Fiddler crabs are social organisms that engage in elaborate mating displays before copulation. Males use their large claw to attract mates through a series of waving motions and acoustic drumming, also used to ward off potential competitors. Waving displays are often characteristic of a certain species, but usually occur at the mouth of the burrow in all crabs. In U. rapax, the large claw creates a weak circular, almost lateral movement, with a distinctive slow progression and jerking motion (Crane 1975). Jerking is always present, and the number of moves ranges from 8 to over 30, creating display series lasting up to 13 seconds each. Often, acoustic drumming and other sounds are produced by the claws and legs to attract females (Crane 1975). Mudflat fiddler crabs court and mate both during the day and at night. In daylight, waving displays by males are likely most important; whereas, acoustic signals predominate during nocturnal courtship. Once the male has attracted a mate, she usually follows him into the burrow for copulation, and few to no reports of aboveground mating have been documented for in U. rapax (Crane 1975). The resulting fertilized eggs are carried in a clump, often called a sponge, on the abdomen of the female until hatching. Ovigerous, or egg-bearing, females were seen throughout the warmer months in Brazilian populations (da Silva Castiglioni et al. 2007), and individuals were considered mature at carapace widths of about 1.4 and 1.2 cm for males and females, respectively (da Silva Castiglioni & Negreiros-Fransozo 2006).
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