive season; hence, the encounter rates between males and
females were lower in H.sinensis. The length of breeding season,
long breeding season in H. takanoi and short breeding season in H.
sinensis, could also aect the chance of reproduction of females.
By extending their receptive periods, females of H. sinensis may
enhance the possibility of encountering preferredmales.
Our study thus suggests that interspecific dierence in life history
traits, especially operational sex ratio based on female receptivity
and population density, is responsible for interspecific dierence
in the degree of sexual dimorphism in social behaviors and mor-
phological features. Sexual size dimorphism is considered to be
mediated by female fecundity selection (Andersson, 1994), but we
did not investigate the female fecundity of the two species, which
should be compared between the two species in the future study.
ACKNOWLEDGMENTS
We are grateful to Prof. Y. Yusa (Nara Women’s University) for
valuable advice. We also thank members of the Laboratory of
Population and Community Ecology at Nara Women’s University
for their support and encouragement. Careful reading of the early
draft by Dr. C.McLay is also acknowledged. We also want to thank
Dr. A. Baeza and two anonymous reviewers for their valuable
suggestions.
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Understanding of animal social and sexual evolution has seen a renaissance in recent years with discoveries of frequent infidelity in apparently monogamous species, the importance of sperm competition, active female mate choice, and eusocial behavior in animals outside the traditional social insect groups. Each of these findings has raised new questions, and suggested new answers about the evolution of behavioral interactions among animals. This volume synthesizes recent research on the sexual and social biology of the Crustacea, one of the dominant invertebrate groups on earth. Its staggering diversity includes ecologically important inhabitants of nearly every environment from deep-sea trenches, through headwater streams, to desert soils. The wide range of crustacean phenotypes and environments is accompanied by a comparable diversity of behavioral and social systems, including the elaborate courtship and wildly exaggerated morphologies of fiddler crabs, the mysterious queuing behavior of migrating spiny lobsters, and even eusociality in coral-reef shrimps. This diversity makes crustaceans particularly valuable for exploring the comparative evolution of sexual and social systems. Despite exciting recent advances, however, general recognition of the value of Crustacea as models has lagged behind that of the better studied insects and vertebrates. This book synthesizes the state of the field in crustacean behavior and sociobiology, and places it in a conceptually based, comparative framework that will be valuable to active researchers and students in animal behavior, ecology, and evolutionary biology. It brings together a group of experts in fields related to crustacean behavioral ecology, ranging from physiology and functional morphology, through mating and social behavior, to ecology and phylogeny. Each chapter makes connections to other non-crustacean taxa, and the volume closes with a summary section that synthesizes the contributions, discusses anthropogenic impacts, highlights unanswered questions, and provides a vision for profitable future research.
Article
Theory predicts marked sexual dimorphism in terms of body size and body structures used as weapons (e.g. chelipeds) in gonochoric species with intense male sexual competition for receptive females and reduced or no sexual dimorphism in species where competition among males is trivial. We tested this hypothesis using a pair of closely-related species of symbiotic porcelain crabs as a model. In one species that inhabits sea anemones solitarily, competition among males for receptive females is unimportant. In a second species that dwells as dense aggregations on sea urchins, male-male competition for sexual partners is recurrent. We expected considerable sexual dimorphism in body size and weaponry in the urchin-dwelling crab and reduced sexual dimorphism in the anemone-dwelling crab. In agreement with expectations, in the urchin-dwelling crab, male body size was, on average, larger than that of females and males invested considerably more to cheliped length than females. Also supporting theoretical considerations, in the anemone-dwelling crab, sexual dimorphism in terms of body size was not detected and differences between the sexes in investment to cheliped length were minor. Interestingly, chelipeds were more developed both in males and females of the anemone-dwelling crab than in the urchin-dwelling crab as a result of the importance of these structures for monopolization of their naturally scarce anemone hosts. Another difference between the studied species was the existence of two clearly distinguishable ontogenetic phases in males of the urchin-dwelling crab but not in males of the anemone-dwelling crab. Whether the two different male morphs display different male reproductive strategies in the urchin-dwelling crab remains to be addressed. Other conditions that might additionally explain the observed differences in sexual dimorphism (e.g. female mate choice) between the studied species remain to be explored.
Article
Although individuals with larger chelipeds can feed on a wider range of prey species and sizes, the proportion of hard-bodied prey in the natural diet of C. maenas was small. Bigger chelae, however, confer selective advantages by increasing success in mate competition and intraspecific agonistic interactions. Field collections of mating pairs of C. maenas indicated that 90% of mating males had chelae that were above average size for the male population as a whole. Cheliped size and meral spread was probably more important than overall body size in determining the response pattern of the subjects. -from Authors
Article
The two forms of Hemigrapsus penicillatus (de Haan, 1835), recently distinguished on the basis of electrophoresis and differences in the size of the setal patches on the male chelae, are here recognized as two distinct species. The individuals having smaller setal patches are H. penicillatus, whereas those with larger setal patches are described herein as a new species. Living and fresh material of the two species are also clearly separated by the size and distribution of dark spots on the cephalothorax, abdomen, third maxillipeds, and chelipeds. Morphology of the male first pleopod is different between the two species. Females of the two species are morphologically identical and cannot be easily separated once colour has faded.