Semidome Building as Sexual Signaling in the Fiddler Crab Uca Lactea (brachyura: Ocypodidae)

During the mating season, males of the fiddler crab Uca lactea build semidomes of mud at the entrances of their burrows to which they attract females for mating in the upper intertidal zone. Related species build similar structures which either reduce aggression between neighboring males or attract sexually receptive females. Male U. lactea did not build disproportionately more semidomes as density increased, suggesting that these structures do not modulate aggression. Larger males built higher and wider semidomes as would be expected if the semidomes are a courtship signal. When the high tides were too low to cover their habitat and the sediment dried, males were unable to build new or repair existing semidomes. Towards the end of the mating season more small males built semidomes perhaps because large males prevented them from courting earlier when most females mated. We made two experimental enclosures, added males to one and males and females to the other, and monitored semidome building. Males built significantly more semidomes in the enclosure with females. Overall, our observations support the hypothesis that the semidomes of U. lactea are a sexual signal. Many species of intertidal brachyuran crabs build mud or sand structures on the surface of the sediment near or at their burrow entrances, such as hoods (Zucker, 1981; Clayton, 1988), pyramids (Linsenmair, 1967), mounds (Wada et al., 1994), chimneys (Wada and Murata, 2000), and mudballs (Oliveira et al., 1998). In many species, only reproductively active males build structures, which have been shown to reduce aggression between neighboring males (Zucker, 1974; 1981; Clayton, 1988; Wada et al., 1994) or to attract females to males’ burrows for mating (Crane, 1975; Christy 1988a, b, 1995; Oliveira et al., 1998; Christy et al., 2001, 2002). During the June to August mating season, males of the fiddler crab Uca lactea construct low semidomes at the entrances to their burrows in the upper intertidal zone (Fig. 1). Reproductively active males of 17 species of fiddler crabs (approx. 100 species) are known to build similar structures variously called hoods, pillars, domes, and semidomes, depending on their size and shape (Christy, 1988a; Christy et al., 2001). Semidome building by U. lactea in Japan was first reported as ‘‘shelter building’’ (Yamaguchi, 1971). Recently, Kim and Choe (2003) and Kim et al. (2004) considered semidomes to be indicators of courtship activity, although neither the function of these structures nor their physical characteristics were described in detail. Here we describe the semidomes of U. lactea and aspects of semidome building behavior in relation to the possible function of these structures for reducing aggression or attracting females. Some other fiddler crabs usually build their structures in the mid intertidal zone, where they are destroyed daily by tides (e.g., pillars in Uca beebei, see Backwell et al., 1995; hoods in Uca musica, see Christy et al., 2001). In contrast, the semidomes of U. lactea are built in the upper intertidal zone in the Korean intertidal mudflat above the reach of the tide for several days during the neap tides. We tested whether sediment moisture content affects structure building by destroying semidomes on days that the habitat was, and on days it was not, covered by the tide. We evaluated whether semidomes reduce aggression by monitoring semidome building over the natural range in male density. If males build semidomes in response to declining nearest-neighbor distances, then their numbers should increase disproportionately with density (Zucker, 1981). For example, given a random dispersion of burrows, the mean nearest-neighbor distance 1⁄4 1/2pdensity 673 JOURNAL OF CRUSTACEAN BIOLOGY, 24(4): 673–679, 2004