The Perplexing Fiddler Crab, Uca perplexa, is found throughout most of the islands of the western Pacific, from Japan down to Malaysia across Indonesia, the Philippines and Papua New Guinea, the eastern coast of Australia and out toward remote islands such as Fiji, Samoa, and French Polynesia. It’s name comes from the confusion that arose in trying to separate this species from closely related forms, particularly Uca annulipes and Uca lactea with which its range overlaps in many areas. In the field these species can be told apart somewhat by color: the major claw in Uca perplexa tends to be more yellow while that of Uca annulipes is more red, although there are other similar species which may also have red and/or yellow claws so its not an absolute indicator.
Because of its spread over so many remote Pacific Islands, in terms of raw area (as well as longitudinal range), it is one of the most widespread fiddler crab species, likely second only to Uca tetragonon (which resides in much of the same area, but is also common throughout the Indian Ocean as well). Given that most of its range area is open ocean, it’s an open question as to whether it actually occupies more shoreline than a widespread continental species such as Uca tangeri.