Fiddler crabs are easily recognised by their distinctive asymmetric claws. This specimen was captured in May 1835 when the Beagle arrived in Mauritius.
Image: Oxford University Museum of Natural History [larger view].
The University of Oxford Museum of Natural History has electronically catalogued Charles Darwin's crabs that had been collected by the famous naturalist while he was making his voyage around the world on the HMS Beagle from 1831 to 1836.
These crustaceans were brought back to England where they eventually came into the possession of zoologist Thomas Bell, who was helping Darwin classify the Galapagos tortoise collection. Finally, the crab collection was purchased in 1862 by John Obadiah Westwood, Oxford's first Hope professor of zoology. The newly-purchased collection was housed at the Oxford Museum of Natural History, where it was neglected until very recently, when museum staff digitized it, along with the original labels that were hand-written by Darwin himself, and made this entire database searchable online for the public to use.
Most of these specimens were also referenced in Darwin's diaries, at least some of which have been published, where more information about some of the specimens is available.
free PDF from the University of Oxford Museum of Natural History. Includes images.
The fiddler crabs in our neighborhood often congregate on the street during the spring and summer. They are especially plentiful at the entrance where as many as 100 may be crushed by incoming and departing cars. My research on the internet has turned up interesting facts about these crabs but no mention of this behavior.