The Bonin Islands Fiddler Crab, Uca boninensis is fairly unique in that it is both endemic to the Ogasawara Islands of Japan (“Bonin Islands” is a historical name) and is not sympatric with any other species, being the only fiddler crab found on these islands. The only other fiddler crab with no sympatric fiddler associates is Uca tangeri, which has an immense range across most of the west coat of Africa and even the southern edge of Europe. There are a few other species endemic to small islands, but each of these overlaps with one or more other fiddler crabs.
Until its recent description, this species was thought to be Uca crassipes. Shih et al. (2013) identified the new species through clear genetic differentiation between it and other populations of U. crassipes; they also found some minor morphological differences between these two very similar species, although nothing so striking as to allow easy identification. Little is known about the ecology of Uca boninensis since almost all previous work simply noted its presence on these islands. It is a medium sized fiddler crab, found in the upper intertidal zone of the estuaries of small rivers, in fragmented rock and muddy-sand substrates.
Because this species is known from only a few remote islands in the Pacific Ocean, its existence is likely threatened by climate change and is a candidate for conservation evaluation and protection. The Ogasawara Islands contain many endemic, unique, and endangered species across all forms of life and were added to the UNESCO World Heritage List in 2011.