J. Zool., Lond. (2005) 266, 197–204 C© 2005 The Zoological Society of London Printed in the United Kingdom doi:10.1017/S0952836905006783

Sexual differences in behaviour during the breeding seasonin the soldier crab (Mictyris brevidactylus)

Satoshi Takeda

Marine Biological Station, Tohoku University, Asamushi, Aomori 039–3501, Japan

(Accepted 16 November 2004)

AbstractThe soldier crab Mictyris brevidactylus breeds in winter. Large females spawned over a few days in early winter aftermost had decalcified their vulvar opercula. Some females spawned in their first winter, less than 1 year after settle-ment. The crabs probably copulate during a few days before spawning while the vulvar opercula are decalcified.During the daytime low tide in the breeding season, large males emerged and fed on the sediment surface. Females,on the other hand, made a sand roof above themselves using sand pellets from which they had sorted out foodparticles, and they fed while hiding under it. After the males had ceased their surface activities, they entered the sandtunnel made by the female. The roof was connected with a hollow shaft, which had been made when the crab hadascended to the surface. Before the habitat was submerged, the shaft was plugged from within with sand to form an airchamber, which suggests that the male and female descend together in the air chamber and copulate there. It is possi-ble that the crab uses the sand roof as a visible signal to attract mates.

Key words: mating system, behaviour pattern, burrow structure, soldier crab, Mictyris brevidactylus

INTRODUCTION

The mating system of a population is affected by phylo-genetic constraints, inter- and intrasexual selection, andenvironmental factors of its habitat. Semi-terrestrial crabs(Grapsidae and Ocypodidae) inhabiting tidal flats havedeveloped good sight (e.g. Zeil, Nalbach & Nalbach, 1986;Zeil & Hofmann, 2001), and have used visual signals,such as waving of the chelipeds, in mating displays (e.g.Crane, 1975; Kitaura, Wada & Nishida, 2002). Similarmating behaviour in different families is thought to reflecta convergent adaptation to the flatness of their habitats(Kitaura et al., 2002).

The soldier crabs of the family Mictyridae Dana, whichconsists of one genus with four species, are distributedin the Indo-Western Pacific region (McNeill, 1926;M. Takeda, 1978). Two (Mictyris livingstonei McNeill,1926 and M. platycheles H. Milne Edwards, 1852) occuronly in Australia, one (M. longicarpus Latreille, 1806) inAustralia and South-east Asia, and one (M. brevidactylusStimpson, 1858) in South-east Asia only (McNeill, 1926;M. Takeda, 1978). They are common on tropical andsubtropical sandy shores, and are an important ecologicalcomponent in these habitats; they influence other benthicfauna, especially the assemblage of meiobenthos, throughtheir feeding and burrowing activities (Warwick, Clarke &Gee, 1990; Dittmann, 1993).

When their habitat is submerged, the crabs reside in bur-rows in the sand (Cameron, 1966; Kelemec, 1979; Quinn,

E-mail: stakeda@mail.tains.tohoku.ac.jp

1986 for M. longicarpus. Cowles, 1915; Takahasi, 1935;Yamaguchi, 1976; Nakasone & Akamine, 1981 forM. brevidactylus. Sleinis & Silvey, 1980; Kraus & Tautz,1981 for M. platycheles). When low tide exposed theirhabitat, they ascend to near, or emerge onto, the surfaceof the sand to feed (Yamaguchi, 1976; Quinn, 1986;S. Takeda & Murai, 2004). They feed on deposited matterin the sand in two ways (Yamaguchi, 1976; Kelemec, 1979;Nakasone & Akamine, 1981; Quinn, 1986; Dittmann,1993; S. Takeda & Murai, 2004). One method is to feedinside a single tunnel-like vault that is roofed with sand andruns along the sand surface (Cameron, 1966; Yamaguchi,1976; Quinn, 1986; S. Takeda & Murai, 2004). The crabscover themselves entirely with the sand roof. This con-struction, which is termed a ‘hummock’ by Cameron(1966) and a ‘tunnel’ by Yamaguchi (1976), is found inthe upper part of their littoral habitat, where the watertable falls well below the surface at low tide (S. Takeda &Murai, 2004). After feeding in the tunnel, the crabs enter ashaft connected with the tunnel that was made when theyemerged onto the surface of the sand (Yamaguchi, 1976;S. Takeda & Murai, 2004). Then they plug the shaft withsand masses, and descend within the shaft with a largeamount of air (S. Takeda & Murai, 2004).

Another feeding method is used on the sand surface,as by most ocypodid crabs (e.g. Fielder, 1970). Thesoldier crabs feed on the sand surface while walkingforward in droves, especially near the waterline (Cameron,1966; Yamaguchi, 1976; S. Takeda & Murai, 2004). Afterfeeding on the sand surface, M. longicarpus returns tothe area where many tunnels are made and retreats into

198 S. TAKEDA

the sand (Cameron, 1966); M. brevidactylus retreats intothe moist, semi-fluid sand slightly above the waterlinein the daytime of the non-breeding season (Yamaguchi,1976; S. Takeda & Murai, 2004). They bury themselves by‘corkscrew-style’ digging, subsequently form a chamberenclosing a small quantity of air, and then descend withthe air chamber (Maitland & Maitland, 1992; S. Takeda &Murai, 2004). They emerge for the next surface activitytogether with the air chamber (Cowles, 1915; Yamaguchi,1976; Maitland & Maitland, 1992; S. Takeda & Murai,2004).

Researchers have studied the mating system of ocypo-did crabs inhabiting the same biotope, and their reproduct-ive strategies and sexual selection, by observing theirmating behaviour on the surface (e.g. Christy & Salmon,1984; Nakasone & Murai, 1998). The mating behaviourof soldier crabs has never been observed (Dittmann,1998), although one pair of M. brevidactylus was ob-served copulating in a subterranean burrow (Yamaguchi,1976). These facts strongly suggest that the matingsystem of soldier crabs differs from that of the ocypodidcrabs. Nakasone & Akamine (1981) reported thatM. brevidactylus females change their behaviour patternand rarely emerge onto the surface of the sand in the breed-ing season (from December to early March in Japan).Dittmann (1998) reported that M. longicarpus femalesemerge less and less frequently as they mature to matingsize. Nakasone & Akamine (1981) and Dittmann (1998)supposed that the change in the behaviour patterns isrelated to the mating system. However, the lack of notice-able sexual dimorphism in M. longicarpus (McNeill,1926) and their year-round reproduction (Cameron, 1966)make them unsuitable for clarifying the mating system ofsoldier crabs. On the other hand, M. brevidactylus shouldbe suitable, because the populations in the Ryukyu Islandsand Taiwan reproduce annually in winter (Yamaguchi,1976; Nakasone & Akamine, 1981; Shih, 1993), so anychange in behaviour pattern will become apparent throughcomparison between the non-breeding and breedingseasons. Clarification of the mating system of soldier crabswould be useful not only in demonstrating variations in themating systems of semi-terrestrial crabs inhabiting tidalflats, but also in revealing the mechanism of coexistenceof the many species.

In this paper, a description is given of the behaviouraldifferences between females and males in their breedingperiod, which was determined from the proportion offemales with decalcified vulvar opercula and eggs on theirpleopods. In addition, the time and location of mating andmanner of encounter is inferred based on differences inbehaviour patterns.

MATERIALS AND METHODS

Breeding period

To identify the breeding period, the sequential change inthe proportion of females carrying eggs externally(ovigerous females) was investigated. The investigationwas performed every 2 days between 16 November and

16 December 2000 and between 13 November and13 December 2001 on the tidal flat at the mouth of theOkukubi River, Okinawa Island, Japan (26◦27′N,127◦56′E). After the crabs had ceased their surface activit-ies in the daytime low tide, a 50 × 50 cm quadrat wasplaced on an area where many tunnels were usually made,and the crabs in the sand were collected to 20 cm deepusing a sieve with a 2.5-mm mesh. The collected crabswere taken to the laboratory, where sex, carapace length(nearest 0.1 mm), the presence of eggs on the pleopods andin the ovaries were recorded. For the females collected in2001, the calcification of the vulvar opercula was deter-mined under a binocular microscope. Brachyurans havetwo basic patterns of mating: some breed only followingecdysis, while others are capable of breeding during thehard-shelled stage through the decalcification of the vulvaropercula (Hartnoll, 1968, 1969; Henmi & Murai, 1999).

Crabs on the sand surface

To clarify whether the mature females engaged in theirsurface activities during the breeding season, > 50 indivi-duals that emerged on the sand surface in an area wheremany tunnels were made were collected. The collectionwas carried out c. 1 h before the lowest tide in the daytimeand at night-time every 5 days between 17 November and17 December 2000, and between 12 November and12 December 2001. At the same time, air and water tem-peratures were recorded. The collected crabs were taken tothe laboratory where the same variables listed above and,in 2001 only, calcification of the vulvar opercula wererecorded.

Surface and subsurface activities

The activity patterns of the crabs were investigated basedon the marks of burrows in the breeding season. A 50 ×50 cm quadrat was placed 10 times in an area where manytunnels were usually made soon after low tide. Tunnels,holes, elongated burrows, and newly constructed burrowswere counted in each quadrat. Tunnels indicated thatcrabs were feeding below the surface; holes indicatedthat crabs had emerged and elongated burrows that theyhad not emerged on the sand surface. Newly constructedburrows indicated that crabs had retreated into the sandafter ceasing their surface activities (S. Takeda & Murai,2004). The elongated and newly constructed burrows weredistinguished by the colour and shape of the sand clods; thesand clods that were pushed out to elongate the burrowswere compact and darker than the surface sand, and thosethat were dug out to construct burrows were loose and ofa similar colour to the surface sand. At the same time, airand water temperatures were recorded. These observationswere carried out at the daytime low tide every 2 daysbetween 13 November and 13 December 2001.

Behaviour on the sand surface

The behaviour of individuals on the surface was observedin an area where many tunnels were made, because they

The mating system of Mictyris brevidactylus 199

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1402000 (n = 409)

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Fig. 1. Frequency distributions of the carapace lengths of the male and female Mictyris brevidactylus collected in the sand where manytunnels were usually made. White bars, ovigerous females.

engaged in their surface activities in a different area attimes other than the breeding season. Observation beganbefore the crabs emerged. Usually the crabs emergedslightly after exposure, and then fed on the surface whilewalking about separately. In the daytime low tide, theyceased their surface activities individually about the timeof low tide, and then retreated into the sand.

Several times during the day, the period between thebeginnings of sequential pauses after walking was mea-sured, and the mean value calculated. The positionswhere a crab paused after walking about and the positionwhere it retreated into the sand after ceasing its surfaceactivities was recorded, in relation to the burrow structuresmade on the sand surface. At night-time, the crabs wererecorded on an 8-mm video camera recorder under infra-red illumination. The daytime observations were carriedout between 26 November and 11 December 2000, and thenight-time observations on 4 and 5 December 2000 and on24 November 2001. The mean coverage of the tunnel andthe sand clods deposited onto the sand surface was about45% of the surface area in the daytime observations, and20% in the night-time observations.

The shaft connected to the tunnel was examined to seewhether it was open or closed (i.e. whether a crab that

had entered a tunnel was able to enter the shaft or not)4 times: (1) c. 1.5 h before the lowest tide when the crabswere feeding on the sand surface; (2) while they frequentlyintruded into tunnels or sand clods dug out on the sandsurface; (3) when a few crabs remained engaged in theirsurface activities; (4) slightly after they had retreated inthe sand. This investigation was performed during daytimebetween 7 and 12 December 2001.

RESULTS

Breeding period

The densities of the crabs in the quadrat did not changewidely within each year, but differed between years (mean± SD: 2000, 48.3 ± 8.5; 2001, 24.4 ± 2.9). The proportionof females was 53.0% (409/772) in 2000 and 55.6%(217/390) in 2001. No individuals had a soft-shelled bodyafter ecdysis.

The carapace length of the males ranged from 4.2 to13.9 mm in 2000, and from 4.6 to 14.6 mm in 2001; andthat of the females from 4.3 to 12.0 mm in 2000, and from3.9 to 11.7 mm in 2001 (Fig. 1). The carapace length of themales showed a bimodal frequency distribution (Fig. 1):

200 S. TAKEDA

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100

12 14 16 18 20 22 24 26 28 30 2 4 6 8 10 12 14 16

DateNovember December

Per

cent

age

Fig. 2. Sequential changes in the percentages of ovigerous female Mictyris brevidactylus (circles, 2000; squares, 2001) and females withcalcified vulvar opercula (triangles, 2001) to the mature-sized females >6.5 mm in carapace length.

the first mode was 5.0–5.5 mm (range 4.0–6.5 mm)in 2000 and 6.5–7.0 mm (4.5–8.0 mm) in 2001, and thesecond mode was 8.0–9.0 mm (6.5–14.0 mm) in 2000and 11.0–13.0 mm (8.0–14.5 mm) in 2001. The carapacelength of females also showed a bimodal frequency distri-bution (Fig. 1): the first mode was 5.0–5.5 mm (range 4.0–6.5 mm) in 2000 and 6.0–7.5 mm (3.5–7.5 mm) in 2001,and the second mode was 7.5–8.0 mm (6.5–12.0 mm)in 2000 and 9.5–10.5 mm (7.5–12.0 mm) in 2001. Thecrabs in the first mode were aged <1 year after settlement,and those in the second mode were aged >1 year aftersettlement (Nakasone & Akamine, 1981).

The carapace length of the ovigerous females was> 6.5 mm in both years (mature-sized female); this inclu-ded several females aged < 1 year after settlement in 2001(Fig. 1). Almost all of these females had eggs in the ovariesor spawned eggs (2000: 351/355 = 98.9% (ovigerousfemales 94); 2001: 175/184 = 95.1% (39)). The ratio ofmature-sized females with eggs in the ovaries or spawnedeggs to all mature-sized females was constant during theinvestigation period in each year. The ovigerous femalesappeared in late November (Fig. 2). The ratios of ovigerousfemales to all mature-sized females rapidly increased inearly December, reaching c. 60% in mid-December ofboth years. In contrast, the ratio of mature-sized femaleswith calcified vulvar opercula to all mature-sized femalesdecreased rapidly from mid- to late November, reachingc. 10% (Fig. 2). All 39 ovigerous females collected in2001, including a female that was spawning, had decalci-fied vulvar opercula. These results indicate that thedecalcification of the vulvar opercula occurredsynchronously within several days c. 1 week beforespawning. Among the nine mature-sized females withouteggs in the ovaries, five females (55.6%) had calcifiedvulvar opercula.

Crabs on the sand surface

The individuals that emerged on the sand surface engagedseparately in activities in the area where many tunnelswere made. Very few individuals occasionally fed on thesurface of the waterlogged sand near the waterline.

The crabs emerged on the sand surface during everynight-time investigation, but not during every daytimeinvestigation owing to the higher water levels in the day-time compared to the night-time low tide (Table 1). Thecarapace length of the males that emerged on the sandsurface was > 7.3 mm in 2000 and 7.5 mm in 2001, andthat of the females was > 6.0 mm in 2000 and 6.7 mmin 2001 (Fig. 3). The sex ratio was skewed to males withgreater skewness in the night-time than in the daytime(Table 1) (2000: χ2 = 44.6, d.f. = 1, P < 0.0001; 2001:χ2 = 36.4, d.f. = 1, P < 0.0001).

Only one ovigerous female was collected in the daytimelow tide, on 7 December 2000. The ratio of mature-sizedfemales without eggs in the ovaries (excluding an oviger-ous female) to mature-sized females was 12.5% (7/56)in 2000, and 9.8% (4/41) in 2001. The ratio of mature-sized females with calcified vulvar opercula to mature-sized females was 59.1% (13/22) before and 42.1%(8/19) after commencement of spawning; no significantdifference was recognized (χ2 = 0.61, d.f. = 1, P = 0.44).Among four mature-sized females without eggs in theovaries in 2001, one had calcified vulvar opercula.

Tunnel and emergence hole

The lowest air and water temperatures at low tide in theinvestigation period were 17.7 and 18.9 ◦C, and the highestwere 24.5 and 24.7 ◦C (Fig. 4). The changes in the number

The mating system of Mictyris brevidactylus 201

Table 1. Number of crabs Mictyris brevidactylus collected on the surface of the sand in relation to water level (cm) at the lowest tide(Japan Meteorological Agency, 2000, 2001)

Daytime Night-time

Date No. of males No. of females Water level (cm) No. of males No. of females Water level (cm)

200017 Nov 0 0 101 46 6 1922 Nov 37 17 56 44 11 4227 Nov 24 4 72 51 1 52 Dec 0 0 99 52 4 397 Dec 53 4 68 54 2 48

12 Dec 50 6 66 61 0 −2017 Dec 31 0 84 63 1 23

Total 95 31 471 25

200112 Nov 48 17 43 60 0 3817 Nov 0 0 72 52 1 −322 Nov 0 0 110 53 4 5327 Nov 0 0 68 53 3 432 Dec 48 9 73 52 2 −77 Dec 0 0 94 52 2 29

12 Dec 0 0 64 55 3 12

Total 96 26 377 15

of tunnels or emergence holes did not closely relate to airand water temperatures, or to the water level at low tide(Fig. 4). There were no tunnels or holes on 21 Novemberor 5 December, because the sand was not emersed. Threeactivity peaks were recognized on 13 and 23 Novemberand 1 December when the total number of tunnels andholes was equivalent to the number of crabs collectedfrom the quadrat (Fig. 4). The number of tunnels changedwidely, but the number of holes did not. A hole was notmade when no tunnel was made, but in some tunnels upto 12 holes were made (Fig. 4). No new burrows and fewelongated burrows were observed in this investigation.

Behaviour on the sand surface

In the daytime at low tide, 72 individuals were observed.The mean period between one pause and the next was10.1 s (SD = 6.2). These 72 crabs paused 354 times, 308times (87.0%) adjacent to the tunnel or the sand clods,and only 46 times (13.0%) on flat sand, even though thecoverage of the tunnels and sand clods was about 45% ofthe sand surface area.

The 72 crabs entered a tunnel or sand clods 99 times.Twelve (12.1%) subsequently went out onto the sandsurface with another crab. Finally, 65 of the 72 crabs(90.2%) entered a tunnel, five (6.9%) entered a hole andsubsequently made a burrow, and two (2.8%) made a newburrow in the flat sand. These results were consistent withthe fact that few burrows were made (Fig. 4).

In the daytime low tide, the proportions of tunnelsthat connected with a hollow shaft were 91.5% (65/71)before crabs entered the tunnels, 74.6% (47/63) whencrabs frequently intruded, 38.6% (22/57) slightly beforecrabs had retreated into the sand, and 0% (0/64) after theyhad retreated.

In the night-time low tide, 39 individuals were recorded.Ten crabs (25.6%) entered a tunnel, 10 (25.6%) entered

a hole, 13 (33.3%) entered sand clods, and six (15.4%)made a new burrow in the flat sand.

The proportion of the sites where the crabs retreatedinto the sand differed between daytime and night-time(χ2 = 51.8, d.f. = 3, P < 0.0001). The difference wasmainly owing to the difference in the proportion thatretreated into a tunnel.

DISCUSSION

The objective of this study was to reveal the matingsystem of Mictyris brevidactylus. The period when theycopulated was estimated by investigating changes inthe proportion of mature-sized females with calcifiedvulvar opercula and with eggs on the pleopods. Thevulvar opercula were calcified at the start of the invest-igation, but were decalcified before spawning (Fig. 2),and a hard-shelled female with decalcified opercula spaw-ned in its burrow. On the other hand, Yamaguchi (1976)observed one pair copulating in a burrow early in thebreeding season. These facts indicate that hard-shelledfemales of M. brevidactylus copulate within a short periodjust before spawning while their vulvar opercula aredecalcified. Thus, the above events probably occurredbetween late November and early December, althoughalmost mature-sized females had eggs in the ovaries a fewmonths before spawning (Nakasone & Akamine, 1981;Shih, 1993).

How did the soldier crabs get their mates? Duringthe study period, mainly large males emerged onto thesurface of the sand (Table 1, Fig. 3), and never showedobvious mating behaviour or copulation on the surfaceduring either the daytime or the night-time low tide. Thedensity of males with carapace length > 8 mm collectedin the sand, which corresponded with the size of the malesthat emerged onto the surface, was 6.3 (SD = 2.5, n = 16,

202 S. TAKEDA

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Fig. 3. Frequency distributions of the carapace lengths of male and female Mictyris brevidactylus collected on the surface of the sand.

range 2–11) individuals per quadrat in 2001 (Fig. 1).This density was less than the density of the emergenceholes, c. 9/quadrat, when many individuals emerged ontothe surface (Fig. 4). These results indicate that almostall large males emerged onto the surface. In addition,when the crabs were actively engaged in their surfaceand subsurface activities, the total number of tunnelsand holes was equivalent to the number of individualscollected from the quadrat. These facts indicate that thelarge females engaged in their subsurface activities whilemaking a tunnel, which is consistent with the observationthat few matured-size females emerged before and duringthe breeding season (Nakasone & Akamine, 1981).

As mentioned earlier, the behaviour pattern at low tidewas different between the large males and large females

in their breeding season; the males engaged in surfaceactivities, while the females engaged in subsurface activ-ities while making a tunnel, as in M. longicarpus(Dittmann, 1998). The large males intruded into tunnelsbefore the females had plugged their ascending shaft con-nected with the tunnel. In this case, no burrow wasnewly made in the tunnel, which indicates that theintruder male entered the ascending shaft. Perhaps theirunderground copulation (Yamaguchi, 1976) is related tothe intrusion of the male into the female’s ascending shaft,which allows both crabs to make a large enough burrowstructure (air chamber) for copulation (S. Takeda &Murai, 2004). In the breeding season, the tunnels madeby the females may function as a guide to the presence offemales.

The mating system of Mictyris brevidactylus 203

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Fig. 4. Sequential changes in the water (circles) and air (triangles)temperatures (top), water level of the daytime low tide (JapanMeteorological Agency, 2001) (middle), and the numbers of crabsMictyris brevidactylus (solid circles), tunnels (open circles), andemergence holes (triangles) (bottom). Break in the gashed and fulllines indicates a change in the investigation time from evening tomorning. N, new moon; F, full moon.

The proportion of females on the sand surface was lessin the night-time than in the daytime (Table 1). Moreover,the number of tunnels in the night-time low tide was clearlyless than that in the daytime (pers. obs.). These findingsindicate that a small number of females engaged in theirsubsurface activities in the night-time, although the waterlevel at low tide was lower then than during the daytime.The inactivity of the females during the night in the breed-ing season supports the possibility that they use visualsignals when mating. They never perform mating displayson the surface, however, unlike ocypodid and grapsidcrabs (e.g. Crane, 1975; Kitaura et al., 2002). The lackof a mating display in soldier crabs may be related to theabundance of visual predators, such as birds, because theforward-walking mictyrid crabs do not move as quickly assideways-walking crabs (Takahasi, 1935; Sleinis & Silvey,1980; S. Takeda & Murai, 2004).

Cameron (1966) observed that M. longicarpus repro-duces all year round, and that large individuals makeburrows and then retreat into the sediment withoutentering the tunnels. These observations suggest thatM. longicarpus does not use the tunnel as a guide formating. Their mating system might be more effective inrelation to their year-round reproduction.

The reproductive behaviours of the remaining twospecies of mictyrid crab have not been reported. Mictyrislivingstonei, however, is thought to hide, owing to its re-markably small eyes (McNeill, 1926). This considerationalso suggests that the mating system of M. livingstonei isdifferent from that of M. brevidactylus (this paper).

Ocypodid crabs inhabiting the same biotope displayvarious manners of mating (e.g. Crane, 1975; Christy &Salmon, 1984; Nakasone & Murai, 1998). In particular,the fiddler crab Uca beebei Crane and the ocypodid crabOcypode saratan Forsk build structures from the sedimentnear the burrow entrance that function as a guide to othercrabs (Linsenmair, 1967; Christy, 1988, 1995). The matingsystem is considered to relate to the phylogenetic constra-ints, inter- and intrasexual selection, and environmentalfactors of the habitat. To clarify the evolutional process ofthe mating system of the semi-terrestrial mictyrid, ocypo-did, and grapsid crabs, further information on the relation-ship between the mating system and reproductive ecologyis required.

Acknowledgements

I thank Dr M. Murai, Mrs. Y. Nakano and S. Nakamura andother staff of Sesoko Station, Tropical Biosphere ResearchCenter, University of the Ryukyus, for their help andfacilitating my work there. I am grateful to Dr M. Muraifor his invaluable suggestions for this study and two ano-nymous reviewers for reviewing the manuscript. Thisstudy was partially supported by a Grant-in-Aid forScientific Research (C) from Japan Society for thePromotion of Science (no. 12640605).

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