CRUSTACEAN RESEARCH，NO. 37: 74-79，200 8 Cheliped differentiation and sex ratio of the fiddler crab Ucαα rcuαta Takao Yamaguchi and Yasuhisa Henmi A bstrαct.--We analyzed atotal of male crabs were dominant among the adults 3，784 young fiddler crabs of Ucαα rcuα- with aCW between 15.1 and 20.5 mm. tα(D e Haan，1833) with carapace Johnson suggested that male bias in U. pugi- width s(CW) between 1. 45 and 9.95 lator could be the result of di旺' e rential mor- mm，collected from asite on the Ohno tality rates between sexes. Frith & River in Central Kyush u ，Japan. All Brunenmeister (1980) examined the sex crabs with CW< 3.0 mmpossessed two ratios and distributions of five fiddler crab small symmetrical ch elipeds. Young species at Phuket Island in Thailand and male crabs with aCW between 3.0 and found two to three times more males than 6.0 mmexhibited one of four stages of females (2.35 in Ucα urvillei (Milne- cheliped differentiation: 1) two small Edwards，1852) (94 males/40 females); 2.2 chelipeds; 2) two large chelipeds; 3) in Uca lactea (De Haan， 1835) (396 only one cheliped; and 4) asymmetrical males/181 females)). They concluded that it chelipeds (on elarge and one small). was necessary to know the sex ratio at the Weidentifie d646 male and 632 female hatching stage to understand sex ratio crabs with aCW between 6.0and 9.95 imbalances. mm. No statistical difference from the It is very important to know the sex ratio 1:1 ratio is evident. We conclude that of the fiddler crabs at the earliest possible the basic sex ratio for U. αrcuαt αi s stage. Without knowing such aratio ，it is unity. impossible to discuss and evaluate the sex ratios of adult crabs. However，in the earliest fiddler crab stages secondary sexual charac- Introduction ， teristics are undeveloped. Young Ucαc r a b s Johnson (2003) researched the sex ratios possess two small symmetrical chelipeds of fiddler crabs and published alist of and are impossible to separate sexually results from 25 studies that demonstrated (Morgan， 1923; Yamaguchi， 1977; male bias in most populations. To deterrnine Yamaguchi & Henmi，2001). Morgan (1923) apossible experimental source for this bias ， found agroup of somewhat larger juveniles Johnson compared the sex ratios for crabs of with two large chelipeds and assumed that Uca ρugilator (Bosc，1802) collected using they were males and that males developed pitfall traps (126 males:95 females) and cheliped asymmetry after losing one of their quadrat excavation (179 males: 144 females) . chelipeds (Morgan，1924). Yamaguchi The sex ratio was clearly male-biased for (1977) examined the cheliped condition of a both methods but it was also found to be sig- large numberof young U. lacteα( D e Haan， ni:fi cantly related to crab size. Females were 1835) and confirmed the existence of four dominant among the juvenile crabs with a types of cheliped combinations: 1) two small carapace width 5and 10 mm， chelipeds; 2) two large chelipeds; 3) one the sex ratio was close to unity for crabs cheliped only (either right or left cheliped with aC 羽T between 10.1 and 15 mm，and being lost); and 4) asymmetrical chelipeds SEX RATIO OF UCA ARCUATA 75 200 150 的 門 N =3 784 戸 ， υE ち 」 t E A AA ω u U 3 5 = z 50 。 2 3 4 567 8 9 10 Carapace width (mm) 亡二二コ with two small chelipeds (n =2 ，130) wi 出 two large chelipeds (n =240) ・園田with only one chelipeds (n =182) 園田園with asymmetrical chelipeds (n =1 ，232) Fig. 1. Cheliped composition of young crabs of Uca arcuata (De Haan，1833) collected at the Ohno Ri ver，Central Kyushu，Ja pan. (one large and one small). 官le young crabs crabs over the minimum size at which the with only one cheliped regenerated asmall di 宜erentiation of large cheliped is finished， cheliped in place of the lost one.百le remain- we might find atrue ，unbiased sex ratio. It ing cheliped continued to grow and became seems reasonable that no large sex-baised alarge cheliped，and thus the crabs attained di旺'erence in habitat preference or behavior cheliped asymmetry. Yamaguchi & Henmi existed between the two sexes of young (2001) also confirmed the presence of four crabs. The study of Yamaguchi & Henmi types of cheliped combinations in Uca arcua- (2001) dealt with too small sample size (total ta and showed that young males in the tran- n= 645; n= 87 > 5.6mm C"乃to assess sex sitional stages of cheliped development had ratios ，so we performed this quantitative aCW between 3.0 and 5.6 mm. The males sampling study of young U. arcuata to deter- with aCW > 5.6 mmwere fully di旺erentiat- mine reliable sex ratios . ed，with one small and one large cheliped. It is not possible to determine the sex Materials and Methods ratio of young crabs correctly before the di 壬 ferentiation of cheliped asymmetry in males Collection of young Uca arcuata was car- iscomplete. However，by examining young ried out at the Ohno River in Central 76 T. YAMAGUCHI &Y. HENMI Kyushu (Otani et αl. ，1997; Yamaguchi & Henmi，2001) from late August to mid Discussion October in 1997，2000 ，and 2004 through 2007. Young crabs were collected by exca- In aprevious paper (Yamaguchi & vating randomly selected burrows. After col- Henmi，2001) ，we examined cheliped d出er- lection ，crabs were put in jars containing 80 entiation in Uca arcuαta but could not ana- %alcoho l. Soon after retuming to the labora- lyze sex ratios because of the small sample tory ，we examined the cheliped condition size for the young crabs collected at two di 壬 and determined the sex from the cheliped ferent sites. In the current study，we exam- characteristics. The C 羽T was measured ined amuch larger number of crabs (n = using astereomicroscope equipped with an 3，784) collected at asingle site. From our ocular micrometer. Although the size di 宜er- quantitative study it became clear that the ence between the large and small chelipeds development of two large chelipeds could was not so prominent as in adult crabs，the occurred before the loss of one cheliped，the two chelipeds were distinguishable from smallest crab that developed two large che- each other: the manus of the large cheliped lipeds had aC 羽T of 3.05 mm，and the small- was well developed and broad ry amaguchi， est crab with only one cheliped had aCW of 2001; Yamaguchi & Henmi，2001). There 3.4 mm. However，not all young males devel- was no di 血culty to separate small males that oped two large chelipeds. Approximately 50 just finished cheliped di 旺erentiation from %of the crabs with only one cheliped kept a the crabs with two small or two large che- small cheliped. They dropped one of che- lipeds. lipeds before developing into two large che- lipeds condition. Otani et al. (1997) studied the population Results s甘ucture and growth of U. αrcuata at Ohno Theresults of the various cheliped condi- River. They conducted monthly collection of tions are presented in Fig. 1. Wecollected a the crabs in 1989 and 1990. Ovigerous total of 3，784 young crabs with aCW females were collected on 18 June 1989 and between 1. 45 and 9.95 mm. Of these，2 ，1 30 10 June 1990.τbey obtained small sex-unde- crabs had two small symmetrical chelipeds termined crabs on 17 August 1989 and 18 and were thus undifferentiated (male or August 1990. Wecollected the young crabs female). Two hundred forty crabs exhibited during the period from late August to mid two large symmetrical chelipeds and 182 October. Weestimate the age of young crabs possessed only one cheliped; these crabs obtained by us did not exceed four were males in atransitional stage of develop- months. They were small compared to the ment. The size of the smallest male with adult crabs and the large chelipeds of the completed di 旺erentiation ，reflected by che- males were not so large. Otani et al. (1997) liped asymmetry，had aCW of 3.4 mm. No reported that the minimum size of the young crabs with two large chelipeds or only ovigerous female was 19 .4 mmCW and the one cheliped had aCW of >6.0 mm. largest male and female had aCW of 36.8 Crabs with aCW between 6.0 and 9.95 mmand 33.6 mm，respectively. It is expect- mmconsisted of 646 males and 632 females ed that no large sexual di旺'erences occur in and sex ratio showed no significant devia- the habitat preference and behavior in the tion from unity (p > 0.05 ，Pearson's chi- crabs with aCW between 6.0 and 9.95 mm. square test). 百1 I s result demonstrated that Wedetermined that the basic sex ratio of U. the basic sex ratio of Uca arcuata is 1:1 and arcuαt αi s 1: 1. Furthermore，one of us no sex bias exists in this species of fiddler (Yamaguchi) examined atotal of 13 ，913 crab. young crabs of Ucαlactea (Yamaguchi， 1977) and also confirmed the four types of SEX RATIO OF UCA ARCUATA 77 cheliped conditions for this species. He narrow fronts; the other four，broad-front showed that all young crabs with aCW く 2.3 species. Beinlich & von Hagen (2006) per- mmhad two small chelipeds. The males formed an extensive study of 出e taxonomy with aCW between 2.3 and 5.3mm exhibit- of fiddler crabs. According to them the ed transitional stages with varying propor- genus Uca comprises about 94 species. They tions of undifferentiated males.