What is the natural history...evolution...of fiddler crabs?

Hi! Thanks for the question.

There is a great deal of information about the natural history of
fiddler crabs at the Fiddler Crab Info website. I will provide here
some snippets from the links mentioned here so as to save you time but
I highly suggest that you read the whole article so as to get a better
perspective.

“The earliest description of the type species of Uca is from a picture
by Seba (1758), which he called Cancer uka una, Brasiliensibus. A
number of authors subsequently used this same picture as a basis for
naming the species (Manning and Holthuis, 1981). Cancer vocans major
Herbst, 1782; Ocypode heterochelos Lamarck, 1801; Cancer uka Shaw and
Nodder, 1802; and Uca una Leach, 1814, are all objective synonyms,
because they are all based on the picture and description from Seba
(1758). Because of this, the official type specimen of the genus Uca
is Cancer vocans major. The earliest description of this species based
on actual specimens and not on Seba's drawing was Gelasimus
platydactylus Milne-Edwards, 1837.”

“Fiddler Crab Systematics”
http://www.fiddlercrab.info/uca_systematics.html 

Other interesting and important facts about the fiddler crab could be
found in the following links:

“Fiddler Crab Species”
http://www.fiddlercrab.info/uca_species.html 

“Fiddler Crab Phylogeny”
http://www.fiddlercrab.info/uca_phylogenetics.html 

“Common Names of the Genus Uca”
http://www.fiddlercrab.info/uca_names.html 

One of the most fascinating questions about the evolution of the
fiddler crab has to do with the size of its claws.

“One of the most fascinating questions in evolution is why some
species have evolved extremely large secondary sexual structures, such
as the antlers of the extinct Irish elk or the horns of beetles. In
all fiddler crab species the males have one enormous major claw,
nearly half of the body weight of the animal, which is used in combat
with other males and in complex waving displays that attract females
to the male's burrow. Why so large a claw? Does the claw detract from
efficient movement of the animal? Does the claw function in damaging
opponents, or is it merely a display device, with little function in
combat? The male's other claw is small and is used for feeding only.
Females have two feeding claws, which gives them an advantage in
gathering food.”

“Evolution of the Major Claw in Fiddler Crabs”
http://www.mhhe.com/biosci/genbio/raven6b/graphics/raven06b/studentpapers/s-research31.htm

A good easy to understand profile of the fiddler crab could be found
in the University of Washington website.

“So what's a Fiddler Crab”
http://faculty.washington.edu/~viscido/fiddler_crabs.shtml 

“Frequently-Asked Questions about Fiddler Crabs”
http://faculty.washington.edu/~viscido/fcfaq.shtml 

Search terms used:  
“fiddler crab” “natural history” evolution
 
I hope these links would help you in your research. Before rating this
answer, please ask for a clarification if you have a question or if
you would need further information.
 
Thanks for visiting us. 
 
Regards, 
Easterangel-ga 
Google Answers Researcher

Great answer..thanks!!